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Module 6 Materials and Hardware

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Owerview of major Materials and Hardware used in Aviation.

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Page 1: Materials and Hardware

Module 6 Materials and Hardware

Page 2: Materials and Hardware
Page 3: Materials and Hardware

CONTENTS

Page

Definitions Ferrous metals

Allay steels Non ferrous metals

Aluminium alloys Chemical abbreviations Identification of metals Practical tests Heat treatments - plain carbon steels

Case hardening Heat treatments - alloy steels Heat treatment - aluminium alloys

Destructive testing of metals Corrosion Non metallic materials

Plastics Fibre reinforced plastics Resins Cores Manufacture of composite components

Adhesives - general Destructive testing of composites Degradation of composites Sealants and bonding agents

Page 4: Materials and Hardware
Page 5: Materials and Hardware

MATERIALS - GENERAL

You should have a good knowledge of the following terms which are used throughout this book, so, using a piece of paper define each of their meanings. Take no more than about 30 minutes. The answers are given below.

* Stress * Density * Fatigue * Toughness * Brittleness * Hardness * Softness * Ductility * Malleability * Resistivity (p)

Stress. Defined as force per unit area. In the SP system it is the Pascal (Pa) and is defined as a Newton per square meter N/m2. In the imperial system it is pounds per square inch (psi). The higher the stress levels a material can take the better. Note that stress units are the same as pressure units,

Density. This is the amount of "substance" in a material. It is defined as "mass per unit volume" ie, Density = kgs/m3. Dense material such as lead is said (incorrectly) to be heavy. A kg of lead is no heavier than a kg of feathers. As an example aluminium has a density of 2700 kg/m3 and steel is 7900 kg/m3. For aircraft the less dense a material is the better - provided is retains those desired properties such as high strength etc.

Fatigue. Fatigue is associated with cyclic stress. All materials should be resistant to fatigue. Fatigue is serious and has been the cause of many aircraft accidents. Normally the stress level that causes fatigue failure is well below that required to cause the part to fail under normal tensile stress.

Toughness. This is the ability of a material to absorb an impact load. Rubber is tough - ordinary glass is not. Toughness is a good quality, without it metals would fracture at the slightest knock.

Brittleness. The opposite to toughness.

Hardness. The ability to resist scratching and indentation. Glass is hard, wood is not. Bearings and piston rings for example should be hard so as to resist wear.

Softness. The opposite to hardness. When two surfaces are in rubbing contact with each other, such as some bearings then one is usually made softer than the other so it will wear first - usually the easier one to replace.

Page 6: Materials and Hardware

Ductility. The ability of a material to be permanently deformed by the application of a tensile load. Wire is drawn into shape by being pulled through a series of dies and is said to be ductile (Drawn - Dies - Quctile).

Malleability, The ability of a metal to be permanently deformed by the action of a compressive load - hammering for example. Rivets are malleable as they are formed by compression.

Resistivity. This gives the resistance of a body in terms of its dimensions. It is called (p) rho. The resistance of an object can be found from the equation

Where p is in ohm metres, L is length in metres and A is cross sectional area in m2. Copper has a resistivity of 1.7 ohm metres whereas steel has a resistivity of 15 ohm metres. Copper is a better conductor than steel.

METALS

Metals can be divided into two main groups - ferrous and non ferrous.

FERROUS NONFERROUS

Fig. 1 METALS

Ferrous (Fe) Metals

These metals have an iron base and include all the plain carbon steels, allc steels, cast irons and wrought iron. A plain carbon steel is a steel which contains only iron (Fe) and carbon (C) between about 0.15% and 1.4% C.

WROUGHT LOW CARBON HIGH CARBON CAST IRON IRON STEEL STEEL

I I I I I .--

0 0.02 0.1 5 1.4 4.5 % CARBON

Fig. 2 PERCENTAGE CARBON IN STEELS & IRONS

Page 7: Materials and Hardware

Fe metals can be divided into 3 main groups - irons, plain carbon steels and alloy steels.

Fe METALS

IRONS PLAIN CARBON ALLOY STEELS STEELS

Fig. 3 Fe METALS

The following pages contain tables relating to properties and uses of metals used on aircraft. Some metals are almost never found on aircraft - such as cast iron - but they have been included because they are found in aircraft related engineering.

TABLE 1 - FERROUS METALS

MATERIAL PROPERTIES USES

Cast iron Brittle, weak, casts well, resists Machine beds, frames up to 4.5% C crushing. Good anti-friction and details. General

properties, self lubricating. castings, bearings. Good vibration damping Pistons, Piston rings. qualities. Density 7700kglm3.

Wrought iron Ductile, malleable, soft, easily Cores of dynamos, 0.02% C magnetised, easily welded. lifting chains, crane

Density 7800kg/m3. hooks. .................................................................................................

Mild steel Ductile, less malleable. Bolts and nuts. (low carbon) Stronger and harder than General workshop 0.15 to 0.3% C wrought iron. Easily forged, machined components.

welded, machined or stamped Girders, forgings, car to shape. Density 7800kg/m3. body panels. p = 1 5 o h m m .

Medium Higher strength than mild steel Leaf springs, wire ropes carbon steel and responds readily to heat general tools, axles, 0.3 to 0.5% C treatments to increase its crankshafts. Used in

toughness and hardness. high strength areas - fuselage joints, bolts, hinge pins etc.

Page 8: Materials and Hardware

TABLE I CONTINUED

High carbon More expensive than medium Cutting tools. Coil steel. carbon steel. Tougher and springs. 0.5% to 1.4% C harder.

Alloy steels By adding other elements the Chromium increases (See table properties of plain carbon steel hardness - ball bearings "Alloy Steels") can be altered. Nickel increases

strength and toughness also resistance to fatigue. Tungsten helps the steel to retain its hardness at high temperatures.

Alloy Steels

The main difficulty when studying alloy steels is that there is such a wide range of alloys that, trying to commit the details to memory, or even a small part of them, would be difficult. For this reason the included table is of the more commonly used elements used in steels to produce particular properties.

TABLE 2 - ALLOY STEELS

ELEMENT Yo QUALITIES USES .................................................................................................

NICKEL (Ni) 3-5 Increased hardness Case hardened parts. without loss of ductility. Easily worked.

27 Non-magnetic almost non-corrodible.

36 Non-magnetic. Has a Precision instruments. low co-efficient of "Invar" steel. linear expansion.

.................................................................................................

CHROMIUM (Cr) 3 Great hardness. Ball and roller bearings. 12- 17 Nearly non-corrodible.

MANGANESE (Mn) 1.5 Greater strength than Welds easily - acts as 5% nickel and harder a purifier. than 3% chromium.

12 Very tough. Parts exposed to "wear and tear".

Page 9: Materials and Hardware

TABLE 2 CONTINUED - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -. - - - - - - - - - - - - - - - - - - - - - - - - - - - - .- - - - - - - - - -

TUNGSTEN (W) U p to Very hard up to 600°C. 14% tungsten is used 20 in high speed steel

drills. Work at higher speeds and temperatures.

COBALT (Co) 12 With tungsten. Used in drills etc working at temperatures higher than 600°C.

35 Easily magnetised. Permanent magnets.

VANADIUM (V) 20 Increase strength Chrome-vanadium without loss of ductility. steels for valves and

other springs.

MOLYBDENUM (Mo) 2-4 Similar effect to tungsten. .................................................................................................

NICKEL & 1-2 Stainless steel Magnetic. CHROMIUM 18-8 Stainless steel None magnetic.

3-5 Great strength toughness Gears, crankshafts engine and airframe parts.

-

INVAR Contains 36% Ni, and Precision instruments 64 Fe. Has a low and gauging systems. co-efficient of linear expansion (0.9). (Mild steel has a co-efficient of 15.0).

STAINLESS STEEL Almost zero rate of Structures - where heat corrosion. Typically resistance is required. contains 18% Cr & 8% Ni, Pipelines. though other grades of "non corrodible steel" are available.

AUSTENITIC STEELS & There are several Same uses as above. IRONS austenitic steels but

most are based on 18:8 stainless steel. Besides the qualities of stainless steel they are non magnetic.

Page 10: Materials and Hardware

TABLE 2 CONTINUED

VALVE STEELS For aero engines, usually Valves. contain 13%Ni, 13% chromium and 3% tungsten. Good resistance to scaling at dull red heat temperatures.

HIGH SPEED STEELS Typically contain 18% Drills. tungsten, 4% chromium Hacksaw blades. and 1% vanadium. Will work at higher temps. than high carbon steel without affecting the temper.

PERMANENT MAGNET May contain up to 35% Permanent magnets. STEELS Cobalt. Various trade

names are available eg Columax contains 8% Al, 14% Ni, 23% Co and 3% Cu.

-------------------------------------------------------------------------------------------------

HIGH PERMEABILITY Soft iron was used Transformer cores. STEELS but metals such as (Those that can be Permalloy (78% Ni) and magnetised and Mumetal (75% Ni) are de-magnetised easily) now more common.

TABLE 3 - NON-FERROUS METALS

MATERIAL PROPERTIES USES ................................................................

TITANIUM ALLOYS High strengthlweight ratio. Good physical properties and corrosion resistance. Density 4500kg/m3. p = 2.6 ohm m. Tensile strength up to 1300MPa. Works at temp. up to 480°C.

Used to replace steel with a saving in weight. Used for compressor and fan blades in turbine engines. Fire proof bulkheads. Heat shields.

Page 11: Materials and Hardware

TABLE 3 CONTINUED

NICKEL Hard, ductile, Anti corrosive. Corrosion resistant. Withstands high temps.

NICKEL ALLOYS Good strengthlweight ratio. Turbine blades and hot Corrosion resistant at high end fittings. temperatures. Monel tensile strength up to 1 170MPa. Some alloys contain 80% Ni 20%Cr.

MAGNESIUM Soft. Poor corrosion Bombs and flares. resistant. Light alloys.

MAGNESIUM ALLOYS Cast well. Prone to Aircraft wheels, and corrosion. Alloyed to give airframe structures. it strength as pure magnesium is weak and soft. Density 1800kg/m3. Will burn in under some conditions, particularly when in powder or swarfe form.

COPPER Tough, ductile, malleable. Tubing. Electrical High thermal and conductors. Used as electrical conductivity. a base for brass and resistant to corrosion bronze. Solders well. Density 8900kg/m3. p = 1 . 7 o h m m Weak - about 200 to 400MPa. Different coppers classified by CDA (Copper Development Association).

BRASS Contains copper, zinc, tin, Lightly stressed manganese, lead, nickel, castings, pipe fittings, aluminium, and silicon. tubing, filter elements, Good wearing, anti-friction bushes, electrical and corrosion resistant. contacts. Density 8500kgIm3. Some brasses have a tensile strength up to lOOOMPa

Page 12: Materials and Hardware

TABLE 3 CONTINUED

BRONZE Copper, tin, nickel and Bearing bushes Lead dloy. Similar properties to brass.

PHOSPHOR Copper, tin and Bearing bushes. BRONZE phosphorous. Stronger

and good in compression. .................................................................................................

TUNGUM Contains copper, zinc, Pipe lines. aluminium, nickel, silicon. Radiator matrix. Resistance to fatigue and Not in common use. corrosion. Strong and ductile.

---

LEAD Soft, weak, ductile. Counter balance a n ~ Density 1 1300kg/m3. mass balance weigh. , .

Alloyed to make solder.

TIN Soft, ductile corrosion Used for tin plating. resistant. Alloyed to lead to make

solder.

SOLDER Tin and lead. Low melting Soft soldering. point. Density 9000kg/m3.

ZINC Soft. Good corrosion Protection of steel resistance. Density parts. 7 1 OOkg/m3.

.................................................................................................

ZINC ALLOYS Low cost low melting Inexpensive comme* :a1 point castings. small parts.

DEPLETED Hard. M a s s balance weights, URANIUM Density 19,00Okg/m3. now removed for safety

reasons.

GOLD Soft. Used for plating some Density 19,300 kg/m3. electrical contactors. p = 2.4 ohmm Gold film windscreens. Good corrosion resistance.

MONEL METAL Contains 70% Ni, and 30% Some structural uses Cu. Resistant to corrosion. and tucker pop rivets.

Page 13: Materials and Hardware

TABLE 3 CONTINUED

CADMIUM Corrosion resistant. Anti corrosive plating. - - - - - - - - - - - - - - - - - - - - -, -. . - - - - - - - - - - - - - - - -, -. - -. - - - - .- - - - - - - - - -. - - - - - - . - - - - - - - - - - - - - - - - -. - - - - - - - - - - - - - - - - -.

ELEKTRON Magnesium, Aluminium Wheels, crankcases 1 I.%, Zinc 3.5%, Manganese 2.5%. Requires heat treatment. May be cast or wrought.

------------------------------------------ ------------ ---- ------ ..

ALUMINIUM See table 4 Wheels, castings, and its alloys. aircraft structure

Super Alloys - also non-ferrous

This class of metals is mainly based on nickel and cobalt (Inconal for example) with strengths u p to 1450MPa. They are expensive, difficult to form and machine but meet the needs for strength and operating conditions

Aluminium Alloys - non-ferrous

These are supplied in the wrought or cast form and may be heat treatable or non heat treatable. The British Standards cover:

BS 1470 to 75 - Wrought BS 1490 - Cast BSL Series - Aircraft DTD Specifications. - Aircraft

(DTD = Directorate of Technical Development).

An American coding system for wrought alloys is based on the main alloying element as follows:

CODE MAIN ALLOYING ELEMENT

None - 99% pure aluminium Copper Manganese Silicone Magnesium Magnesium 8r, silicon Zinc Other

Page 14: Materials and Hardware

The first digit indicates the main group, the second digit indicates any modification to the original alloy and the last two digits indicates the actual alloy in the group or the impurity level.

Example 1. Duralumin and any suffix after the forth digit would indicate, for example:

20 1 7.- 0 = Annealed wrought Duralumin. - T2 = Annealed cast Duralumin. - T6 = Solution treated and artificially aged Duralumin.

Example 2. 2025-H4 indicates aluminium copper alloy (2xxx), original alloy (xOxx), with 4.5% copper, 0.6% manganese and 1.5% magnesium (xx24) and strain hardened (xxxx- H4).

Strain hardening is not used much on A1 alloys used on aircraft. To modify +lie properties of A1 alloys heat treatments are used.

7 series alloys have a strength approaching that of steel and are widely used on aircraft. Aluminium-lithium alloys are now being developed that have a 10% lower density (lighter) and are up to 20% stronger than existing A1 alloys. (this would make a weight saving on the construction of a Boeing 747 for example of about 14,000 Ibs (6400kg).

The codings are specified on the metal specification (packets for rivets), drawings etc and details of what they mean found in tables.

Some A1 alloys will increase their strength with time after heat treatment (age hardening), others require precipitation heat treatment to bring on the process and some alloys will not age harden at all. (Refer to the section on Heat Treatments in this book).

A1 alloys generally have the following properties:

* Good strengthlweight ratio. -k Fatigue limited (see the section in this book "Testing of Metals"). * Notch sensitive (a small scratch is liable to develop into a crack). * Less corrosion resistant than aluminium. * Less malleable and ductile than aluminium. * Good thermal and electrical conductivity (p = 5). * Up to 8 times stronger than aluminium with little or no increase in

weight. (Density = 2800 kg/m3, aluminium = 2700 kg/m3).

Page 15: Materials and Hardware

TABLE 4 ALUMINIUM & ITS ALLOYS -. - - - -. - - - .- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - .- -- - - - .- - - - - - - - .- - - - - - -. - - - - - - - - - ,- - - - - - - - - - - - - - - .- - - - - - - - - -. - -

MATERIAL PROPERTIES USES

ALUMINIUM Soft, malleable, corrosion resistant. Used in light alloys as High electrical and thermal a base material, and conductivity. Little strength. used for cladding. Used Density 2700kg/m3. (Compare as a conductor. mild steel at 7800kgIm3).

DURALUMIN Nearly as strong as mild steel. Structural parts. (Wrought) Density about 113 that of steel. Sheets, rivets, tubes.

Must be heat-treated. "Age Hardens". Aluminium. Copper 4.5% Magnesium 0.7% Manganese 0.7% Silicon 0.7%

ALCLAD Dural sheet with coating of (Wrought) aluminium. Corrosion resistant

Heat treatment as above.

ALPAX Aluminium. Silicon 13.0% Intricate castings. (Cast) Iron 0.6% Manganese 0.5% aircraft and engine

Zinc 0.1% Good for casting. parts. Strong. Low thermal expansion. Fair corrosion resistance.

"Y" ALLOY Aluminium Copper 4.5% Pistons and cylinder (Cast) Nickel 2.3% Magnesium 1.7% heads,

Resistant to corrosion and fatigue. Must be heat treated. "Age hardens". Withstands relative high temperatures.

HIDUMINIUM Copper 2.5% Nickel 1.5% Aircraft structures. Magnesium 1.2% Iron 1.5% Pistons and cylinder Silicon 1.0% heads. Strong as mild steel. Requires heat treatment and low temp. heat treatment to age harden.

2000 SERIES Damage tolerant. Used in critical Al ALLOYS structural areas.

7000 SERIES Main alloying element - zinc. Used in strength critical areas.

Page 16: Materials and Hardware

TABLE 4 CONTINUED ---- --------- -------

LITHIUM Improved strengthlweight Several types being BASED ratio. produced for newer A1 ALLOYS aircraft to replace

both the 2000 and 7000 series.

Chemical Abbreviations

These are used extensively within the industry and while you need not remember them specifically you should have some knowledge of the more commonly used terms eg:

Aluminium Carbon Cadmium Cobalt Chromium Copper Iron Magnesium Manganese Molybdenum Nitrogen Nickel Oxygen Lead Tin Titanium Vanadium Tungsten Zinc

IDENTIFICATION MARKINGS ON METALS

The CAA specifies that materials used in the manufacture of aircraft parts shall comply with at least one of the following specifications:

* British Standard Aerospace Series (BSAS) Specifications. * DTD Specifications. * Specifications approved by the CAA. * Specifications prepared by an organisation approved by the CAA.

Page 17: Materials and Hardware

BSAS and DTD specifications make provision for the material to be marked by the inspector as well as other markings to ensure full identification.

Marlung Methods

Materials during manufacture should be marked as soon as possible during their production run with one or more of the following methods:

(a) Metal stamp marking (not usually on titanium). (b) Markings produced by a die or mould used in the shaping

of the metal. (c) Marking by rubber stamp, roller, or printing machine. (d) Using a colour scheme.

The marking should not be easily removed and should not damage the metal. Stamp marking should not be used on:

* Stressed parts where the stamp might cause stress concentration. * Thin section metals. * Metals of hard surface finishlspecial surface finish. * Parts or materials machined to close tolerances.

Standard Colour Scheme

A widely used system for the identification of metals is the standard colour scheme. The scheme is additional to any identification requirements laid down in the various specifications. If the colour scheme has not been applied by the manufacturer then it should be applied by the operator before the metal is placed in bonded store.

An alternative method to colour coding is overall marking. The metal - usually in sheet form - is printed all over with the material specification eg, BSL 72 (L72). The metal must, of course, be printable and the metal must not be affected by the print.

The colours may be applied as a band or bands across the corner of sheet metal bearing the identification stamp. On some sheet metals the bands may be painted near one edge of the metal and at right angles to it.

Strip material will have the bands painted on one end, or in some cases on both ends. Some sheet metals have a coloured disc 3" (76mm) diameter painted on them with additional colours added as concentric rings 1.5" (38nlm) wide. For material in coil form the colours will be marked a t intervals.

Page 18: Materials and Hardware

Protective Film Treatments

All metals are required by regulation to be capable of storage without deterioration. This means that if there is any chance of corrosion etc occurring during storage then the metal must be given an anti-corrosive treatment sufficient to protect it during the expected storage life.

This means that most metals are required to have an additional protective film treatment applied as soon as possible after production. This may be a clear film, but if i t is coloured, such as red lanolin resin, then an additional band of black paint is added to the colour scheme and the protective film is added up to the black band.

Some metals which differ only in surface condition or intended usage but are the same basic material are given the same colour code. Metals with the same specification but with different heat treatment conditions or properties havc different colour codes.

When the specification number of a material is changed eg, from a DTD number to a BS number, then the colour code will not be changed unless there is a significant change in the material itself.

Colours

Current colours used are: black, blue, brown, green, red, white, yellow (and violet for aluminium rivets).

Heat Treated Material

Material that is released in a heat treated state other than that stated in th specification must be marked in red with the appropriate term to denote thp condition. (See Heat Treatment Symbols in the book entitled "Drawing" in ~ l l i s series). The Approved Certificate must also be annotated.

Examples of the terms used.

(a) AS ROLLED (b) ANNEALED - material in its softest condition.

(c) NOT AGED - material solution treated and requires precipitation treatment. (See heat treatments).

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The Identification Marking

The marking should contain the following information:

A The specification number. A The inspection stamp (where necessary). * The manufacturer. A Batch number (and cast number where appropriate). * Test report.

General

Always use material specification as laid down in the aircraft maintenance/repair manual (SRM). If the correct material is not available check the alternative spares list, and if that does not help contact the aircraft manufacturer.

2. Always positively identify the material from the colour coding /specification numbers. If in the stores also check specification with the Approved Certificate/EASA Form 1 and/or other documents from the manufacturers.

3. If the material has to be cut (sheet or strip) and used in smaller piecesthen always cut from the sidelend furthest from the identification. This does not apply to "all over marking" material.

4. If in doubt about the identification of a piece of metal then it is not to be used on aircraft.

5. The following two tables are practical workshop tests for the identification of metals and are not to be used for the identification of metals to be used on aircraft.

blank

Page 20: Materials and Hardware

PRACTICAL TESTS

TABLE 5 - PRACTlCAL TESTS FE METALS

NOTE WHEN BEHAVIOUR APPEARANCE TYPES OF METAL DROPPED ON WHEN OF SPARK

ANVIL CHIPPED FMCTURE FROM A GRINDlNG WHEEL

Grey No ring Chips easily Dark grey Dull Cast crystals of red Iron uniform size non-

bursting. ............................................................................................ --

Wrought Low pitch Easily chipped. Course Bright Iron ring. Chips bend. fibrous grain. yellow

non- bursting.

Low Medium Bright silvery Bright Carbon pitch ring. as above large crystals yellow few Steel carbon

bursts.

High High pitch Harder to chip Pale grey, fine Bright Carbon ring. than low carbon crystals. yellow, all Steel steel, but chips bursting.

bend.

Tungsten Very high Will not chip. Silky blue grey Red no1 Steel pitch ring. fine crystals. burstin.

following the wheel.

Austenitic Steels Non magnetic .-----

Stainless Steel Copper is not deposited when copper sulphate solution is applied.

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TABLE 6 - PRACTICAL TESTS NON FE METALS

METAL TEST

Aluminium White in colour, light and mon-magnetic. Soft and bends easily. Caustic soda turns the surface white.

Alclad More springy than aluminium. Caustic soda turns the surface white and the edge black.

Duralumin The same properties as Alclad except that the application of caustic soda turns the surface black.

Magnesium White in colour, lighter than aluminium and non magnetic. Alloy Fillings ignite in a flame. Copper sulphate causes

effervescence and the surface to turn black.

Solder White, heavy and soft. Non-magnetic and melts with a soldering iron. Will mark on paper and crackles when bent.

Titanium Lighter than steel. White sparks when held against a grinding wheel.

HEAT TREATMENTS

Metals may have their properties changed by alloying. Alloying can give a metal:

* Better anti-corrosive properties. A Better strength and fatigue resistant properties. * Better macbineability, casting and heat treatable properties.

The heat treatment of a metal normally involves heating the metal to a specific temperature and then cooling at a specific rate. Heat treatments can produce the following properties:

* Increase strength. * Increase hardness. * Increase softness. * Increase toughness. JC Increase "springiness".

Some heat treatments can affect the anti-corrosive properties of a metal though they are not normally heat treated for this reason.

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HEAT TREATMENT OF PLAIN CARBON STEELS

QUESTION What does "plain carbon" mean? (2 mins)

ANSWER A steel containing Fe and C only

When steel is heated its temperature increases steadily until it is momentarily checked at the "critical" or "arrest" point. At this point the metal absorbs heat and changes occur in the structure of the metal, without temperature rise. After this period has passed the temperature continues to rise as before.

If steels having different carbon contents are heated in this way and the "arrest" points plotted on a graph, and if all these points are joined an Iron Carbon Equilibrium Diagram is produced (figure 4).

Line AEB of figure 4 represents the lower arrest points and is called the Low.,r Critical Point (LCP), and line DEC represents the higher arrest points and i, called the Upper Critical Point (UCP). Most of the heat treatments that are carried out on plain carbon steels relate to the LCP and UCP temperatures on the iron carbon equilibrium diagram.

0 0.2 0.4 0.6 0.8 1 .O 1.2 1.4 1.6

% CARBON

Fig. 4 IRON CARBON EQUALIBRIUM DIAGRAM

Micro Structure of Steels. When viewed under the microscope the micro structure of plain carbon steel looks similar to the views shown in figure 5. The ferrite is pure iron and cementite is iron carbide, Pearlite gets its name from its pearl-like appearance (under the microscope) and is made up of fine plates of cementite and ferrite. When heated to just above the LCP (line AEB on the graph) about 700°C, the pearlite changes to austenite. The ferrite and cementite does not change.

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AT 0.87% C ALL PEARLITE

CEMENTITE

ABOVE 0.87% C

Fig. 5 MICROSTRUCTURE OF STEELS

When heated to higher than the UCP (line DEC on the graph) the metal goes into what. is called Solid Solution where the whole structure becomes austenite.

Hardening. Produces a hard brittle steel. Heat to just above the UCP for steel up to 0.87% C, and just above the LCP for steels with a higher carbon content. Quench in water. Slower quenching produces a tougher (and not so hard) steel.

Tempering. Relieves the brittleness in a hardened steel. Reheat a hardened steel to between 200 to 300°C and cool or quench. (The cooling rate is not critical). Some steels are heated to 600°C which produces a high strength steel, tough and with good ductility. In general the higher the temperature the less the hardness and the greater the toughness.

Examples :

* Some structural steels 600°C (Tough). -k Springs 300°C. J; Drills, taps and dies 240°C. (Hard)

Annealing. This will refine the structure of the steel and convert it to its softest possible state. Heat to the same temperature as for hardening but cool as slowly as possible by leaving the part in the ashes or furnace and allowing the furnace to cool naturally.

Normalising. This process allows the structure to be refined back to its normal condition after working. When the metal is cold worked internal stresses are set up which make it weak and brittle, to relieve this condition normalisirig is carried out. The steel is heated to its annealing temperature and allowed to cool naturally in still free air. On low carbon steel Low Temperature Normalising may be carried out using a temperature about 500°C.

Page 24: Materials and Hardware

Refining. Prolonged heating above the UGP can cause the grain structure to coarsen (the grains to get bigger and the structure more brittle), so refining will reduce the size of the crystalline structure, reduce the brittleness and increase the toughness.

This process is usually carried out on steels that have been case hardened.

Heat to about 900°C and quench. Repeat the process 2 or 3 times but with a lower temperature each time.

CASE HARDENING

Applied to low carbon steels to produce a hard wearing "outer skin" whilst still retaining a tough inner core. The process is normally carried out in the following sequence:

1. Carburising (eg introducing extra carbon into the "outer skin" c- the metal).

2. Annealing - slow cooling from the carburising temperature. 3. Refining (as described above). 4. Hardening. 5. Tempering - as necessary.

Methods of Carburising

Open Hearth. Hezt the part to a cherry red and dip in a box of carburising compound (Kasenite). Repeated 3 or 4 times to give a "case" of about 0.005in (0.13mm) thick.

Box Process. The parts are packed in Kasenite (a carbon rich compound) in sealed metal box and heated u p to 900°C. Four hours at this temperature produces a case thickness of 0.040in (1.02mm).

Cvanide Hardening. The part is placed in molten sodium cyanide at 920°C to produce a case of about 0.0 loin (0.25mm).

Nitriding. Used on certain alloy steels containing aluminium and chromium called Nitriding Steels. The parts are heated to 500°C in a box through which is passed ammonia gas. Produces a case thickness of 0.030 in (0.76mm) after 90 hours. The low temperature and the fact that there is no quenching required means that there is less likelihood of distortion.

Page 25: Materials and Hardware

HEAT TREATMENT OF ALLOY STEELS

There is such a wide range of alloy steels that it is difficult, if not impossible in a single book, to describe all the heat treatments that may be carried out. The following table attempts to give some idea as to the range of alloy steels and the Hardening and Tempering heat treatments,

TABLE 7 - HEAT TREATMENTS OF ALLOY STEELS

METAL APPROXIMATE HARDENING COOLING TEMPERING COMPOSITION TEMP ("C) ("c)

Pearlitic 1.5% Mn 840 OIL 650 Manganese 0.4% C Steel 0.5% Mo

Austenitic 14% Mn 1000 WATER Manganese Steel 1% C

Pearlitic 5% Ni 860 OIL 600 Nickel Steel 0.4% C

Pearlitic 810 OIL 150 Chromium 1.4% Cr Steel (En 3 1) 1% C

Low Alloy 5% Ni 850 OIL 600 Nickel 1.5% Cr Chrome Steel 0.35% C

Stainless 13% Cr 950 Steel 0.4% C

OIL 600

.................................................................................................

Stainless 18% Cr 950 OIL 650 Steel (S80) 2% Ni

0.1% C

Stainless 18% Cr 950- 1075 OIL 100-750 Steel (1818) 8% Ni

High Speed 18% W 1320 OIL 550 Steel 4% Cr

1% V, 0.6% C

Again, the heat treatment is carried out using an Equilibrium Diagram which is more complex than the Iron Carbon Equilibrium Diagram shown above.

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Note. Nickel-chrome steels are prone to a defect known as "temper brittleness" when being tempered through the range 250°C to 400°C. The problem - which is not fully understood - causes a marked reduction in the toughness of the metal and, to make things more difficult, can only be revealed by destructive testing of test pieces after the heat treatment process.

HEAT TREATMENT OF ALUMINIUM ALLOYS

The heat treatments that can be carried out to A1 alloys are as follows:

(a) Solution Treatment. Initially makes the metal soft but allows the process of age hardening to occur.

(b) Precipitation Treatment. Only carried out after solution treatment and accelerates the process of hardening.

(c) Annealing. Makes the metal soft for working.

The process of heat treatment requires the metal to be heated for a specifie~ time at a specified temperature then cooling or quenching in a specific way.

IMPORTANT NOT ALL ALUMINIUM ALLOYS CAN BE HEAT TREATED AND THOSE THAT CAN MUST BE HEATED TO SPECIFIC TEMPERATURES WITHIN SPECIFIC TOLERANCES. TO HEAT TREAT A PARTICULAR ALUMINIUM ALLOY REFERENCE MUST ALWAYS BE MADE TO THE APPROPRIATE SPECIFICATION/PROCESS DOCUMENTS.

Example: To heat treat L'72 refer to British Standards BSL72. It will specify treatments, temperatures and cooling methods. For example, Solution Heat Treat to 495°C + 5°C. The soaking time will be specified as well as the quenching process. Note the temperatures here are quite specific, with most ferrous metals, temperatures may be approximate to 30" or so.

Solution Treatment

NOTE. The term has n0thin.g to do with putting the metal into a salt bath or any other type of solution - other than for cooling/quenching. The metal maq be heat treated in a salt bath but it is more convenient to use an electric oven.

This will soften the metal for a short period only and will allow the metal to age harden - with an increase in strength. The metal is heated to a specific temperature usually within the range 460°C to 540°C for a period of time then quenched in cold or boiling water.

Rivets so treated must be formed (used) within 2 hours of treatment. They will attain their design strength in 2 to 4 days (see graph). Metals can be lightly fabricated/ bent within this period.

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PRECIPITATION TREATED

t STRENGTH

TIME --t

Fig. 6 GRAPH OF STRENGTH AGAINST TIME

Precipitation Treatment

This process, where specified, will greatly accelerate the rate of age hardening. The metal may attain its design strength within 2 to 20 hours. After precipitation the strength of the metal is greater than if it is allowed to age harden naturally (see figure 6).

Precipitation heat treatment temperatures are low, usually within the range 100°C to 200°C and so&ng times may be up to 20 hours. Cooling may be by quenching in cold water or cooling in still air.

Annealing

This permanently softens the metal for working (unless heat treated further). In many cases it also makes the metal more prone to corrosion.

In general the metal is heated to a specific temperature within the range 360°C to 420°C and after the soaking time, allowed to cool in still air.

Refrigeration

To slow down the process of age hardening the metal may be refrigerated immediately after solution treatment. For example, rivets previously solution treated can be kept in a cold storage cabinet next to where the work is being carried out. Rivets removed from the cabinet must be used within 2 hours. Storage time will depend on temperature eg, minus 20°C, storage time up to 150 hours.

I t is common in the industry to use a domestic freezer

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Doing it this way means that when doing a big repair a large quantity of rivets can be heat treated in the heat treatment shop, which may be the other side of the airfield. The rivets can then be put in the freezer close to where the work is being carried out and used, 2 hours worth, at a time.

QUESTION If a rivet has to be heat treated, can you work out what sort of heat treatment would be carried out? (1 0 mins).

ANSWER Lets analyse the wrong answers first. We cannot heat treat rivets that have already been formed. Made up parts, riveted plates etc, must not be heat treated because they will warp due to contraction/expansion. This means we cannot anneal the rivet so as to make it soft for working as it would be required to solution treat after forming - and that's not possible. If we precipitated the rivet it would make it too hard to form.

So the only treatment we can carry out - if allowed by the specification - is solution treatment. And the rivet must be used within 2 hours - or put in a refrigerator straight away. It must be used within 2 hours of removal from the refrigerator.

Soaking Times

This is the time the part is kept in the over/salt bath a t the specified temperature. In general the larger the part the ionger the soaking time - but do not over soak. Times, for exaxple are:

26 SWG sheet (0.18" 0.0457mm thick) 10 mins rivets 15 mins

16 SWG sheet (0.64" 0.1163mm thick) 25 mins

Quenching

Always quench or cool in accordance with the specificatian. The quenching methods listed below start with the fastest method first.

1. Brine (salt water) 2. Cold water (not warmer than 20°C). 3 . Hot water. 4. Oil. 5. Still air. 6. Warm oven.

Most cooling/quenching for aluminium alloys is 2 or 5 above.

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Methods of Heating

1. Thermostatically controlled electrically heated ovens. 2. Air heated furnaces. 3 . Salt baths, These use salts that melt at high temperatures and

have significant safety issues attached to their operation.

Limitations on Heat Treatments

Clad aluminium alloy sheet should not be heat treated more than 3 times. Riveted up, bolted and joined sections should not be heat treated. Only heat treat A1 alloys where it is laid down in the specification for that metal.

Cleaning

[t is most important that parts treated in a salt bath should be thoroughly cleaned after treatment (the salts are highly corrosive). The parts should also be thoroughly cleaned prior to putting in a salt bath because a dirty part can cause a violent reaction with the molten salts (effectively a small explosion). If splashed with molten salts wash off immediately and seek medical advice. Some salts can be up to 600°C and will cause severe burns.

Also parts quenched in brine must be thoroughly washed and dried as it is also corrosive.

Rivets

These are usually placed in a wire basket for treatment. If any treatment is dlowed for a specific rivet it will be solution treatment.

Identification Of Heat Treated Conditions

Immediately after the material has been heat-treated, it should be marked with the appropriate symbol denoting the treatment to which it has been subjected. Rivets should be put in a bag and labelled.

There are two identification systems in general use in the UK ie, that recommended in British Standards 1470 to 1477 and that recommended by in SP4089.

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Identification System Recommended in British Standards 1470 - 1477

Material in the annealed condition.

Material in the "as-manufactured" condition, e.g. as rolled, as extruded, straight and/or drawn to size, or a s forged, without subsequent heat treatment of any kind.

Material which has been annealed and lightly drawn (at present applicable only to rivet, bolt and screw stock).

Material which has been solution-treated and requires no precipitation treatment.

Material which has been solution-treated and will respond effectively to precipitation treatment.

Material which has been solution-treated and precipitation- treated.

Material which has been drawn after solution treatment (at present only applicable to wire).

Material which has been precipitation-treated only.

TESTING OF METALS

Various tests are carried out on metals (and other materials) to ascertain the material's properties in terms of strength, toughness, hardness, etc. The te: - are normally destructive in that they damage the metal in some way. Each process normally tests for one property. The tests are carried out in a laboratory with special test equipment and qualified personnel.

THE TENSILE TEST

An accurately machined test piece is placed in a machine and stretched under a tensile load until it breaks. This test provides data on:

A Ultimate tensile strength (UTS). J; Yield point. x Elastic limit. * Modulus of elasticity etc.

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When the test piece is stretched, during the early stages it behaves elastically. In other words if the load were removed the test piece would return to its original length. In the later stages it behaves plastically - in other words it takes on a permanent stretch (or permanent set) so that if the load were removed the test piece would stay at its "new" length.

A graph is plotted of load (stress) against extension (strain), and from this graph certain facts can be ascertained.

For mild steel the elastic limit is well defined, as is the yield point where the metal takes on a permanent set. Of course in normal use the part will not be loaded past its elastic limit.

The ultimate tensile strength of the test piece is shown where the graph is at its highest. This is the highest load the metal will take before it breaks.

-PLASTIC DEFORMATION I I (PERMANENT DEFORMATION) ULTIMATE TENSILE STRENGTH

LOAD OR

YIELD POINT

STRESS

ELASTIC EXTENSION The test piece will return to its original length when the load is removed

EXTENSION OR STRAIN

Fig. 7 GRAPH OF STRESS AGAINST STRAIN FOR MILD STEEL

Proof Stress

Some metals, when tested, do not show a marked elastic limit and yield point, therefore it is difficult to compare the test results of one specimen with another. For this reason values are recorded of Proof Stress.

Proof stress is that stress that is required to produce an elongation of the test piece by 0.1% of its original length (0.1% Proof Stress). For 0.2% Proof St-ress the change in length is 0.2%.

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0.1% OF GAUGE LENGTH

Fig. 8 METHOD OF DETERMINING 0.1% PROOF STRESS

HARDNESS TESTING

QUESTION Define Hardness (2 mins) .

ANSWER Hardness is the ability to resist scratching and indentation.

There are several different test methods available, and they all rely on indenting the surface of the metal with an "indentor" and measuring the indentation size or depth.

The Brinell Hardness Test

This uses a hard steel ball and is covered by British Standards 240.

A special machine presses a small steel ball into the surface of the test piece for a period of 10 - 15 seconds with a certain force and the Brinell Hardness Number (HB) is found from the formula (there is no need to remember it):

where F - the force in kg. -

D - - diameter of the ball in mm. d - - diameter of the indentation in mm. (measured

using a graduated microscope)

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Because different values can be obtained by using different diameter balls on the same test piece, it is usual when quoting the HB number to quote the ball diameter as well as the force applied eg:

where HB = Brinell Hardness Number. 10 = 1 0mm ball.

3000 = force in kg.

For very hard materials, ball deformation becomes a problem, and it is better to use another method such as the Vickers Hardness Test.

The Vickers Hardness Test

This is covered by British Standards 427 and uses a diamond head. This shallow pyramid shaped head is pushed into the surface of the material for a period of 15 seconds. A force is applied of between 5 to 120kg.

The diagonals of the indentation are measured and the Vickers Hardness Number (HV) is either calculated or found from tables. When quoting the HV number it is usual to specify the load used eg:

where HV = Vickers Hardness Number 30 = 30 kg force 650 = hardness value

The Rockwell Hardness Test

rhis is covered by British Standard 89 1 and unlike the others it measures the depth of indentation of a standard indentor.

Nine scales of hardness are available and the amount that the indentor moves into the metal is measured by a Dial Test Indicator (DTI) fiied to the test equipment.

The test value would be found by calculation or from tables and quoted as:

HRB = 60 HR = Rockwell Test B - - Scale B 60 = Hardness number

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The Shore Scheroscope Test

This involves the dropping of a small diamond pointed hammer onto the surface to be tested and measuring the height of the re-bound.

The height of the rebolxnd is measured against a special graduated scale - the higher the rebound the harder the metal.

This test leaves no visible impression.

TOUGHNESS TESTING

QUESTION Define toughness. (2 mins)

ANSWER This is the ability of a material to absorb an impact load. It is opposite to brittleness.

Most tests involve hitting the test piece with a mass of known energy and ascertaining how much energy is used to break the test piece.

A heavy pendulum is supported at a set height by a latch an the impact testing machine. The amount of energy in the pendulum is known as a function of Potential Energy (PE) in Joules.

where PE = Potential Energy (Joules). m - - the mass in kg.

g - - acceleration due to gravity (9.8 1 m/s2).

h - - datum height in metres.

When the pendulum is released it swings down and breaks the test piece clamped in a special jaw at the bottom of the swing arc. The test piece must break for the test to be valid.

The pendulum will continue on its swing to reach a certain height on the other side of the test machine.

The height that it would have reached had there not been a test piece in the way is already known, so the height that it reaches after striking a test piece is an indication of the amount of energy taken out of the swinging pendulum to break the test piece.

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PENDULUM RELEASE CATCH

SCALE 8 POINTER

BRAKE TO STOP PENDULUM AFTER AFTER TEST 1;

Fig. 9 IMPACT TESTING MACHINE

The lzod Test

This uses a notched test piece supported vertically in a vice like jaw. The end is broken off in the test.

The Charpy Test

This uses a notched test piece laid across a gapped jaw. This test piece is snapped in the middle by the swinging pendulum.

CREEP TESTING

Clreep is the slow plastic deformation of metal, subjected to prolonged loading often at high temperatures. It is a problem with:

* Jet engine turbine blades. * Structures subject to aerodynamic heating during high speed

flight.

Creep is tested for by using several test pieces (of the same metal) and subjecting each test piece to a particular load and temperature. Each test will normally produce a graph of Creep Strain against Time (figure 10).

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CREEP STRAIN

TIME

Fig. 10 GRAPH OF CREEP STRAIN AGAINST TIME

During primary creep the metal is "settling in" and hardening is occurring. Secondary creep occurs over the life time of the component and is generally very slow.

Tertiary creep is dangerous because it can lead rapidly to lose of appropriate clearances and component failure.

FATIGUE TESTING

QUESTION What is fatigue? (5 mins)

ANSWER It is the cyclic stressing of a part. The stress level is normally well within the elastic limit level and therefore it could be considered to be harmless. IT IS NOT. Failure can occur due to fatigue at strt , levels well within the design maximum normal stress.

All fatigue testing involves the loading and unloading of a test piece a number of times until it breaks. The test cycles (N) are then recorded against the load (stress) on a graph.

The test machine can vary but a common method is to use a rotating test piece loaded downwards so that one revolution of the test piece will produce one load reversal.

A bar (test piece) of circular cross section is clamped in a chuck which is rotated by an electric motor. A bearing is fitted at the free end of the test piece with a mass carrier fitted to the bearing.

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REV COUNTER BEARING TEST PIECE \

/ \

\ 0

C MOTOR

- -

MASS CARRIER

' I' CUT-OFF POWER SW'TCH SUPPLY

Fig. 11 FATIGUE TESTING MACHINE

The mass carrier always hangs vertically downwards so that when the motor rotates the test piece, the test piece is put through one complete cyclic loading for each revolution. Effectively being bent up and down once every revolution.

A heavy mass is placed on the mass carrier. The cage over the machine is put in place and the motor switched on. The test piece rotates. When the test piece fails, the mass carrier falls down, contacts the cut-off switch and stops the motor. The rev counter on the motor shows the number of revs (and hence cyclic loads) that has occurred to failure. This value (N) is plotted on a graph against stress (o).

Another (identical) test piece is fitted, the mass is reduced slightly and the whole process is repeated. The result is an increased value for N. This is also plotted on the same graph. After many tests, each with a slightly reduced load on the mass carrier, all the points on the graph are joined up, and a graph, as shown in figure 12 is produced.

Tested in a corrosive atmosphere producing corrosion fatigue

STRESS

IT

0 0

CYCLES ( N )

Fig. 12 GRAPH OF STRESS AGAINST CYCLES (PLAIN CARBON STEEL)

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As you can see, for plain carbon steels, if the stress level is kept low enough then failure will not occur under normal conditions.

If the stress is raised too high then failure will occur. - the higher the stress the sooner the failure.

Fatigue Limit

Some metals do not exhibit a fatigue limit and no matter how low the stress level fatigue failure will occur at some time.

With structures made of metals with no fatigue limit then special inspection/ tests are carried out on the structure at regular intervals whilst in service. The structure might be "lifed" and after a certain life span withdrawn from service.

Figure 13 shows the graph produced by metals that do not have a fatigue 1jlB,it.

0 CYCLES (N)

Fig. 13 GRAPH OF STRESS AGAINST CYCLES (Non Ferrous Metals & Austenitic Steel)

CORROSION

Corrosion results from the fact that most metals will try to revert to their natural or more stable state. Generally metals are inherently unstable in their commercial form and fairly readily combine with other elements to degrade the metal. For example, metals react with oxygen to form oxides, acids and alkalis combine with metals to form salts, hydroxides etc. Some metals, however, are very stable and strongly resist corrosion eg, gold, platinum, titanium, silver etc.

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Although there are a large number of reactions that may occur between metals and their environments, the reactions may be broadly divided into two:

* Oxidation or "dry" corrosion. The reaction between a metal and its environment without the intervention of an electrolyte.

k Electrochemical or "wet" corrosion. Requires an electrolyte such as

impure water, water vapour, or some other electrically conducting liquid.

Oxidation

This term is applied to corrosion where no electrolyte is present. It can occur where metals are in contact from combustion products from internal combustion engines, gas turbine engines, etc.

OXIDE THICKNESS t

TIME --+-

Fig. 14 OXIDATION RATE COMPARED TO TIME WHEN TEMPERATURE VARIES

The oxide film that forms on metals generally tends to protect them from further corrosive attack.

Oxygen reacts instantly with bare metal to form a film that adheres to the metal surface. This forms a barrier (for some metals) that prevents further attack by the oxygen on the metal. The rate of oxidation depends on the environment and the nature of the oxide film. Some films are more permt:able than others and some adhere more strongly to the metal than others. It has been noted that the rate of oxidation falls sharply with increase in film thickness. A general curve of oxidation rate with time is shown in figure 14. Jt can be seen that as temperature increases so does the oxidation rate.

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Electrochemical Corrosion

This is the most commonly met with category of corrosion. It can take many forms but usually always takes place in the presence of water or water vapour with traces of other substances. A pd (potential difference measured as voltage) exists between two surfaces, or two areas within the same surface. One of the areas or surfaces becomes anodic (+) and the other becomes cathodic (-). The anodic area usually corrodes while the cathodic area has material added to it. The electrolyte provides the current path.

CURRENT FLOW

Fig. 15 SIMPLE CORROSION CELL

The corrosion cell, as shown in figure 15, is minute in size but will join with other cells to attack large areas, or form deep pits, or follow grain boundaries inside the metal.

The main factor affecting the rate of corrosion attack is the pd between the two joined metals or between two areas of the same metal. The pd between two metals can be measured with a sensitive voltmeter and recorded and a list drawn u p of all metals, known as the galvanic series.

The Galvanic Series

The Galvanic Series lists metals in pd order with the least noble a t one end and the most noble at the other. It is usual to specify the electrolyte used with the table (the most common being seawater).

This means that, when joining any two metals together, the metal that is likely to corrode out of the two can be found by reference to the Galvanic Series. For example, joining copper t.o low carbon steel would result in the low carbon steel corroding if corrosion started. (The table shows low carbon steel to be less noble than copper).

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The least noble end may be called the Active end and the most noble end may be called the Passive end.

Those metals marked with a r ~ asterisk (") may be found in more than one position in the table depending on their actual composition.

TABLE (PART) OF THE GALVANIC SERIES IN SEA WATER

MOST NOBLE END (pd = +0.2V)

Graphite Platinum

Ni-Cr-Mo Alloy Titanium (pd = OV) Stainless Steels*

Ni-Cu Alloys Silver Nickel

Ni-Cr Alloys Lead

Bronzes Brass* Copper

Tin Cast Iron

Low Carbon Steel Cadmium Al Alloys

Zinc Magnesium (pd = -1.6V)

LEAST NOBLE END

Types of Corrosion

Corrosion rarely occurs in one form only, since one type of corrosion invariably leads to another, often more serious.

Surface Corrosion. Appears as a reddish brown rust on steels, a whitish grey powder on aluminium and its alloys and magnesium alloys, and as a green discoloration on copper and its alloys. It occurs on the surface of metals but can develop into pitting corrosion.

Pitting Corrosion. The pd locally in the metal causes the corrosion to develop into the metal forming pits, sometimes very deep. Often starts with surface corrosion, can be very serious and may develop into Stress Corrosion and Fatigue Corrosion.

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PITTING CORROSION

SURFACE CORROSION CORROSION

STRESS / CONCENTRATION

Fig. 16 STRESS CORROSION

Stress Corrosion. Metals under stress generally corrode more rapidly than unstressed metals. Stress will be increased by cracks and Pitting Corrosion because they reduce the amount of good metal left which is capable of taki- , the load. With the development s f a pit the stress level a t the end of the pit increases. With an increase in the depth of the pit the amount of remaining metal is reduced so increasing the stress level which will open up the pit more to allow further corrosive attack. This is turn leads to a deepening pit and even higher stress levels.

The process is a continuous cycle (a form of positive feedback) that will eventually lead to the failure of the part.

Corrosion Fatigue. This is similar to stress corrosion but the loads are cyclic. The definition of fatigue is "Cyclic stressing of a part - often at stress levels well below the ultimate stress level the part will fail at". For many metals fatigue will eventually cause failure but with corrosion present in the pit failure occurs significantly earlier.

Of course, it goes without saying that, if stress/fatigue corrosion is found iL, ~ t s early stages then appropriate rectification (usually replacement of the part) will prevent failure.

Identification of these types of corrosion is not easy so Non Destructive Techniques (NDT) are used such as X-rays etc.

Galvanic Corrosion. Can develop where metals are in contact. The main areas of attack are the faying surfaces (contact surfaces), so the corrosion may not be readily visible externally. Though it may be seen around the faying edges. Can occur between two different metals in contact (see the Galvanic Series) or between two identical metals having had different heat treatments.

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CORROSION /

LESS NOBLE NOBLE METAL

Fig. 17 GALVANIC CORROSION

Where metals have to be joined, always try to join metals that are the same material, and, ideally having had the same heat treatment. But at any rate always use the correct jointing compound (check the Aircraft Maintenance Manual -- AMM, or the Structure Repair Manual - SRM). Signs of the corrosion should be looked for along the faying edges of skin panels, around bolt heads, rivets, metal to metal joints etc.

Intercrystalline Corrosion. This is a most serious form of corrosion as it is very difficult to detect. It usually occurs between the grain boundaries of alloys and is within the metal. It may develop close to the surface, in which case a crack or small blisters may become visible. On the other hand it may not develop near the surface and external indications may never appear - until it is too late - when the part fails. Internally it can be detected by using X-rays or ultrasonic testing - if it is suspected that it is there in the first place.

GRAINS ROSlON OR CRYSTAL

Fig. 18 HIGHLY MAGNIFIED SECTION SHOWING GRAIN STRUCTURE AND INTERCRYSTALLINE CORROSION

Fretting Corrosion. Occurs in bolted joints, riveted joints and other assemblies subject to fretting (slight rubbing movement between the joined parts). The most usual cause of fretting is vibration and this can be induced into the airframe or components by the engines, electric/hydraulic/pneumatic motors/pumps and it can also be induced aerodynamically by propellers, rotor blades and flutter. If assemblies are not attached securely enough to each other and are subject to vibration then fretting corrosion can occur.

The heat and friction developed promotes oxidation which is rubbed into a powder called "cocoa" powder. The combined action of the corrosion process and the fretting will cause rapid deterioration/wear of the joined parts locally

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Joints should be correctly and securely assembled with jointing compound as specified in the AMM (and correctly lubricated for splined shafts etc), and assemblies should be checked for signs of cocoa staining.

Crevice Corrosion. Occurs in crevices and areas where a lack of ventilation prevents the metal maintaining its natural protective oxide film. Also the areas remain damp longer than open areas.

CORROSION

Fig. 19 CREVICE CORROSION

Filliform Corrosion. Corrosion penetrates the outer layer (cladding) of the metal either via a damaged area, pitting or rivet holes, and spreads radially along the boundary of the cladding and the parent metal. May be impossible to see unless it becomes severe. Affects alclad Al alloys.

Exfoliation Corrosion. This corrosive attack occurs along the grain boundaries within the metal. It is found in rolled A1 alloys and tends to follow the direction of the rolling. The effect of severe exfoliation corrosion is to produce a quilt like texture to the surface of the metal, hence the name for the condition (figure 20) - Quilting or Pillowing.

Fig. 20 T H E RESULT OF EXFOLIATION CORROSION

a0 -

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Microbiological Corrosion. Occurs in aircraft fuel tanks due to the growth of micro-organisms which require the water content of kerosene fuels for their development. They will give off corrosive substances such as ammonia, sulphides, and acids.

The growth collects as slime on the tank walls affecting the electrolyte concentration locally. These areas become anodic and a corrosive attack begins. The slime can also affect the operation of the fuel system comporlents by clogging fuel filters etc.

AcidIAlkali Corrosion. Caused by spilt acids and alkalis and will cause serious damage unless quickly neutralised. Mercury spillage also causes rapid and serious chemical change in aluminium alloys which will normally require replacement.

Erosion Corrosion When corrosion occurs in the presence of a fast moving fluid the rate of corrosive attack may be much higher than would occur in a slow moving or still environment. The fluid may be in the form of a powder, liquid or gas.

Metal may be removed from the material surface either as dissolved ions or as solid particles. Commonly found on propeller leading edges, rotor blade leading edges, compressor and turbine blades and aerofoil leading edges. The initial action on most of these components is the removal of the protective/outer layer by the abrasive action of the air - compounded if the air contains particles such as water droplets or dust particles - as happens when an aircraft flies through a dust cloud - part of a sand storm or a cloud thrown u p by a volcano.

Prevention/darnage reduction on engine components is usually achieved by the use of hard, erosion resistant coatings. On propellers an erosion strip may be fitted. The best preventative measures are the identification and frequent inspections of suspect areas and prompt rectification of any damage found.

Cavitation Corrosion. In certain fluid systems cavitation can occur within the fluid. This is caused by a sudden drop in pressure which allows gas bubbles to form. My happen occasionally because of rapid fluid system pressure drop or may be nearly continuous a t positions in the system such as spur gear pumps where the teeth inter-mesh.

The result can be that material is worn away (of the gear teeth) and if the atmosphere is corrosive then corrosion will occur. The combined effect of cavitation erosion and corrosion can cause rapid metal removal with decreased machine efficiency and eventual failure.

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Anti-Corrosive Treatments

The following is a list of anti-corrosive treatments, most of which are applied by the manufacturer/overhaul facility only.

For anti-corrosive treatments, repairs and anti-corrosive measures applied/taken by the maintenance engineer you are referred to the appropriate book on the subject in EASA module 7.

PROCESS APPLlCATION

Electro - Plating The surface of the part is covered with a thin layer of metal by being exposed to a solution of a metallic salt which is decomposed by electrolysis. The part is placed in an electrolyte bath and a current is passed through. Copper, nickel, chromium, lead, cadmium, tin, zinc, an precious metals are used for plating. Cadmium platin: is used extensively for steel parts on aircraft.

Hot Dipping The part is immersed in a bath of molten metal thereby acquiring a covering of that metal. Plating metals for this process have relatively low melting points, eg tin (tinning) and zinc (galvanising).

Cementation The part is coated with a plating metal by being heated whilst in contact with a dust or powder of that metal, eg aluminium (calorising) and zinc (sherardising).

Metal Spraying Heated particles of the plating metal are sprayed onto the part (like paint spraying). The particles impinge upon the work to form an adherent coating. Aluminium, brass, copper, nickel, and zinc are used as spraying metals.

Phosphating The part is immersed in a bath of boiling acid phosphate solution. The solution reacts with the surface of the metal to form a metallic phosphate which is highly anti- corrosive. The process is applied to ferrous metals anc may be known by various names eg, parkerising, walterising, etc. A surface conversion process.

Anodic Oxidation Usually called anodising but may be known by other names. The part is placed on the anode bar of an anodising bath and immersed in the electrolyte. With current flowing the surface of the part is chemically converted to an oxide layer. This layer prevents corrosive attack in service. Used extensively on aluminium and its alloys. A surface conversion process.

continued

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Alodising An anti-corrosive treatment for A1 alloys which also increases the paint bonding qualities. The metal is cleaned with an acid, washed in clean water and then given a coating of Alodine (a propriety chemical similar to Alochrome). This turns the surface a greenish colour. The metal is again washed in clean water and then given a rinse in a Deoxylyte bath (also a propriety chemical solution). Used at user unit level.

Chromate The part is placed in a bath of chromating solution which treatment produces a protective chromate film on its surface.

Applied to magnesium alloys and zinc exposed to humid atmospheric conditions. A surface conversion process.

Cladding A mechanical process of rolling one metal onto another eg, a thin layer of aluminium is rolled onto both sides of duralumin sheet to produce alclad.

Paints, enamels, May consist of protective compounds held in suspension etc in a suitable liquid (eg chromates in primers) which dries

out after application. Applied by brushing, spraying, dipping or rolling and are often used as additional protection to those listed above. Used at user unit level. Oils, greases, lanolin, jellies etc are often used as temporary, or semi permanent processes and sometimes as an additional process to those listed above. Used at user unit level.

Organic treatments

NON METALLIC MATERIALS

In this book we deal with the following materials:

* Cloth * Wood * Plastics * Rubber * Fibre reinforced composites

Fibre reinforced composites are covered to a greater depth than the other materials. Cloth, wood, plastics, and rubber are mentioned briefly because they have their uses in the aircraft industry.

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CLOTH

Used in aircraft construction for the covering of some light aircraft and for furnishings. Fabric used for aircraft covering may be:

* IJnbleached Irish linen * Madapollam. Madapollan. * Polyester cloth.

Unbleached Irish linen and Madapollam/ Madapollan are tautened by doping whilst polyester is tautened by the application of heat. Cloth used for the covering of aircraft seats and berths is usually made from man made fibres and must conform to current fire and smoke blocking regulations.

WOOD

Used extensively in older aircraft for all parts of the structure and in the manufacture of propellers. Still used in some composite constructions. Used on some comparatively modern aircraft eg, the fuselage of the de Havilland jet fighter - the Vampire.

TABLE 8 - WOOD COMPARED TO AL ALLOY

MATERIAL DENSITY (kg/ rn3) LONGITUDINAL TENSILE STRENGTH (GPa)

Wood (Spruce) 600 0.05

A1 Alloy 2700 70

Its strength and density can vary considerably depending on the type of woud selected and, of course, it can rot and be attacked by insects, fungus etc. It is easily worked and repaired. Wood is stronger in tension along the grain than across it.

For more details on wood and wooden structures (for the mechanical person) see the book in this series entitled "Wood and Fabric Structures".

PLASTICS

Strictly speaking plastics should be called polymers. Polymers can be man made or natural. Natural polymers include rubber (from trees) and shellac (the excrement from a South American ant).

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Man made polymers can be divided into two main groups - Thermoplastics and Thermosetting Plastics.

Thermoplastics. These soften on heating and harden on cooling, and the process is capable of repetition. Examples include Perspex (polymethyl methacrylate) and Nylon.

Fibre reinforced thermoplastics include PPS, PEEK etc.

Thermosetting Plastics. These become plastic on initial heating but become permanently set on cooling. They can not be softened again by further heating.

A good example is Bakelite (phenol formaldehyde), Formica, Ebonite and Epoxy resins. The term thermosetting also includes those polymers which set by the addition of a curing agent and do not necessarily need heating eg, epoxides.

Fibre reinforced thermosetting plastics include polyesters, vinlyesters, epoxy 2tc.

Various fillers can be added during manufacture, for example:

* Asbestos - resistant to high temperatures. * Carbon - improves conductivity.

Colour

Various pigments and dyes can be added to plastics in the production stage to give an "all through" colour.

Rubber

A naturally occurring thermosetting plastic obtained from the sap of trees. Natural rubber is normally vulcanised with sulphur to produce a tough elastic material. Used in anti vibration mountings; drive belts; shock absorbers (simple bungee cord types) and of course tyres. It can be made electrically conductive by adding carbon.

FIBRE REINFORCED COMPOSITES

These are increasingly being used in the aircraft industry for structures and components because they exhibit a high Specific Strength (SS) (strength/density). Example: the tensile SS of carbon fibre is 4 to 6 times the tensile SS of A1 alloy or steel. (Airbus have already tested a complete airliner size wing in carbon fibre).

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Additional advantages include:

Does not corrode. Easy to shape - double curves etc. High level of integration possible with other structures, eg skin to stiffeners, formers, frames etc. High fatigue limits with load cycles much higher than with metals. High resistance to chemical attack and weathering. Can be made radar transparent. High impact resistance. Can be made as a insulator; a conductor and a dielectric. Good thermal properties and a fire retardant.

In this section we will deal with the materials and all general aspects of composites and should more information be required, particularly on structures, the reader is directed to the book in this series entitled "Aircraft Structures".

GFRP k

AFRP = Aramid fibre reinforced FAIRING plastic (Kevlar). CFRP \

CFRP = Carbon fibre reinforced plastic.

GFRP = Glass fibre reinforced plastic.

GFRPRADOME 1 k

5 7 v- 7- CFRP

/A & SPOILERIAIRBRAKES

TRACK FAIRINGS

a CFRP

Fig. 21 USE OF COMPOSITES ON THE A310

Types of Composites

k Fibre reinforced plastics - polyesters, PPS etc. * Sandwich structures with the outer layers of metal or fibre, and

the core using honeycomb made of nomex, A1 alloy, carbon etc. * Fibre metal laminates such as ARALL (Kevlar fibres) and GLARE

(Airbus A380) . k Metal Matrix Composites (MMCs) using aluminium, titanium etc.

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* Glass Matrix Composites. k Ceramic Matrix Composites. * Ceramic Ceramic Composites. * Carbon Carbon Composites.

A note on GLARE

This is a new material and is used in the construction of large parts of the Airbus A380 fuselage.

GLARE is made up of alternating layers of aluminium foil and glass fibre polymer prepreg layers and, size for size, is 25% stronger than A1 alloy and 20% lighter. It can be made as sheets or complete structures (with stringers, frames bulkheads etc) in an autoclave.

It is inspected in the normal way for external defects and requires a specialist .qDT ultrasonic technique for the detection of internal defects.

Fibres

Various fibres are used as reinforcing elements within a resin for fibre reinforced plastics. They include:

Glass. These are continuous glass filaments 6 to 15pm in diameter (0.000,006 to 0.000,O 15m) [This is called a micrometre or sometimes, incorrectly, mu-- metres. Mu is the Greek letter p].

The fibres are usually coated with a lubricant to improve handling and may have other coatings to improve bonding etc. The fibres are supplied in different forms: A glass; C glass; S glass etc, E glass is currently the most popular.

Aramid Fibre. This is an organic fibre. Supplied as Kevlar (Du Pont). Kevlar 29 used for cordage and ropes. Kevlar 49 is supplied for plastics reinforcement.

Carbon Fibre (HT and NM). There are some 12 manufacturers world-wide making over 40 different carbon fibres. The fibre is manufactured as a tow and the finer tows have up to 12,000 filaments in each tow.

Carbon fibres are strong in tension and are often coated to improve handling and bond strength.

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Hybrid Fibres. Hybrid fibres can be made up in many different forms and can include:

(a) Two or more different types of fibres layered together within a resin matrix.

(b) A mixture of two or more different types of fibre weave within a resin mix.

The following table shows the comparisons of density and strength of different materials used on aircraft with properties of some fibres given in the next table,

TABLE 9 - COMPARISON TABLE, STRENGTH & DENSITY

MATERIAL DENSITY LONGlTUDINAL TENSILE (kg/m3) STRENGTH (GPa)

Wood (example) 800 A1 Alloy 2700 Aramid Fibre 1380 Glass Fibre E 2000 VHM Carbon Fibre* 1690

* VVHM = Very High Modlllus

TABLE 10 - PROPERTIES OF FIBRES

MATERIAL SPECIFIC FATIGUE RELATIVE TENSlLE FAILURE COST STRENGTH STRESS @ (£1 (GPa) 106 CYCLES

(MPa) E Glass 0.54 260 1 Carbon 1 .O 860 40 Aramid 0.83 980 20

CHARPY IMPACT TEST (kg / m2

Although aramid fibres have good fatigue strength, aramid reinforced composites don't. This is because of bond fracture between the resin and the fibre and is causing trouble in service because of the moisture absorption by the microcracks.

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Make up of Fibre Cloth

Layers of fibre cloth are layered u p within a resin so that when cured the resultant structure is solid with good strength properties. Fibre cloth may be supplied untreated or pre-preg in a variety of grades and weaves to BS 3369 or MIL-C-9084 standards. Glass cloth may be supplied in one of three basic forms:

* Chopped strand mat. The yarns are in a random direction and of comparatively short length. Not common.

* Weftless weave. Continuous yarns only going one way, with an occasional yarn going at right angles to keep all the others together. May be called Unidirectional weave.

* Plain weave or weft and warp weave. With continuous yarns going u p and down and, weaved in between, yarns going from left to right.

Zarbon fibre is made u p into sheets of varying thicknesses either unidirectional or plain, pre-preg or untreated.

Most cloths are made u p of yarns going up and down (warp yarns) and weaved between them yarns going from left to right (weft yarns) (weft to right).

With a weftless weave the fabric is stronger along the yarns than across them, so if a composite is made u p using this type of fabric weave and if it was to fail it would be more likely to fail along the fibres (ie cracks along the fibres) that across them. Fabric with a weft and warp weave is likely to tear 'one yarn at a time' so the tear will propagate usually in the form of an L shape.

WARP YARNS SUPPORT YARNS

/

PLAIN WEAVE

WEF YAR

UNIDIRECTIONAL WEAVE

Fig. 22 WEAVES

Now try the following questions to see if you have understood the information in the tables.

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QUESTION What does GPa mean? (5 mins)

ANSWER Pa means Pascal and is the unit of stress and pressure. It is small (nearly 7000 to lpsi). G means giga ie, lo9, or in other words 1,000,000,000. GPa is spoken as Giga Pascal.

QUESTION What do you understand by the term "Fatigue failure stress @ 106 cycles"? (1 5 mins)

ANSWER Fatigue is the cyclic stressing of a part and if the stress level is low enough (for some materials) the part will never fail. For most materials if the stress level is raised then failure occurs sooner. In the table above the part is put through 106 cycles (1,000,000). If the applied stress is low enough failure does not occur.

If the stress level is raised so that the test specimen fails at exaptly 1 million cycles then we have a comparison of the materials resistance to fatigue failure.

You can see from the table that while aramid fibre is not as strong as carbon fibre it is significantly better when it comes to fatigue resistance (as a fibre only).

RESINS

The fibres (like string) are very good in tension but poor (very poor) in compression. To make them more rigid and able to withstand bending and compressive loads they are bonded together using resin. Various resins are available for bonding laminates and as an adhesive for the adhesive bonding of metal to metal, metal to wood, metal to polymer etc. A few are described below.

Unsaturated Polyester Resins. Used with glass reinforced plastics (GRP). Tb-y have good strength and chemical resistance. They tend to shrink on curing and do not like temperatures above 150°C.

Vinyl Ester Resins. These are similar to the unsaturated polyester resins above.

Phenolic Resins. Used for aircraft interiors because of their low smoke emission properties.

Epow Resins. These are a thermosetting resin. They are versatile, have a low shrinkage rate with high strength and good chemical resistance. They are used widely in engineering and are usually supplied as a two part mix.

Polyamide Resins.. These are suitable for use up to 300°C and are available in films, varnishes, powders, laminates etc.

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General

Hardening occurs through the reaction of curing agents, hardeners, catalysts or activators and some epoxylhardener combinations will cure at ambient temperatures - while others will require heat to cure (refer to the manufacturers literature).

When dealing with the mixing of resins for composites or adhesives the catalyst/accelerator is added and mixed into the resin to start the cherrlical reaction process. Once mixed it will have a "pot life" which will be shortened if the ambient temperature is high. Once the composite is "laid up" a curing time is required which will be shorter if heat is applied. Some resins are cold cured and do not require the application of heat whilst others must be heated to allow the bonding process to reach its full strength.

Pot life is stated in the manufacturer's literature.

TABLE 11 - PROPERTIES OF SOME RESINS

RESIN TENSILE TEMP. WATER SOLVENT STRENGTH LIMIT RESISTANCE RESISTANCE (MPa) ("C)

Unsaturated Up to 90 180 GOOD FAIR Polyester EPOXY 105 220 GOOD GOOD Vinyl Ester 85 180 GOOD FAIR Polyarnide 120 400 LOW ------

The choice of resin is important as an incorrect resin can have an adverse effect on the material it is being used with. It may not be strong enough or fail due to heat or age.

When using resins it is important to maintain strict cleanliness during the mixing and bonding process as any dirt, dust etc will seriously adversely affect the joint strength.

Always follow the resin manufacturer's instructions.

CORES

With all structures subject to bending it is the outer layers (actually called fibres) of the structure that take most of the stress (compressive and tensile).

Figure 23 shows a cantilever beam (cantilever = supported at one end only) but the same principle applies to non-cantilever structures such as floor panels, skin panels etc.

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The centre portion of the beam takes very little stress (except for shear) and in a uniform monolithic structure this centre is almost so much "dead weight". The drawing shows a beam and the principle applies to all structures whether it is a main spar of an aircraft, the skin of the airframe, a helicopter rotor blade etc.

I FORCE

LOW DENSITY CORE

OUTER FIBRES

Fig. 23 BENDING

In each case the outer fibres of the material take all the tensile and compressive stress with the centre fibres taking very little. Many compositt structures are therefore designed having a low strength, low density core to reduce the weight of the overall structure, with most of the stress being taken by the outer fibres.

The core may be made of honeycomb, foam, or some other low density material while the outer fibres are made of metal, fibre composites etc. The core then is of low density, designed mainly to resist shear and compressive loads and include the following:

Balsa Wood Not used much these days but was used as a core on several aircraft including the de Havilland Mosquito (plywood/ balsa wood/plywood sandwich fuselage skin).

Honeycorrlb Used extensively as core material in aircraft floors, structures, control surfaces, helicopter blades etc. Can l-- made of aluminium, glass fibre or composite.

Foam (Polyvinyl chloride) PVC is used as the core of some composite structures.

Micro balloons Within a resin mix.

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TABLE 12 - COKE MATERIALS

MATERIAL DENSITY COMPRESSIVE SHEAR k / m 3 STRENGTH (MPa) STRENGTH (MPa)

Balsa Wood 96 5.2 1.3 Nomex Honeycomb 64 2.9 1.7 Aluminium Honeycomb 118 7.6 5.2 Foam (PVC) 100 1.4 1.1

Note. Nomex is made from aramid fibres bonded with phenolic resins.

MANUFACTURE OF COMPOSITES COMPONENTS

Several methods are used to manufacture composite components and this section is included for interest only. There is no need to commit this to memory, although some of the general principles are used when repairing composite aircraft structures.

Compression Moulding

Usually uses pre-preg fibres (fibres impregnated with resin) in sheet, tape or woven form.

Individual plies of pre-preg are laid one on top of the other to produce the required thickness. This "preform" is then laid in the bottom half of a mould. The top half is then closed and secured and heat applied.

The heat and pressure allows the resin in the pre-preg to flow and bonds the ?lies into a single structure to the shape of the mould. On cooling the mould is opened and the item removed. Trimming and finishing is then carried out.

Vacuum Bag/Autoclave Moulding

The most common method is to use a pre-preg lay-up similar to that used for compression moulding. Once the lay-up is completed a vacuum bag is placed over the complete assembly and evacuated of air. Thus atmospheric pressure produces the necessary force to push the plies together.

For components/structure repairs where a bag cannot be placed over the complete assembly a plastic sheet is used - with a suitable valve attached - which is sealed with special adhesive tape around the edges of the repair area.

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Heat may be obtained by:

(a) Placing the assembly in an oven (autoclave) at 30Q°C and 1.4MPa pressure (about 200psi).

(b) Using a heater blanket. (c) Using heaters within the mould.

To allow the resin to flow an absorbent membrane is placed between the vacuum bag and the lay-up material. Temperature sensing bulbs are usually placed inside the vacuum bag close to the laid-up material to automatically control the temperature of the heater elements.

Mandrel Wrapping

Involves wrapping a mandrel with layers of pre-preg material. After heating ~d curing the mandrel is removed. Used for tubes and hollow sections.

Pultrusion

This is a continuous process for the production of rod, tubes and long sections. The fibre (glass, Kevlar or carbon) is drawn from a spool through:

FIRST a resin impregnation tank, THEN through a pre forming die, THEN through a curing die (heated), TO emerge as a continuous section to be cut to length as

required.

Filament Winding

Separate filaments are accurately wound onto a mandrel of the appropriate shape after first being impregnated with resin (or pre-preg is used). The complete assembly is heated to cure the resin then the mandrel is collapsed/dismantled and removed. In some cases the mandrel may be left in place and form an integral part of the component. Used in the manufacture of pressure vessels.

Adhesive Bonding

Used in the process of metal to metal joining; metal to composite joining; composite to composite joining and honeycomb joining. To manufacture a cored composite structure the two "skins" are manufactured either by compression moulding or auto-clave moulding.

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The two "skins" are then bonded either side of the core by using resin adhesives. The structure is heated in a pressurised aut.0--clave.

The resins may be a two part mix resin (epoxy) or it may be supplied in film form.

This process, in general, produces a strong structure, without any stress risers (such as rivets, bolts etc) with a good strength/weight ratio. However, i t is difficult to know if the bond joint is satisfactory. With a riveted joint, for example, the formed rivets can be inspected for shear, correct forming etc.

With an adhesive bonded joint there is no sign that the joint is satisfactory .- it. looks the same after the bonding process as before. This means that special checks must be carried out. These include:

* Complete cleanliness and scrupulous attention to detail when preparing the materials and carrying out the process.

* A thorough inspection of the joined parts to see if there has been any relative movement and to check any visible bond lines for signs of the bonding agent.

A The destructive testing of test pieces manufactured at the same time using the same materials and techniques as employed with the original work.

ADHESIVES - GENERAL

Many theories exist as to why adhesives work. Why does the adhesive "stick" to the surface (the adherent)? Several theories have been suggested including chemical reactions, intermolecular forces (absorption) and intermolecular electrical forces. Text books differ on the subject.

The advantages of adhesive bonding include:

.!T No holes to weaken the material, J; No high temperatures involved during the manufacturing process,

unlike welding. * Smooth surfaces. Ideal for external aircraft skins. * The adherends are sealed.

The disadvantages include:

* Long curing times. * Careful joint preparation required. * Some materials are dangerous to handle. * Difficult to inspect the finished joint. * Joints not suitable for high working temperatures.

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. [ ADHEREND I

ADHESIVE < ADHEREND

Fig. 24 ADHERENDS

Classification of Adhesives

Adhesives can be classified as either organic or inorganic, with the organic range split into two - synthetic and natural. Synthetic adhesives can be div;*bd into thermoplastic, elastomeric and thermosetting.

ADHESIVES s INORGANIC ORGANIC c

SYNTHETIC NATURAL

L 'THERMQPLASTIC ELASTOMERIC THERMOSETTING

Fig. 25 ADHESIVE CLASSIFICATION

Adhesives

Inorganic. Such as sodium silicate based. Not used for metal bonding.

Natural. Rubber (from trees), shellac (from an ant), cellulose etc. Used for things like paper and wood.

Thermoplastic. Made from thermoplastic resins. Are softened by heating which can be repeated. Used were great strength is not required though hot melt thermoplastics can have a strength up to 18MPa.

Elastomeric. Rased on synthetic rubber they produce and instant stick when the two adherends are brought together. They set by the evaporation of the solvents. For structural work thermoplastic and thermosetting resins are added.

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Urmoset t ing. Includes epoxide and urea resins. Provides a strong joint and used in the manufact.ure of structural components. The process of making the joint usually involves a curing agent. When the resin and agent are brought together curing takes place which involves a chemical reaction.

Testing The Joint

After a joint is bonded and after the appropriate curing time the test specimen should be tested. Depending on the materials and the type of joint made these tests can include a Tensile Test, a Shear Test, a Peel Test and a Cleavage Test (for thicker materials).

Special testing machines are provided that provide a calibrated load and this can be plotted on a graph against extension/deformation/breaking of the test piece.

i'he cleavage test would only be suitable for thicker non-flexible test pieces, whilst the peel test would only be suitable for thinner flexible material.

FORCE TENSILE TEST

SHEAR TEST

' CLEAVAGE TEST PEEL TEST

Fig. 26 JOINT TESTING

DESTRUCTIVE TESTING OF COMPOSITES

This section deals with testing of composites. Many of the tests are similar to those used on metals, but many composites can prove difficult to test and get valid results. Remember, with destructive testing of metals the results are only meaningful if the test piece is destroyed during the test.

For non destructive testing (NDT) refer to the book Non Destructive Testing in module 7 in this series.

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Testing of any material/joining process can be divided into:

1. Visual examination (a form of NDT). 2. Conventional NDT methods. 3 , Destructive methods - workshop. 4 . Destructive methods - laboratory or manufacturer

Items 1, 2, and 3 will be dealt with later and item 4 (for metals) has already been covered, but the laboratory destructive testing of composites has produced its own problems because their properties do not lend themselves readily to the "standard" methods of testing used on metals.

Some of the tests are similar, though the results may not be as good or as definitive as one would like. Where the tests are similar reference will be made to the section on The Testing of Metals.

When evaluating the results of tests of composites it is important that care taken because the results can vary. This variation can be caused by:

* Minor variations in the batch being tested. * Fine variations in the preparation of the test specimen. * Small variations in the actual test method.

Testing is carried out to British Standards (BSI) and to standards set by the American Society of Testing of Materials.

Safety

Considerable energy can be stored up in a test piece during the test. This energy can be released in explosive form and can be very dangerous. All testing must be carried out behind safety screenslshatter proof guards, and by qualified staff.

Flexural Test

This measures centre point deflection as a function of load. Tests may involve a three point test or a four point test with the load increased in increments and at each stage the amount of deflection measured. A graph is then plotted of load against deflection.

Tensile Test

This is carried out in a similar way to tensile testing of metals ie, the test piece is "stretched" in a tensile testing machine and its extensionlbreaking point is measured against the load applied.

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FORCE

O -, TEST

SPAN -----c 1 FORCE

Pig. 27 THREE POINT FLEXURAL TEST

C FORCE

SPAN

Fig. 28 FOUR POINT FLEXUFWL TEST

The load is progressively increased and at intervals the value of the load is recorded and the extension of the test piece measured. At the end of the test a graph is drawn of load against extension.

'Test pieces have to be thin because of their high tensile strengths and it is often very difficult to satisfactorily attach the test specimen to the machine due to its plasticity - the test piece deforms and slips out of the chuck 01- jaws.

Compressive Test

'The test piece is placed in a similar machine to the tensile test machine but the machine is selected to "squash" the test piece under a compressive force. Like all compressive test pieces it has to be of a reasonable diameter to prevent buckling. If it buckles the test is invalid.

Again, this test has its problems as failure often occurs due to "transverse delamination" - not what is being testing for.

As with the tensile test measurements are taken regularly of load and size of deformation and will all the readings obtained and a graph is plotted of load against reduction in size.

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Shear 'T'est (figure 29)

[Jsually applled to tubes and round sections and difficult to test for. In general the test is as follows:

X Clamp the test piece at one end to a torque measuring device. i Rotate the free end slowly (about half a radian per minute - about

I complete revolution in 12 minutes) and note the torque (Nm) a t the fixed end at regular time intervals.

% The relationship between the indicated torque a t the fixed end and the rotated amount at the free end is an indication of the amount of s h e w stress in the test piece.

r\ FIXED END ROTATING END

1 TEST PIECE

Fig. 29 SHEAR OR TORSION TEST

Impact Testing

The following tests are used but none have proved totally satisfactory

(a) Izod pendulum test. (b) Charpy pendulum test. (c) Drop weight test. (d) Ballistic impact test, (e) Slow bend test.

For details of (a) and (b) refer to the section in book 1 on The Testing of Metals. The other tests have been listed for reference only.

DEGRADATION OF COMPOSI'I'ES

LJnlike metals, composites do not corrode but they do have their problems.

Galvanic Corrosion

Galvanic corrosion can occur to A1 alloys and cadmiurn plated steel if attached r o CFC (Carbon Fibre Caxnposite).

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The pd (potential difference) can be as high as 1 volt. Special jointing cornpounds are used as is the use of epoxy paint treatment.

Surface 0xidat.ion

This is not an important factor with cornposjtes though surface changes can occur when combined with uv light and rain.

Frost

Will damage any composites where water has ingressed into the material. When water gets into a composite and then freezes it expands - this causes delamination and de -bonding.

UV Radiation

This will degrade glass more than carbon - but at any rate - uv absorbing additives should be used on the outer surfaces of all composites. Aramid fibres are seriously affected and must be protected.

Erosion

This can come from many sources but with aircraft it is usually airstream driven rain and debris (insects, dust etc). It affects wing leading edges, engine compressor blades, engine intakes, rotor blades, etc.

Glass fibres are more resistant to this sort of damage than carbon and boron.

Lightening Strikes/ Static Electricity

Carbon epoxy resins are 3 times more insulative than A1 alloys - this leads to very high field levels on the surface of the material. Various processes have been tried to reduce the problems including metal meshing within the composite. Aluminium surface foiling is used on carbon composites.

Fire

Inorganic resins will not withstand high temperatures and soon give off inflammable gases and thick black smoke. To reduce this problern the ou t e r lays of the composite should be glass fibre and the surface should be treated with a fire retardant coating - particularly cabin furnishings.

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SEAIAN'I'S, BONDING AGENTS & COMPOUNDS

A wide range of non- metallic materials is used for the maintenance, repair and overhaul of aircraft. 'They include: compounds, greases, oils, detergents, fillers, jo~nting compounds, cleaning agents, pre-treatments, anti corrosive agents, paints, paint strippers, fuels, fuel additives, hydraulic fluids, anti-ice fluids, lacquers, adhesive tapes, bonding adhesives, disinfectants, storage preservatives, powders, et c.

The AMM for each aircraft type will have a comprehensive list of these "consumables". This is published in chapter 20-3 1-00. It is important that you consult this chapter before using any compounds from oils to paints, to greases and speed tapes.

'The AMM will list all the compounds that can be used on the aircraft, with their specifications (eg, British, US, German, NATO code, etc) if applicable. Some compounds may be listed as "local purchase" whilst others may be supplied by specific manufacturers. Some may be listed under a trade narr,,, eg Loctite.

Where fuel additives are listed the actual percentages may be quoted. In some cases the ratios are stated as "ppm" (parts per million).

For large aircraft the tables in chapter 20-3 I list literally hundreds of non- metallic materials. Below, are tables of some of the materials that are available. They are for reference only and not included are:

* Fuels. * Fuel additives. * Hydraulic fluids. * De-icing fluids. * Paints and paint strippers. * Extinguishants.

These will be dealt with in the appropriate book in the LBP series covering that particular topic.

The information under the SPEC column includes those countries that have local specifications to meet that required by the equipment manufacturer, and/or a brand name product.

There should be no need to corrirnit the details to memory but you should have a some knowledge of the more c:ornmon sealants and bonding agents used.

blank

- 62 -

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TABLE 13 - GREASES

/ DESIGNATION I SPEC 1 USES 1

Mineral based USA High temp Bushes, roller & ball bearings

Graphited, mineral based IJK USA France

General purpose 5% graphite

Graphited, thread compound UK USA France

Anti seize grease for threads. 50% mineral jelly - 50% graphite

Synthetic, high pressure For certain applications. Temp range - 54" C to 1 2 1 '

- - -- --

Fuel & oil resistant Used in engine fuel and oil USA systems France

Silicon

Vaseline or petroleum jelly

USA

UK USA France

Lubrication for metal and rubber in pneumatic systems -- -- -- - - -- - --

Synthetic rubber seals Electrical bonding faying surfaces

Anti fretting UK USA

Used as an anti fretting compound

/ Mineral

- - - -- -- -- Corrosion preventative

--

Silicon, insulating &, sealing

Lubricant O2 systems 1----

UK USA France

USA

- -- - -. - - -.

UK USA

USA

General use at normal temperatures

-- - -- Used as a corrosion prevention layer

- -- - - - - -

Metal to metal sealing against moisture ingress

.-

Thread lubricant for oxygen systems

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TABLE 14 - LUBRICANTS

DESIGNATION SPEC USES

Rust inhibitor Dinatrol USA Germany

Solid film Molycote USA

Air drying solid film lubricant

Used as a n assembly aid during component overhaul and a t lubrication points of aircraft systems.

Prevents locking of screwthreads

General Various Grades

Anti seize USA

TABLE 15 -. LACQUERS

DESIGNATION SPEC USES

- . - .- - .. - -. . --- - -

Clear epoxy varnish with 1 Astral 1 Electrical lacquer catalyst

Transparent lacquer Sikkens For covering metal labels such as landing gear labels

Corrosion preventative Rustban395 USA

Corrosion preventative lacquer

TABLE I. 6 - BONDING AND AUF-IESIVES

SPEC USES

General purpose adhesive honeycomb filler

Araldite 106 USA Germany

Composite repairs I

-- .-

Permanent thread compound

Loctite270 USA

General purpose dimethacrylate compound

. --- - .- - - -

High temperature sealant Thread locking compound (occasional removal)

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TABLE 16 - BONDING AND ADHESIVES cont

. - - -- - - . -- - -- --- -- - --- - - Sealant 1 ~ ~ ~ 7 3 2 +

Primer 1 USA

IJSES

-. -- .- - -. -. - - - - -. . --- . - -- .

For toilets and galleys

/ Solvent based nitril rubber / adhesive

Con tact adhesive

-- . --- - - - - - -- - -- -- -

Two-part epoxy adhesive I---- Araldj te Adhesive for PTFE cloth

1 Adhesive film I FM73-M-06 I Structural adhesive I

Self adhesive aluminium tape

Glass fibre tape

Sound damping tape

bonding -. . - - - - - - - -- -

Scotch425 Temporary protective cover Germany USA

- - -- - - - -- -

Scotch36 1 Temporary repair of cargo hold fire proof panels I-

-.

Permacel Aluminium backed cotton Germany tape for sound & thermal USA insulation

Polytetrafluoroethylene anti- seize tape

- - - High temperature adhesive AF 143 Metal to metal - honeycomb

to metal bonding

121 USA

TABLE 17 - SEALANTS

For use on liquid & gaseous oxygen systems

----

Polysulfide brush consistency

Polysulfide fillet consistency

Polysulfide sealants - general

-- -. .

PR1422A2 UK USA

PR1422B2 UK USA Various UK USA Germany

Brush on, fuel tank and pressure cabin fuselage sealant

-- -- - -- - -- -

Fuel tank and pressure fuselage fillet sealer

-- - -- -

Various different sealants supplied for sealing (a) along edges of joined structures (b) individual

I I nut and bolt assemblies, & I (c) applying to faying surfaces prior to assembly

Page 70: Materials and Hardware

TABLE 18 -- CLEANING AGENTS - - - -- - - . --- - 1 DESIGNATION

I SPEC

-- - - - -

Aircraft exterior - ----- ---- -.

General purpose aircraft exterior cleaner

Liquid detergent concentrate Ardrox6025 USA

Cleaner and stain remover

-. ----- . - ..- --

Varsol/white spirit - --- - - -.

Cleaning solvent for mechanical parts

UK USA

Trichloroethane (Methyl chloroform)

Genklene USA

Cleaning solvent

Trich lorotrifluoroethane Cleaning oxygen system pipe lines

Isopropyl alcohol Air3660 France USA

- -- . - --. - Altupol

/ General cleaning

- ! Cleaning rain repellent off windscreens

--. -- - - - - - - Rain repellent cleaner

Safety solvent USA Odour free solvent cleaning agent

Carpet & fabric cleaner A USA - . - - . . . . -- - -. - - -

Cabin window cleaner I USA

Plastic polishing compound (fine grade)

--

VDU cleaner

PP-560 USA

Paste for polishing Plexiglas

Alglas V Anti static flight-deck Visual Display Unit (CRT) screen cleaner

TABLE 19 - MISCELLANEOIJS

-- - - - - - -. - -- 1 ~ 7 0 1 Removal of Skydrol fluid

spillage Hydraulic fluid removal powder

Microballoons Used as a filler when carrying out composite repairs

Aluminium metal polish

- - -- - -- ---- - -- - - -- -

Abrasjve polish for polishing out scratches in aluminium

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TABLE 19 - MISCELLANEOUS cont.

- --- - - inhibitor WD4Q 1 USA

Corrosion preventative Moisture repellent

-. .- -. . -- . .. -~

r g e n . -- - . - .- -. -. . leak .. detector .- IEi1-2 I .- -. - .. - -- - - . - .. . .- . -. - . . . ---.--a - - ... - . -. - - -- ..

Toilet deodorant AMS 1476 Non formaldel~yde based toilet deodorant

-. .- -. .- . -. - - - . - - -- -

Drinking water system Calcium hypochlorite disinfectant disinfectant for the potable

I i 1 water system I

~t is not possible (within the confines of this publication) to specify the storage conditions for all the materials listed above. But in general the following points should be noted.

1 . Keep all containerised materials in their original sealed containers. 2. Open slatted shelving is recommended. 3. Follow the storage instructions on the container and/or in the

material manufact urers5 literature. 4. Keep records of materials in store - batch numbers; date of receipt;

manufacturer etc. File all manufacturers' documentation, Release Certificates1 EASA form 1 s.

5. Rotate stock - first in - first out. 6. Note any storage life/use-by-date. Discard any out-of-date material

in accordance with manufactures' jnstructions/local regulations. 7. Store inflammable materials in non-combustible lockers/buildings

away from workshops, hangars and aircraft. 8. In general storage areas should be clean, dry, secure, and frost free.

The materials should not be in direct sunlight. The area should be well ventilated and the temperature should be kept a s even as possible.

9. Specific temperatures may be specified for certain materials by the manufacturer, eg -20°C for pre-preg carbon fibres; 7 to 23°C for paints and dopes, etc.

10. Specific (m,wimum) relative humidity levels may also be spec~lied for certain materials.

For storage details of specific materials (eg batteries, paints etc) refer to the appropriate book in the LBP series.

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Page 73: Materials and Hardware

CONTENTS

Page

Glass fibre repairs 1 Carbon fibre repairs 5 'rspection of composite structures 1'7

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Page 75: Materials and Hardware

COMPOSITE REPAIRS

Repairs to composite structures is generally considered to be more difficult than repairs to metal structures. Of course, all repair information, cornposlte and metal, is given in the repair manual (SRM) a n d most operators will use a "composites" qualified person to carry out repairs, However, a s a licensed engineer you are required to know how this is done as the composites person will report to you on completion of the repair.

GLASS FIBRE

It is most important when carrying out a repair to follow the repair manual instructions. This usually specifies that the same type of core is fitted to that which has been removed during the repair process. Remember on radomes/dielectric covers the repair should be 'radar transparent'. 'The repair should also follow, as far as possible, the original contour and shape of the rigirlal component.

Preparation and Mixing of Resins

In general always:

(a) Wear protective clothing, including goggles. (b) Work in a well ventilated area. (c) Mix the chemicals in accordance with the manufacturer's

instructions. (d) Wash the area thoroughly if chemicals come in contact with the

skin. (e) Irrigate the eyes immediately with water if the chemicals come into

contact with the eyes - and seek medical advice.

Mixing

The ingredients should be stored (normal maximum time 12 months) at temperatures less than 10°C and be allowed to come to room temperature before mixing and all materials, working areas, tools and utensils must be kept thoroughly clean and dry.

The resin and additives should be carefully measured into a glass container in the correct proportions as specified in the manufacture's instructions. These proportions may be specified as percentages by weight.

The catalyst should be thoroughly mixed into the resin before adding the accelerator and any additional material such a s fillers etc.

Page 76: Materials and Hardware

GLASS CLOTH PATCHES

CORE

Fig. 1 TYPICAL REPAIR TO CRACKED SKIN

Repatrs to be at feast 10" \ GLASS CLOTH (25mm) apart with dimensions A and Bat a maximum of 2.5

PATCHES

to 7" (63.5mm to 117.8mm) depending on type of repair (round or square).

GLASS CLOTH

Fig. 2 TYPICAL REPAIR TO DELAMINATED SKIN

Pot Life

Once mixed the resin begins to cure and may have a pot life of between a few minutes and several hours before it begins to gel. Always ensure the resin is used well within it's pot lifetjme. Djscard (in accordance with local regulations) all time expired materials.

Page 77: Materials and Hardware

Curing

Most mixed resins will cure at room temperature within a few hours, but may take several days to cure completely It may be necessary to use heat to cure the resin, sa check the Repair Manual (SRM) for details. Heating may be carrled by the use of lamps, electric heaters, electric blankets or ovens. Temperature control may be by a thermostat or by marking the part with a special pencil that changes colour at a specific temperature.

Film Adhesives

Some adhesives are supplied in film form and the amount required is simply cut from a large sheet. They are generally easier to apply than liquid or powder adhesives, but once the protective backing is removed it is most important that the adhesive film is not touched as this will severely affect its adhesive properties,

Each patch is 0.7" GLASS CLOTH larger than the next one PATCHES

OUTER SKIN

PATCH PLATE

thickness as inner Dimensions A 8 B skin) are a max of 1 to

5" (25.4 to 127mm)

whether the repair is round or sguare

INNER SKIN

t--"--i

SKIN SECTION I 11 11 11 11 11 1 I

Fig. 3 TYPICAL REPAIR WHERE BOTH SKINS & CORE ARE DAMAGED

Figure 1 shows an exarnple of a patch repair to a crack on the outer skin. The ends of crack are stop drilled using a 31 1 6 t h (4.8mm) twist drill. Glass cloth patches (3) are cut as per SRM and using the mixed resin bonding agent are cemented into position. Pressure is applied and this can be done using a vacuum sheet stuck with double sided sticky tape to the skin. A parting layer is used between the patches and the vacuum sheet and vacuum is applied from a vacuum pump via a valve in the vacuum sheet.

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Figure 2 shows a typical repair where the outer skin is damaged and has to be repaired by insertion. The skin is cut away without damaging the core using a router (not easy as the skin arld core are bonded together). Two glass cloth inserts are cut'and (using the rnixed resin) placed into position. The glass cloth patches are placed in the same way. Again, pressure is applied as before.

Figures 3, 4 and 5 shows repairs where the core has been damaged and requires replacement. A s with the other repairs a router is used for material removal, and sometimes wood chisels and the like are used to remove old resin -- which is difficult to do.

Limits are specified in the SRM as to the maximum length of crack/size of damage, the minimum distance between repairs and the minimum distance from the repair to the edge of the panel.

OUTER SKIN

MIN LAND 0.5" (13mrn)

Fig. 4 EXAMPLE OF CORE REPAIR 3" DIA MAX

General Repair Considerations

1. Ascertain the exact extent of the damage and classify the repai using the repair manual (negligible - repairable - replacement .,,c).

2. Support or jury rig the structure if necessary.

3. Check the effect of the repair on radar transparency - if applicable.

4. Mix and use the resins in a warm dry atmosphere (min 20°C).

5. Remove resins from store and allow to attain room temperature for at least 24 hours.

6 . Remove paint from the area by sanding, then clean with acetone or MEK and allow to dry.

7. Cut out the damage to a regular shape, stepped or otherwise, a s per the SRM dimensions.

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8. Sand area if specified in the SRM.

9. Lay up the repair using cloth and resins in accordance with the repair manual. Cloth plies normally in the same direction as the original lay.

10. Apply pressure to the repair using weights, clamps ox vacu ~rxn bags.

11. Use a mould, for more complex shapes, made fi-om wood or other similar material.

12. Use a parting agent on the mould to prevent the resins from adhering to the mould.

13. Remove all traces of parting agent from the repair.

14. Inspect the repair, repaint and carry out functional check to check for radar transparency.

15. If a control surface check weight and mass balance and carry out control system check plus an independent check.

16. Record all work done and clear Log Book.

All materials & dimensions similar to the previous drawing

Fig. 5 EXAMPLE OF SMALL CORE REPAIR (1.5" BIA MAX)

CARBON FIBRE COMPOSITES (CFCs)

There is an increasing use of CFCs in the construction of aircraft. The advantages of this material over conventional metals are many and include:

k Good strength/weight ratio. .k Resistance to impact damage - often difficult to detect if it has

sustained damage. k Non-corrodible. A Easily moulded to complex shapes.

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9, 1s not aff'ect.ed by hydraulic and other fluids. * Does not suffer from cracking and has vexy good fatigue strength.

Like GRP it is made up of layers of fibre but carbon and not glass. It may be pre-preg (already pre-impregnated with resin) or may be carbon fibre material requiring a bonding agent between the layers. Once the layers are made up the resin is allowed to cure - usually using heat and pressure (vacuum bags).

Materials

(a) Resins and other chemicals. Stored at -18°C usually has a shelf life of 12 months - refer to manufacturers literature.

(b) CFC and Kevlar material stored in a dark room in their original plastic containers. Kevlar is affected by uv light.

(c) CFC pre-preg is stored at - 18°C and again may have a shelf life nf

12 months. May have a life of one month out of cold store.

All materials should be allowed to reach room temperature before being worked on. This usually means keeping at room temperature for a period of 24 hours.

Types of Structure

Sandw&Construction. Not unlike the sandwich construction of GRP. It is designed to have a light, reasonably weak centre with strong outer fibres. The outer fibres being in tension or compression with the centre being in shear.

Many combinations of composite (metal and non metal) can be used. Figure 6 is a typical example. The sandwich is usually made up of a honeycomb centre with multiple plies of composite pre-preg cloths laid at different angles to each other and cured under pressure in an autoclave.

HONEYCOMB CORE

Fig. 6 TYPICAL SANDWICH STRUCTURE

jVJonolithic Structux. Structural components such as sheet skin, angles, ribs, frames, top hat sections etc are made from monolithic material in a similar way to the build up of the outer layers of the sandwich structure.

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Figure 7 shows an A310 spoiler made from glass fibre reinforced plastlc (GFRP) skln panels and ribs. With the fittings being made from metal.

Mixed Structure. Figure 17 shows the construct~on of an A320 flap. It 1s a mixed structure with some monolithic and some sandwich components

Fig. 7 MONOLITHIC STRUCTURE

SANDWICH STRUCTURE MONOLITHIC PANELS

Fig. 8 EXAMPLE OF MIXED STRUCTURE

Like GRP, damage that does occur may be difficult to detect. It is therefi~re important that if damage is suspected then a thorough investigation is c arried out aver the whole area.

The cfarrlage is usually associated with impact and the inspection procedure is similar to that used with GRP.

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AREA OF IMPACT

I I SPREAD OF DELAMINATION

-\ /----- BROKEN INNER LAYERS

Fig. 9 IMPACT 'SPREAD' ON CFC SKIN

X--rays may be used to check for internal darnage/delamination on sandwich structures and ultra-sonics may be used on monolithic structures. When using ultra-sonics a couplant must be used between the probe and the part being tested (oil or grease on metals). For CFCs a rubber tyred wheel or wat js used.

Thermal Pulse Thermography (TPT) may be used. This process involves the use of a high intensity thermal pulse and the rate of diffusion is measured. An image of the thermal pattern is then displayed on a screen and a change in the pattern will indicate a defect,. Modern TPT systems will involve the use of computers for storage and analysis of data.

Repair

The repair process is similar to that which is employed with GKP structures.

Equipment

The equipment will vary depending on the type and level of the repair bein- carried out, but the following is a typical list of the equipment required:

* A CFC bay with everything kept scrupulously clean. * Repair heaters - electrical heater mats thermostatically controlled. * Vacuum pressure bags - to put the repair under pressure when

curing. * Temperature probes - to monitor the temperature of the repair

when curing. k Cold storage equipment. A Various tools including diamond coated saw blades and diamond

tipped drill bits. * Breathing equipment and a dust extraction plant. CFC particles

and dust are dangerous if breathed in and fumes from the chemicals are toxic.

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Repair Methods

These will be laid down in Chapter 51 of the SRM and may involve the use of infill, metal patching, GRP lay-up, CFC lay-up, core replacement etc,

Damage (and the repair) can be divided into three main groups:

* Negligible damage. May be repa~red/modified for cosmetic reasons or to stop the damage getting worse.

* Structural damage. Has to be repaired to maintain the integrity of the structure. May be a standard repair in the manual, or may require the approval of the aircraft manufacturer.

k Replacement damage. Severe damage that requires the replacement of the component.

When assessing the damage always inspect an area much larger than the 'obvious' damage as the impact shock can travel through the material and .how up some distance away. For example - if it is damage to a panel, check for security and damage at the panel attachments and check for transmitted shock into the surrounding structure.

Of course, all these types are damage are laid down in the SRM, as are the repair schemes.

In general the repair materials should be the same as the original component unless specified otherwise.

General Repair Procedure

Clean and dry the repair area. Remove the paint (by sanding) in the area taking care not to damage the fibres. Remove all traces of dust. Remove the damage. Check that all the damage has been removed. Scarf the edges as specified in the manual. The scarfed edge may have a taper of 20: 1. The core is removed by the use of a router. Check the repair limitations in the repair manual. The fibre layers are laid u p by hand and usually involve the use of pre-preg material. This may be laid up at 0°, 45" and 90" Use might be made of 'in-fill', an insert, blind rivets, bolts, metal patches etc. Allow to 'cold cure7 - use a vacuum bag or heat in an autoclave. Inspect the repair and repaint if necessary. Depending on what has been repaired check the system and sign for all the work done.

Page 84: Materials and Hardware

,BLIND RIVETS, COMPOSITE DOUBLER

Fig. 10 SKIN REPAIR USING RIVETS ADHESIVE & DOUBLER

COVER PLY PLY 6

PLY 5 PLY 4

PLY 3 PLY 2

PLY 1 ADHESIVE FILM

Q 0 -C-)- STEPPED CUT-BUTS

Completed lay-up before hot bonded cure

Fig.. 11 SKIN REPAIR HOT CURE USING PRE-PREG

Two basic methods of repair:

(a) 'Cold Cure'. Using room temperature (20°C min) or heater blankets. Curing can take up to 7 days but with heater blankets using temperatures of about 80°C the time can be reduced to less than an hour - depending on materials, type of repair etc.

(b) 'Hot Cure'. This process uses an autoclave with temperatures u p to 180°C and curing times as short as 45 minutes, again depending on materials and type of repair.

Page 85: Materials and Hardware

Repairs to Sandwich Structure

The damaged core is usually removed and the void filled with a mixture (of adhesive and thickening agent), or a core plug of honeycomb is bonded into position. The skin is then repaired in the same manner as already described.

COVER PLY

PLY 6 - PLY 5

PLY 4 PLY 3

FLY 2 - PLY 1

Completed wet lay-up before cure

Fig. 12 SKIN REPAIR USING COLD CURE WET LAY-UP

ADHESIVE / COMPOSITE DOUBLER

Fig. 13 COLD CURE REPAIR USING DOUBLER AND VOID FILLER

Delamination and Debonding

Delamination occurs when two or more plies become separated frorn each other - often due to impact. They may be repaired by layering or by injecting adhesive through the rivet holes (drilled iaw the repair drawing) and riveted up using blind rivets.

Page 86: Materials and Hardware

COMPOSITE DOUBLER HONEYCOMB ADHESIVE, ,-------- --PLUG

Fig. f 4 HOT BONDING USING A HONEYCOMB INSERT

Debonding occurs when the honeycomb core separates from the outer skin. Repair can be carried out by injecting adhesive into the honeycomb through holes drilled in the skin. Pressure should be applied to the skin to ensure a good bond between the skin and the core material.

DELAMINATION BLIND RIVETS

ADHESIVE INJECTED THROUGH RIVET HOLES

Fig. 15 DELAMINATION REPAIR

Metal Patching

The metal patch may be bolted or bonded into position. Metal patching does not attempt to restore the structure to its original strength or contour but is a quick method af repairing small cracks or limited damage to non-primary structures.

-, *, b, , '*, ., \, '., HYPODERMIC , \ ' \, *, *, ,.-*\ \ a,\.':..*-'~,, . . <, ...' .-

,.-2'i '', ..*' -*' AREA OF . DEB 0 N Dl

\ \ \

ADHESIVE '5

\ ',' , ', \' ,\

Fig. 16 BEBONDING REPAIR

Page 87: Materials and Hardware

POROUS PARTING FILM FIBRE GLASS SANDING PLY

BLEEDER CLOTH PLIES \ FIBREGLASS / GRAPHITE REPAIR PLIES

BREATHER PLIES ArER BLANKET SILICON RUBBER SHEET

ELECTRICAL VACUUM BAG

VAClJUM HOSE

TEMPEWTURE

/ ~ ~ 9 5 6 1 ~ 3 GRAPHITE FIBRE GLASS \ ADHESIVE FILLER CORE REPAIR \ PLIES CAP HONEYCOMB

PLUG

Fig. 17 TYPICAL REPAIR TAKEN OF A HONEYCOMB STRUCTURE USING 'COLD CURE' WITH A HEATER BLANKET, VACUUM BAG

& TEMPERATURE PROBES

Figure 17 shows an example taken from an SRM. Study the drawing and note the following:

* The repair plies, insert core and adhesive. * The vacuum bag - stuck down around the edges with bag seal

(double sided sticky tape), J; Parting films - to stop the bleeder cloth plieslrubber sheets from

adhering to the repair. * Bleeder cloth layers - to allow all air to be evenly drawn away from

the repair. * Silicon rubber sheet to allow an even pressure over the whole area

of repair. * The thermostatically controlled heater blanket with its electrical

supply. * The temperature probes (thermostats).

Void Filler -- Honeycomb Section

W h e n repairing honeycomb section where the honeycomb is removed the void must be filled with a core plug or filler compound. The type of filler will depend on the size of the void. In general small diameter voids are filled using:

A A n adhesive and thickener.

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* A resin mixed with micro balloons. The micro balloons are small phei~olic resin hollow spheres that help to produce a low-density (light-weight) filler.

* Foam etc.

For larger holes a core plug is manufactured from the same material as the original honeycomb and cemented into position with a resin mix/resin micro balloons mix. The minimum size of hole where a manufactured plug must be fitted is stated in the repair manual eg:

* ........................... A32013 10 above 2" in diameter * .......................... Boeing 737 above 0.5" in diameter * DC9 ................................... above 2" in diameter

Electrical Bonding

Some composite structures are electrically bonded to allow for an electrica: path. Aluminium foil may be used and external metal discharge strips. Ararnid (Kevlar) has an aluminium foil ply.

EXTERNAL DOUBLER Same thickness as skin

/ ADHESIVE PLUS THICKENER

Fig. 18 EXAMPLE OF A REPAIR TO DAMAGE NOT MORE THAN 2" DIAMETER ON THE A320

blank

Page 89: Materials and Hardware

EXTRA REPAIR PLY OR After fitment and when dry sand taper edges that are in "-.4, the a~rf low

Determine number of pl~es,

Apply resin mlx 3 to upper face orientation, and original material

of core plug just prior to fitment from component structure

of repair plies

CORE PLUG 2 plies of BMS 9-3 type H-2 or H-3 - Same material as original (or 4 plies of BMS type 9-3 type D) saturated with resin mix 2

\ Apply resin mix 3 to lower face & around core plug just prior to fitrnent

Taper sand or step

Fig. 19 REPAIR TO DAMAGE ON A 737 GREATER THAN 0.5" DIAMETER TO ONE SKIN & THE HONEYCOMB CORE

Apply XEA9390 & phenolic microballoons mixture to bottom & sides of both core plug 8 hole

TEFLON TAPE

\

Fig. 20 TYPICAL PLUG FIWING USED ON SOME AIRCMPT FOR DAMAGE GREATER THAN 1" DIAMETER

Page 90: Materials and Hardware

Cover plies are glass fibre if original

en use carbon

\

118" (3.2mrn) After damage cleaned out UNDERCUT core hole filled flush with

resin mix 4 or 5

Fig. 21. REPAIR TO DAMAGE 0.5" OR LESS TO ONE SKIN & HONEYCOMB CORE - 737

SPLICE STRIP

CLASS 350, GRADE 1 OR 2 ALUMINIUM FOIL PLY

ADHESIVE FILM

EXTRA REPAIR PLY

REPAIR PLIES

ADHESIVE FILM

'OR PLIES

Fig. 22 DETAILS OF A REPAIR TO MAINTAIN AN ELECTRICAL PATH ACROSS A FOIL COVERED SURFACE - B739

Page 91: Materials and Hardware

INSPECTION OF COMPOSITE STRUCTIIRES

To some extent composites can be more difficult to inspect for flaws than metal structures.

When subject to impact damage they can 'spring-back' and show little or no sign of impact, Certain NDT techniques will not work with conlposites, eddy current, magnetic particle etc, and whilst X-ray interpretation of negatives on metals can be difficult the results of csrnposite X-rays can be more so.

Defects in composites include:

Cracks. Bulges. Splitting - particularly inside the panel. Delamination. Debonding. Moisture/water ingress. UV (ultra violet) degradation. Erosion. Lightning strike damage. Fire damage. Signs of bowing and signs of damage to systems/eq~zipment inside the panel.

Bulges may be a sign of delamination or debonding and may be accompanied by water ingress.

Splitting is usually a sign of impact damage.

Debonding is the failure of the bond joint between 2 composite parts or between a composite part and a metal part. May be the result of impact damage or more likely, poor quality of the initial bonding process.

Delamination is similar to debonding but occurs between the layers of a built- up material .

Moisture ingress can result from impact damage or from a poorly made joint. Once in, the water can increase the damage area particularly if subject to freezing conditions. Can show up as stains on the surface.

Where cracks occur they are likely to run jnline with the weft or warp plies of the material. The likelihood of a crack occurring is considerably reduced by constructing composites of a weftlwarp material or laying consecutive weftless plies at right angles to each other.

UV radiation will degrade some composite fibres more than others, but al any rate UV absorbing additives should be used on all outer surfaces of composite build- ups.

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Erosion will occur on all leading edge surfaces (mainplanes, tail-planes, fins, propellers, rotor-blades etc) irrespective of the material they are made of. It is caused by small particles in the air such as rain, dust, insects, etc.

Lightning strikes show up as surface damage to the material, not too unIike impact damage, usually with signs of burning. Check a n y lightning conductor strips for security and damage. Carry out an electrical bonding check.

Electrical discharge damage to radomes may be difficult to detect. One method is to pressurise the radome (off the aircraft and in a safety cage) to about 3psi (20kPa) and check for leaks using uncured resin on the outside - which will bubble if there is a leak.

Resins will not usually withstand high temperatures and when burning will give off inflammable gases and thick smoke. When burned-off will leave the fibre yarns behind.

Visual Inspection

The area should be inspected in a good light for those defects listed above. The structure should be inspected both sides as splitting may occur on the inside of a panel where the only evidence of damage on the outside is a scuffmark.

Additionally if damage is suspected the edges of the panel/area should be inspected for signs of transmitted shock, The transmitted shock may show up as damage to an adjacent panel/area or to damage and looseness of attaching bolts, screws etc. It is important to note that if the panellarea has suffered impact damage it could have moved in sufficient to damage systems/services within the aircraft, so check these as well.

Coin Tapping

Where delamination is suspected a small metal object can be used to tap the area and check for a change in the sound when tapping good structure to when tapping un-sound structure. A coin about 1" (25mm) in diameter is ideal.

Tap lightly at the side of the area where delamination is suspected and continue tapping while moving across the area. Any delamination will be indicated by a change in the sound.

A tool called a Woodpecker can be used. This electronic tapping device has a small tapping head than can be moved over the area and the sounds observed as before. Two small feet allow the tool to be rested against the surface to be tested giving the tapping head the correct distance from the surface for best results. The tool can be connected to a CRT screen where the feedback signal can be displayed.

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Moisture Meter

A moisture meter ma.y be used for checking for slgxas of moisture ingress. The pencil size probe is held against the suspected surface and moisture is indicated either on a dial or an LED display,

Infra-Red Thermography

This has been developed by Airbus Industries for detecting water ingress in composite-sandwich structures.

Infra-red thermography is based on the principle that an object emits electromagnetic radiation, the intensity of which is related to its temperature. When a structure is heated and allowed to cool, water contaminated areas cool more slowly than 'dry' areas and these area can be detected using an infra-red camera. (The specific heat of water is 5 times higher than composite materials).

The infra-red camera converts the thermal radiation into an electronic signal, which is displayed in colour on a video screen.

WATER INGRESS AREAS

FASTENERS

Fig. 23 CRT SCREEN DISPLAY - INFRARED THEWOGRAPHY

Method

1. Clean and dry the area to be inspected (both sides). 2. Heat the area using a special electric blanket. The electric blanket is

temperature and time controlled so that it heats up slowly, taking at least 15 minutes to reach 60°C. The temperature is held at this value for a further 5 minutes.

3 . The blanket is removed and the airframe allowed to cool. As it cools the slower cooling 'wet areas' show up on an infra-red scanning camera.

4. One engineer operates the scanning camera while another views the output on a video monitor. Wet/damp areas are indicated in colour as shown in figure 23.

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5. When a wet area is shown the viewing operator tells the camera operator. The camera is held still and the area on the panel is marked for further investigatlor~ /repair.

Note. A typical system is the Agema Infra-red Systems Thermovision 2 LO which will detect a difference of O.l°C at 30°C to an area down to about lOmm x 1 Omm contaminated with 1 0% water - and located on the opposite side of the skin.

Ultra-Sonic Testing

lJsed mainly for detecting below-surface voids but also for surface flaws at a point some distance from the place of accessibility,

High frequency sound waves, when transmitted through solid material, are reflected by any discontinuity such as a void or a flaw. This reflection is converted into a signal on a cathode ray tube (CRT), which can be interpre by a trained operator.

These sound waves are above the audible frequency of the human ear and can be transmitted in three different forms:

a) Longitudinal . in the same direction as the motion of the sound. b) Transverse - perpendicular to the motion of the sound. c) Surface - transverse waves along the surface of the material.

The pitch of the sound is controlled by its frequency and its speed through the material by the characteristics of the material.

Each probe comprises a quartz crystal and sound damping material. When the crystal is fed with an ac supply, it vibrates at the frequency of the received input. These vibrations are passed into the material in a direction related the shape of the probe.

STRAIGHT TIR ANGLED TIR

PLASTIC WEDGE

COMPONENT SOUND WAVES

FLAW

Fig. 24 ULTRA SQNIC PROBES

- 20 -

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The receiver crystal is vibrated by the received sound waves and generates an ac supply, which is fed into the vertical axis of the CRT. The result is a line on the CRT with a number of vertical 'blips'.

To prevent any signal coming from the air gap between the probe and the surface a couplant such as oil is used.

CRT DISPLAY SHOWING NO FLAW

B O ~ J DARIES

FLAW

CRT DISPLAY SHOWING INTERNAL DEFECT

QUALIFIED NDT INSPECTOR

Fig. 25 ULTRA SONIC TESTING

On material without any flaws there will be a vertical to represent the top surface and another the bottom surface; the distance between them related to thickness of material (distance travelled by the sound waves).

"oid within the material will reflect the sound waves earlier and erect a ,xnaller vertical on the CRT between the first and second verticals at a position related to its distance from the surface.

Note that the screen displays shown in figure 25 show a clear indication of a defect. In reality the indications may be difficult to see and interpret. Also expertise is needed to operate the probe as defect orientation may require several passes using different sides of the material.

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Fig. 26 USING AN X-RAY MACHINE

Radiography

A user-unfriendly system that produces X-ray pictures to be analysed. Using either X or Garrlma rays which can pass through almost all materials and which are extremely dangerous to humans (as well as animals). This system is similar to photography.

X-RAY TUBE

Fig. 2'7 X-RAY EQUIPMENT

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These are generated in an electron tube needing 250,000 Volts to gve a better picture quality than gamma rays. The electron tube is relatively large gnving lirriitetl access.

Ganima Hays

Self-generated by radioactive isotopes, each isotope being about the size of a overcoat button. Access into small spaces is easier - into shafts, etc. Has a poorer picture quality than X-rays.

In general the process is a s follows:

(a) Set up equipment with X-ray tube on one side of the part to be checked and the (light sealed) negative on the other side.

(b) Place test piece in front of negative (this provides a density comparator on the negative so that comparisons can be made between it and the rest of the image).

(c) Check exposure times and distance of tube from part (distance measuring rod supplied).

Id) Clear hangar of personnel and place warning signs around aircraft. (e) From remote control panel switch on tube and monitor the area. (f) After the correct exposure time switch off tube and remove

equipment/ signs. Allow maintenance personnel back on aircraft, (g) Develop negative and analyse results.

The process requires a high level of expertise both in setting u p the equiprnent, calculating the exposure times and interpreting the X-ray results. The equiprnent is also dangerous to use. Operators require a regula- medical check-up and wear a personal radiation dosimeter. Always stay out of roped off weas.

QIJESTION If a defect was found using any of the above methods, what action would you take? (10 mins)

ANSWER If the crack or a void is in a component then it will normally require replacement, but check the manual first - some cracks might just be allowed if they run in a certain direction and/or are in a certain area and/or are below a certain length, but will normally need stop drilling. If a crack or void is in a structural member it may be classed as negligible (check the repair manual-- the same parameters may apply as above) and stop drill the ends of the crack. If the defect is outside the negligible limits then the area must be repaired in accordance with the S R M , or the part replaced.

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If porosity is found then check the repair manual, but in general the component is replaced, or the area is repaired.

QUESTION What does 'stop drill' the end of the crack rnean and why is it carried out? (5 mins)

ANSWER The exact. end of the crack is located (often very difficult) and a drill is used to drill a hole right through the cracked material. This has the effect of reducing the stress concentration at the crack end to a lower concentration on the wall of the drilled hole, so (hopefully) stopping the crack from propagating. Always inspect the crack a t a later date to see that it has not developed further.

QUESTION If an NDT team was to carry out an inspection on your aircraft, what would be their relationship to you as a licensed engineer? (5 mins)

ANSWER They would be requested by you or the senior engineer of the company to carry out the NDT test. Their findings would be recorded and signed for using their own documentation and they would report back to you (or the senior engineer). They would hand over their recorded findings and you would clear the defect in the log-book (if no defect was found or after rectification carried out) making reference to the NDT report.

QUESTION What parts of the aircraft would you carry out an NDT test. on and when? (5 mins)

ANSWER Those parts/components that the ChA/aircraft manufacturer or your company tells you to or a part that you are highly suspicious of. Airworthiness Directives/Service Bulletins will be sent from the CAA/manufacturer to all operators of your aircraft to carry out a particular check. The instructions will normally indicate a tim limit and if it says 'before next flight' it effectively grounds the aircraft. In some cases a report has to be sent back to the CAA/rnanufacturer of the findings.

Note. The student is advised to read Airworthiness Notice 94 - NDT Testing - Qualifications

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CONTENTS

Page

Wood 1 Timber 2 Seasoning 4 Diseases and defects 5 Aircraft woods 8 Adhesives 9 Aircraft wooden structures 16 Inspection of wooden structures 20 Repairs to wooden structures 29 Fabric covering 39

Mat erlals 40 Preparation prior to fabric covering 43 Covering methods 44 Joining fabric to fabric 45 Hand sewing 47

Repairs to fabric 54 Doping 61

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Wood h a s beer1 the main constructioll material for man for thousands of years and, hence when aircraft were invented (about 100 years ago), i t was used for a~r f rame construction

Compared with metal it has many advantages including:

* Light (density of spruce between 300 and 600kg/rrl" ((aumirlium 2800kg/m3). But wood densities vary - some car] be so dense that they are over 1000kg/m"rid will not float on water.

* Readily available, inexpensive and a renewable resource.

k Easily machined, drilled, filed, screwed, planed, sanded

Easily joined using wood-screws, nuts and bolts, nails/panel pins, adhesives, staples etc.

* Good thermal insulation.

* Nearly as strong a s alumirhiurn - but some aluminium alloys can be over 10 times stronger than wood, and steel can be over 6 0 times stronger.

Ilisadvantages include:

* Quality not consistent. Ever1 taking a specific type of wood, say sitka spruce. Depending on where the tree was grown and the rate of growth in any one year the wood quality can vary.

* Quality within a single plank of wood can vary due to knots, grain inclination, defects etc.

* The mechanical properties of wood are said to be anisotropic. In other words the strength and elastic properties are different whether they are measured along the grain or across the grain. Wood is much stronger in tension and compression along the grain than across it and stronger across the grain in shear.

* Wood can shrink and warp, is liable to rot, can deteriorate with age and is subject to insect attack.

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TIMBER

Timbers for commerce are obtained from exogenous trees (grow outwards in plan view, by the addition of layers or rings) Timber is divided into softwoods (coniferous or vvergreen trees) and hardwoods (deciduous). Trees are dormant In winter and cornmenee their growth in sprlng. The sap ascends the tree causing the gro~utkl of the 'springwood' and causing the tree to bud. The sap undergoes chenlical changes in the leaves due to the action of the sunlight and the carbon dioxide In the air. The sap returns down the tree during summer and wlnter and t h ~ s causes the growth of the 'autumn-wood'.

ANNUAL GROWTH RINGS

HEARTWOOD

MEDULLARY RAYS

Fig. 1 CROSS SECTION OF A TREE TRUNK

The tree trunk is rnade u p from Sapwood - the unripe part of the woody layers, porous and full of sap, h a s little strength, sugaraxy and invites decay and insects. The Bark - which consists of two layers - the outer layer being the protective cork-like covering and the inner layer termed the inner bark or phloem which is soft. Between the phloem and the wood is a skin-like layer termed the cambiun~. The tissue of the cambiurn combines with the rising and falling sap to form new growth rings each year. It is most active during the spring; the wood forming during that time is light in colour and of open texture. During the auturrin the cambium is less active and the wood formed is darker and denser. The difference between the autumn and springwood is clearly visible in trees such as firs and pines but is hardly noticeable in tre such a s teak and mahogany.

Medullary rays convey the moisture from the sapwood to the heartwood while the tree is growing. These are thin sheets of cellular tissue that radiate from the pith and extend lengthwise through the timber. The rays that extend right across (from pith to the bark) are termed primary rays and those that extend partially across are termed secondary rays. Medullary rays are more pronounced in such woods a s oak and beech.

Felling

'I'rees sllould be felled at the beginning of wir~ter in temperate climates and during the t l r j season in tropical climates.

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During these t i~nes the sap is at rest. At other times the wood contains too much sap which is difficult to dry out wlthout damaging ihe propertics o f the wood. When a tree is felled is governed by its state of maturity, which can be found by the state of its foliage Oak matures a t 120 - 200 years Firs and pines mature a t 70 - 100 years. Trees that are felled too young or too old have timber of inferior quality.

A tree 1s felled and stripped of its branches. Some logs are squared for ease of transportation. These are termed 'baulks'. Sawing logs and baulks into planks, deals, battens etc is termed conversion. The timber is square sawn.

Maxinlum shrinkage occurs along the lines of the annual rings. Timber used in aircraft is rift sawn to lesson the possibility of shrinkage.

MAXIMUM

G RAl N

indicated at ;ic l

Fig. 2 SHRINKAGE

RIFT SAWN YI

SLASH SAWN

QUARTER SAWN

Fig. 3 TYPES OF CUT

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SEASONING

On felling a tree may contain u p to 50% molsture Most of this must be rerrioved to obtain erio~lgh hardness, stiffness and resistance to decay, and to reduce shrinkage to acceptable levels. The degree of seasoning is measured by thc rnolsture content of the timber and is expressed as % moisture content of the dry weight of the timber. The moisture content may be measured using the Marconi Moisture Meter (or other commercially ava~lable meters) or by the following method:

1 . A small sarrlple of the timber is removed, weighted arid dried in an oven a t 1 OO°C (2 12°F) until two successive weightings are the same. Thc (YO moisture is then calculated as:

loss of weight x 100 = O/o moisture content <,

dry weight

'There are several ways of seasoning timber including the 3 listed below

Natural Seasoning - Slow but gives the best results. Planks are stacked undercover in such a manner as to allow maximunl ventilation and shielded from wind, s u n and rain. The wood is stacked in a Dutch barn (a roofed barn without sides), the first planks laid on wooden skids (keeps planks away from damp grass etc), and successive layers are interspaced by slats placed above one another to prevent warping of the planks. To prevent the ends of the planks splitting a s the timber dries, strips of hoop iron or wooden slats are nailed on. 'This seasoning takes from 1 to 9 years depending on the size of the planks and whether it is softwood or hardwood. I t is an expensive process.

Water Seasoning -- This is applied to logs or baulks and although quick is liable to diminish the strength and durability of the wood. The timber is put in a strearn of fresh sunning water with one end of the log towards the flow. Thus some of the sap is washed out by the force of the water going through. This takes about 10 days. After removing the log the internal water evaporates, '' : timber is then cut u p and seasoned the natural way for half the normal per-qd.

Artificial Seasoning - Kiln dried (hot air). Softwood boards can be seasoned in 10 to 14 days. The stacked planks are placed in a kiln in which the temperature is raised to 80 - 220°F (27 - 105°C) according to the type of timber. 'The steam heat (from pipes), warm air currents and the humidity is controlled to prevent the timber from drying too quickly and developing shakes (a type of split).

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DISEASES AND 1)EFECTS

Trees, subject a s they are to the hazards of nature, develop imperfect~ons during growth. Defects which occur in sawn timer can rnostly be elirnlrlated by usirig correct methods of conversion, seasoning, storage and preservation, but d~scases may be present in the tree and remain active after the timber has beer] converted anti seasoned. These diseases are caused by the action of fungi. A fungus is a kind of plant which can only live by feeding on organic material. These thread llke cells penetrate the wood, boring minute holes invisible to the eye, and absorb the substance of the wood as food, wkieh disintegrates the wood to a state called decay. Attack by fungi may be identified by discoloration of the timber, by mildew, by the reduction of the wood to a powder or by the wood turning into a soft spongy mass.

Dry Rot

This does not attack a living tree but attacks timber subjected to humid conditions combined with poor ventilation. Sapwood and unseasoned timber are most susceptible to this disease, which turns wood to a powder. This disease spreads rapidly and may be identified by a fungoid growth on the surface of the wood.

Wet Rot

This may occur in a living tree or sawn timber and is a decomposition of the fibres. In a living tree this may occur by water finding its way through the bark and in sawn timber by subjecting it to alternate wet and dry conditions. Wet rot transforms the timber into a soft spongy mass. A similar disease to wet rot is Druxiness, but in this instance the water does not enter into circulation with the sap bu t becomes stagnant, setting u p decomposition of the surrounding wood.

Causcd by a fungus growth. May be present in unseasoned timber and remains active after seasoning. It appears a s a stain or a group of speckled patches and reduces the wood to a very soft state. It is contagious and spreads through thc timber rapidly. When detected, the affected part most be rerrloved and burnt. If found in spruce planks intended for aircraft construction, the wood either side of the infection must be removed for a distance of at least 8 inchcs (203mm) in a longitudinal direction either side of the dote area

This 1s thc decay of over-rnature trees. C)n converted t~rnber it appears ;is a reddish brown stain.

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Kind Galls

These are swellirlgs on the trunk and branches of a tree caused by the growth of new layers over a wound made by the attack of lrlsects or by a branch having been br-oketl off. Rind galls reduce strength because they cause divergence of the grain.

Knots

These are the roots of the branches of the main tree trunk.

Live knots arp the roots of braches which were growing when the tree was felled ancl although all knots are a source of weakness, wood containing live knots can he accepted provided judgement is used to determine whether it is suitable for the work in hand. In sawn timber a dead knot can be identified by a dark ring of wood around its outer edge. If dead knots cannot be eliminated the timber should not be used.

Other types of knots include bud knots, pin knots arid spike knots.

All knots should be no more than 0.25" in diameter and if clustered too close together the wood should not be used.

Karnmy Grain

This is the narne given to wood w ~ t h a curly grain. Such wood is difficult to work and unsuitable fro structural members.

Incorrect Grain Inclination

The limit of grain inclination for spruce is 1 in 15 for grade A and 1 in 12 fr grade B. Grade A is used for aircraft structural work and the inclination sh*\uld be checked to ensure that the above limit is not exceeded. The most usual method of determining the inclination of the grain is by examining the flower- face of the timber to find the resin ducts. It will readily be seen whether they are straight or inclined. If the inclination exceeds the limits specified, the timber should be classified to a lower grade.

Cross and Spiral Grain

Cross grain is caused by a bcnd in the tree, a knot or incarrcct conversion. To prevent this through the latter reason, timber should be sawn so that the grain runs a s nearly a s possible parallel to the edge of the material. (Timber is stronger along the grain than in any other direction). Spiral grain is caused by high winds twisting the trunk.

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Heart-Shake

Shakes are small splits In the tirriber and should not be present in sectlons of timber intended for structural use on atrcraft

A heart shake usually follows the course of a sap duct long~tudinally anti is usually visible on the tangential surface 'The use of a small size feeler gauge will assist in finding the depth of the shake The defect sho~lld he cut out of the timber.

Ring-Shake

This defect is indicated by a parting of the annular rings. Ring-shakes are usually caused by frost, particularly after a heavy rainfall. The defect should also be cut out of the timber.

Compression-Sha ke

This defect appears on a cross-section and usually takes the form of a thin wavy line. Compression-shakes are most dangerous as they are a partial fracture of the timber and any future loads may cause the fracture line to spread.

Pitch Holes

There are two kinds of pitch holes, one being the horizontal type which usually appears at the base of a knot, and the other the vertical type which is sometimes referred to as a gum pocket. Gum pockets may either be 'alive' (the gum-seam has not dried out) or 'dead', and in the case of the latter, the timber should be rejected. 'rests on 'live' gum pockets have shown that the timber in the region of the gum pocket usually gives a better result than the remainder of the timber.

Blue Stain

This defect only occurs in sapwood which should not be used in aircraft parts.

Insect Attack

Shows up as the timber having small holes in the surface. Such woods rrlust not be used on aircraft.

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AIK(:KAFVT WOODS

Sitka Spruce

A soft~rood frorn Canada and U S A . Bright-brownlsh yellow with little or no odour. Straight grained and easy to work. Its stiffness, bending strength, hardness and its resistance tc) splitting are hlgh in relation to its weight. Because of this it is used on aircraft (spares, struts, longerons, ribs etc). It is seasoned to a rnoisture coritent of 15 to 17%.

Ash

Hardwood. Grown in the UK. Whitish-yellow, close and fairly straight grained. Tough and strong and has good shock resistance qualities but is not a s light a s Sitka Spruce. Used for longerons, trestle beams, bearing blocks etc. Moisture content 15 - 16%.

Mahogany

A hardwood from Central America. Reddish-brown to dull red in colour. Straight grained, strong and elastic with good bending strength, stiffness and corripressive strength along the grain. Also resistant to shrinking, swelling and lvarplng arid good glue retaining properties. Used for propellers, bearing blocks, rigging boards etc. Moisture content 14%.

American Black Walnut

A hardwood from Canada and USA. The heartwood is a rich chocolate brown colour and the sapwood is nearly white. It is a medium dense wood, hard and mostly straight grained. Good weather resistant properties and retains its shape well. Used for propellers and bearing blocks etc. Moisture content rr -st not exceed 13%.

Douglas Fir

A softwood from Canada and USA. Colour from pale reddish-yellow to deep orange-brown. Has prominent growth rings and mostly straight grained, is somewhat resinous and has a distinctive odour when worked. It is strong and tough and is used in aircraft coristruction that is highly stressed in bending and compression. Moisture content 10 - 17%.

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A hartJwood from Tropical America Soft mrit11 no strength I t js the lightest tirnber in general use and is pinkish-white to pale brown in colour. Must be stored carefully as moist conditions cause rapid deteriorat~on. Used sometimes for the core of sandwiched ply. About a quarter of the weight of other woods.

Note The weight of the timber (density) is governed by rate of development, moist~xre content and part of the tree from which it was cut

ADHESIVES

Adhesives are better for joining wood than, say, wood screws because its use avoids the stress concentration that the screws would produce. Also the wood is not damaged locally. The disadvantages of using adhesives are that subsequent dismantling of the joint is not possible and stricter process control is required to produce a satisfactory joint.

Glues fall into two main groups:

h Casein glues * Resin glues

Any glue that meets the requirements of the approved specification a s laid down by the C M and other authorities such as the FAA is satisfactory for use in civil aircraft. Synthetic resin adhesives should comply with British Standard 1204 in the UK for Weather and Boil Proof (WBP) and Moisture Resistance (MR). In all cases, glues are to be used strictly in accordance with the glue manufacturer's recommendations.

Casein glues are water-based and made from milk; they were used widely in wooden aircraft repair work. Modern casein glues for use in aircraft should contain suitable preservatives, such as chlorinated phenols and their sodium ialts, to increase their resistance to organic deterioration under high-humidity conditions. Most casein glues are sold in powder form ready to be mixed with water a t ordinary room temperatures, but some are supplied in liquid form.

Synthetic resin glues are more widely used now and usually consist of a two part nlix - a resin and a hardener. Once rriixed there is a chemical reaction that callses the adhesive to commence to harden. Synthetic resins are better in that they retain their strength and durability under molst conditions and after exposilre to water. The most comalonly used synthetic resin glues :ire the phenol -formaldehyde, resorcinol-formaldehyde and urea- formaldehyde types.

'The rc.sorcino1-formaldehyde type glue is recommended for wood aircraft applic a t ' lons.

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Inert fillers are often added by the glue manufacturer to the resin, such a s walnut shell flour, to give better working characteristics and joint-forxning properties (increased viscosity and gap filling properties).

The most suitable curing temperatures for both urea-formaldehyde and resorcinol glues are from 21°C (90°F) to 24°C (75°F) (room temperature). Temperatures below 15°C (60°F) are not recommended and electric blankets (80°C - 176°F) can be used to provide rnore rapid setting times. Gluing times can take u p to 2 to 3 weeks

Some terms used are as follows:

Cold Setting Adhesive. An adhesive which sets and hardens a t room temperature, ic 10°C to 32°C (50°F to 86°F) within a reasorlable period.

Close Contact Adhesive. A non gap-filling adhesive suitable for use only with those joints where the surfaces to be joined can be fabricated accurately and brought into close contact by means of adequate pressure and where glue 1 :s exceeding 0.005in (0.125mm) can be avoided.

Closed Assembly Time. The time between the assembly of the joints and the application of pressure.

Double Spread. The spread of adhesive equally divided between the two s ~ ~ r f a c e s to be joined.

Gap-filling Adhesive. An adhesive suitable for use in those joints where the surfaces to be joined may or may not be in close or continuous contact, owing either to the impossibility of applying adequate pressure or to slight inaccuracies in machining. Unless otherwise stated by the manufacturer, gap- filling adhesives are not suitable for glue lines exceeding 0.050in (1.25mm) in thickness.

Glue Line. The resultant layer of adhesive joining any two adjacent wood layers in the assembly.

Hardener. A material used to promote the setting of the glue. It may be supplied separately in either liquid or powder form, or it may have been incorporated with the resin by the manufacturer. It is a n essential part of the adhesive, the properties of which depend on using the resin and hardener as directed.

Open Assembly Time. The period of time between the application of the adhesive and the assembly of the joint.

Single Spread. The spread o f adhesive to one surfa:ace only

Spread of Adhesive. Thc amount of adhesive applied per unit area. Expressrd as g /ml or lb/ lOOft? Can be asccrtained by weighing a piece of scrap plywootl t~eforc application and n-cigh~ng after application

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Synthetic Resin. A synthetic resm (phenolic) is derlved from the reaction of a phenol with an aldehyde A synthetic resin (amino plastlc) is derived from the reaction of urea, thiourea, melamine or allied compounds with formaldehyde.

Synthetic Resin Adhesive. A composition substantially consisting of a synthetic resin, either the phenolic or amino type, but including any hardening agent or modifier which may have been added by the manufacturer or which must be added before use, according to the manufacturer's instructions.

Synthetic resins can be obtained either in liquid or powder form. In general, powder resins have the longest storage life, since they are less susceptible to de terloi-ation from high ambient temperatures.

Powder resins must be mixed with water in accordance with the manufacturer's instructions before they can be used in conjunction with a hardener. To obtain satisfactory results, it is essential that they be properly mixed. Once mixed, the adhesive must not be diluted unless this is permitted by the manufacturer's instructions. In many instances, manufacturers specify 3 definite period of time which must elapse between the mixing and the application of the adhesive. During this period, the adhesive should be covered to prevent contamination. When resins are supplied in liquid form, they are ready for immediate use in conjunction with the hardener. Liquid resin must not be diluted unless this is permitted by the manufacturer's instructions.

When mixing the hardener with the resin, the proportions must be in accortlance with the manufacturer's instructions. Hardeners should riot be permitted to come into contact with the resin except when the adhesive is mixed prior to use.

Any utensils used in the hardener should not subsequently be used in the resin and vice versa. After use utensils should be washed in water containing 5% sodium carbonate (washing powder). Typical synthetic resin adhesives include:

4raldite. General glue and used for bonding timber to metal or fibreglass. Supplied in two parts - a liquid resin and a liquid hardener. When mixed in the correct proportions is applied to both surfaces, the surfaces are clamped togethcr and setting time depends on temperature.

Aerodux. Also supplied in two parts, a liquid resin and a liquid or powder hardener. The joint is made as for Araldite but curing times can be long.

Aerolzte. The resin is supplied in powder form to be mixed with water or already in liql~id form. The hardener is an acid and comes in three strengths. Medium strcngth (coloured green) is usually used. The resin is applied to one surface, the hardener to the other and the surfaces are brought together and clamped. C~~rir ig tjme can be as short a s one hour when heating is applied.

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G L U I N G

The surface to be p ined rnrlst be clean, dry and free from grease, oil, wax, paint, etc. It is important that the parts to be joined have approximately the same rno is t~~re content, since variations will cause stresses to be set up because of swelling or shrinkage which rnay lead to the failure of the joint. A safe range for moisture content js between 8 and 16%.

The wood to be glued should be at room temperature. The surfaces to be joined should not be overheated since this affects the surface of the wood and reduces the efficiency of most synthetic resin adhesives.

Synthetic resin adhesives are sensitive to variations in temperature. The usable (pot) life of the adhesive, proportion of hardener to use and clamping time all depend largely on the temperature of the room a t the time of gluing.

The Wood Surface

Plywood surfaces should be lightly sanded either in the direction of the gral,~ or diagonally across it.

Timber surfaces should be sanded using a medium grade glass paper or a wood scraper. To ensure a good fit the parts can be assembled first, dry with a layer of chalk on one surface. It the joint is a good fit the chalk will transfer over the whole area to the other surfzce. The chalk must be completely removed before the glue is applied.

Glue Application

It is generally desirable to apply adhesive to both surfaces of the material. This applies particularly where the glue line is likely to be variable or when it is not possible to apply uniform pressure.

Adhesive can be applied by a brush, glue spreaders or rubber rollers that b - e slightly grooved. The amount of adhesive required depends largely on the type of wood and the accuracy of machining. Dense wood requires less adhesive than soft or porous types. Adhesive should be applied generously to any end grain. Smooth, side-grained surfaces may be satisfactorily glued with a thinner spread. The general rule is that the adhesive should completely cover the surfaces to be glued and remain tacky until pressure is applied to the joint.

Difficult gluing conditioiis may occur when a soft wood is to be glued to a much denser wood because the adhesive tends to flow into the more porous wood. I n such instances, rxnless otherwise specified by the manufacturer of the adhesive pre-coating and partial drying of the softer surface, before normal spreading, is recomrnenclec-l .

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It is advised that the joint is first assembled dry t o check for correct assembly, glue line clearances, clamp positions etc - then disxnantled and assem bled correctly using glue. The interval between the applicat~on of the adheslve to thc surfaces and the assembly of the joint should be kept a s short as possible. Sorne adhesives contain solvents which should be allowed to evaporate before the joint is assembled If this is not done, bubbles may be created and result in a weak joint. For adhesives of this type, the manufacturer will specify a time interval which should elapse before the joint is closed.

To ensure that the two surfaces bind properly, pressure must be applied to the joint. This pressure should be applled evenly over the complete joint using clamps and blocks of wood to provide an even pressure and prevent local compression damage to the joint itself.

The pressure is used to squeeze the glue out into a thin continuous film between the wood layers, to force air from the joint, to bring the wood surfaces into intimate contact with the glue and to hold them in this posation during the setting of the glue.

Pressure should be applied to the joint before the glue becomes too thick to flow and is accomplished by means of mechanical clamps, hydraulic clamps, screw presses, electric power presses, brads (a sort of nail), nails and screws.

Non--uniform gluing pressure commonly results in weak and strong areas within the same joint. The amount of pressure required to produce slrong joints may vary from 125 to 150psi for softwoods and 150 to 200psi for hardwoods. Insufficient pressure and/or poorly machined contact surfaces results in thick glue lines, which are weak and should be avoided.

On small joints such as those found in wood ribs, the pressure is usually applied only by nailing the joint gussets in place after spreading the glue. Since small nails must be used to avoid splitting, the gussets should be comparatively large in area to compensate for the relative lack of pressure. At least four nails (cement-coated or galvanised and barbed) per square inch are "o be used and in no event must nails be more than 3hin (19mm) apart. Small orass screws may also be used.

Use handspring clamps only when gluing softwood. Because of their llnlited pressure area, they should be applied with a block of wood at least twice as thick as the member to be clamped.

High clamping pressures are neither essential nor desirable, provided good contact between the surfaces being joined is obtained. When pressurt. is applied, a small quantity of glue should be squeezed from the joint. 'Th~s should be wiped off before it dries. The pressure mrlst be mainta~ned dur-ang the full setting time. This is important since the adhesive will not reunite if disturbed before it is fully set.

Clamp tightness should be re-checked 10 minutes after the joint is assernblecl.

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The se t t~ng time depends on the temperature a t which the operation is carried out. An increase in temperature results in a decrease in tlic setting period.

Full jolnl strength and resistar.lce to moisture will develop only after conditioning for at least '2 days In some cases a period must elapse of u p to 3 weeks for the chemical reaction ta be fully completed. Again, this depends on the amb~en t temperature and tlre type of hardener used. Usually when repairs are made, the joint will be of reasonable strength after 1 day.

When gluing large areas (areas of ply for example) the drawing may specify drillings at intervals in one ply rriernber to allow any trapped air to escape.

Local warmth may be applied using electric blankets, electric fires, electric lamps, kilns etc. Remember, DO NOT EVER HEAT the joint - this can scorch the wood and/or bubble the glue - in either case a weakened joint results.

Testing Glued Joints

Glued jo~n t s are impossible to examine properly. The only access to the joirlL, once assembled, is along the glue line - and only then if it is visible. So, just like the adhesive bonding of metal structures, strict control of the gluing process is required a t all times with test pieces produced - to be tested to destruction to ascertain the strength of the joint.

Ideally, the test piece should be cut from the actual cornponent being assembled (make the part that much longer to allow for the removal of the test piece).

The test sample should be 1 inch (25mm) wide and a t least 2 inches (50mm) long. The test pieces should be joined with an overlap of Y2 to 3/4 inch (13 to 19mm). The glued test sample should be placed in a vice and the joint broken by exerting pressure on the overlapping member.

FORCE A WOOD FRACTURE

DRAWING FROM CAP 562 Fig. 4 TYPICAL SATISFACTORY BROKEN TEST PIECE

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Ideally the wood should break and not the glue line, but at any rate the fractured glue face should show at least '75% of the wood fibres broken evenly disl ributed over the glue surfaces (figure 4).

Where repairs are to be made on old aircraft in which the wooden structure is joined with a casein cement, all traces of the casein cement must be removed from the joint, since this material is alkaline and is liable to affect the setting of a synthetic resin adhesive. Local staining of the wood by the casein cement can, however, be disregarded.

Wet Tests

When specified, wet tests should be rnade for testing the efficiency of the adhesive after immersing the test samples in water at different temperatures and for different times. Such tests are prescribed in British Standard 1204, but the results are only valid if BS 1204 test pieces are used. However, testing joints, in a manner similar to that already outlined, after immersion in cold water (15" to 25°C [GO0 to 77"FI) for 24 hours, will give a good indication of whether they are satisfactory. Such tests should only be carried out on joints which have been conditioned for 2 to 3 weeks.

Failure of Glued Joints

Glued joints are designed to provide their maximum strength under shear loading. If a glued joint is known to have failed in tension it is difficult to assess the quality of the joint, as these joints may often show an apparent lack of adhesion. Tension failures often appear to strip the glue from one surface leaving the bare wood; in such cases, the glue should be examined with a magnifying glass, which should reveal a fine layer of wood fibres on the glued surface, the presence of which will indicate that the joint itself was not at fault.

If examination of the glue under magnification does not reveal any wood fibres but shows an imprint of the wood grain, this could be the result of either pre- a r e of the glue prior to the application of pressure during the manufacture of the joint, or the use of surface-hardened timber. This latter condition is particularly common with plywood and with other timbers which have been worked by high-speed machinery and have not been the surface correctly prepared.

If the glue exhibits an irregular appearance with star-shaped patterns, this m a y be an indication that the pot-life of the glue had expired before thc joint was made or that pressure had been incorrectly applied or maintained. In all such instances other jo~rlts in the aircraft knowri to have been made at the sarne time should be considered as suspect.

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Old Aircraft Repajrs

Where repairs are to be carried out on old aircraft in which the structure is joined with a casein glue, all traces of the casein should be removed from the joint since this material is alkaline and is liable to affcct the setting of a synthetic resin adhesive. Local staining of the wood by the casein can, however, be disregarded Where urea formaldehyde (UF) glues are to be used, the surface should be wlped wath a solution of 10940 w / w acetic acid in water, and allowed to dry before the glue 1s applied.

Note. This process must. only be used with urea formaldehyde (UF) glues. If used prior to the application of, for example, resorcinol formaldehyde (RF) glues, the joint strength could be seriously impaired.

Storage

Mixed adhesives have a very limited pot-life and any spare mixture left over after the completion of a task should be discarded straight away.

Unmixed resins and hardeners have a shelf life and this should not be exceeded. Resins in powder form which show signs of caking or corrosion of the container and liquid resins which show signs of 'gelling' or have become excessively viscous, should be rejected even if shelf life has not been exceeded.

Glues and resins should be stored in their original containers in clean dry conditions out of direct sunlight. The temperature should not exceed 2 1 "C ('70°F). Glues and resins should be used on a "first in first out7' basis.

AIRCRAFT WOODEN STRUCTURES

The basic structure of an aircraft made of wood is not too unlike a n aircraf made from metal or composite - in principle.

The structure can either be:

* Non-monocoque -k Monocoque A Semi-monocoque

Non monococpe structures are those built on the beam principle. The fuselage, for example, is made up of longerons and struts made of wood. These are compression members. Any tensile loads in the strrrct ure are normally accommodated by tension wires. The whole structure is covered with fabric (natural or synthetic) to provide an aerodynamic shape. Secondary structure may be added to improve the stxeamlining.

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Figury 5 shows a typical wood structure rear fuselagt. whcre all the strength is taken by the longerons and vertical and horizontal cross nlembers. Figure 6 shows a typical wing structure with a frorit and rear spare (with somctirnes an intermediate spar) to take the rnain bending loads and ribs to give strength and shape to the aerofoil. The whole wmg is covered with fabr~c and it is quite common to cover the leading edge with plywood.

Figure 7 shows two examples of main spars. A s with all spar construction the main principle is to get as much structure a t the top and bottorn of the spar, To this end spars may be constructed using a web to support to support cap strips or flanges a t the top and bottom - or the spar constructed as a box with the main strength (spruce) members separated by ply webs.

Some spars may be made as a sirnple rectangular cross section - less expensive but with a poorer strengthlweight ratio. Tailplanes and fins will normally be constructed similar to the mainplanes.

FABRIC COVERING

PLYWOOD COVERING\

HORIZONTAL. SPACEHSISTRUTS

Fig. 5 NON-MONOCOQUE FUSELAGE

FRONT SPAR MAIN SPAR ATTACH M EN1 ATTACHMENT

WING - FABRI COVERED

Fig. 6 WING CONSTRUCTION

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Figure H shows two examples of the construction of ribs. These are both truss type ribs made u p of square cross section spruce cap strips glued and p~nned to each other using ply gussets. Some ribs may not be of open construction (top rib ln figure 8) b u t may have a complete ply covering - in some cases with lightening holes to reduce the overall weight.

E B

CAP STRIPS OR FLANGES

pizmzq (IBEAiSPARl

Fig. 7 SPAR CONSTRUCTION

TRUSS TYPE RIB

PLY COVERI~G TO BOTH SIDES OF RIB STRUCTURE

Fig. 8 RIB CONSTRUCTION

Monocoque structures are rare but do exist - a t least for fuselages. Normally made of plywood which is formed into a hoop to provide all the structural reqrrire~nents of the fuselage as well a s all the aerodynamic requirements. With rnonococlue structure there is no internal bracing.

The de-Havilland Mosquito's rear fuselage is made of a plywood-balsawood -

plywood sandwich construct ion that forms a monocoque structure with no internal support (the same principle a s a chicken's egg). The inside of the fuselage is quite smooth except where brackets and 0thr.r fittings are attached to take supports for flying control cables, electrical and radio cables, ecluiprr~ent etc.

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CROSS SECTION OF FUSELAGE.

LOW DENSITY BALSA WOOD INFILL

Fig. 9 MONOCOQUE FUSELAGE

Semj monocoque structure (where the skin takes some of the load) is common with metal aircraft. For wooden aircraft i t would involve the aircraft's skin k i n g strong enough to take some of the load and this could only happen if the skin was made of plywood with some internal support such a s frames and stringers.

HORIZONTAL & VERTICAL SPACERS OR STRUTS

Fig. 10 SEMI-MONOCOQUE STRUCTURE

Figurc 10 shows the rear fuselage of the de-Havilland Rapide. It has four longerons with vertical and horizontal spacers/strnts. I t is basically syllnre in cross- section with a complete covering of plywood.

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tiat~r-~i: Covering

'I'kte joining of wooden parts of the structure has already been dealt with but little has been said of h o w the fabric is attached. It rnay be fitted to the skeleton of the airframe by:

Tying on WI th string. * Fitting thr fabric covering a s a 'sock' over the wing/fuselage. + Clamplrlg on with special metal clamps.

Once the fabric is fitted on the airframe it is tautened by doping or the application of heat and weatherproofed - using paints.

Fabric covering - and repairs -- will be dealt with in more detail later.

What follows is a general guide a s to the checks and inspections to be carried out on wooden structures.

INSPECTION OF WOODEN STRUCTURES

When inspecting wooden structures it is most important that the relevant aircraft maintenance manual be consulted.

'This part of the book gives guidance on the inspection of wooden aircraft structures for evidence of deterioration of the timber and glued joints. I t should be read in conjunction with the relevant aircraft manuals, approved Maintenance Schedules and manufacturer's instructions.

Glued Structures

Provided that protective varnish was applied to all exposed wood surfaces after gluing and the aircraft satisfactorily maintained, deterioration of the timber and glued joints is unlikely. However, deterioration is possibly for many reasons and the structure should be inspected regularly. Factors which m:Ad cause deterioration include:

a) Chemical reactions of the glue itself due to ageing or moisture, or to extremes of temperature or to a combination of these.

b) Stresses set u p due mainly to timber shrinkage.

c) Development of mycological growths (ie fungus).

(1) Oil corltamination from the engines, hydraulic systems etc.

c) Fuel coritamination due to fuel system leaks or spillage in the tank bays.

f) Rain water jrigress and blockage of drainage holes.

Alr-craft which are exposed to large cyclic changes of temperature and humidity arc. especially prone t o timber sl-~rinkage which in turn rnay lead to glue dctc-rioration.

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The amount of movement of timbers due to these changes varies with the volume of each structure member, the rate of growth of the tree from wh1c.h the timber was cut and the way in which the timber was converted. Thus, turo large members secured to each other by glue, are unlikely to have identical characteristics and differential loads will, therefore, be transmitted across the glue joint due to humidity changes. This will impose stresses on the glued joint which, in temperate zones, can normally be accommodated when the aircraft is new and for some years afterwards. However, with age the glue tends to deteriorate, even when the aircraft is maintained under ideal conditions and these stresses may cause joint failure.

In most wooden aircraft of monoplane construction the main spars are of box formation consisting of long top and bottom transverse members (ie spar booms) joined by plywood webs. The spar booms may be built up from laminations glued together and at intervals vertical wooden blocks are positioned between the two booms to add support to the plywood sides.

The main spars carry most of the loads in flight and are, a t times, subject to flexing. The glued joints should, therefore, be free from deterioration but, unless the spar is dismantled or holes cut in the webs, internal inspection may be virtually impossible.

Long exposure to inclement weather or strong sunlight will tend to deteriorate the weatherproofing qualities of fabric coverings and of surface finishes. I f fabric covered ply structures are neglected under these conditions the surface finish will crack, allowing moisture to get to the wooden structure resulting in deterioration through water soakage.

Aircraft General Structural Survey

Before commencing a detailed exarnination of the aircraft structure, the structure should be inspected externally for signs of deformation, such as warped wing structures, tail surfaces out of alignment or evidence of obvious structural failure. It may be prudent to carry out an airframe rigging check (see the appropriate book in module 7).

The aircraft should be housed in a dry, well-ventilated hangar and all inspect.ion panels, covers and hatches removed. It may be necessary to rernove sections of fabric. (There is a CAA requirement [AN501 that all older wooden aircraft are dismantled/opened-up from time to time to inspect a representative sample of the wooden structure and any unserviceable wood replaced.) The aircraft should be thoroughly dried out before examining glued joints or carrying out repairs.

Should any defects be found in the opened-up section of the airframe thYn further parts will have to be inspected by removing fabric covering fron~ niore parts of the airframe. It is possible that, if significant deterioration is fourrtl, the aircraft will have to be completely uncovered and, after suitable rectificat Ion, completely recovered.

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Immediately on opening an inspection panel, or any enclosed area a check should be made for smell. Each component should be sniffed. A musty smell indicates fungoid growth or dampness and, if present, necessitates a further examination to establish which areas are affected.

DRAWING FROM CAP 562 Fig. 11 DOUBLE SKIN FUSELAGE STRUCTURE

Where the wings, fuselage or tail unit are designed as integral stressed structures, such as inner and outer ply skins glued and screwed to s t r u c t ~ .1 members (figure 11) no appreciable departure from the original contour or shape is acceptable.

Where single skin plywood structures are concerned, some slight sectional undulation or panting between panels may be permissible (check SRM) provided the timber and glue Joints are sound. However, where such conditions exist, a careful check must be made of the attachment of the ply to its supporting structure. To check this, apply a moderate force by hand to push the ply from the structure. A typical example of a single skin structure is illustrated in figure 1 2.

STRUCTURAL MEMBERS

SCREWS PLYWOOD \

Fig. 12 SINGLE SKIN STRUCTURE

The contours and alignment of leading and trailing edges are susceptible to deformation and should be checked carefully. Any distortion of these light ply and spruce structures could indicate deterioration and a careful internal inspection should be made. lf a general, check for security and any deterioration - if found check the main wing structure also.

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Where there are access panels or inspection covers on the top surfaces o I fuselages, wings or tailplane, check that water has not entered. If it has, ( heck for internal deterioration and when refitting the inspection panels ensure that they are waterproof.

Splits in the proofed fabric covering on plywood surfaces should be invest gated by removing the defective fabric in order to ascertain whether the ply is serviceable. It is common for a split in the ply skin to be the cause of a s~rnilar defect in the fabric covering.

Fabric having age cracks and thick with repeated dopings, may indicate that the structure underneath has not been critically examined for some time. Insertion patches in the fabric could also indicate that structural repairs liave been made a t that point.

Whilst a preliminary external survey may be useful in obtaining a general assessment of the condition of the aircraft, it should be remembered that timber and glued joint deterioration often takes place inside a structure without any external indications. Where moisture enters a structure, it will cend to find the lowest point, where it could stagnate and promote rapid deterjoration.

Inspection of Timber and Glued Joints

Assessment of the integrity of glued joints in aircraft structures presents difficulties since there is no positive NDT method of examination which will give a clear indication of the condition of the glue and timber inside a joint. The position is made more difficult by the lack of accessibility for visual inspection.

The inspection of a complete aircraft for glue or wood deterioration will necessitate checks on remote parts of the structure which may be known, or suspected trouble spots and, in many instances, are boxed in or otherwise inaccessible. In such instances, considerable dismantling is required and it may be necessary to remove all the fabric and to cut access holes in ply structures to facilitate the inspection. This must be done only in accordance with approved drawings or the Structure Repair Manual (SRM) for the aircraft concerned and, after the inspection has been completed, the structure must be made good and re-protected.

All known or suspected trouble spots must be closely inspected regardless of log book records indicating that the aircraft has been well maintained ancl properly housed throughout its life.

Note. Where access is required and no approved scheme exists, approval should be obtained from the aircraft manufacturer or an organisation approved by the C M for such work.

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Ply Access Holes

In general, access holes are circular in shape and should be cut with a sharp trepanning tool to avoid jagged edges. I t is essential to avoid applying undue pressure to the cutting tool, especially towards the end of the cut, otherwise damage may be caused to the inner face of the panel by stripping off the edge fibres or the ply laminations.

Where rectangular access holes are prescribed care is necessary to ensure that they are correctly located and that corner radii are in accordance with drawing requirements.

NUT & BOLT

PLYWOOD SKIN

DRAWING FROM CAP 562 Fig. 13 GLUE LINE CHECKS

The edges of all access holes must be smoothed with fine glasspaper, prefc ,bly before inspection is commenced, since contact with the rough edges may damage fingers (cuts and splinters) and cause wood fibres to be pulled away.

It is important that the whole of the aircraft structure - front fuselage, rear fuselage, tailplane, fin, elevators, rudder, ailerons, flaps, slats, struts, etc - is inspected in detail before any decision is reached regarding its general condition.

Remember, when cutting an access hole it is most important that damage is not done to structure (or components) the other side of the hole.

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Glue Line

When checking a glue line (at the edge of the glued joint), all protective paint coating should be removed by careful scraping. It is important not to damage the wood in any way nor to mark or damage the glue line.

A good source of light is needed together with a magnifying glass, feeler gauges and remote viewing mirrors, intra- scopes etc,

Where the glue line appears to tend to part, or where the presence of an actual glue line cannot be detected or it is suspect, then, providing the wood is dry , the glue line should be probed with a thin feeler gauge and, if any penetration is possible, the joint should be regarded as defective.

Notes

1. It is important to ensure that the surrounding wood is dry, otherwise a false impression of the glue line would be obtained due to closure of the joint by the wood swelling.

2. Where pressure is exerted on the joint, either by the surrounding structure or by bolts or screws, this pressure should be relieved so a better assessment of the glue line may be made.

The choice of feeler gauge thickness will vary with the type of structure, Gut a rough guide is that the thinnest possible gauge should be used. Figure 13 indicates the points where checks with a feeler gauge should be made.

Timber Condition

Dry rot and decay are usually easy to detect. Dry rot is indicated by small patches of crumbling wood, whilst a dark discolouration of the wood surface or grey streaks of stain running along the grain are indicative of water penetration. Where such discolouration cannot be removed by light scraping +he part should be rejected or repaired as per the AMM. Staining of the wood ~y the dye from a synthetic adhesive hardener can be disregarded.

Water Penetration of Structure

If this js suspect in an area where there are some wood screws remove one or two and check if they are corroded (figure 14).

Slight corrosion of the screw due to the adhesive may occur during original construction, so the condition of the screw should be compared with that of a similar screw, removed from another part of the structure known to be 11-ee frorn water ingress.

Excess corrosion will warrant further investigation as to the cause.

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BULKHEAD FRAME SKIN SPACER

WOODSCREW

CORROSION caused by moisture ingress and possibly indicating glued joint failure

DRAWING FROM CAP 562 Fig. 14 CHECKING FOR WATER INGRESS

Note. Plain brass screws are normally used for reinforcing glued wooden members, although zinc coated brass is sometimes used. Where hard woods such as mahogany or ash are concerned, steel screws are sometimes used. Unless otherwise specified by the aircraft manufacturer, it is usual to replace screws with new screws of identical length but one size larger.

The removal of bolts, bushes, support brackets, metal fittings etc can also provide a means whereby water ingress can be checked. Be careful to ensure that any items attached to the airframe by these bolts etc are properly supported before the bolt, bush etc is removed.

Main/rear spar bolts/ bushes may be removed (again ensuring adequate support of fuselage/wing, tailplane etc). Primary joints may have bushed holes and the bushes should also be withdrawn. Corrosion on the surface of these bolts and bushes and timber discolouration, will provide a useful indicatiol ,f any water penetration. Bolts and bushes should be smeared with an appr zd protective treatment before being refitted through wooden members.

Note. When refitting bolts it is important to ensure that the same number of shrinkage washers are fitted as were fitted originally.

Experience of a particular aircraft will indicate those parts of the structure most prone to water penetration and entrapment (eg a t window rails or the bottom lower structure of entry doors), but it must be remembered that this is not necessarily indicative of the condition of the whole aircraft.

All drain holes should be kept clear of debris, paint, dope etc.

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Water Penetration of Top Surfaces

The condition of the weather-proofed fabric covering must be checked (set. later text). If in any doubt about its weather-proofing or if t.here are any signs of poor adhesion, cracks or other damage, it should be peeled back to allow a more thorough inspection.

Where the fabric covers a plywood layer the condition of the exposed ply surface should be examined and if water penetration has occurred, this will be shown by dark grey streaks along the grain and a dark discolouration at ply joints or screw countersunk holes, together with patches of discolouratior~. If these marks cannot be removed by light scraping, or in the case of advanced deterioration, where there are small surface cracks or separation of the ply laminations, then the ply should be replacedlrepaired iaw the SRM.

The fabric can be replaced/repaired after the ply repair.

Qther Defects

Of course, water/moisture penetration is not the only defect that can occur to wooden structures. Below are listed others that should be examined for.

Shrinkage. This can induce stresses in glued joints and cause looseness of metal fittings or bolts and, if fluctuating loads are present, can result in damage to the wood fibres a t the edges of the fittings or around the bolt holes. Shrinkage can be detected by removing any paint or varnish as described above and attempting to insert a thin feeler gauge between the timber and the fitting or bolt head.

Elongated Bolt Holes. All bolt holes should be examined for elongation or local surface crushing of the wood fibres. The bolts should be removed to facilitate the examination and, in some cases, the bolt itself may be found to be strained. Rectification of elongated bolt holes is carried out in accordance with the SRM, '-he usual method being to open out the holes and fit steel bushes.

Remember, when removing bolts to support the structure that the bolt is holding.

Bruising and Crushing. Bruising or crushing of structural members can be caused by over-tightening of bolts, excessive loads being placed on the structure during maintenance etc. Repair schemes for such damage arc governed by the extent and depth of the defect and given in the SRM.

Compression Failures. Sometimes referred to as compression shakes, art: due to rupture across the wood fibres. This is a serious defect which at times is difficult to detect and special care is necessary when inspecting any wood(:n member which has been subjected to abnormal bending or compressive loads which may occur during a heavy landing.

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In the case of a member having been subjected to an excessive bending load, the failure will appear on the surface which has been compressed, usually at a position of concentrated stress such as at the end of a hardwood packing block. The surface subject to tension will normally not show any defect. For a member taking an excessive direct compressive load, the failure will usually show on all surfaces.

Where a compression failure is suspected, a strong light source shone along the member, in line with the grain, will assist in revealing the disruption of the grain lines.

Previous Repairs. Not really a defect, but when carrying out a structural examination always inspect repairs carefully for integrity.

,Joint Failure

A glued joint may fail in service a s a result of an accident, poor workmanshi-, or due to excessive loads being imposed. It is often difficult to decide the n? ire of the load which caused the failure, but it should be borne in mind that g l ~ c d joints are generally designed to take shear loads.

If a joint is designed to take a tension load, it will be secured by a number of bolts or screws (or both) fairly closely pitched in the area of the loading. If a failure occurs in this area, it is usually difficult to form an opinion of the actual reasons for it, due to the break-up of the timber occurring close to the bolts.

In all cases of glued joint failure, whatever the direction of loading, there should be a fine layer of wood fibres adhering to the glue, whether or not the glue has come away completely from one section of the wood member. If there is no evidence of fibre adhesion, this may indicate glue deterioration, but if the imprint of wood gain is visible in the glue this is generally due to 'case hardening' of the glue during construction of the joint and the joint has always been below strength. If the glue exhibits a certain amount of crazing or sta- shaped patterns, this indicates a too rapid setting time, or the pot life of th, glue has been exceeded. In these cases, the other glued joints in the aircrh,, should be considered suspect.

Damage caused by a heavy landing may be found some distance away from the landing gear attachment points. Secondary damage can be introduced by transmitted shock from one end of a strut or bracing to the other, causing damage well away from the point of impact. A thorough inspection of the existing paint or varnish a t suspected primary or secondary impact points may reveal, by cracks or flaking, whether damage has actually occurred.

Note.

If the aircraft is stored outside or in hot dry conditions then special checks will be required for deterioration of wood, joints, fabric and painting and doping finishes.

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REPAIRS TO WOODEN STRUCTURES

Whenever wooden parts sustain damage, a detailed inspection must be carried out - firstly around the primary damage area (where the initial impact occurred) and then the surrounding area to check for secondary damage Secondary damage in the form of cracks, bowing and splitting sometimes occurs a distance away from the primary damage area caused by shock transmission along members.

Repairs are carried out to damaged areas and to areas where deterioration has occurred strictly iaw the repair manual (SRM). If a repair scheme is not specified by the manufacture for a particular part of the structure/particular type of damage then the manufacturer should be contacted for detail of how to proceed.

The purpose of a repair is to obtain a structure at least a s strong as the original. Severe damage may require replacement of the entire damaged assembly, but minor damage can be repaired by cutting away the damaged members and replacing them with new sections. This replacement is accomplished by glue, or glue and nails, or glued and screw splicing.

Damage may be classed as:

* Negligible - small areas of wood damage that might just need blending out and a varnish treatment.

* Damage repairable by patching. A plywood patch or length of timber applied over the damaged area (after the damage has been cut to a regular shape). Fitted using glue, nails and/or screws.

* Damage repairable by insertion. The damage is cut to a regular shape and an insertion is spliced in.

* Damage repairable by replacement. The whole section is removed and a new section fitted.

Tools

Standard wood working tools are required to include hammers, mallets, saws (hand and powered), wood chisels, planes, spoke shaves (a sort of small hand plane) drill bits, screwdrivers, sanding equipment, scrapers, rasps, glue pots, glue mixing equipment, clamps etc.

Materials

These include various sizes of nails, panel pins, woodscrews and glue resins and hardeners.

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Safety equipment includes overalls, gloves, breathing equipment and goggles.

Several types of wood are commonly used. Solid wood such as beams or planks will be needed and also various thicknesses of plywood (sometimes just ply or laminated wood) will be required.

Laminated wood is an assembly of two or more thin layers (veneers) of wood, which have been glued together with the grain of all layers approximately parallel.

Plywood is usually made of an odd number of veneers with the grain of each layer at an angle of 90" with the adjacent ply or plies. High-density material includes compreg, impreg or similar commercial products, heat stabilised wood or any of the hardwood plywoods commonly used as bearing or reinforcement plates. The woods listed below are used for structural purposes. For interior trim, any of the decorative woods such as maple or walnut can be used.

WOOD USES (Only where specified in the SRM)

Spruce All structural members.

Douglas Fir May be used as substitute for spruce in same sizes or in slightly reduced sizes.

Noble Fir May be used as substitute for spruce.

Western Hemlock May be used as substitute for spruce.

Northern White Pine Cannot be used as substitute for spruce without increase in sizes to compensate for reduced strength.

White Cedar May be used as substitute for spruce in same sizes or in slightly reduced sizes.

Yellow Poplar Should only be used as a substitute for spruce after accounting for reduced strength properties.

All wood and plywood used in the repair of aircraft structures must be of aircraft quality. Ideally the wood used to repair a part should be the same as that of the original whenever possible. If it is necessary to substitute a different wood, always follow the recommendations as laid down in the SRM.

Maximum grain inclination shollld not exceed 1 : 15

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Permitted Defects

Ideally the wood should be defect free but often this is not possible and some defects will be present. Some defects are allowed - other are not.

Cross grain. Spiral grain, diagonal grain or a combination of the two is acceptable providing the grain does not diverge from the longitudinal axis of the material more than 1: 15. A check of all four faces of the board is necessary. The direction of free-flowing ink will assist in determining grain direction

Wavy, curly and interlocked grain. Acceptable if irregularities do not exceed limitations specified as above.

Hard knots. Sound hard knots up to 3/8" (10mm) diameter are acceptable providing: (1) they are not in projecting portions of the I-beams, along the edges of rectangular or bevelled unrouted beams, or along the edges of flanges of box beams (except in lowly stressed portions) and (2) they do not cause grain divergence a t the edges of the board or in the flanges of a beam more than 1 : 15.

They should not be in the centre third of the beam and should not be closer than 20" (508mm) to another knot or other defect (applies to lOmm knots - smaller knots may be proportionately closer).

Pin knot clusters. Small clusters are acceptable providing they produce only a small effect on grain direction.

Pitch pockets. Acceptable in the centre portion of a beam providing they are at least 14" (356mm) apart where they are in the same growth ring and do not exceed 1%'' (38mm) in length by %" (3mm) width by 1/8" depth and providirig they are not along the projecting portions of I-beams, along the edges of rectangular or bevelled unrouted beams, or along the edges of the flanges of box beams.

Mineral streaks. Acceptable providing there is no decay.

Defects Not Permitted

Spike knots. These are knots running completely through the depth of the beam perpendicular to the annual rings and appear most frequently in quartersawed lumber.

Checks, shakes and splits. Checks are longitudinal cracks extending, generally, across the annular rings. Shakes are longitudinal cracks usually between two annular rings. Splits are longitudinal cracks induced by induced stress.

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Compression wood. This defect reduces the strength of the wood and is difficult to recognise. It is characterised by its high specific gravity (heavy) and it has the appearance of an excessive growth of summer wood. In most woods it shows little contrast in colour between the spring wood and the summer wood, If in doubt reject the wood, or subject samples to a toughness test.

Compression failures. This is caused by the wood being overstressed in compression by natural forces during the growth of the tree, felling trees on rough or irregular ground, or rough handling of logs. Compression failures are characterised by a buckling of the fibres that appear as streaks on the surface substantially at right angles to the grain and can show as pronounced failures to very fine hairlines. In doubtful cases carry out a toughness test.

Decay. Examine all stains and discolorations to determine whether or not they decay. All wood must be free from decay.

Spliced Joints

This a process of inserting a piece of solid wood or ply into an existing woo~cn member.

The scarf joint is generally used in splicing structural members. The two pieces to be joined are cut a t a n angle (bevelled) and glued. The slope of the bevel should be not less than 10 to 1 in solid wood and 12 to 1 in plywood. The scarf is cut in the general direction of the grain.

PRESSURE I I PART BEING

/ PART BEING JOINED

SCARF CUT

Fig. 15 SCARF JOINT

When making the scarf it is important to ensure that the two mating edges are in close contact. The best method for doing this is to cut the two scarf mating edges separately then clamp them together using G clamps and 2 strong timbers (two by four - 2in x 4in). Then a fine toothed saw is run down the joint to act similar to a router.

The process may need repeating after tapping the timbers closer together. The edges are then given a light plane.

The scarf may not be exactly 1 in 10 (though it should be close) but at least the two mating surfaces will be exactly parallel to provide a sound glued joint.

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If softwood, subsequent sanding should not be carried out but is recomnlr.nded for some hard plyu~ood surfaces, wood that has been compressed through exposure to high pressure and temperatures, resin-impregnated wood (irr~pr-eg and compreg), or laminated paper plastic (papreg)

It is recommended that no more than 8 hours elapse between final surhc:e finishing and gluing.

Plywood Skin Repairs

Most skin repairs can be made using:

* The surface or overlay patch - a rectangular, triangular, oral c.)r round patch fitted over the cleaned out damaged area.

* The splayed or flush fitting patch - for use with smaller damaged areas.

x The scarf patch - similar to the splayed patch but uses a scarf of 1 in 12 and used for bigger repairs.

* A plug patch - similar to an insertion repair for metal airframes.

* A fabric patch - for very small holes.

The Surface Patch

Should not be used on plywood over l/s inch (3mm) thick and the general procedure is:

1. Consult the SRM for repair details and the AMM for details of systems and equipment that may need removing to gain access.

2. Bring aircraft and materials into hangar for 24 hours to allow to get to correct temperature for gluing (if in cold climate).

3. Trim the damage rectangular or triangular shape depending on the location of the damage relative to other structure such as frarnes and formers. The corners of the cutout should be rounded with a radius of a t least five times the thickness of the skin.

4. Classify the damage - this is always carried out after cleaning out the damage to a regular shape.

5. Manufact~x-e the backing plates (doublers) from ply a t least a s thick as the skin. These are reinforcements placed under thc edge of the hole inside the skin.

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The doubler should extended from one framing member to another and are strengthened at the ends by saddle gussets attached to the frames.

6. Cut the patch to extend at least 12 times the skin thickness beyond the edges of the opening from material of the same kind and thickness as the original skin. The edges of the patch are bevelled (scarfed).

7 . Apply glue to a.11 surfaces (refer to the adhesives section in this book) and nail to prevent any movement. Clamp together if possible -- if not apply weights to ensure surfaces are held tightly together.

8. After the glue has dried the area should be covered with fabric if on the outside of the aircraft. The fabric should overlap the original ply skin by a t least 2 inches (5 1 mm).

9. The fabric should be doped and any paint schemes reapplied.

10. Any disturbed systems refitted and function tested.

11. The appropriate documentation cleared eg the CRS completed.

PLY SKlN DAMA

PLY SKlN PATCH

DOUBLER / BEHIND

HOLE CUT TO SIZE SKIN

Fig. 16 TYPICAL PLYWOOD PATCH

BEVELED EDGES T PATCH PLYWOOD SKIN \ \

3T 114" MIN

i --

Fig. 1'7 SECTION A-A TYPICAL PLYWOOD PATCH

The leading edge of a surface patch should be bevelled with an angle of at least four times the skin thickness. The face-grain direction of the ply patch must be in the same direction as the original skin.

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The Splayed or Flush Patch

A splayed patch is a patch fitted into the plywood to provide a flush surface. The term 'splayed' denotes that the edges of the patch are tapered, but thy slope is steeper than is allowed in scarfing joints. The slope of the edges is cut at an angle of five times the thickness of the skin.

Splayed patches are used for small holes where the largest dimension of the hole to be repaired is not greater than 15 times the skin thickness and t.hy skin thickness is not more than 0.1" (2.5mm).

After trimming the damage to a regular shape, tack a small piece of plywood under the hole to provide a centre point for a compass. Draw two concentric circles around the damaged area on the aircraft skin. The difference between the radii is five times the skin thickness. The inner circle marks the limit of the actual hole and the outer one marks the limit of the taper.

Cut out the inner circle and taper the hole evenly to the outer mark with a. -,hisel, knife or rasp. Prepare a circular patch, cut and tapered to match the nole. The patch is of the same type and thickness as the plywood being repaired.

Apply glue to the bevelled surfaces and place the patch into place with the face-grain direction matching that of the original surface.

After the patch is in place, a pressure plate cut to the same size of the patch is centred over the patch, with waxed paper between the two and pressed firmly against the patch with a weight or clamp to provide pressure. Do not use excessive pressure. After the glue has set, fill, sand and finish the patch to match the original surface.

WEIGHTS 114" PLY T = 0.1" OR OR CLAMP PRESSURE PLATE

LESS / WAXPAPEROR

I 4 I- \ PATCH

\ 5T PLYWOOD SKIN

Fig. 18 SPLAYED PATCH

Scarf Patch

Scarf patches are preferred for most skin repairs as they provide a smooth outer finish. The scarf patch differs from the splayed patch in that it car1 be larger (limits laid down in SRM) and the edges are scarfed to a 10 to 1 slope instead of the 5 to 1 used with the splayed patch. The scarf patch also uses reinforcements under the patch where the glue joints occur.

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Scarfed patches are used on flat surfaces or curved surfaces provided they are not too curved (greater than 100 times the skin thickness). Backing blocks or other reinforcements must be shaped to fit any skin curvature.

3T 114 MIN /

SCARF / 1

Fig. 19 SCARF PATCH

Whenever possible, the scarf edge of the patch should be supported internally.

A backing block is shaped from solid wood and fitted to the inside surface of the skin and is temporarily held in place with nails. A hole, the same size as the inside circle of the scarf patch, is made in the block and is centered ove- the trimmed area of damage. The block is removed after the glue on the pa' -1

has set, leaving a flush surface to the repaired skin.

When the back of a damaged plywood skin is not accessible, it should be repaired as follows: After removing the damaged sections, install backing strips along all edges that are not fully backed by a rib or spar. To prevent warping of the skin, backing strips should be made of a soft textured plywood, such as yellow poplar or spruce rather than solid wood. All junctions between backing strips and ribs or spars should have the end of the backing strip supported by a saddle plywood gusset.

I f needed, nail and glue the new gusset plate to the rib or frame. It may be necessary to replace the old gusset plate with a new saddle gusset, or it may be necessary to nail a saddle gusset over the original.

Attach nailing strips to hold backing strips in place while the glue sets. Us, 3

bucking bar if necessary to provide support when nailing. After the glue SF. i,

fill and finish to match the original skin.

Plug Patches

Similar to an insertion patch, they may be oval or round and are used on plywood skins. They are used only for damage that does not involve the supporting structure under the skin.

The plug patch is made up of a plug or insert with edges cut square and a backing piece or doubler.

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BUTT JOINT

/ PLUG PATCH

\ PLY S K I N

/ / \ /

PLYWOOD DOUBLER

Fig. 20 PLUG PATCH

The skin is cut out to a clean round or oval hole with square edges. The patch is cut to the same size and when installed, the edge of the patch forms a butt joint with the edge of the hole.

A round patch can be used where the cutout is no larger than 6" (152mrn) in diameter. The general procedure is not too unlike that described for a si-lrface repair with the following main points of difference:

1. A plug patch is cut of same material and thickness a s the original skin with square edges.

2. Cut insert and hole in skin the same size.

3 . Cut the doubler or backing piece of %" (6mm) plywood.

4. Apply a coat of glue to the outer half of the doubler surface where it will bear against the inner surface of the skin.

5. Centre doubler a t back of skin hole. Nail in place using a bxlcking bar or similar support for backing and clamp.

6. After the glue has set, apply glue to the centre surface of the doubler and insert. Place the insert in hole and screw with No 4 wood screws at 1" (25mm) pitch.

7. Apply pressure to patch by means of a pressure plate. Place waxed paper or cellophane between plate and patch to prevent glue from sealing plate to the patch.

9. After the glue has set, remove pressure plate, waxed paper, nails and screws. Fill nail and screw holes, sand and finish to match the original surface.

The steps for making an oval plug patch are similar to those for making the round patch. The maximum dimensions for oval patches are 7" by 5" (178mrn x 127mm).

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Fabric Patch

Small holes that do not exceed more than 1" (25mm) in diameter, after being trimmed to a smooth outline, can be repaired by doping a fabric patch on the outside of the plywood skin. The edges of the trimmed hole should first be sealed and the fabric patch should overlap the plywood skin by at least 1". Holes closer than 1 inch to any frame, or in the wing leading edge or frontal area of the fuselage, should not be repaired with fabric patches. The patch should have a serrated edge.

Spar And Rib Repairs (Solid Wood Repairs)

For minor damage the web members of a spar or rib can be repaired by applying an external or flush patch, provided the damaged area is small. Planks of spruce or plywood of sufficient thickness to develop the longitudinal shear strength can be glued to both sides of the spar. Extend the planks well beyond the termination of any damage - as laid down in the SRM.

If more extensive damage has occurred, the web should be cut back to structural members and repaired with a scarf patch or joint. Not more than two splices should be made in any one spar.

A spar may be spliced at any point except near highly stressed areas such as wing attachment fittings, landing- gear fittings, engine mountings, or lift and inter-plane strut fittings. Splicing under minor fittings such as drag wires, anti-drag wires or compression strut fittings is acceptable provided that the reinforcement plates of the splice should not interfere with the proper attachment or alignment of the fittings. For other fittings any measurements as to proximity/overlapping of reinforcing backing pieces etc are not exceeded.

Always splice and reinforce plywood webs with the same type of plywood as the original. Do not use solid wood to replace plywood. Plywood is stronger in shear than solid wood of the same thickness because of the variation in grain direction of the individual plies. The face-grain of plywood replaceme. . webs and reinforcement plates must be in the same direction as that of the. original member to ensure that the new web will have the required strength.

Bolt and Bushing Holes

All bolts and bushings used in aircraft structures must fit tightly into the holes. Looseness allows the bolt or fitting to work back and forth which will enlarge the hole. In cases of elongated bolt holes in a spar or cracks in the vicinity of boltholes, splice in a new section of spar or replace the spar entirely.

Holes drilled to receive bolts should be of such a size that the bolt can be inserted by light tapping with a mallet. If the hole is so tight that heavy blows are necessary to insert the bolt, deformation of the wood may cause splitting or unequal load distribution.

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Well-sharpened twist drills produce smooth holes in both solid wood and plywood. The twist drill should be sharpened to approximately a 60" cut t~ng angle.

Bushings made of plastic or light alloy provide additional bearing surfacv area without any significant increase in weight. Sometimes steel bushings are used to prevent crushing the wood when bolts are tightened.

Rib Repairs

A cap strip of a rib can be repaired using a scarf splice. The repair is reinforced on the side opposite the wing covering by a spruce block, which extends beyond the scarf joint not less than three times the thickness of the strips being repaired. The entire splice, including the reinforcing block, is rei~lforced on each side by a plywood plate.

When the cap strip is to be repaired at a point where there is a joint between it and cross members of the rib, the repair is made by reinforcing the scarf joint with plywood gussets.

When it is necessary to repair a cap strip a t a spar, the joint should be reinforced by a continuous gusset extending over the spar.

Edge damage, cracks, or other local damage to a spar can be repaired by removing the damaged portion and gluing in a properly fitted block, reinforcing the joint by means of plywood or spruce blocks glued into place.

The trailing edge of a rib can be replaced and repaired by removing the damaged portion of the cap strip and inserting a softwood block of white pine or spruce. The entire repair is then reinforced with plywood gussets and nailed and glued.

Compression ribs (the members fitted between the top and bottom of a rib) come in many different forms and their repair will be specified in the SRM.

Ideally smaller items structural members such as glue blocks, filler blocks, compression members, braces and rib diagonals should be replaced.

FABRIC COVERING

The fabric covering of an airframe is to provide an aerodynamic airtight and weatherproof covering (achieved after doping and painting). The fabric has some strength in tension but non in compression.

If a large area of the aircraft is to be covered (or the whole aircraft) an opportunity presents itself for the inspection of the complete skeleton of t /le

airframe and a visually inspection of all the systems, pipelines, cables, c-o t i trol runs etc.

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All foreign matter is removed and protective treatments (as prescribed in the relevant drawings/AMM) must be applied. It may be necessary to install flying control cables, electric cables, fuel tanks and other syst~ems/components before covering large areas and these should be inspected as necessary and checked for security. The most suitable conditions for fabric covering are - room temperature [16"C to 2 1 O C (6 1 O F to 70°F)] and a relative humidity of not more than 70%.

MATERIALS

This part of the book describes the materials used in the covering of UK manufactured aircraft. Non UK fabric-covered aircraft use these or similar materials manufactured in accordance with equivalent specifications.

Natural Fabrics

Supplied in bolts, rather like large toilet rolls. These fabrics are woven frorr spun threads or 'yarns'; those running lengthwise are termed the Warp YalLis and those running crosswise are termed Weft Yarns (they run from weft to white - a play on words 'Left to Right').

After manufacture the fabric is inspected by being passed over a light-box and any defects noted. These are marked by sewing a small piece of red cotton on the selvedge of the fabric. The fabric is then wound on a spindle to form the bolt.

The selvedge is the non-fraying edge of the fabric where the weft yarns are 'turned around' during the weaving process.

When in use the bolt is hung from a steel bar suspended from the ceiling and the fabric is pulled down in a similar way to how toilet paper is pulled from a toilet roll. Where a defect is noted (by the red cotton on the self-edge) that 2--a is cut away and is not used for aircraft work.

The number of yarns per centimetre (or per inch) varies with different weights of fabric and is not necessarily the same in both the warp and the weft directions.

When an unsupported fabric covering is required to carry air loads, unbleached linen to British Standards (BS) F1 is normally used, but some aircraft have coverings of cotton fabric complying with BS F8, BS F57, BS F116 or DTD 575A.

A light cotton fabric complying with BS F 1 14 (referred to a s Madapolam) is generally used for covering plywood surfaces. This acts as a key to the doping scheme, giving added strength, weather proofing and improved surface finish.

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BOLT OF FABRIC

DEFECT ---

INDICATOR WEFT YARNS

SELVEDGE I

FRAYING EDGE ' \ SELVEDGE

Fig. 2 1 A BOLT OF FABRIC

Tapes

Linen tapes complying with B8 F1 and cotton tapes complying with BS F8 are available in various widths. They are used to cover leading edges, trailing edges, ribs, stitching and for repair work. They are usually doped into position - the dope acting as an adhesive. The tapes are supplied with serrated edges sometimes called pinked edges.

If linen tape is not available then I t may be cut from a bolt of fabric using a soft pencil and rule for marking out and cutting using pinking shears (serrated edge scissors). If serrated edge scissors are not available the edges of the fabric must have their wrap yarns removed (teased away) to leave only the weft yarns for a %" (6mm) on each side.

The reason why the edges of the tape are serrated is that the zigzag edge effectively lengthens the edge (compared to if it was straight) - and this provides a longer edge to give better adhesion.

Cotton tape complying with BS F47 (referred to as 'Egyptian Tape') is generally used on those members where chafing may occur between the structure and the fabric and is also used externally to protect the fabric against damage by the stringing cord (stringing = tying the fabric onto ribs etc).

Egyptian tape (which is quite expensive) has three thread inclinations - weft, warp and bias - with the bias thread being woven at 45'. Both edges are selvedges and therefore it cannot be made up by cutting from the bolt but must be ordered in from stores.

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Thread. Used for hand or machine sewing. Linen thread complying with BS F34 is normally used. For hand sewing, No 40 thread (minimum breaking strength 3 kg [7 lb]) is used double, or No 18 thread (minimum breaking strength 7.25 kg [ l 6 Ib]) is used single. For machine sewing, No 30 thread (minimum breaking strength 4.5 kg [ l o lb]) or No 40 thread is used.

Sewing machines are not too unlike domestic sewing machines - but often have a longer arm to allow for sewing greater amounts of material. They are used to sew together pieces or fabric prior to putting on the aircraft.

Stringing

Flax cordage complying with BS F35 or braided nylon cord (coreless) complying with DTD 5620 is normally used. Used to tie the fabric to the structure.

Eyeleted Fuselage Webbing

On a number of older aircraft, cotton-webbing braid with hooks, or lacing eyelets and kite cord, are used for securing the fabric to the fuselage.

Man-Made Fabrics

Natural fabrics, such as cotton or linen, deteriorate in use as a result of the effects of sunlight, mildew or atmospheric pollution and may require replacement several times during the life of the aircraft.

Man-made fabrics are rlow approved and used extensively on many aircraft which makes fabric recovering less frequent.

The two main types of materials are polyester-fibre and glass-fibre, which are marketed under various trade names (Dacron etc). The general procedure for the use of these fabric is given below but, of course, you should always cor, ~ l t the AMM/SRM for the aircraft concerned -- and follow the fabric manufacturer's recommendations.

Polyester-Fibre Materials. These may be attached to the structure by the methods described later under the heading "Covering Methods" or by use of pre-sewn envelopes ("sock" method) or by use of an approved adhesive at the points of contact with the structure. The materials used for attachment and stringing must be compatible with the main fabric.

Before stringing, polyester fibre covers are tautened by the application of heat, the degree of shrinkage being proportional to the heat applied. The most common method of applying heat is a household iron set a t about 120°C ('wool' setting) and used in an ironing motion. Care is necessary to prevent the application of excessive heat as this may melt the fibre, or overtauten the fabric: and distort the airframe structure.

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Where non-tautening dope is used after fabric fitting, the fabric may be fully tautened prior to doping (check by tapping the fabric with the fingers - it should be tight - similar to a drum, but not too tight), but where tauteri~rrg dope is used the initial shrinkage should leave the cover fairly slack, since tautenir~g will continue over- a period of months after the dope has bee11 applied.

(If in doubt as to how much tautening is needed - make up a wooden frame about a metre square - cover it with fabric - and use it a s a test piece. One can always do this some days before the planned time for the actual doping so as to be ready when the time comes.)

Repairs within the specified limits may be carried out (as described later), and/ or patches may be stuck on, using a suitable adhesive. Large patches should be tautened in the same way as the main covering fabric.

Glass-Fibre Materials. Glass-fibre fabric is normally fitted to mainplanes and tailplanes in a spanwise direction, being attached a t the leading and trailing .edges with a 50mm (2 in) doped seam. Fuselages may conveniently be covered using four pieces of material at the top, bottom and sides, doped seams again being employed. Some glass-fibre material is pre-treated to make it compatible with cellulose acetate butyrate dope and is not suitable for use with cellulose nitrate dope.

Glass-fi'bre material is only slightly tautened by doping and must be a good initial fit, after which glass-fibre stringing should be fitted in the appropr-ia.te manner.

Repairs within the specified limits may be made by cutting out the damaged area of fabric and doping on a cover patch which overlaps 50mm (2 in) a11 round.

Storage

All materials used for fabric covering should be stored at a temperature of' about 20°C (68°F) in dry, clean conditions and away from direct sunlight. When required for use, the materials should be inspected for possible flaws (eg iron mould discolouration, signs of insect, rodent or other damage) and any affected parts rejected.

PREPARATION OF STRUCTURE PRIOR TO COVERING

The structure should be prepared by removing all sharp edges from any parts which will be in contact with the fabric. Wood should be lightly sanded a11d metal edges taped with Egyptian tape to prevent chafing. Where any covering tape is wound on structure it is important to ensure that the covered parts are suitable protected from corrosion (metal parts) or deterioration generally (wooden parts) - and all carried out in accordance with (iaw) the SRM.

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The structure to be covered should be inspected as outlined above. Corners, edges, projections, bolt or screw heads etc should be suitable protected. Where - - -

serious chafing may occur and a strong reinforcement is required, a canvas or leather patch may be sewn to a fabric patch, then doped into position.

In order to prevent dope from reacting with any protective treatment and to prevent fabric from adhering to wooden structure (where it should not adhere), all aerofoil members whlch will be in contact with the fabric are normally covered with adhesive cellulose or aluminium tape, or painted with dope- resistance white paint. Exceptions to this requirement are described later.

On some aircraft, which have a tubular metal fuselage frame (primary structure), the fuselage shape is made up with wooden formers attached to the main framework and to these wooden formers are secured stringers onto which the fabric covering is doped. This secondary structure must be inspected for security and any sharp edges removed with fine glass paper.

Where stringing is likely to be chafed by parts of the structure, protection should be provided by wrapping such parts with Egyptian tape. Before the tape is applied the structure should be treated with varnish to protect it fr~.,n corrosion should the tape become wet.

Flying/other controls and cables should be tensioned to assume their normal positions and secured by cont.ro1 locks.

COVERING METHODS

An aircraft fabric may be fitted with the warp or weft running a t 45" to the slipstream, or in line with the slipstream. The former (bias) method is generally considered to be stronger and more resistant to tearing, but the latter method is used on most light aircraft. Two covering methods are described below, but the actual method used will depend on the SRM.

The Prefabricated Envelope

Sometimes called the "sock method" where a fabric envelope is made up on the bench using machine sewing etc. Each envelope is made up from a pattern using accurate measurements (rule, pencil etc). An envelope is made up for the for the mainplanes, fuselage, tailplane, fin, flying control surfaces etc.

The envelopes are made loose enough (but not too loose) to facilitate slipping them over the structure and to achieve the proper tautness after doping. They are attached to the structure by stringing or other approved methods. Some fixtures may be fitted and the material doped, painted etc.

On mainplanes the envelope is drawn over the wing tip and gradually pulled down towards the root, generally keeping the spanwise seam in line with the trailing edge.

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When the cover is located it is secured (by stitching, cementing, or retaining strip) to the inboard end of the mainplane, and necessary openings for cables, struts, tank caps, etc are cut and stringing is applied as necessary.

For the fuselage the envelope may be open, or partially open, a t the bottom, to simplify fitting. The fin envelope is usually fitted first; then the fuselage envelope is stretched forwards over the fuselage and secured in the same way as the original fabric. The cover is usually cemented or doped to the fuselage formers.

Control surface envelopes are usually left open at. the hinge line, where tliey are secured by cementing, doping or stitching.

The Blanket Method

With this method the fabric is cut to shape, and machined together to forrn larger areas and then attached to the structure.

$or the mainplanes and tailplanes the cover is normally made-up from lengths of fabric machine stitched together. This is wrapped around the mainplane from front to rear starting and finishing a t the trailing edge and joined by hand stitching using the Trailing Edge stitch. On some aircraft with light alloy structure, hand stitching is dispensed with and the edges are doped into position. The fabric is then attached to the ribs by stringing.

A number of different methods are used to attach fabric to the fuselage. The fabric is not normally attached in one piece, but usually consists of several pieces (eg sides, top and bottom), which are doped separately onto the frame or sewn together a t their edges. Joins or seams are covered with doped-on tape. Since the air loads on the fuselage are not as great a s on the mainplanes, it is not usual to employ stringing, although it may be specified in some instances.

Control surfaces are covered in a similar way to the mainplanes and usually require stringing. The fabric is normally folded round the hinge line and sewn cogether round the remaining contour of the surface at the trailing edge.

JOINING FABRIC TO FABRIC

This may be carried out by a sewing machine (off the aircraft) or hand sewing (off or on the aircraft). Obviously machine sewing is significantly faster arltl more accurate, but when it comes to accuracy all sewing is carried out by first marking out with a rule and soft pencil. A line is drawn on the fabric along which the line of stitches is to run and, for hand stitching, the pitch of eac:h individual stitch is marked (not too unlike the marking out used when riveting).

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Before commencing the actual stitching the two pieces of fabric are 'tack stitched' together. This entails the temporary stitching of the fabric at fairly widely spaced intervals just to hold the fabric pieces in place. As the normal stitching progresses so the tack stitches are removed.

Hand stitching (and stringing) requires a lot of patience.

Seams

The seams in the fabric covering should be either parallel to the fore-and-aft line of the aircraft or on the bias, depending on the covering method used. With the exception of trailing edge or leading edge joints (where such action cannot be avoided) seams should never be made at right angles to the direction of airflow. Two types of machine seams are employed, the balloon seam and the lap seam.

The balloon seam or French Fell (figure 22), is normally specified for all fabr'- joints. To make the seam, the edges of the fabric are folded back 16mm (0 .A5 in) and are then fitted into each other as shown, tacked together and then machine sewn with four stitches per centimetre (nine per inch) in two parallel lines 9mm (0.375in) apart and 3mm (0.125in) from either edge.

DRAWING FROM CAP 562 Fig. 22 THE BALLOON SEAM

After completion, the seam should be examined over a strong electric light (preferably a light-box) to ensure that the inside edges of the fabric have not been missed during sewing.

The lap seam (figure 23) should only be used when specified in the SRM. Unless the selvedges are present, the edges of the fabric should be serrated with 'pinking' shears. The edges should overlap each other by 31mm (1 -25") and should be machine sewn with four stitches per centimetre (nine stitches per inch), the stitch lines being 12mm (0.5") apart and 9mm (0.375") from the edges. After stitching, a 75mm (3") wide serrated-edge fabric strip should be doped in position. Note the conversion discrepancies.

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75mm TAPE POSIT

COVERING

'ION

EDGE

DRAWING FROM CAP 562 Fig. 23 THE LAP SEAM

HAND SEWING

Hand sewing includes:

* The Trailing Edge stitch. * Stringing. A The Herringbone stitch. * Darning. k The Boot stitch.

The first two will be dealt with in this section with the remainder being dealt with in the section headed Fabric Repairs.

Beeswax

All threads used for hand sewing and all cord used for stringing (when not pre- waxed), should be given a liberal coating of beeswax. This protects the thread, facilitates sewing and reduces the likelihood of damaging the fabric or :nlarging the stitch holes when it is pulled through.

The thread is waxed by holding the beeswax in one hand (it is not unlike a bar of soap) and pulling the thread or cord over the bar. It will wear a small grove in the bar and the process is repeated 2 or 3 times to ensure complete waxing.

Overhand Stitch

Sometimes called the Trailing Edge stitch (figure 24) and is used at trailing edges, wing tips and wherever a sudden change in cross section occurs. Sufficient fabric should be allowed for, for turning under before the fabric is cut. 12mm (0.5in) turn-under is usually sufficient. An even gap of about 6mm (0.25in) (usually) should be allowed for pulling up the two edges to obtain the correct fabric tension, but this figure can only be determined finally by experience.

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. . . . . - . FABRIC

TRAILING EDGE

OVERHANDOR TRAILING EDGE STITCH

DRAWING FROM CAP 562 Fig. 24 OVERHAND OR TRAILING EDGE STITCH

The sewing should follow the contour of the component evenly to ensure a good finish after doping. The number of stitches should be three per centimetre (eight per inch), with a lock stitch being included about every 50mm (2in). ' : lock is shown as the last stitch in figure 24.

Stringing

Flax cord complying with BS F35 is normally used for stringing purposes and is generally applied in single strands as shown in figure 25. As an alternative, but only when approved by the manufacturer, doubled No 18 thread may be used during repair work.

SINGLE KNOT

ANTI CHAFING

SINGLE KNOT

STRINGING C

BOTTOM RIB .- PLY GUSSETS BOOM

ORD

DRAWING FROM CAP 562 Fig. 25 TYPICAL STRINGING

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When the fabric covering of the component has been completed, cotton ariti- chafing tape to BS F47 is stretched centrally over the fabric along each rill, top and bottom and stitched into position at the trailing edge.

Using a stringing needle (if access cannot be obtained to the rib inside thy aerofoil the needle must be long enough to pass through the thickness of the aerofoil) and commencing a t the top surface, the stringing cord should be passed through the tape and fabric as close to the rib as possible, out through the bottom fabric and tape, round the lower rib boom and back up through both surfaces again. A double knot is used to secure the first and last stringing loops and after each 450mm (18") section. In between, single knots are used.

The stringing pitch is normally 75mm (3") but in the slipstream area, or on aircraft of more than 9 10kg (20001b) weight, the pitch is often reduced to 38mm (1.5").

Variations from these may be stipulated in the relevant SRM and it may be necessary to vary the pitch in order to avoid internal structure or system components.

After completion a strip of serrated tape, 37mm (1.5") wide, should be doped over the stringing line on both surfaces, care being taken to ensure that no air is trapped under the tape and that the tape is securely attached to the main fabric.

Note. The knots shown in figure 25 are typical but different knots may be specified in the SRM.

Boom Stringing

This type of stringing is used on deep aerofoil sections where it might be difficult to thread the cord the long distance from the top of the wing to the bottom. The procedure is similar to that described for ordinary stringing, except that the cord is passed round the rib boom at the top and bottom

of round the entire rib.

Top and bottom fabric are therefore attached separately and the inside of each boom must be taped to prevent chafing of the stringing cord. Alternate rill and boom stringing is sometimes used on aerofoils of medium depth, ie between 150 and 300mm (6 to 12").

Care must be taken to ensure that all stringing is maintained at a satisfactory tension and that it is not so tight as to cause distortion of the ribs.

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The Slip Stream

For stringing purposes, the slipstream area is considered to be the diameter of the propeller plus one rib on either side. In the case of multi-engined aircraft, the entire gap between the slipstreams, regardless of its width, is also considered to be slipstream area.

Miscellaneous Methods of Fabric Attachment

In addition t.o the standard methods of fabric attachment, other methods maybe employed. Some methods are described below.

Strip Attachment. Attachment of the fabric by wrapping it around a light alloy strip or rod, which is then secured in a channel or groove is sometimes used with metal structures (figure 26).

METAL RIB

\ FABRIC COVERING THIS JOINT SHOWN PRIOR

TO TIGHTENING UP

ATTACHMENT

LIGHTENING HOLES /

DRAWING FROM CAP 562 Fig. 26 METAL STRIP ATTACHMENT OF FABRIC

Special Boom Attachment or Special Stringing. These methods can vary depending on the aircraft. The process shown in figure 27 involves pressing the fabric into special rib booms using aluminium alloy channel pieces with the covering fabric protected top and bottom by protective tape. The metal strip is attached to the boom by screws and the "channel" produced by the fixing is covered over by a doped on length of tape.

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CHANNEL ATTACHMENT SCREW SERRATED OR FRAYED Screws into caged (anchor) nut fixed

/ EDGE STRIP DOPED ON to underside of rib \

ALUMINIUM ALLOY CHANNEL

PROTECTIVE TAPE

FABRIC COVERING

PROTECTIVE TAPE

DRAWING FROM CAP 562 Fig. 27 SPECIAL STRINGING

4dhesives. On some small aircraft, where air loads are light, stringing is not used on the wing and tail surfaces and the fabric is fixed to the structure by means of a proprietary adhesive. This method produces a much smoother surface on the fabric and saves time during construction and repair.

Attachment of Fabric to Plywood

Dope is generally used for the attachment of fabric to plywood, but before the fabric is applied, the wood surface should be smoothed with fine glass paper and any cavities, such as those caused by the countersinking for screwheads, filled and allowed to set. The filled area should be kept to the absolute minimum because of the reduced adhesion of the doped fabric onto filler.

The wood surface should be treated with one coat of tautening dope, followed by a further coat after the first one has dried. After the second coat of dope has Iried, the fabric should be spread over the wood and stretched evenly to avoid wrinkling. A coat of tautening dope should then be brushed into the fabric making sure that it penetrates through the fabric. For this purpose a fabric pad is useful for rubbing in the dope.

After the dope has dried it should be lightly rubbed down to remove small spikes that might have formed using 'wet and dry' rubbing paper (grade 0 or 00). Then the required paint finishing scheme is applied (see later notes on doping).

Attachment of Fabric to Metal Surfaces

Where aluminium alloy is used as part of the structure (such as the leatf~ng edge profile) the fabric is generally doped into position. Alternatively, a thermoplastic adhesive may be used and guidance on the use of this xn;itt.rial may be obtained from the SRM.

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To ensure satisfactory adhesion of the fabric, the metal surfaces should be thoroughly cleaned and primed with an etch primer.

Drainage and Verl tilation

Drainage and ventilation holes are necessary on all aircraft particularly fabric covered ones to minimise corrosion of metal parts, rotting of wood, fabric, etc.

Drainage holes are usually positioned on the lower surfaces of fuselages, nacelles, mainplanes, tailplanes, control surfaces etc and the AMM will show their location. After fabric covering the aircraft, these must be replaced by punching the correct diameter hole in the fabric and doping on a drainage eyelet. It is common practice to clear the eyelet using an ice pick once the final finish has dried.

When holes are used for ventilating purposes, the holes may be located in sheltered positions regardless of drainage qualities.

Drainage eyelets are usually oval or circular in shape and are doped onto t,,; surface of the fabric. In some cases they may be secured by stitching through pre-pierced holes in the eyelets before the finishing scheme is applied.

Shielded or shrouded eyelets may be used to improve either the drainage or the ventilation, or to prevent the ingress of driving rain or the entry of sea spray (on marine aircraft). These eyelets must only be used in positions laid down in the SRM/AMM and must not be used as an alternative to standard eyelets. It is also important that the shroud is facing in the correct direction - usually rearwards for draining and forwards for ventilation - but not necessarily so.

DRAWING FROM CAP 562 Fig. 28 PLANE & SHROUDED EYELETS

Inspection Panels

Inspection panels are usually cut into the fabric after the completion of fabric covering. The actual panels employed will vary on the aircraft. Three methods commonly used are described below.

Woods Frame. These are light circular or square frames, made from celluloid sheet, which are doped onto the fabric covering at the required positions. The fabric covering is then cut away from inside the frame and a serrated edged fabric patch doped over the whole area a s shown in figure 29.

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FABRIC PATCH DOPED OVER REPAIR

12mm RADII

DRAWING FROM CAP 562 Fig. 29 WOODS FRAME INSPECTION PANEL

To use the inspection panel the patch is removed and after the inspection is carried out a new patch is doped on and the finishing scheme re-applied.

Zip panels. These consist of two zips machine sewn into the fabric in the form of a vee, the open ends of each zip being a t the apex of the vee. This type of access is suitable for positions where frequent inspection or servicing is necessary.

Sips tend to get clogged up by dopes, paints etc and can be very difficult to open so care should be taken to keep the sips clean at all times.

Spring Panels. This is particularly suitable for use on light aircraft. It consists of a circular plastic ring and a dished light alloy cover. The ring is doped into position in the same way as the Woods Frame and the fabric cut away from the inside. The cover is fitted by pressing the centre of the cover with the thumbs whilst holding it in both hands.

PLASTIC RING DISHED COVER RIVETED TO CLIP

FABRIC COVERING METAL CLIP ACCESS HOLE

DRAWING FROM CAP 562 Fig. 30 CROSS-SECTION OF SPRING PANEL

The dish shape is reversed away from the clip allowing the clip to be inserted diagonally in the hole. The complete cover with the clip is rotated to align the clip under the ring and the pressure is released from the cover. The dishr:d cover reverts to its normal shape and closes onto the plastic ring as shown in figure 30.

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REPAIRS TO FABRIC

If the fabric has been damaged extensively, it is usually impractical and uneconomical to make repairs by sewing and patching. The extent and location of damage to the fabric that may be repaired will be detailed in the SRM, but extensive damage is often made good by replacing the complete fabrlr: panel. However, the replacement of a large fabric panel, particularly on one side of a component, may lead to distortion of the structure and it may be advisable to completely re-cover the component.

Before commencing any repair, the cause of the damage should be ascertained and rectified - if possible. The internal structure should be inspected for direct. damage and secondary damage (damage caused by transmitted shock). The inspection should also include a check for loose objects such as stones (thrown up by tyres), remains of birds, insects, etc. These should be removes and any structural damage made good.

All dope should be removed by using thinners from the fabric surrounding +%e damaged area before any stitching is carried out, since doped fabric will t e ~ if any tension is applied to the repair stitches.

Repair to Cuts and Tears

If a straight or L shaped tear it may be repaired using the herringbone stitch doping a length of tape over it afterwards. If the damage is larger it may be repaired by darning, if larger still it may be repaired by an insertion repair. Outside these limits (all laid down in the SRM) the area will have to be recovered.

Herringbone Stitch

The herringbone stitch (also known as the Ladder Stitch) should be used fc-- repairing straight cuts or tears, which have sound edges and for insertion repairs. The stitches should be made as shown in figure 3 1, with a lock k - _ ~ t every 150 mm (6").

There should be a minimum of two stitches to the centimetre (four stitches to the inch) and the stitches should be 6mm (0.25") from the edge of the cut or tear. The thread used should be that as stated earlier in the book and needles should be just big enough to allow the thread to be threaded through the eye. Some needles are curved to allow for stitching back u p through the material.

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1. Start with 2. Pass thread CUT OR TEAR Finish with a a thumb under fabric IN FABRIC thumb knot FABRIC knot , / / \ /

3' Then 4. Then through cut & under etc over

DRAWING FROM CAP 562 Fig. 3 1 THE HERRINGBONE STITCH

After the stitching has been completed on a straight tear, a 25mm (1") wide serrated edge tape should be doped over the length of the stitching.

After a patch repair using the herringbone stitch a square or rectangular fabric serrated edge patch should be doped over the whole repair, ensuring that the edges of the patch are parallel to the warp and weft of the fabric covering and that they overlap the repair by 37mm (1.5").

In both cases the original doping scheme (and paint scheme) should be restored.

Repairs using Woods Frames

This is a recognised method of repair. Damage greater than simple cuts and tears which cannot be repaired using the herringbone stitch can be repaired by using the Woods Frame method. The process is similar to that described for fitting a Woods Frame as an inspection panel. Repairs of up to 50mm (2in) ;quare may be made, provided they are clear of seams or attachments by a distance of not less than 50mm (2in). The affected area should be cleaned with thinners or acetone and repaired as follows:

The Woods Frame should be doped into position surrounding the damaged area and, if the frame is of the square type, the edges should be parallel to the weft and warp of the covering. When the dope has dried, the damaged portion of the fabric is cut out and the aperture covered by a fabric patch as previously described.

If a Woods Frame is not available one can be made from cellulose sheet 0.8mrn (0.030in) thick with a minimum frame width of 25mm (lin). In the case of the square type the minimum corner radii should be 12 mm (0.5in). In somt. cases, aircraft manufacturers use 2mm plywood complying with British Standard V3 for the manufacture of the frames, in which case it is import- dn t to chamfcr the outer edges of the frame to blend with the aerofoil contour.

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Repair by Darning

Irregular holes or jagged tears in fabric may be repaired by darning provided the hole is not more than 50mm (2") wide at any point. The stitches should follow the lines of the warp and weft and should be closely spaced as shown in figure 32. The first darn (weft) should follow the fabric weft yarns as near a s possible picking up on sound fabric about 0.25" (6mm) away from the edge of the damage. The second darn (warp) should follow the warp yarns of the fabric and the first line should pass OVER - UNDER - OVER etc the weft darns. The second line should pass UNDER - OVER - UNDER etc the weft yarns with each successive line alternating the OVER - UNDER - OVER sequence.

FABRIC PATCH DOPEDOVER \ REPAIR

DRAWING FROM CAP 562 Fig. 32 REPAIR BY DARNING

The whole repair should be covered with a serrated fabric patch in the usual way, with an overlap of 37mm (1.5") from the start of the edge of the darn.

FABRIC /COVERING

DARN YARNS IN-LINE WITH WEFT & WARP

Repair by Insertion

For damage over lOOmm (4") square, insertion repairs are generally used. I NO

methods are described.

Note that when cutting the fabric for repair the corners are not radiised (as in metal repairs) and, except for round holes the edges of any cuts are in line with the weft and warp yarns of the covering material. All square and rectangular patches are cut parallel to the weft and warp yarns.

Normal Insertion Repair

The damaged area of the fabric is cut out to form a square or rectangular hole with the edges parallel to the weft and warp. Each corner of the hole should then be cut diagonally (at 45O), to allow a 12mm (0.5") wide edge of the covering fabric to be folded back under the fabric. This should be held in position with tacking or hemming stitches.

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The patch should be made 25mm (I") larger (in both length and width) than the cut-out area and each edge should be folded under for 12mm (0.5") and tacked in position in a manner similar to that described above. In this condition the size of the insertion patch should be similar to, or slightly smaller than, that of the cut-out area. Note that none of these edges have pinked edges .

The insertion patch should be held in position inside the cut-out area with a few tacking stitches and then sewn in position using a herringbone stitch of not less than two stitches to the centimetre (four stitches to the inch), as shown in figures 31 and 33. A 25mm (1") wide tape should then be doped over the seams.

Important. Before commencing the cutting away of the damage you shoc~l(l work out the exact size of the repair on a piece of paper noting the pitch of the stitches being %" (very similar to how a metal repair is carried out). Once worked out the SRM should be consulted to see if the resulting cut away is within the repairable (repair by insertion) limits. If it is outside the limits then there is no need to proceed with the insertion and recovering the whole area should be considered.

For small repairs a square or rectangular cover patch, with frayed or serrated edges, is doped in position to overlap the edge of the tape by 3 lmm (1.25"). Where the size of the insertion is more than 225mm (9") square, a 75mm (3") wide fabric serrated edge tape is used. The tape should be mitred (a 45" cut) a t the corners and doped in position.

The original finish is then restored.

ALTERNATIVE COVER 25mm COVERING PATCH OVER WHOLE AREA

Pitch of 114" is modified at the corners 8 the diagonal stitch forms a figure of 8 & the inside hole is used

12mm FOLDED UNDER

DRAWING FROM CAP 562 Fig. 33 NORMAL INSERTION REPAIR

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An Alternative Insertion Repair

Consists of cutting away the damaged fabric a s described above, but the edges of the covering fabric as well as the edges of the insertion patch are turned upwards (12mm for the covering fabric and 37mm for the insertion).

The insertion is tack stitched in position and a boot stitch is used to stitch it in correctly. The boot stitch (figure 35) is hand sewn taken along the folded edges a t Y4" (6mm) pitch (stage 1 in figure 34).

EDGES DOPED DOWN p i z q / , r I

A

-1 37mm

INSERTION PATCH

DOPED ON

ISTiEET-1 FABRIC PATCH

lf - r ' . . - ' ! - - , 1

r

DRAWING FROM CAP 562 Fig. 34 ALTERNATIVE INSERTION REPAIR

, .' :ITCH , , , , " , I J

Stage 2 entails laying the edges down outwards from the centre of the repair (folding down) and doping in position. A fabric patch is then cut with a 2 5 ~ ~ overlap on all edges and doped into position.

Edges of insertion repair and patch should be frayed or pinked.

No 18 waxed thread to BS F34 is used for boot stitching. Two threads with two needles are used crossing past each other through the same hole (or very close to). The threads are tied together a t the ends and with a lock knot every 150mm (6").

Checking Fabric Condition

The fabric covering of a n aircraft will deteriorate with time. The rate of deterioration depends on the type of operation, climate, storage conditions and the maintenance of a satisfactory surface finish. Because of water penetration, oil contamination, chafing and local wear, the covering will deteriorate quicker in some areas than others.

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TWO THREADS

START KNOTS

Fabric & stitches shown before the stitches are pulled tight

DRAWING FROM CAP 562 Fig. 35 THE BOOT STITCH

In some cases an arbitrary life may be placed on the fabric, but fabric coverings should be checked at the periods specified in the maintenance schedule and prior to renewal of the Certificate of Airworthiness.

4 visual examination is carried out on the fabric, inside and out, as far as possible checking areas where deterioration is likely to occur, or is known to occur on that particular aircraft. Unless defects are found this is usually sufficient to warrant acceptance of the condition of the fabric as a whole. I f the strength of the fabric is in doubt then further tests will be necessary.

Fabric Strength Testing

A "rule of thumb" test for checking the strength of the fabric is to push the fabric hard with the thumb (on an open area of unsupported fabric). If the thumb pushes through, then the fabric is definitely too weak.

If the thumb moves the fabric in, causing the paint covering to crack, then further tests are required. If there is little movement of the fabric then it is likely to be satisfactory. Warning - this method is not reliable and not satisfactory as a definitive test.

Note. Any locally cracked paintldope finish can be locally repaired by removing with an approved solvent and the re-application of the doping/painting scheme. Make sure the fabric is serviceable first.

A more reliable method is to use a portable tester such as the one shown In figure 36. These testers are, generally, only suitable for checking the condition of fabric where the dope finish has penetrated the fabric. Finishes such as cellulose acetate butyrate dope do no normally penetrate the fabric and experience has shown that the absorption of moisture in humid conditio~~s can produce unreliable test results.

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In addition, butyrate dope, even when some penetration of the fabric has occurred, produces a finish which hardens with age, a s a result the conical point on the tester will not readily penetrate the covering and the test will tend to indicate that the fabric is stronger than it actually is. Thus where butyrate dope is used, or where the dope has not penetrated the fabric, laboratory tests should be tests should be carried out.

For a laboratory test (see later text in this book) a piece of fabric is cut from the aircraft and the dope is rerrloved using a suitable solvent where necessary. The test piece is given a tensile test and if it has a strength of a t least 70% of the strength of new piece of fabric to the appropriate specification then it is considered airworthy.

Portable Tester. The tester shown consists of a spring loaded penetrating cone and plunger housed within a sleeve. When pressed against a surface the cone is forced u p through the sleeve against the spring and the plunger projects through the top. The tester should be used on single layer unsupported fabric only and should be held at 90" to the surface with pressure applied toward? the fabric in a rotary motion, until the sleeve flange touches the surface (fip. ~ . e 36).

The amount of penetration is indicated by the length of plunger showing above the sleeve and is marked by coloured bands or a graduated scale.

A table is provided with the tester giving the colour or scale reading required for a particular type of fabric.

Note. This tester is of American manufacturer and the table supplied refers to fabric complying with American specifications (AMS, TSO and MIL). It can be adapted for use on fabrics complying with DTD and BS specifications by comparing the strength requirement specifications of US and UK fabrics.

The test should be repeated at various positions locally on the aircraft and the lowest reading obtained should be taken as representative of the fabric a s s whole.

All punctures produced by the tester should be repaired with a 50 or 75mm (2 or 3") diameter doped fabric patch.

Laborato y Tests. These are normally associated with testing for tensile strength and uses tensile tests and bursting strength tests.

Tensile tests are used on new fabric and require the use of six warp and six weft samples, each 62mm x 300 to 400mm (2.5in x 12 to 16in) in area. These test are not generally used for fabric coverings on aircraft, as they would necessitate significant areas of fabric removal (and partial re-covering of the aircraft) - and the fabric might turn out to be serviceable.

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COLOURED BAND INDICATOR

SPRUNG LOADED SMALL HOLE IN FABRIC PENETRATING CONE

Fig. 36 PORTABLE FABRIC TESTER

On aircraft, therefore, it is recommended that the portable tester be used first. and if the results are not satisfactory, or in-conclusive, samples of fabric should be sent to a laboratory for bursting strength tests in accordance with the specification for that particular type of fabric. These tests require small samples approximately 87mm (3.5") in diameter.

Bursting strength tests can be carried out on a machine operating on the principle of applying force to a polished steel ball of 25.40mm ( I ") diameter, the ball being in contact with the test sample, which is clamped between two circular brass plates having coaxial apertures of 44.45mm (1.75") diameter. The load is applied at a constant rate and the load a t the breaking point of the fabric is the bursting strength of the fabric.

An Instron machine, which operates on this principle, is suitable for conducting tests on used aircraft fabric. A s an alternative, a machine operating on hydraulic principles can be used. In this machine, hydraulic pressure is applied at a constant rate to a rubber diaphragm, which is positioned to expand through a clamp aperture of 30.99mm (1.22") diameter, exerting a force gain st the fabric sample held between the clamps.

Note. The test methods referred to above are in accordance with the American Federal Test Method Standard No 191, Methods 5120 and 5122 respectively. All tests must be carried out by an approved test establishment.

DOPING

This particular subject, doping, is not actually specified in the syllabus but the CAA has informed us that it is considered as an integral part of structural fabric covering (which it is of course) so questions will be included in the C:AA examination paper.

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Natural fabrics, such as cotton or linen, deteriorate in use as a result of the effects of sunlight, mildew and atmospheric pollution. Ma-n-made fibres resist some of these agents better than natural fabrics but still require protection.

The dope film provides following functions:

a) Tautening of natural fabrics. b) Waterproofing. c) Air-proofing. d) Light-proofing.

Materials

Dopes. Dope consists of a number of resins dissolved in a solvent to permit application by brush or spray. This is modified with plasticisers and pigments to add flexibility and the required colour (see figure 37). There are two types of dope in use, namely, cellulose nitrate and cellulose acetate butyrate. The former is usually known simply as nitrate dope and the latter as butyrate 0.:

CAB dope. The main difference between the two is the film base.

In nitrate dope a special cotton is dissolved in nitric acid, whilst in butyrate dope cellulose fibres are dissolved in acetic acid and mixed with butyl alcohols. The plasticisers in the two dopes are also different, as are the resin and solvent balances. Dope must be stored under suitable conditions and has a tendency to become acid with age. If old dope is used it will quickly rot the fabric. Only fresh dope should be used, preferably buying it in for the job in hand.

Dope-Proof Paints. Due to the nature of the solvents used in dope, many paints will be attacked and softened by dopes. Dope-proof paint must be used to coat structure, which will be in contact with the doped fabric. Spar varnish is used for wooden structure and an epoxy primer is suitable for metal structures.

Aluminium Dope. To make the fabric lightproof, preventing damage from ult--7- violet radiation, an aluminium dope is used. This is usually supplied readv mixed but can be prepared by mixing aluminium paste or powder in clear , - ~ p e but it is essential that the materials are obtained from an approved supplier and mixed in accordance with the manufacturer's instructions.

Thinners. Dopes are formulated so that the solid constituents are suspended in the appropriate solvents. For spraying purposes it will normally be necessary to thin (reduce the viscosity) of the dope.

It is important that only the thinners as recommended by the manufacturer of the dope be used. The amount of thinners required is specified by the manufacturer and modified by experience t.o take account of the equipment used, atmospheric conditions etc.

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The viscosity can be measured by using a Viscosity Cup, which a cup with a small hole in the bottom. In use, the cup is dipped into the dope to fill it with dope. It is then lifted in the air to let. the dope flow out. The flow is timed from when the cup is lifted from the container to the first break in the flow.

In this way subsequent batches of dope can be mixed to exactly the same viscosity as the first batch. It is important that nitrate and butyrate dopes are mixed only with their own specialised thinners. A retarder, or anti-blush thinners, is a special type of thinners with slow-drying solvents. By drying more slowly they prevent the temperature drop and consequent moisture condensation that cause blushing in a dope finish. In use, the retarder replaces some of the standard thinners and can be used in a ratio of up to one part retarder to four parts of thinners.

FILM FORMERS

Fig. 37 DOPE CONSTITUENTS

Cleaning Agent. Methyl-ethyl-ketone (MEK) is a solvent similar to acetone. I t is used as a cleaning agent to remove wax and dirt and to prepare surfaces for 2ainting or re-doping. It is also used for cleaning spray guns and other equipment.

Fungicides. Since natural fabrics can be attacked by various forms of mildew and fungus, it may be necessary to provide protection for cottons and linens when doping. This is achieved a fungicide being added to the first coat of dope. The dope is usually supplied ready mixed but can be prepared by using a fungicidal paste obtained from an approved supplier (mixed with the clear dope in accordance with the manufacturer's instructions).

The first coat of dope should completely penetrate the fabric.

Caution. All fungicides are poisonous. Avoid contact and do not inhale the fumes -- this applies to all solvents, paints etc anyway.

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Tack Rags. A tack rag is a rag dampened with thinners and is used to wipe a surface clean after it has been sanded to prepare it for the application of the next coat. Proprietary cloths are also available.

Sandpaper. Sanding is carried out using wet-or-dry (sometimes called "wet-and -dry7') paper. This is a waterproof sandpaper supplied in various grades - eg: 280, 360 and 600 (the finest grade).

Safety Precautions

The storage and use of dopes in the UK is covered by various Government regulations made under the Factories Act.

Solvents are highly flammability. They have a low flash point and the vapour produced is heavier than air. Once ignited a serious fire, which can spread rapidly is produced.

A common cause of ignition is the shorting discharge of static electricity

Static electricity can be generated by brushing, sanding and wiping large areas of fabric (and many other materials) as when applying dopes, paints and sanding down and cleaning. Ordinarily this may not be a problem but when doping etc there are usually large amounts of inflammable fumes in the atmosphere ready to ignite with the smallest spark.

For example: if an operator is sanding a large area and wearing rubber soled shoes and not earthed in any way he/she will be at the same electrical potential as the surface. Should the charge on the operator be lost through bodily contact with some earthed metal part in the hangarlspray shop and he/she touches the aircraft structure being worked on the static charge will jump to earth creating a spark and igniting the fumes.

The best way to prevent happening is to eliminate the static charge by grounding the structure being doped. An earth wire connected between the structure to a clean metal part of the spray shoplhangar will do the job.

Clothing that is made of synthetic fibres will build up a static charge more readily than clothing made from cotton. Leather soled shoes will allow the static charge to earth to ground.

When spraying (particularly nitrate dope) ensure that the spray gun, the operator and the structure being doped are all grounded together.

In the spray shop, floors should be kept clean by being doused with water and swept whilst still wet. Remember, spontaneous combustion can occur if dope and zinc chromate oversprays are mixed.

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The fumes created during the spraying process are hazardous to health ;is well as being a fire risk. Correct operator protection must be provided as recommended by the dope manufacturer's. At the first sign of any irritation of the skin or eyes, difficulty in breathing or a dry cough, the operator should stop work and seek medical advice.

All electrical equipment used in the shop must be designed so that it canriot ignite any fumes. Lead lamps must be of the explosion proof type.

Working Conditions

It is important to control both the temperature and humidity of the air in the spray shop. It is also necessary to maintain sufficient airflow through the shop to remove the fumes.

Electric driven explosion proof extractor fans are installed a t floor level in the shop to extract all the fumes. The rate of airflow is dictated by the size of the spray shop and is the subject of various Government regulations. The discharge of the vapours may also be the subject of further requirements and the advice of the Factory Inspectorate should be sought. The air inlet to the spray shop should preferably be via an adjoining room, or behind a baffle to reduce draughts to a minimum.

Ideally the air and humidity of the incoming air can be controlled in the adjoining room before it enters the spray shop (cooling with an air conditioning unit to remove the moisture then reheating to obtain the correct temperature).

Many problems associated with doping can be traced to incorrect temperature or humidity of the air and/or the dope. Dope (and any other materials) brought into the shop from the outside store-room must be allowed to stand overrlight in the temperature controlled shop. The air temperature should be between 2 1" and 26°C (70" to 79°F). If the temperature is too low the rapid evaporation of the solvents will lower the temperature of the surface to the point where moisture will condense and be trapped in the finish. Too high a temperature causes too rapid drying of the dope, which can result in pinholes and blisters.

Ideally the relative humidity should be between 45 and 50% but satisfactory results can be obtained with relative humidity a s high as 70% or a s low as 20% depending on temperature and airflow, but doping is more difficult at these values.

Relative humidity can be measured with a hygrometer and although direct reading instruments are available, the wet and dry bulb type is still the most common. In this instrument two thermometers are mounted side by side, the bulb of one being kept wet by water evaporating through a wick.

To take a reading of relative humidity, both thermometers should be reat1 and the difference between them noted; the wet bulb thermometer will be the lower.

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Wet Bulb reaaing away Irom Lne ury D U I D reaulrlg giv~r~g Lrlc ucprcaslurl V ~ ~ U G

of the Wet Bulb.

The Wet Bulb reading and the Depression Value reading are read off against each other on a table - where the two columns meet will indicate the % relative humidity.

All brushes, spray equipment, cups, containers etc should be kept clean using thinners before the dope has had time to dry. Oil and water traps in the airlines should be cleaned regularly and air bottles drained of accumulated moisture. If equipment has any dried dope, it should be dismantled and the parts soaked in methyl-ethyl-ketone or a similar solvent. Packings and seals should never be soaked in solvents or they will harden and become useless.

Preparation Prior To Doping

The area to be doped should be thoroughly cleaned. The correct temperatur and humidity should be achieved with the atmosphere and all materials.

An inspection should be made of the fabric-covered component to verify the following points:

a) The structure has been painted with dope-proof paint where required.

b) Correct and secure attachment of the fabric to the structure. c) Correct allowance for tautening of the cover where this is a

natural fabric such as cotton or linen. If the cover is too slack, no amount of doping will rectify this, if it is too tight, the structure could easily be distorted.

d) All dust has been removed from the fabric. e) The fabric has reached the correct temperature.

f) Plastics components, such as windows and windscreens, are adequately protected against solvent attack. Use solvent proof masking and masking covers,

With the dope at the correct temperature, it should be mixed and thinned to the correct consistency for brush or spray application as appropriate.

Whilst the dope is in storage the solid materials tend to settle and the purpose of mixing is to make sure these are brought back into suspension.

To mix the dope, half the contents of the tin are poured into a clean tin of the same size. The remaining dope is stirred until all the solid material is in suspension. The contents of the first tin are then poured into the contents of the second and a check made that all pigment has been loosened from the bottom. Then the dope from one tin is poured into the other and back again, until it is thoroughly mixed.

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Application to Natural Fabric

The best-looking and most durable film is produced by using multiple (.o;its of a dope that is low in solids. A large number of thin coats, however, requires a great deal of time and modern dope schemes tend to use fewer, but thicker: coats than the earlier schemes. The dope scheme is a schedule listing the number and order of coats of each type of dope.

The standard aircraft doping schemes are 752 (medium tautening), 751 (lrght tautening - used on light structures that would be distorted by over tautening) and 753 (heavy tautening - used where an extra taut cover is required).

Priming Coats. This first coat of dope provides the foundation for all the subsequent coats. It forms a mechanical attachment by the dope encaps~rlating the fibres. The dope should be thinned by 25 to 50% and applied by brush. The dope is worked into the fabric to ensure adequate penetration, but it should not drip through the other side.

4 fungicide should be added this first coat. When applying the first coat to the wings, the entire wing should first be doped on both sides aft of the front spar. The dope should be allowed to shrink the fabric before doping the 1eadir:g edge. In this way the fabric will tauten evenly and adjust itself over the leading edge cap without forming wrinkles.

There are three tautness levels available; a low tautness scheme, a medium tautness scheme and a high tautness scheme. The main difference being the number of coats of dope. Given below, a s an example, is shown the medium tautness scheme.

MEDIUM TAUTNESS SCHEME (BSX26/752)

Dope Weight Normally obtained in the g/m2 oz/yd2 following number of coats

*ied oxide tautening dope 68 2.0 Aluminium tautening finish 34 1.0 Pigmented non-tautening finishes 34 1.0

Where an aluminium finish is required the scheme should be:

Red oxide tautening dope 102 3.0 Aluminium non-tautening finish 34 1.0

Where a glossy finish is required follow with:

Transparent non-tautening finish 34 1.0 1 or 2

Note: A tolerance of + 20% is permissible on any of the weights given above.

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After a 1 hour rninimum drying time, apply tapes, drainage eyelets, grommets, inspection panel rings etc. A heavy coat of nitrate dope should be brushed on where required and the tape laid on, working it down onto the surface and rubbing out any air pockets a s the tape is laid. A further coat of clear dope is brushed over the top of the tapes. Drainage eyelets or grommets and inspection rings are attached in a similar manner.

To ensure good adhesion eyelets, grommets and rings may be soaked in dope thinners for no more than two minutes to allow them to soften. Holes in eyelets and rings are opened with a sharp, pointed knife after doping is complete. Taping is followed by a further coat of clear dope, which may be butyrate and may be applied by a spray gun.

Filling Coats. When the first butyrate coat has dried, the fabric will feel rough due to the short fibre ends (the nap) standing up and being hardened by the dope. This nap can be sharp and should be lightly sanded off, using dry sandpaper. The surface should then be rinsed clean with water and dried.

Two full wet cross-coats of butyrate dope should now be applied one sprayel I

on in one direction and the other a t right angles to it - before the first coat dries. These in turn should be followed with one good cross-coat of aluminium dope after light sanding of the clear dope to improve adhesion. The aluminium coat is in its turn wet sanded lightly to produce a smooth surface and the residue rinsed off with water. Once the aluminium coat has dried, it should be checked for continuity by shining a light inside the structure. The film should be completely lightproof.

Finishing Coats. The finishing coats of pigmented butyrate dope may now be sprayed on. The number of coats should not be less than three. A high gloss finish is obtained by lightly sanding each coat when dry and spraying multiple thin coats rather than several thick coats.

The use of a retarder in the colour coats will allow the dope to flow out and form a smoother film. The final coat should be allowed to dry for at least a month before it is polished with rubbing compound and then waxed. The surface should be waxed a t least once a year with a hard wax to reduce th. possibility of oxidation of the finish.

Application to Polyester-Fibre Fa.bric

Polyester-fibre fabrics are being increasingly widely used for covering aircraft because of their long life and resistance to deterioration. For this reason it is important that the dope film is of the highest quality so that its life will match that of the fabric.

Priming Coats. Polyester-fibre fabrics are heat shrunk to provide a good smooth finish and tautening of the fabric is not a function of doping, although all dopes will tauten to some extent.

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The most notable difference in doping a synthetic cover is the difficulty, when compared with natural fabrics, of obtaining a good mechanical bond betmreen the dope and the fibres. Unlike natural fibres the polyester filaments are riot wetted by the dope and the security of attachment depends on them being totally encapsulated by the first coat. This must be nitrate dope thinned In the ratio of two or three parts dope to one part thinners. This is then brushed into the fabric to completely encapsulate the fibres.

The dope should form a wet film through the fabric but it should not drip through to the opposite side. The initial coat should be followed by two rnore brush coats thinned to a brushing consistency.

Since polyester is not organic, there is no need for a fungicide to be added to the first coat.

Filling Coats. Taping and attaching of drainage eyelets or grommets and inspection rings follow the same procedure as for natural fabrics. Priming coats should be followed by spraying two full-bodied cross-coats of clear butyrate dope. After these have dried they should be lightly sanded (400 grit) and cleaned with a tack rag. One full cross-coat of aluminium dope should then be sprayed on and lightly wet sanded when dry, the residue being rinsed off with water. This coat should be tested to verify that it is lightproof.

Finishing Coats. The finishing coats should now be applied in the same manner a s for natural fabrics. It should be noted that with a properly finished polyester cover the weave of the fabric will still show through the dope film. Any attempt to completely hide them with additional coats will result in a finish that does not have sufficient flexibility to resist cracking.

Application to Glass-fibre Fabric

Glass-fibre fabric has a loose weave, which tends to make it difficult to apply to aircraft structures. To overcome this problem it is pre-treated with butyrate dope and the covering and doping must be carried out in accordance wit.h the manufacturer's instructions.

Priming Coats. Nitrate dope must not be used under any circumstances with this type of fabric. The first coat of clear butyrate dope is sprayed on with the dope being thinned only enough to permit spraying. The atomising pressure must be set to the lowest possible that will permit atomisation without the dope being blown through the fabric. The coat should be heavy enough to thoroughly wet the fabric and soften the dope in the fabric, but must not be so heavy that it causes the dope to run on the reverse side of the fabric.

If the dope is allowed to run in this way an orange peel finish will develop and the fabric will not tauten properly.

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After the first coat has dried, further coats of butyrate dope should be sprayed on, each a little heavier than the one before, until the weave fills and the fabric tautens; this may take as many as five coats.

Tapes, drainage eyelets/ grommets and inspection rings are applied with a coat of butyrate dope.

Filling Coats. Once the fabric is taut and the weave has been filled, two full- bodied brush coats of clear butyrate dope should be applied and allowed t.o dry. The film should then be carefully sanded, making sure that it is not sanded through to the fabric.

Whilst the fabric is not damaged by ultra-violet radiation, the clear dope can deteriorate as a result of exposure and therefore, a coat of aluminium dope should be sprayed on for protect.ion and lightly wet-sanded smooth.

After the aluminium dope has been sanded, the residue should be removed '-v washing with water and then the surface dried.

Finishing Coats. These are applied in the same manner a s for natural fabrics. Several thin, wet coats of coloured butyrate dope will allow the surface to flow out to a glossy finish.

Doping Problems

If not carefully controlled some doping faults can occur (some of these faults can also occur with painting). These are listed below.

Poor Adhesion. Adhesion may be poor between the fabric and the first coat of dope and between the aluminium coat and subsequent coats. Adhesion to the fabric, particularly polyester fabric, is largely dependent on the technique used to ensure the encapsulation of the fibres. Adhesion to the aluminium coat r q y be impaired if too much aluminium powder was used or if the surface was r?dt thoroughly cleaned after sanding. The use of a tack rag to finally clean a surface before applying the next coat is always recommended.

Blushing. Blushing is a white or greyish colouration that forms on a doped surface. If the humidity of the air is too high, or if the solvents evaporate too quickly, the temperature of the surface drops below the dew-point of the air and moisture condenses on the surface. This water causes the nitrocellulose to precipitate out. Moisture in the spray system or on the surface can also cause blushing. Blushing can be controlled by reducing the humidity in the air (raising the temperature by several degrees may help) or by using a retarder in the place of some of the thinners.

A blushed area can be salvaged by spraying another coat over the area using a retarder instead of some of the thinners; the solvents attack the surface and cause it to flow out.

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Bubbles or Blisters. Caused by the surface of the dope drying before all i.11i:

solvents have had time to evaporate. This may happen if a heavy coat of tiope is applied over a previous coat that has not dried.

Dull Finish. The gloss of butyrate dope may be improved by the addition (.)i' up to 20% retarder in the last coat. Excessive dullness may be caused by I-xolding the spray gun too far from the surface so that the dope settles as a semi-dry mist. Small dull spots may be due to a porous surface.

Fisheyes. These are isolated areas which have not dried due to contamir~ation of the surface with oil, wax or a silicone product. Cleanliness is important with all wax removed using a suitable solvent before re-doping the area.

Orange Peel. Caused by insufficient thinning of the dope or holding the spray gun too far from the surface. It can also be caused by too high an atomising pressure, use of thinners that dries too fast or by cold damp draughts.

Pinholes. Smaller versions of a blister. Apart from the causes listed above, they can be caused by water or oil in the spray system. An air temperature that is too high can also be a cause.

Roping. This is a condition in which the surface dries a s the dope is being brushed on causing an uneven surface. Common when the dope is cold. When applying with a brush, dope should not be over-brushed. The pressure applied to the brush should be sufficient to ensure the penetration of the dope through the fabric.

Rough Finish. Dirt and dust on the surface, insufficient sanding and too low a working temperature can all cause a rough finish.

Runs and Sags. This type of defect is caused by too thick a coat, especially on vertical surfaces. This causes the dope to run and sag.

Seneral Considerations

The weight of the dope applied to the fabric is an indication that the scheme has been correctly applied. In the BS X26 doping schemes the weight per unit area is given and should be checked by doping a test panel a t the same time as the structure. The fabric is weighed before doping and then again after doping, the difference being the weight of the dope film. Mil Specs call for a minimum dope weight of 16 1g/m2 (4.75 oz/yd2) with a tolerance of + 20%.

When an aircraft is re-covered and re-doped it is essential that it is re-weighed and a new Weight and Centre of Gravity Schedule raised, it is also import.ant that control surfaces are balanced and checked against the AMM.

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CONTENTS

Springs Bearings

Internal clearance Lubrication Sealing and protection

Gears Zontrol chains Drive belts

Page

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SPRINGS

A spring is a device that will deflect under load (storing that energy as a strain) and return to its original length once the load or force is removed.

Springs are normally made of spring steel (high carbon steel hardened and tempered and chrome vanadium steels) supplied in sheet or wire form (typical specification SAE 6 150). Sheet form is used to manufacture leaf springs and wire is used for the manufacture of coil (helical) springs. Coil springs can be used as compression springs or tension springs.

Springs are used in the aviation industry for:

* Control springs - in instrument force/ torque balance systems. -f; Lock springs - in over-centre geometric locks. J; Return springs -- in hydraulic/pneumatic valves to return the valve

to its original setting. A Force pressure springs - to produce a pressure within a

hydraulic/ pneumatic valve for operation/ control purposes. * Shock absorber springs - to absorb energy eg, kinetic energy to

strain energy - spring shock absorbers on some tail-skids. * Force springs - spring balance etc.

Hooke's Law

Robert Hooke English physicist 1635 - 1703. Hooke's law of elasticity states that, up to the elastic limit, the strain (change in length) of a material is proportional to the stress (force per unit area). By their very nature springs are designed to work within this law. This means that if 1 unit of force is applied to a spring it will deform 1 unit of length and if the unit of force is doubled the change in length will double.

The spring balance shows this. Support, say, a lkg bag of sugar on the balance and it will extend a certain amount and if the mass is doubled to 2kg the balance extension will double.

Terms Used For Coil Springs

Free Length - The length of the spring without any load applied. When checking this length is should be within the limits as laid down in the appropriate maintenance manual.

Pitch - The distance between the centre of one coil of the spring and its adjacent coil - without any load applied.

Coil Distance - This is the distance between two ad-jacent coils - without any load applied.

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MEAN COIL FREE LENGTH DIAMETER

\ 1 2 3... ... 13 COILS ,

OUTSIDE INSIDE COIL COIL PITCH WIRE DISTANCE (AT BOTH EN DS) DIAMETER DIAMETER DIAMETER BETWEEN COILS

Fig. 1 COIL SPRING TERMS

Wire Diameter -- The diameter of the wire from which the coils are made.

Outside Coil Diameter - The outside diameter of the unloaded spring (OCD).

Inside Coil Diameter - The inside diameter of the unloaded spring (ICD).

Mean Coil Diameter - The average between the OCD and the ICD.

Tip Thickness -.-. The thickness of t.he ground section of the end of the spring (Compression coils only).

Compression Springs

May be wound left or right handed and are usually finished off top and bottom by grinding the coils flat to provide a surface on which the spring can act. The coils are usually made of round section wire but they may also be made of square section bar and the coil diameter is usually large compared to its free length.

Tension Springs

Again, these may be left or right handed and the coils are terminated, usually, by the ends being bent to provide a fixing to the component. Coil wires are usually of round cross-section with the coil diameter being small compared to the free length.

Springs may be designed in several forms:

* Helical (coil springs). Very common. * Ream spring. Absorbs a large amount of energy but has a limited

amount of movement.

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Leaf spring. Similar to a beam spring but is thinner and may be built up with several layers.

* Special springs. To include cup spring discs built up into a slc]ck providing an effect similar to a compression spring.

Maintenance

There is little maintenance requirement for springs and almost all rectifjcation is by replacernent. Further details or maintenance checks are in module 7.

BEARINGS

Bearings are designed to reduce the friction between moving parts - usually rotational movement of a shaft within a housing. They may be classified as:

J; Air Bearings. The rotating parts are kept apart by a thin film of air pumped under pressure between the journal and the rotating shaft. In some systems the air pressure is applied before the shaft starts to rotate and friction (and hence wear) rates can be very low.

Journal Bearings. (High Friction Bearings). The shaft rotates within a bush usually supplied with oil under pressure.

Roller/Ball Bearings. Sometimes called Low Friction bearings. Use is made of balls or rollers running between inner and outer races. Lubricated with oil or grease. Most are radial bearings.

RADIAL BEARINGS

THRUST BEARING

ANGULAR CONTAC BEARINGS

Fig. 2 BALL RACES

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Ball bearings employ balls, sometimes caged, which rotate within grooved tracks. Roller bearings use cylindrical, tapered or spherical rollers running in suitably shaped tracks. Both types run contir~uously.

Inner and outer races, and balls and rollers are made from high-grade carbon chromium steels, which is resistant to wear (the most important factor) and allow rotary motion while absorbing axial and thrust loads. The metal is also corrosion resistant. Metal with a high chrome vanadium content is used such as SAE 6195.

Selection of Bearing Type

In the selection of the correct type of bearing for any particular part of a transmission system the following factors must be taken into consideration:

Magnitude of the load to be carried. Direction of the load or loads. Available space. Rotational speed. Precision accuracy. Alignment requirements. Axial displacement requirements. Noise requirements (silent running?). Rigidity. Bearing life.

Taking each point in turn:

Load magnitude is usually the most important factor in determining the size and type of bearing. Ball bearings are usually used for light to medium loads, whilst roller bearings are better able to cope with heavier loads.

RADIAL LOAD + Fig. 3 RADIAL LOADS

RADIAL LOAD RADIAL LOAD

FLANGED OUTER RACES

-___ UNFLANGED INNER RACES

Fig. 4 ROLLER & NEEDLE BEARINGS

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Loud direction is important as cylindrical roller bearings having only onc, race without flanges and needle roller bearings can carry only radial loads. A combined load comprises both radial and axial forces acting simultaneously. The most important factor affecting the ability of a bearing to carry an ax131 load is its angle of contact (A).

CONTACT ANGLE

OUTER CAGE THRUST RACEWAY C E GROVE

\

FLANG

Fig. 5 EXAMPLES O F BALL AND ROLLER BEARINGS

RADIAL LOAD RADIAL LOAD RANGE RANGE t

AXIAL LOAD

Fig . 6 RADIAL & AXIAL LOADING BALL & SPHERICAL ROLLERS

RADIAL LOAD

AXIAL LOAD

INNER RACE ONLY AGAINST THE TAPER

Fig. 7 RADIAL & AXIAL LOADING TAPERED ROLLERS

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The greater the angle (A) the more suitable the bearing is for axial loading. Single and double row angular ball bearings and taper roller bearings are mainly used for combined loads. Deep groove and spherical rollers may also be used.

Single row angular ball and taper bearings carry axial loads in a single direction only. Where t.he direction of axial loads vary, two back-to-back (or face-to-face) bearings can be used. In the case of high axial loads, separate thrust bearings (eg deep groove ball) and support bearings (roller) are used.

Thrust ball bearings are suitable for moderate axial loads, and are designed to be either single or double acting.

Spherical roller thrust bearings can carry heavy axial loads but smaller radial forces.

LOAD

Fig. 8 SINGLE & DOUBLE BALL THRUST BEARINGS

THRUST I

RADIAL. LOAD

II)- RADIAL

/ OUTER RACE

Fig. 9 SPHERICAL DOUBLE ROLLER BEARING

Spherical control rod bearings are not revolving bearings and are used where movement back and forth is required. They are used on flying control rod systems for example. They may be used as rod end bearings and may be located in threaded fixtures to enable control rigging to be carried out. Spherical bearings also allow for slight misalignment of control rods and components during normal operation.

Available space is determined by the component design. Deep groove balls are normally used on small diameter shafts while cylindrical or spherical rollers can be considered on larger shafts. Needle bearings can be used where radial space is limited (such as Hardy Spicer constant velocity joints), whereas single row cylindrical or deep groove ball thrust bearings are used in areas of limited i ~ ~ i a l space.

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(A Hardy Spicer coupling is used in shaft power transmission systems to allow limited angular movement about the shaft/s centre line. I t uses four needle bearings as part of a centre cross piece 01- universal joint to allow limited angular movement between the two rotating shafts.)

BOLTED JOINT BETWEEN THE TWO SHAFTS

OUTPUT SHAFT

GREASE NIPPLE

IP RETAINERS

Fig. 10 HARDY SPICER CONSTANT VELOCITY JOINT

BEARING/ HANGAR SELF ALIGNING

OUTER RACE

ROLLING ELEMENTS 'FIXED' INNER

I I RACE

SHAFT

Fig. 11 SELF ALIGNING BEARING

SINGLE ROW DOUBLE ROW RADIAL DUPLEX THRUST

SNAP SHIELDED SEALED SELF MAGNETO WHEEL AXIAL RING ONE SIDE ALIGNING THRUST

Fig. 12 BALL BEARINGS

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Speed limitations are determined by the maximum permissible operating temperature of the bearing, the type of lubrication and cooling available. Low friction bearings (ball or roller) which generate low internal heat are ideal for high rotational speeds.

Precision bearings are used on shafts where stringent demands are made on accuracy ie high-speed shafts; these will include double row angular ball thrust bearings.

Angular misalignment can, for example, be caused by a shaft deflecting under heavy loads or fuselage flexing in flight for a long shaft. Bearings capable of accommodating such movement are self-aligning ball bearings, and spherical roller bearings.

STRAIGHT STRAIGHT STRAIGHT PAPERED TAPERED SEPARABLE SEPARABLE NON DOUBLE ROW OUTER RACE INNER RACE SEPARABLE

BARREL BARREL CONCAVE CONCAVE NEEDLE DOUBLE ROW DOUBLE ROW

Fig. 13 ROLLER BEARINGS

Axial displacement of a shaft by a force (for torque measurement) or expansion or contraction (due to temperature change) is permitted by the use of a 'non- locating' bearing of the single flangeless race roller or needle type. Note that normal bearing configuration consists of a locating (fixed) bearing and a non- locating (free) bearing.

Silent running is sometimes a n important factor in bearing selection (bearings in the vicinity of the flight deck or near passenger compartments). Deep groove ball bearings are normally chosen for this type of application.

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INNER RACE

SHOULDERS

DIAMETER

SEPARATOR

Fig. 14 DEFINITIONS - ROLLER BEARINGS

CUP LENGTH

Fig. 15 DEFINITIONS - TAPER ROLLER BEARINGS

LENGTH A

RE1 LIP

Fig. 16 DEFINITIONS - NEEDLE BEARINGS

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Rigidity of a bearing under load can sometimes be important. Although the elastic deformation of a loaded rolling bearing is very small, roller type bearings deflect less than ball bearings due to the greater contact area between the rolling elements and the raceways.

Bearing llfe is defined as the number of revolutions (or operating hours at a given constant speed) which the unit is capable of enduring before flaking or breakdown occurs on the races or rolling elements. As no two bearings of the same type have identical lives the 'basic life rating' is based on the life achieved by 90% of a test population of identical bearings in laboratory test conditions.

Bearing Elernents

Bearing Rings (Inner and Outer Races) and Rolling Elernents (Rollers and Balls) are made from high-grade carbon chromium steels, allow rotary motion while absorbing axial and thrust loads. The metal is hard, resistant to wear and has good anti-corrosion properties.

Cages. The primary function of these is to keep the rolling elements apart and in separate bearings retain the rolling elements. Made from pressed brass, steel or phenolic materials. May be called a separator.

Seals. May be made of elastomeric material with the seal snapped into position with a ring.

Radial Bearings

Used in all forms of transmission, eg shafts, gears, control rods, pulleys, etc. Manufactured with the balls in single or double rows, normally they are rigid but may be self aligning when accurate alignment may not be maintained during operation. May be sealed to prevent debris from entering the bearing and to retain the lubricant. Balls are normally retained in a cage, but in sc e cases there is a filling slot which enables more balls to be used giving a grt er load capacity.

Angular Bearings

Suitable for radial and axial loads in one direction. The outer race is recessed on one side to allow assembly/dismantling. Where axial Ioads in both directions occur two bearings are used back-to-back. The load capacity depends on the contact area.

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Thrust Bearings

Designed for axial loads only, so are used with either a roller or ball radial bearing. Balls are usually retained in a cage between two grooved races. hlost suitable for heavy axial loads at low speeds.

Instrument Precision Bearings

Manufactured to close tolerances and used in instruments and commurlication equipment.

Cylindrical Rollers

Capable of carrying greater radial loads than ball bearings because of tile greater contact area. Bearings with ribs on both races will also be capable of carrying light axial loads. Most common are rollers where the length is equal to he diameter. Needle roller bearings have lengths several times greater than the

diameter.

Taper Rollers

Designed so that the axes of the rollers form an angle with the shaft axis. Capable of accepting radial and axial forces simultaneously. May be installed back-to-back. The axial loads cause rubbing on the cone lip or flange so adequate lubrication is necessary. Used in helicopter rotor heads, gear boxes etc.

Spherical Rollers

May have one or two rows of rollers running in a common spherical track in he outer race - this gives good self-aligning properties. Can withstand high

radial and axial loads.

INTERNALCLEARANCE

Standard ball and roller bearings are manufactured in four classes of diametrical clearance and are marked to indicate the class of fit. It is important that any bearings replaced are of the same part number and nomenclat~~l-r: (check J A A form 1 and IPC/AMM) and have the same classification of f i t . The marking is generally a series of dots or circles.

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One Dot Bearing - Group 2

This group has the minimum arnount of clearance. Used where minimum axial and radial movement is required - usually precision work. Must not be used where heat is likely to be transmjtted to the bearing and are not suitable for thrust bearings or for high speed.

RADIAL CLEARANCE

4 1- AXIAL CLEARANCE

Fig. 17 INTERNAL CLEAMNCES

Two Dot Bearing - Normal Group

Intermediate range and used for most general applications. Used where only one race is an interference fit within its housing (requires force to be fittedlremoved) and there is little transfer of heat to the bearing.

Three Dot Bearing - Group 3

This group has a larger clearance range and is used where both inner and outer races are interference fits in their housings. Heat transfer is moderate and the bearing is suitable for high speed operation.

Four Dot Bearing - Group 4

These have the greatest clearances. Both races are interference fits and heat transfer is considerable.

LUBRICATION

Provided to reduce friction, dissipate heat and prevent corrosion. For low speeds the bearing is usually packed with grease - which might be anti--freeze grease. For high speeds the bearing may be lubricated by an oil spray from a metered supply - as in some jet engines. It is important that only oils and greases as specified in the AMM are used and lubrication frequencies as stated in the maintenance schedule are adhered to.

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SEALING AND PROTECTION

Bearings must be protected against the entry of moisture and dirt, and t c ~ prevent the loss of lubricant. Considerations affecting the type of seal would be (a) type of lubricant, (b) space available, (c) misalignment of shaft and (d) seal friction.

There are two basic forms of seal:

a) Non rubbing seals. b) Rubbing seals.

Non Rubbing Seuls rely on narrow gaps or radial labyrinths to form the seal. This type has negligible friction and wear and is particularly suited for high speeds and temperatures. Straight or spiral grooved labyrinth seals used in areas of extreme temperature (in conjunction with sealing air pressure) are used on jet engines.

7ubbing Seals rely on the elasticity of the sealing material and maintaining a minimum pressure at the sealing surface. Can be a simple felt or rubber/polymer washer for grease or grit seals. 'V' seals comprise a rubber ring with a 'hinged' rubber lip that is pressed radially against the sealing rotating surface. Used on wheel bearings.

Y' seals are used externally with grease lubrication and internally with oil. Carbon rubbing seals are sometimes used with high-speed shafts.

Shielding and sealed bearings are pre-packed with the correct lubricant and do not normally require lubrication in service eg deep groove or self-aligning ball bearings - but check the AMM.

Page 188: Materials and Hardware

GEARS

A gear is a machine element used to transmit motion between rotating shaftslwheels when the centre distance between the shafts is not too large. They provide a positive drive, maintaining exact velocity ratios between driving and driven shafts.

Power transmission gears are usually made from chromium molybdenum steel (eg E4 130) which provides good toughness and resistance to wear. Some (low power) gears are made from sintered metal (powered metal). Non-power gears can be rnade of almost any material including composites for quieter running non lubricated arrangements.

Most gears are run lubricated either by regular maintenance lubrication or by being run semi submersed in oil or spray lubricated.

J p CRITICAL SECTIONS

OF WHEEL & PINION I

OF WHEEL & PINION

Fig. 18 BASIC GEAR TYPES

There are two basic gear tooth profile forms the origins from which all gear types are derived. They are the involute gear, by far the most common in general use, and the conformal gear, but because of problems that were lar, 'y insurmountable until now, has not been used much in the past. Modern manufacturing techniques have brought about its resurrection and at least one helicopter (the Lynx) now utilises conformal gearing.

An involute tooth is laid out along a curved line which is generated by taut wire as it is unwound from a cylinder. The generating circle is called the base circle of the involute. The involute curve establishes the tooth profile outward from the base circle. From the base circle inward, the tooth flank simply follows a radial line and is faired into the bottom land with a small fillet.

Terms used:

Addendum The radial distance between the Pitch Circle of a gear wheel and the top of the tooth.

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Addendum Circle The circle that passes through the tips of the teeth.

Circular Pitch Length of the arc of the Pitch Circle between the centres of other corresponding points of adjacent teeth. Generally referred to simply as the 'pitch'.

Clearance The difference between the Addendum and the Dedendurn.

Dedendum The radial distance between the Pitch Circle and the Root Circle (depth of wheel tooth below pitch circle).

Dedendum or The circle that contains the roots of the teeth. Root Circle

Face

Flank

That surface of the tooth which is between the pitch circle and the top of the tooth parallel to the axis of the gear.

That surface which is between the Pitch Circle and the bottom land parallel to the axis of the gear (the flank also includes the fillet).

Interference If contact does not occur on the line of action then interference may occur. This is often the case when a pinion with a small number of teeth is in mesh with a gearwheel with a large number of teeth - the faces of the wheel teeth binding with the flanks of the pinion teeth. If this happens the pinion teeth will be undercut at the roots. This will cause debris causing further wear and weakening of the teeth with eventual failure.

Line of Action Contact between the teeth of meshing gears takes place along a line tangential to the two base circles. This line passes through the Pitch Point and is called the Line of Action.

FACE

Fig. 19 INVOLUTE GEAR TOOTH DETAIL

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TOO THlC

. ADDENDUM

DEDENDUM

Pinion

Pitch

FACE WIDTH

\

TOOTH TOP LAND SPPCE / / - a - F

ADDEND1 CIRCLE

- FLANK

\ BOTTOM ROOT \ PITCH LAND ROOT CIRCLE CIRCLE

Fig. 20 SPUR GEAR TERMS - 1

The term applied to the smaller of two mating gears.

Gear teeth pitch may be measured as follows:

Daimetral Pitch is the number of teeth per inch of Pitch Circle Diameter. It is a ratio.

Circular Pitch is the distance between two corresponding points on two adjacent teeth around the Pitch Circle.

Pitch Circle A circle, the radius of which is equal to the distance from the gear axis to the Pitch Point.

Pitch Circle The diameter of the Pitch Circle. Diameter

Pitch Point The point a t which two pitch circles meet - the point of contact which transmits the motion tooth to tooth.

Pressure Angle The angle between the line of action and the common tangent to the Pitch Circles at the Pitch Point.

Top Land Is the top surface of a tooth at the tip or crest. The Bottom Land is the surface between the fillets of each adjacent tooth a t the root.

Root Fillet That bottom portion of the tooth profile where it joins the bottom land. It is usually concave.

Page 191: Materials and Hardware

Toe

DEDENDUM

C D

CICULAR PITCH

CLEARANCE

ADDENDUMOR TIP CIRCLE

INTERNAL SPUR EXTERNAL SPUR

Fig. 21 SPUR GEAR TERMS - 2

That part of a bevel gear that is the shortest part of tht. tapered tooth. I t subscribes the smallest diameter. The heel is the other end of the tooth that subscribes the largest diameter.

Tooth Space Distance between two adjacent teeth measured along the pitch circle.

rooth Thickness The thickness of a tooth measured along the pitch circle.

Working Depth Is the maximum depth that the tooth extends into the tooth space of the mating gear.

Whole Depth Is the sum of the Addendum and the Dedendum.

Types of Gears

Various types of gears transmit power through gearboxes. The type selected for use in a specific application will depend on:

How much power to be transmitted k Is a change of rpm required? t Is a change of torque recyuired?

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* Is a change of angle or direction of drive required? " Is the gear system to be free from feedback (non-reversible)?

Using a smaller driver gear (with less teeth) than the driven gear, will reduce the speed of the driven gear but its torque will be increased. If a larger driver than driven is used the reverse is true.

A gear system, or gear train, is made up of gears that are:

* Driver - a gear wheel that drives another gear wheel.

k Driven - the other gear wheel that is being driven.

* Idler - this is a driven and a driver wheel as it is a wheel between two others. Often used to change the direction of rotation (eg anti-- clockwise to clockwise) or change the speed.

Figure 23 shows an internal and external Spur Gear where either the larger the smaller gear could be the driver so loads in the system would be 'fed-b% i' from the driven to the driver,

Figure 24 shows a Worm Gear where the worm is the driver but the driven gear (Pinion Gear) could not be the driver as any movement of it would not turn the worm (non reversible, no feedback).

Fig. 22 GEAR TRAIN

INTERNAL

Fig. 23 SPUR GEARS

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WORM U Fig. 24 WORM GEAR

Gears are named according to the angle of intersection of the axis and thc shape of their teeth:

A Spur L; Helical * Worm * Hypoid * Bevel etc

Spur Gear

These are classified as external (the most common), internal, and Rack a r ~ d Pinion. External spur gears have teeth which point outward from the cent re of the gear. Internal or annular gears have teeth pointing inward towards the gear axis. A rack (a gear with teeth spaced along a straight line), together with a pinion gear, convert straight-line motion into rotary motion and vice versa.

Spur gears are normally straight toothed (but can be spiral cut - helical gear). Used on shafts that run parallel to one another but not on the same axis. Can Se noisy due to impact of engaging teeth.

PITCH LINE OF RACK

Fig. 25 FWCK TOOTH DETAILS

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Fig. 26 RACK & PINION SPUR GEAR

Helical Gears

These are a development of the spur gear. lnstead of the teeth being parallel to the axis of the gear they lie at an angle (a helix angle).

The main advantage of helical gears over straight cut gears is that more teeth area are in contact at any one time. Meshing takes place along a diagonal line across the faces and flanks of the teeth. Thus one pair of meshing teeth remain in contact until the following pair engage so the load on the teeth is distributed over a larger area. This provides a smoother and quieter drive as well as enabling more pourer to be transmitted.

The disadvantage of helical gears is that they produce a heavy axial load on the shaft. This axial load can be eliminated by the use of double helical gearing (herringbone) but can also be absorbed by thrust bearings that support the gear shaft.

DOUBLE SINGLE

Fig. 2'7 HELICAL SPUR GEARS

blank

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A do~tblc helical gear h a s two sets o f teeth, orit: w ~ t h a rig11t 11:ir1~1 l-ir.lix a l ~ d the other n.ith a left hand helix.

In sorne drive systems from engines to propellers and rotor heatls, the axial thrust loat3 on a shaft fitted with helical spul- gears is utilised fox torque measurement purposes The shaft is allowed a sniall a r n o ~ ~ l ~ l of ( > n d float and , a s torclue is applied, the shaft moves axially (as it rotates). This axial movement pushvs on a small piston t h u s producing pressure in an o~led filled dash pot. The oil pressure is transduced into a n electrical signal for fllght deck indicators readl~ig torque in Nm.

PRE CYL

SPUR GEAR

DRIVING GEAR)

AXIAL THRUST WHEN TORQUE APPL-IED -

HELICAL GEAR

OUTPUT SHAFT

Fig. 28 GEAR BOX SHOWING HELICAL GEAR A S TORQUETRANSDUCER

Spur gears a re found in gearboxes; in epicyclic reduction gear trains; accessory drive trains, and in gear-type oil pumps - for engine oil systems and some older hyclra~~lic systems (giving low pressure/ high flow rates).

For internal s p u r gears, the positions of the adderldum a n d dcclendum are rcvcrsc.ci from those of the external gear but are still relatetl to I l ~ e root a n d tip. This I-csults i11 a different tooth :\ction and less s l~ppage thari with a n equivalent external spur .

'I'hc. iiitcrnal gear makes it suited to closer centre distances t1iat-I could IJ(> used ivith :ti1 external gear of the same size When it is necessary to r r~c t i~~ta in t tie same sense of rotation for two parallel s l~aft s , the, 1nte1-nttl gc1:11 i s t,spec-~:~ily de s i~ :t t~ le becatisc it eljmi~i;~tes thy nced for an idler- gcar 'I'1ic:sc. c.orlditio[ls rnnltc- the internal gear highly adaptable tto ey~icyrlic and pl;t~l(:t;t~ y ~ c ; \ J t r ~ ~ l n s

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11s r r i e ~ ~ t ~ o r l ~ ~ l ; t t x ~ \ l t ~ , ~ C I I C ; I I gears <ire essen l~a l l~ spul gears. ' l ' l le~ run on parallel axes with teeth oblique t o the tooth s~~r fnce , sta1-ting :it one edge ~_roc~edi!lg across the face of thc tooth. This actlon results in reduced irnpact stress and quieter operation, particularly at high speed.

I-ierl-ingbonc- gears are equivalent to two hel~cal gears of opposite hand placed side by slde They are suited to high-spced operation and eliminate the axial thrusi produced hy single helical grars Helical gears are referred to as right or left hand in the same manner a s screw threads. A right hand gear being one on whlch the teeth twist clocktvise as they rerede from the ol~server- looking alo~lg t h c gear- axis

Usecl t o coriiiect shafts in the sarne plane where the centre lines inter-sect and a change of direction is required. The teeth can be either straight cut or spiral cut and its basic form is that of a cone. They are commonly found on intermediate and tail rotor gearboxes on helicopters where a change in the direct~on of drive is required. They are also used in many gearbox accessory drives a t the input stage of the turbine shaft and the accessory drive. Used to change the shaft 'axis clirectiorl andlor change the speed.

GEAR 1 AXIS I PITCH CIRCLE DIAMETER

l-3 * P lg- 29 BEVEL GEAR TERMS

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'The :1ng1~ between tlic shafts is u sua l l~ a right angle hut it rnay Iinve : i l l \ :~ngle u p to 180" The vvlocity ratio is the inverse ratio of the diameters of t h e i ~ t,a.;es or teeth ratios.

TWO bevel gears with equal numbers of teetll and running togethvr s v l t l ~ t i l t ~ i r

shaft axes intersecting at 90" are called Ml t~e gears. Several forrns of l ~ c ~ v c l gears are in use, including straight tooth. spiral anti skewed gear-s.

Exter-rial bevel gears have pitch arlgles lcss than 90'. Internal bi-~vel ge:lr:, have pitch angles greater than 90".

A crow11 gear 1s one havlng a pitch angle of 90'. In n crown geai- thcrefot t., its pitch surface is a plane and the crown gear corresponds in this respeci to a rack and spur gcaring.

The s~nlplest forrri of bevel gear has stl.alght teeth. 'The diametrical p ~ t c . k ~ o f a bevel gcar is constant across the full widtl~ of the teeth.

Because each point on a stralght tooth bevel gear rerr~ains a fixed distance- from the pitch cone apex, there is no sliding along as the tooth engages.

Spiral bevel gears provide a gradual engagement compared to the full line engagement of straight bevel gears. 'Their teeth are curved and oblique. 'J'11ey have greater load carrying ability than with straight teeth gears of t l ~ e sarrie size.

Fig. 30 BEVEL GEARS

Bcvcl Gear Terrris

If the rurved surface of the back cone is viewed normally the tevth hcivc: t i l t .

s a m e profile as the tceth on a spur gear The atlderldum a ~ l d c l e d c * n d ~ i n ~ hi~vc. thc slinie proportions a s spur gear leetli but are mc.as~ired above ancl bt,low the p ~ t c h rircle ~~a ra l l e l to the back cone clriver.

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l J ~ - t , h h ~ i ~ t , :ii~gIcs 1.~11- t.1 gt~'iij 5 ; t i L~ ~ . ~ s u ~ i l l j ~ 14',:~' o r 20L as fo:. spur gears.

At l t f l t lonal t o those terms ~ ~ s t ~ l for spur gears the f'ollowi~~g are used

t ) l : i - h ('oiie Angle The anglt. btltween the axis of t he gear and the p1tc:h cone teeth (-el I trf- When the pitch c-one angle is 45" the gear is a mltre gear

Fac c- A~lgle Angle t)etivethn the ljne at rlght angles to the axis arid the top surfaces of the teeth.

1l:clgt. Angle The angle between a line at rlght arigles to the axis and the top edge of the teeth.

R~lcienclu~n Angle The angle between the gear wlleel and the top surfaces of the teeth.

I)erlerid~ir~l Angle The angle Gc.tween the gear wheel and the bottom surfacc of the teeth.

On some gyar boxes, to establish correct wear patterns, one of the bevel gears m;ly be adjusted forwards or backwards along its axis (by a fraction of a mm). This may be carried out using shims (as per the manual) and will put more or less area of tooth in rrlesking contact.

'These are used where thc centre lines of the two shafts neither intersect or r u r ~ parallel.

They are similar to bevel gears in application and form, but the basic surfaces on which they are cut are hyperboloids instead of cones. The teeth are helical and the axes of the shafts c-lo not intersect.

TOE /HEEL

Fig. 31 HYPOID GEAR

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Used \?.here a large reduction in speed and ari increase i n torcluc. is r e q ~ ~ ~ r e d Gives a 910 back-feed' provisiori and used on systems where i t is required that any load taken by the system is not felt on the input to the worm Used or1 lifting equipment using either a. manual or powered worm input.

These connect shafts a t right angles which lie on different planes. The worm IS

esserlt~ally a screw which may have a singlt., double or triply start thread. These engage with teeth on the pjnion gear. Older teeth or1 pinions were straigl~t but now are usually wasted to give a greater contact area with tht- worm. Worms may be known as Encircling or H~ndley Worms. W ~ t h parr~llel worms the teeth are straight sidecl on a section througli the axis, and have the same proportions a s standard involute track teeth.

The worm is the driver and the pinion is the driven gear. Movement cannot be transmi ttvd the other way.

SKEW GEAR

Fig. 32 WORM GEARS

Gear Trains

A PI-inciple function of gears is to change the speed of rotation and/or their jirectjon. Besides changing speeds the torque can be reduced or increased.

The change in speed of two gears in mesh is calculated a s the Velocity Ratio. Velocity Ratio (VR) is the number of revolutions N1 of the driving gear dividecl by the n t~rnber of revolutions Ny of the driven gear in the same time interval.

For gears with teeth Tl and 7'2, respectively, VR is expressed as the following equation:

Exarn1)lc. I . If a 20 tooth piriiorl (the smaller of a palr of gears) tir~vc~s a 4 0 tooth gear-, thc: piriiori must rotate ~ L V I C ~ ' for eac l~ one revolut~on of t11e gear.

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- - 20 - ' /J or 0.5 (half as fast) 40

Example 2 Driver spur with 200 teeth (2u 40Orprn What is the speed of the dr iven gear with 30 teeth?

- - 200 =- 6.66 (6.66 times faster) 3 0

- - 400rprri x 6.66 = 2666rpm approx.

Stepping up or stepping down the speed of the driven gear will also affect i torque. Stepping up the speed reduces the torclue by the same ratio and st eppir~g down the speed increases the torque by the same ratio.

If the final, or driven gear, in a two gear (external spur) gear-train is to rotate in the same direction a s the driver then an Idler gear i s required between the two. I f tile distance between the driver gear the final gear is large then several idler p a r s may be required The idler gear does not affect the speed ratio.

'I'he most important d~stirlction on classifying gear trains is that between ordinary and epicyclic gear trains In ordinary trains, all axes remain stationary relative to the frame but in epicyclic trains, at least one axis moves relative to the frame.

SUN WHEEL PLANET PINION 1 CARRIER

FIXED ANNULUS

PLANET P~NIONS

Fig. 33 SPUR EPICYCLIC GEAR

'l'hc. rc.ci~~ction in the sprvtl o f the final ds~ve may be ach~eved in several stages ;IS In some helicopter nlain rotor drives. 'I'he first stage is normally corriprisctf of ;in 11711)ut geal ctrivu~g an i r l y ~ r l t driven gear, whic11 11;~s a Larger number of t ( , L l i t i i , , ~ ~ t l l ~ , ~ T - ~ X ~ L I -

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The I-vtiuction achieved across this type o f gearing is expressed a.; a r;it~o, wher-c. the number of teeth on the drive11 are compared with thc nurnbcr of teeth on the driver. Thus a gear train consisting of a driver with 30 teetll and drix-en with 90 teeth would have a reduction ratio of 90 : 30 or 3 : I .

Thc second reduction stage is usually i n the form of a spur ep~c:y<-lic. reduction gear. 'This consists of a central, or Surr gear, ~vhrch revolves insidc a sr:~tionary Ring gear (the ring gear, a fixed annulus, norrnally forms part of the g e a ~ box outer- ~ ~ a s i n g and is internally toothed) Interposed between the sun gear , ~ n d the rlng gear, and meshing with both, are sets of Planetary Pinloris, varjilng in number from three to eight (figure 33 shows 3). The planetary pinions are housed in a carrier to which is secured the output shaft.

As the sun gear rotates, the planetary pinions are made to rotate about their cutis, :iind, because they are in mesh with the ring gear, which is stationary, they "walk" rouiid the gear, taking with them the planetary pinion carrier. This transmits a drive to the output shaft, which rotates in the same direction as the sun gear, but a t a reduced speed.

The reduction achieved in an epicyclic gear assernbly is also expressed as a ratio, but the numbers of teeth o f t h e sun gear and the ring gear only are considered. The actual reduction car1 be found using the following formr-]la:

Number of teeth of S l J N + Number of teeth of RING Number of teeth of SUN

Thus tln assembly consisting of a sun with 40 teeth a11d a ring with 120 teeth \voul(l have the following reduction:

This can be expressed a s a ratio of 4 : 1

From the above it can be seen that the planetary pin~ons are, in fact, ~ d l e r gears and their number of teeth is of no consequence to the actual reductjon ratio. They are, of course, an essential part of the assembly, providing the means of transmitting the output drive power.

In sorrle assemblies, the epicyclic gearing is in two stages (figure 35), wit17 the lower stage output shaft driving the upper stage sun gear. In a single stage assembly the planetary pinion carrier transmits the drive directly to t h c ~na in rotor drive shaft.

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T!!j s gear trair! ronsists rjf two cpposed bevel gears of different diameters.

'Tht. large gear 111 figirrr 34 1s the driving gear and the smaller gear is a fixed stationary gcal l'hree planet gears or pinions, free to rotate, are mounted or1 equally spactld arms whirl-i are part of the output shaft (eg a propeller shaft).

They are situaletli belwevn, arld are in engagement with, both the driving and fixed bevel gears Rotation of the driving gear causes the planet gears to rotate which drives their respectlrie rnounting arrns and the shaft. The assembly allows for high torclue transmission and acts as a r-eduction gear.

FIXED GEAR

3 PROPELL

Fig. 34 BEVEL EPICYCLIC GEAR

Figure 28 shows a two stage speed reduction gear box for a helicopter. Stage one is a spur gear and stage two is a helical gear. This gear also acts a s a torque transducer utilising the fidct that when power is transmitted through the gear the helical teeth produce a n axial movement of the shaft.

Pressure is created in a n oil filled cylinder which is converted to an electric, signal for transmission to cockpit instruments.

Fjgure 35 shows the gear train or transmission system of a twin engined helicopter (based on the Aerospatial Super Puma). There is no need to commit the details to me1mor.y and the figures are approximate - but studying the clrawing will give 311 insight into how gears are used.

Both (jet) engines drive into the system at about 22,000rpm. This goes via a ciouble helical rccluctiori gear to bring the rprri down to about 8000rpm These two drives feed into a common drive via a single helic:~l reduction gear to bring t h e rpm down to about 5000rprn.

'I'his common drivr drives ( h e rnain bevel gear to redlice the r p m again - down to about 20001-pm.

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MAlN ROTOR 265rpm

265rprn EPICYCLIC REDUCTION GEAR

772rpm

FORWARD 30UBLE L H ENGINE HELICAL 22840rpm REDUCTION GEARS

. n

REDUCTION

REAR TRANSMISSION

ACCESSORY DRIVE

MAlN BEVEL OIL PUMP

GEAR

Fig. 35 HELICOPTER TRANSMISSION SYSTEM

The final reduction stages to the main rotor are achieved uslng a doublt. serially mounted epicyclic reduction gear. This brings the final speeci do~vn to 265rpm.

The individual engine drives take spur gears to drive accessories such as oil pumps, hydraulic pumps , generators etc (about 2000 to 3000rprn), arid the comnlon drive is used to drive the tail rotor.

CONTROL, CHAINS

These comply with the requirements of British Standards 228 or IS060h.

C h a i ~ l s ancl sprockets provide a strong flex~ble positive connectior~ in cor~trol systerns and are generally used where it is necessary to change tlirectiori or to conrlcct to a push /pu l l rod system. Used where high loads are c.ncountc.rt.d, eg engine controls, flying controls etc

'The chain is made u p of a series of a pair of liril<s (an inncr llr l lc and an o ~ ~ t e r link). E;lch inner link consists of two:

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A Oi~ t r r plates. A t3enrir1g pins.

'I'he ckialri llas three principal dirrtprisions:

* Pitch The distance between the centre of two adjacent

rollers/pins. ?; Roller diameter. .A 'I'hc ivldth betwerr~ the lnner plates.

These dirr~ei~sions ar-e important for the serviceability of'the chain and for its correct fitrr~rnt around sprocltet wheels, pulleys etc.

OUTER

OUTER LINK

INNER 1

WIDTH BETWE INNER

BEARING \ I

PIN <r I

.-INK ------. 1

ROLLER E N

PLATES I I

I I +"- OUTER PLATE +-t Secured by peening the end of the

bearing pin. If a disconnect point ROLLER would be secured by locked nuts DIAMETER on the bearing pin or other

approved method.

Fig. 36 ROLLER CHAIN CONSTRUCTION

Chain assemblies are supplied from the manufacturer (approved supplier) ; complete proof load tested units and no attempt should be made to dismantle riveted links or attachments. Only the bolted or screwed attachments can be disconnected.

Any peerled nuts and bolts and split pins must be used O I ~ C C only.

'I'he chair) is supplied boxed, lightly oiled and coiled in oil--paper. I t is identified by part rlurnber and name arid shoulcl be accompanied by the appropriate stores release docurnentatiurl (,JAA/ EASA form 1 )

LVheri fittings ~ l r e connected to the end of t he chain they must be fitted in a positivc way using lockecl pins, locked nut arid bolt asscmblics etc. The SBAC standard for. locking a nut and boll asserriblv 1s to peen the bolt end for chams of 8mrn pitch or under a t i c i LISP a split pinned lock nut for larger chains (the out cr ~)l ; \ te of the cl1a113 is tlorrnally tapped)

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CHAIN Taking tensile loads only

END CONNECTOR Allows for attachment to pushlpull rod, cable turnbuckle, cable connector, bellcrank lever etc

\ SPROCKET WHEEL Allows for chain direction change & far component drive

t Rotates chain about its axls by 90 degrees

DRAWING FROM CAP 562 Fig. 37 CHAIN & SPROCKET ASSEMBLY

EXTENSION PIECES / Will not allow chain

I\ to be fitted to the

NON-INTERCHANGEABLE END FITTINGS §POCKET

WHEEL

I \ / sprocket th e wrong

DRAWING FROM CAP 562 Fig. 38 NON-REVERSIBLE CHAIN

The use of cranked links for the attachment of end fittings to chains is riot permitted. Nor is it permitted to use spring clips for the attachment of linlts to join the ends of a looped chain. Positive methods must be used such as bolted joirlts (with the bolts loclted).

Charlgc of direction is achieved by the use of sprocket wheels. And the axls of the ch:tin m a y be changed by 90" hy llie use of a hi-planer block.

7'0 1)rcxvent the chain from being fitted the wrong way round non 1-evers11,lc. chai~i~; arc: ~isecl.

Page 206: Materials and Hardware

These are the samr as the standard chain except that they have extension pieces every othrr lirik and they are fitted to sprocket whecls where there is a guard close tu tile whrel. When fitted to the sprocket wheel the extension pieces pass around t he wheel either sidr of the wheel. I f 11 is attempted to fit the cham to r i ~ c \vheel the wrong way round the extension pieces will be on the outsidr circunifrrrrrt-t of the wheel and will not pass undr r the guard.

Fig. 39 NON-REVERSIBLE CHAIN FITTED TO THE A320 TAILPLANE MANUAL TRIM CONTROL SYSTEM

Chains rnay have handed or rion-intercharrgeable erld fittings, this means that, together with the chain extension pieces and guard it is impossible to fit the chain incorrectly into the system.

Maintenance

This is detailed in module 7.

Page 207: Materials and Hardware

DRIVE RE1,'I'S A N D PIJL1,EYS

These are used to drive conlparativelv lightly loaded componerit s suc.11 ; IS

generators - on some piston engine aircraft, and tirriing rnec%har~isnls C:)rrectly installed and tensioned they provide an illexpensive lightweight dl-ive SJ stem which is easy to maintain. The fabric reinforced rubber belt forms a c-oi~tinuous loop around two (or more) pulleys Note Pulleys are called sheaves in qome publications.

On some systems the belt may go ax-ound more than one pulley with uric. belng the driver and the others being driven. To maintain tension a sprurig loaded or acljustable idler pulley may be fitted (normally in the longcst s t r :light rtln of the belt) between the driver and driver1 pulleys.

This chapter deals with the different types of belts ant1 pulleys that may be fo~lncl in service. For the actual design and maintenance practices of a particular belt drive system you s h o ~ ~ l d refer to the belt drive rr1anufac.turer7s manual and/or the AMM.

Many belt drives are of the "V" type, though there are examples of flat belt drives in use and synchronous belts for applications where it is important that components operate synchronousl.y (cam belts on pjston engines for example.

These are used with flat pulleys with flanges and/or with guides The flanges or guides are to ensure the belt does not come off the ptllley. The flat belt system is cheaper than other belt systems :tnd used where very little load transmission is required. They are of thinner cross section and the specification dirnensions a s for- V belts are less important.

V Drive Belts

These are divided into 2 groups - - heavy duty and light duty. The V design ensurf-s i t sits within the V shaped pulley with no tendency to come off and increases its grip as more tension (power) is applied. The belts are made of rubber or synthetic materials and are strengthened by fabric material, thls provides strength in tension and reduces the belts ability to stretch. 'The rubber provides grip and a wearing surface. It also proterts the fabric from moisture and contamination.

Tlie c1:issical cross section is shown In figur-r~ 4 I . It is sornetinles callvtf 13,lnded Cons tl-uction. The main tension fabric yarns r u n longltudil~ally and I ht. complete belt is enclosed by a fabric covering and a layer of rul-~ber

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MATERIAL

Fig. 40 FLAT BELT CONSTRUCTION

RUBBER OH SIMILAR MATERIAL.

MAIN TENSION MATERIAL

- - - I I SUPPORTING FABRIC --, ,/ Helps keep shape of belt

Fig. 41 VEE BELT CONSTRUCTION

Its loading is higher than the flat belt but the radius of the p~llleys must not be too small. For snialler pulleys where a reasonably load is required a notched belt should be used.

'The Molded Notched V belt is shown in figure 42 with the tension fabric plies in the outcr section - wliere the tension loads are highest. Tlie belt is designed to take sirnilar loads to the Banded V Belt but will accornrnodate pulleys of smaller radii. Notched V belts are usually designated with an X', so a 3V notched belt, for example, would be designated a 3VX.

Size

'Thcre are three measurements that are used to designate the size of a V belt: its Outside Circumference (OC); its Effective Length (EI,) and its Pitch Length (PL) .

Out side Circumference (OC)

This is measured uslng n tape mvasure wrapped around the outside of the belt I t is not very accurate a n d does not provide n measurement of the belt when rlncier tcnsiorl ( i t will stretch slightly unc1c.r load), which it would be under nornl;\l working conditions. Flowcver, ~t does provitle a nonlirlal length w h ~ c h I S

c;lsjr to meastlrt.

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MAIN TENS FABRIC

Fig. 42 MOULDED NOTCHED V BELT

Effc-ctive Length (EL)

This requires a special measuring rig consisting of two pulleys, one fixcd and m e loadable with a n attached measuring scale.

'To rneasure the EL of a belt it is placed around two pulleys with specifieti groove sizes. One pulley is fixed and the other is designed so it can be lo:~ded to stretrh the belt. There is a scale on the loaded pulley to indicate the length b e t w e ~ n the two pulley centres.

Tlie belt is placed around the pulleys and the second pulley loaded to a specified load, the belt is moved through three complete revolutions whilst being subjected to the load. The EL of the complete belt is calculated by taking the indicated measurement of the loaded pulley times 2 and a d d ~ n g t h ~ s to the circumference of one pulley (this equals the two halve c~rcumferences of each pulley).

It is the Effective Length that most manufacturers quote in their specific 'a t . ions.

BELT UNDER TEST FIXED PULLEY

Fig. 43 EFFECTIVE LENGTH (EL) MEASURING RIG

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Wherl the belt helids around a p u l l ~ v thy outside of the belt is lr l tension and the insldt I S 11-1 cornprt.ssion Whe~c, the centre of the tension occurs is called the neutral axis or tensile chord l ~ n e The tensile chord 1s within the belt (towards tile outcr edge) and therefore cannot bt. measuretl. The PL is the length of the tens~lt; chord around the complete belt.

It is calculated using equations but a qualitative approach will serve to indicate its relatioxlship to OC and EL.

Assume we have two identical belts with the same extt:rnal dirncnsions but one belt (belt 2) has a lower tensile chord (it is designed with its fal~i-ic yarns further away horn the outer edge). For the two belts the OC and E L would be the same 1,ut the PL of belt 2 tvould be smaller than the PL of belt 1.

Pcllleys (Sheaves)

These are usually made of steel and supplied in various diameters and groove angles. Diameters specified include outside diameter and pitch diameter and include groove angies ranging from 32" to 38'.

BELT DRIVES FROM HANDWHEELS TO SPROCKET

HANDWHEEL

TOOTHED DRIVE WHEELS

OCKET WHEEL

, CHAIN TO rAlLPLANE

Fig. 44 SYNCHRONOUS BEET SYSTEM - A 3 2 0

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S\7nc-llronous Belts

Thest. are slmllar to flat belts in d c s ~ g n t.xc.ept that t h rv are toothed 'I'lie teeth are mo~rldecl a s part of the inner surface and prcwidr a posltlve d r ~ v e ~v1t11 no slip (the other belts are used wher-e a n y slip, if present, is not a problerl~).

Synchronous belts are used with toc~thrd pulleys and used with t~xnirlg d r~ves such as ignition systems and valve 11ftixig mechanisrns of some pistor] engines. They :ire niore expensive than the other iirlts.

'lgure 44 shows an exarnple of thy use of a synchronous belt system. I t c.onnec.ts the tailplane trim wheel 111 the flight deck of tlie A 3 2 0 to sproc-ket drives under tlie floor. 'These use a chain and cable systern back to thc tailplar~e. Thc system is dup1icate:d.

General

t is important that when replac~ng either a pulley or a belt of any systcrri that ~t is checked for serviceability and also that it is the correct part (chvclc belt markings). Marly pulleys/belts, partjcularly of the V type construction look very sjmilar, and it is important that the IPC/AMM is followed closely ancl docu~nc.nts such a s the JAA/EASA forrri 1 clearly specifies the correct part by name, part number, batch nuniber, serlal nuniber etc.

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CONTENTS

Page

Screw threads Terminology Standard screw threads

Identification of nuts and bolts British nuts and bolts TJnified bolts and screws AS bolts BSF and BA nuts Unified nuts AS nuts American nuts and bolts Locking and retaining devices Special fasteners Screw thread inserts Studs Keys and keyways Rivets and riveting

British solid rivets American solid rivets Selection of rivets

Blind rivets Tucker pop Chobert. Avdel Cherry

Rigid pipes Fuel delivery pipes

Flexible hose assemblies Pipe-line identification

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SCREW THREADS

A screw-thread is a continuous helical groove cut externally into a piece of round section material or internally into a previously drilled hole. Threads are manufactured in many different forms - the majority of which are V form with different angles of V depending on the standard.

Screw-threads are used extensively with nuts, bolts, studs and other fastening devises. They are also used in power transmission systems, precision measuring equipment and many other applications.

CREST

1 MINOR DIAMETER MAJOR DIAMETER

--I ---- ------- EFFECTIVE DIAMETER k--

TWO START THREAD

w

LEAD = PITCH x NO. OF STARTS

Fig. 1 SCREW THREAD TERMINOLOGY

Terms

Single Start Thread - A single continuous helical groove cut either internally or externally. Most screw thread fastening devices (eg nuts and bolts) use this type of thread. With a single start thread the lead is equal to the pitch.

Thread Form -- The profile of the thread form. See figure 2.

Crest - The tip of the thread whether internal or external.

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Root - The bottom of the thread groove whether internal or external.

Flank - The side which connects the root and the crest.

Thread Depth - The radial distance between the crest and the root.

Threud Angle - The angle between two flanks.

Major Diameter - The diameter taken across the crests of an external thread or the roots of an internal thread. May be known as the outside or nominal diameter.

Minor Diameter - Diameter taken across the roots of an external thread or crests of an internal thread.

Pitch - The distance from one crest to the next or from one part of the crest to the same part of the next crest.

Lead - The axial distance moved by the moving part (usually a nut) of a threaaed pair when it is turned through one revolution. On a single start thread, lead equals pitch. On a double start thread lead is twice the pitch

Coarse and Fine Pitch - Two threads may have similar thread forms and major diameters yet have different thread depths. The threads with the deeper thread will have less threads per inch and is said to be coarser. A coarse pitch single start thread has a greater lead than a finer pitch thread of the same major diameter. Therefore, the coarse thread has a smaller minor diameter and the male member is consequently weaker. A fine thread has a stronger root portion, tighter grip, finer adjustment and is more resistance to slackening under conditions of vibration than is a coarse pitch screw of similar major diameter.

Multi Start Thread - These are threads with more than one start (more than one helical grove). A double start thread has two threads of the same size running helically around the shaftlhole together but starting opposite one another. WILA such a thread the lead is twice the pitch. Multiple start threads increase the linear movement of the moving member without changing the pitch of the thread.

Left Hand Thread - In the majority of screw threads, the helical groove is cut in a clockwise direction so that the moving member of a threaded pair, when turned in a clockwise direction, will move away from the operator. En some situations, however, it is necessary to have a left hand thread. In this case the moving member of a threaded pair, when turned in a clockwise direction, will move towards the operator. Most threads are right-handed but some are left-handed such as the threads on oxygen charging equipment and one thread of a turnbuckle.

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STANDARD SCREW THREAD SYSTEMS

These standards should not be confused with standardised 'nut and bolt' systems, although most 'nut and bolt' systems will use standard screw thl-eads.

The following pages give a brief description of most of the standards that you are likely to encounter.

British Standard Whitworth (BSW)

An older UK standard using a symmetrical V-shaped thread form with a 55" thread angle. The thread is rounded equally at crest and root. Threads per inch vary from 24tpi for 3 / 16" BSW to 4tpi for 2 %" BSW. Not used on aircraft but used in general engineering.

BSF & BSW THREADS BA THREADS

UNF, UNC, IS0 & AN THREAD FORM ACME POWER THREAD

PITCH t--+

SQUARE POWER THREAD BUlTRESS POWER THREAD

Fig. 2 THREAD FORMS

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British Standard Fine (BSF)

This has the same thread form as BSW but has a finer pitch, eg %" BSW has 20tpi, %" BSF has 26tpi. It is found largely on older type British aircraft.

Basic sizes are:

3/ 16" to 5/ 16" major diameter by steps of 1/32"

813" to 1" major diameter by steps of 1 / 16"

1%)) to 13/4" major diameter by steps of %"

2" major diameter upwards by steps of %"

British Association (BA)

This is a fine pitch thread of symmetrical V-shaped form with a thread angle of 47%". The thread is rounded equally at crest and root. This type of thread may be found on older British aircraft mainly on smaller electrical and instrument equipment. It is a metric thread and sizes range from 0 BA - 6mm major diameter to 10 BA - 1.7mm major diameter.

American National

The two standard American threads are the American National Fine and American National Coarse. They are similar to BSF and BSW in application respectively. The thread form is different from the British ones with truncated roots and crests - and has a thread angle of 60".

American National Coarse (ANC) - This thread has a pitch to diameter ratio approximately equal to the BSW thread. Sizes range from No 1 (0.073" majoi diameter) to No 12 (0.216" major diameter). Beyond this range the sizes are indicated by stating the major diameter in fractions of an inch (eg 5/ 16" ANC).

American National Fine [ANF) - This uses the same thread form as the ANC but has a pitch which is finer than BSF. The size range is the same as for ANC.

Unified

In order to bring about some standardisation in screw-threads, negotiations between the United Kingdom, Canada and the United States in 1948 resulted in the adoption of unified standard screw threads with metric equivalents. These threads are of two basic series which are Unified Coarse Thread (UNC) and Unified Fine Thread (UNF).

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Unijied Coarse (UNC) - the pitch of these threads is similar to BSW and ANC Sizes range from %" to 4" major diameter.

Unzfied Fine (UNF) - The pitch of these threads corresponds with BSF and ANF. Sizes range from 1/4" to 1 %" major diameter.

International Standards Organisation (ISO) Metric

This thread replaces BA, BSW and BSF thread forms. The thread comes in a coarse and fine series. The coarse series approximates to BSF and is the one for general aircraft use. The fine series is for components needing very fine threads. Sizes range from 1mm to 100mm. The coarse and fine threads are designated as follows:

MI 2 - M - metric, 12 - diameter of thread (coarse series).

MI2 x 0.75 - MI2 as above, 0.75 indicates the pitch of the thread and this is not a standard coarse series.

OTHER THREAD FORMS

There are many other thread forms such as BSP (British Standard Pipe) for pipes; Knuckle Threads for cloth and soft fixings, and special-to-type threads. One group of threads, however, does require a mention and these are the Power Transmission Threads. They are used to convert rotary movement into linear motion and also designed to take significantly higher loads than the V form threads. They may be suitable for power transmission in one or both directions.

Power threads are used on:

* Some aircraft flap systems. * Vices. * Lathe lead screws. * Locking devices on aircraft lifting jacks etc.

The Acme Thread

This is an American power transmission thread which is suitable for transmitting power in either direction. The included angle is 29". This type of thread works well with half nuts used for quick release mechanisms.

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Square Thread

This thread is also used for power transmission in both directions. It has an angle from flank to root of 90".

Buttress Thread

A buttress thread is designed primarily to transmit loads in one direction only - as in a vice. One flank is a t 90" to the axis while the other is a t 45" - this gives an included angle of 45". This thread also works well with quick release half nuts.

IDENTIFICATION OF NUTS AND BOLTS

This section may be a bit tedious, but it is of vital importance to the safety of t. , aircraft that the correct nuts and bolts are fitted.

Remember the incident where an airliner windscreen was fitted using under size bolts? The windscreen blew out when the cabin pressurised and the pilot was sucked out of the aircraft by the rush of air. It was only the quick action of the rest of the flight crew that saued him - by grabbing hold of him - even though he was half way out of the aircraft. The engineer concerned had not checked on the correct size of bolts to befitted and finally fitted the wrong ones.

So it is very important that the correct nuts and bolts are fitted and it is very easy to fit the incorrect ones.

QUESTION Describe how you would check a nut and bolt for correct size, material etc, to be fitted to an aircraft? (10 mins)

ANSWER The first thing to do is to ascertain what is the correct nu t and brl. to be fitted. It is not sufficient just to say 'Check it with the one thaL llas been removed'. It might have been fitted as an incorrect item in the first place. There are several ways of checking:

* Look in the Illustrated Parts Catalogue (IPC). A Check the Aircraft Maintenance Manual (AMM). * Check the spares lists displayed at the stores. JC Check the modification/repair leaflet if being used.

Once you have ascertained the correct type and size required then the verification that you have the correct item can be carried out in several ways:

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j: The part may be in a sealed packet with the part number and identification marked on the packet (small nuts, bolts a r ~ t l screws). The EASA (JAA) form 1 may be available for inspection.

* Larger nuts, bolts and screws can be identified by symbols and codes stamped on the head; colour; anti-corrosive treatnlt:nt etc.

BRITISH NUTS AND BOLTS

BSF and BA Bolts and Screws

The following drawing shows some typical forms of BSF and BA nuts and bolts.

HEXAGON HEAD BOLT

CLOSE TOLERANCE (HIGH TENSILE STEEL)

7

%i? ROUND HEAD

u COLD HEADED HlGH TENSILE

STEEL BOLT

SHEAR BOLT (HIGH TENSILE STEEL)

HEXAGON HEAD HlGH TENSILE STEEL SET

SCREW

CHEESE HEAD

8 9

%? @ RAISED COUNTERSUNK COUNTERSUNKHEAD

HEAD

Fig. 3 TYPICAL FOFWS OF BA AND BSF BOLT AND SCREW HEADS

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Legend for figure 3.

1. Hexagon headed bolt. Natural Colour. Corrosion resistance steel.

2. Cold headed HTS (High Tensile Steel) bolt. Cadmium plated.

3. HTS screw (all the shank threaded = screw). Cadmium plated.

4. Close tolerance HTS bolt. Cadmium plated.

5. HTS shear bolt (has a thin head). Cadmium plated.

6. Cheese head screw. Supplied in various materials, eg

* A1 Alloy - anodic finish. * Corrosion resistant steel - natural finish. * Low tensile steel - cadmium plated. * Brass - tinned.

7 . Round head screw. Supplied in the same materials as 6 above.

8. 90" countersunk screw. Again supplied in the same materials as 6 above.

9. Raised countersunk (90") screw. Again supplied in the same materials as 6 above.

Aluminium alloy bolts can be identified by an = sign or an L sign stamped on the head.

Remember the smaller nuts, bolts and screws are identified by the packet label and the larger ones may be identified by symbol, shape of head, code markinf and colour.

Size Markings

For the smaller bolts and screws the size is indicated by a number following the part number. It indicates the nominal length in tenths of an inch. It is preceded by a letter indicating the diameter of the shank.

blank

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'X' = NOMINAL LENGTH

Fig. 4 NOMINAL LENGTH OF BSFIBA BOLTS/SCREWS

Diameter Code Letter

A -

B - C -

E -

G -

J L -

N -

etc

6 BA 4 BA 2 BA % BSF 5/16 BSF 318 BSF 7 / 16 BSF % BSF

EXAMPLE

A 318 inch A 2 5 bolt of nominal length 3.3 inches would have as a part number:

The A25 indicates the British Standard Specification for the bolt and would specify material, anti-corrosive treatment, colour and shape of head.

UNIFIED BOLTS AND SCREWS

The following drawing shows some typical forms of Unified bolts and screws. The usual identification, if the bolt/screw is big enough is three (or part of) to~lching circles stamped on the head or on one side.

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Fig. 5 TYPICAL FORMS OF UNIFIED SCREWS AND BOLTS

Legend for figure 5.

1. Hexagon headed bolt which may be supplied in high tensile steel cadmium plated or corrosion resistant steel natural colour.

2. Hexagon headed close tolerance bolt. HTS cadmium plated.

3. High tensile steel shear bolt. Cadmium plated.

4. Dogpoint. This is the identification on the shank end of some of the smaller bolts.

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5. Mushroom headed bolt. Supplied in high tensile steel - cadmium plated and corrosion resistant steel - natural colour.

6. Pan head bolt. Supplied in materials the same as 5 above plus Al alloy (green) and LTS (Low Tensile Steel),

7 . 90" countersunk head. Supplied in:

* High tensile steel. Cadmium plated. * Corrosion resistant steel. Natural colour. * A1 alloy. Anodised green.

8. 100" countersunk head. Supplied in materials the same as 7 above.

Other types of screws/bolts are also supplied including close tolerance countersunk heads and double hexagon headed close tolerance bolts. The finish d l depend on the material, eg:

Corrosion resistant steel - natural finish. Steel - cadmium plated. A1 alloy - green. Brass - tinned.

Size Marking

The nominal length of the bolt/screw is the same as for BSF/BA bolts and screws, ie it is the plain shank from under the head - or if countersunk to include the countersunk portion.

All [Jnified screws can be identified by markings/codes on the packaging label.

Larger bolts can be identified with the code (standard and size code) though countersunk screws are not usually marked at all.

Diameter Code Markings

Y -

z -

A -

B - C -

D -

E -

G -

J -

etc

0-80 UNF 2-64 UNF 4-40 UNC 6-32 UNC 8-32 UNC 10-32 UNF 92 in UNF 5/16 in UNF 3/8 in UNF

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BRITISH AEROSPACE COMPANlES 'AS' BOLTS

These cover a range of sizes and shapes of head that are not covered by British Standard Specifications.

Head shapes include:

* Hexagon. * Round. * Mushroom. * Countersunk 90" and 120" * Round flat head. * Double hexagon.

Size Coding

Many of these bolts are not marked in any way and therefore identification would have to rely on packeted-labelled items. The nominal length is measured in the same way as BSF and BA bolts and screws and length identification and diameter coding is also the same as BSF bolts.

The size coding is preceded by the A S number. For double hexagon headed bolts the nominal length includes the complete shank including the threaded portion.

BSF AND BA NUTS

The shape and materials of the nuts is similar to the bolts already described. They are supplied in the following forms:

* Slotted. * Castellated. * Standard. * Thin - for shear bolts.

The size marking and identification is similar to that described for BSF/BA bolts.

UNIFIED NUTS

These are of hexagon form and may be:

* Standard thickness. * Thin - for use with shear bolts. * Slotted. * Castellated.

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They are identified in a similar way to the bolts ---. with larger nuts kiaving the Unified symbol stamped on them and the Rntish Standard number followed by a letter indicating the diameter.

Drawings and label identifications will also include the following letter w1-lele applicable:

P - ordinary nut. S - slotted. C - castellated. T - thin.

Anchor nuts are not normally identified so great care must be exercised when selecting/ using anchor riuts.

Left hand threads are indicated by the letter I, printed on the specification and stamped on one of the flats of the nut.

Fig. 6 EXAMPLE OF A PARTS CATALOGUE - RB211

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Note. Figure 6 shows an example taken from the IPC (Illustrated Parts Catalogue) of the Rolls Royce RB2 1 1. It shows the itemising of all the parts of the bracket detail to include rivets, washers and 3 A S bolts and nuts.

BRITISH AEROSPACE COMPANIES 'AS' NUTS

The double hexagon stiff nuts are supplied in a size range from 8-36 UN,JF (Unified Fatigue Resistant Fine Thread) to 9/ 16 - 18 UNJF. These are made of heat resistant steel and may be identified from the AS number marked on the side of the washer face.

NOTE. The 8 (as in 8-36) indicates the diameter - in this case 8 BA.

Ordinary nuts and anchor nuts are supplied made of HTS or corrosion resistarit steel all with UNJ thread. They are not marked individually but are identified L,

a label in the packet in which they are supplied. The identification will incluc' ,-he AS part number and the size code.

Anchor nuts can be supplied in a variety of fonns, eg

* Double lug. "ingle lug. * Strip etc:.

AGS (Aircraft General Standards) Nuts

These may be supplied in various forms, eg

* Stiffnuts. * Wing nuts. * BSP nuts (British Standard Pipe). * Whitworth threads. * UNC and UNF threads.

Diameter size coding is similar to HSF/BA bolts and material codes are:

i MS (Mild Steel) - cadiniilrn plated. 2 CRS Corrosion Resistant Steel) or rnonel metal -- cadmium plated. 3 A1 alloy - anodised blue. 4 Brass/ bronze - tinned.

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AMERICAN NIJ'TS AND BOLTS

Many of the world's civil aircraft are of American rnaxlufacture, so rt is only natural that we should know something of the nuts and bolts prtsduced xri America.

There are various standards in the U S incltlding.

A- Federal Specifications. k SAE - Society of Automotive Engineers Specifications. k AMS - Aeronautical Matenals Division of SAE. * AN - Air Force/Navy Specifications. t MIL and M S - Military Standards.

-A NAS - National Aerospace Standards - accepted by the majority of aircraft manufacturers. If used on military aircraft they will carry the AN or MS prefix.

Screw Threads

Threads on both aircraft: bolts and machine screws are of the American riativnal Standard type. This standard has two common classes of fit: class 2 - fret. fit and class 3 - medium fit. The Class 3 or medium fit is used most extensively for aircraft bolts and machine screws.

Threads are noted by the number per inch. The NAS standard corltairis the: following three groups of threads: 1 National Coarse, 2 National Fine and 3 National Extra Fine. The National Coarse (NC) and National Fine (NF) are used on aircraft bolts and machine screws.

In general, the National Coarse threads are used in the smaller sizes through to No 8 and the National Fine threads are used in sizes N o 10 and larger.

Jnified Threads are used and these threads make interchangeability poss~ble between American, British and Canadian production. The main difference between the old 'National' threads and the 'Unified' threads is that the former provides a slightly looser fit.

For example, where a National Fine thread of medium fit was defined as 'iVF3' the Unified thread is 'UNF3A' or 'UNF3B'. These threads are designated by the prefixing of the letter 'U' and the suffixing of either the letters 'A' or 'R' depending on whether the thread is external (bolt) or internal (nut)

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AN Bolts

'These are supplied i n two series:

* 3 to 1000 - generally low strength cadmium plated steel or A1 alloy. Ofterl called the Early Series.

* 6 figure series (lQ0,OO and upwards). These are n~ade from higher strength materials and are of' more recent design.

Identification of the nut/bolt can be by reference to the packetlparts list but bolts in particular can be identified by symbols marked on the head.

DOUBLE DASH INDICATES ALUMINIUM ALLOY BOLT

DASH INUlCAlES CORROSION RESISTING

STEEL. 'X' INDICATES \ STEEL

Fig. 7 AN3 to AN20 HEXAGON HEADED BOLTS

AN bolts may be obtained in the following materials. The coding symbols shown follow the basic dash number and identify the material required.

C - - Corrosion resisting steel. DD = Aluminium alloy - anodised.

AN Air Force -- Navy Standard. 4 Diameter = inch. Thereafter code increments by 1 / 16 inch. £3 Bolt with drilled head and shank. 10 Dash No indicates bolt length - related to diameter. A The adding of letter A indicates no cotter pin (split pin) hole.

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Early Series A N Screws

'Fhese differ frorri the bolts in that they are made of lower strength rnaterlal, have slotted or cruciform heads that are either:

* Countersunk (1 00' or 82"). " Washer head (has a small washer machined a s part of a roirnd hea<l). " Raised cheese head (Fillister head), T o u n d head. * M~lshroom head (Truss head).

Cod irig

The AN number indicates the type of head. Size coding can vary depending on type of head and type of thread, eg fine or coarse. In very general terms the code -elates to the diameter in 1 / 1 6eh inch.

The length code represents the shank length - including the head if countersunk - ira I / 1 6 t h inch increrrients.

Supplied in:

Steel. ' Corrosion resistant steel. " Brass - unplated. * Brass - plated. " A1 alloy. " Bronze - unplated. " Bronze - plated (cadmium).

Early Series AIV Nuts

Used with AN bolts and screws and are supplied in various forms, eg

"lain hexagon. * 'Thin hexagon. * Slotted plain hexagon. * Slotted thin hexagon. " Castellated hexagon. A Wing.

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Coding

Those intended for use with AN bolts have the same coding as the bolt. Those intended for use with screws have the same code as the screws. Letters in t?le code represent:

C - - Corrosion resistance steel. DD = A1 alloy -- machine screw nut. D - - Other a1 alloy. B - - Brass. L /R = Left/ Right hand thread.

Late Series AN Bolts

These are supplied as plain hexagon headed bolts with the material marked ox them as a code, eg E l l - alloy steel cadmium plated. The heads may be supr d drilled depending on the AN number. Size codings range from 10-32 to 3/4 -- 16.

Late Series A N screws

Supplied in cheese head or raised cheese head form, drilled or undrilled, with a single screwdriver slot.

Size ranges from 4 - 40 to 10 - 3/13 UNC and UNF.

Late Series AN Nuts

Supplied in plain, slotted, thin or castellated f o m with the material specification code stamped on one flat.

MS Bolts

Are supplied in a wide variety of head types, eg

* Hexagon - plain. * Hexagon - drilled - one hole. * Hexagon -- drilled -- 6 holes. " Hexagon - slotted. * Double hexagon (called a 12 point). Wouble hexagon -- exiended washer face. A Countersunk (1 00"). * Internal wrenching (Allen key).

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In general codings represent the diameter in 1 / 16th inch and the grip length in 1/ 16th inch. Grip length = plain shank portion including the head where it is countersunk.

Because the length code may change with the diameter it is rnost zrnportcant that the complete part number of a particular item be checked by reference to 111 e packet identification and the IPC.

THIS BOLT IS MADE OF NICKEL STEEL, CADMIUM PLATED TO PREVENT CORROSION. THESE BOLTS MAY BE EITHER NF(73) THREAD

OR NC (74) THREAD AS PER CODING.

Fig. 8 M S 2 0 0 7 3 & M S 2 0 0 7 4 DRILLED MEXAGONHEABEDBOLT

EXAMPLE: MS2 1250 - 05 - 07

21250 Identifies a double hexagon (1 2 point) drilled or plain UNF alloy steel cadmium plated bolt.

- 05 Indicates diameter in 1 / 16th inch increments. - 07 Indicates grip length in 1 / 16tll inch increments.

MS Screws

These are supplied in size ranging from N o 4 t.o 1 % inch. Materials include:

* Steel - Cadmium plated. * Steel - Zinc plated. * Steel - Phosphated

Corrosion resistant steel. " Alloy steel - Cadmium plated. " Alloy steel - Phosphated.

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Head Shapes include:

* Hexagon - slotted. * Hexagon. * Pan head. * Countersunk '8 00". * Cylinder head.

Head markings use symbols and code letters and numbers. 'Threads can be UNC or UNF.

MS Nuts

These are supplied in vanous forms in plated or unplated condition. Sizes range from No 4 to 1 inch diameter.

NAS Bolts and Screws

These are supplied in a wide range of specifications. Head shapes include:

I-{exagon - plain. " Hexagon - crown head. * Hexagon - drilled - corner to corner. " Hexagon - drilled - 6 holes. * Hexagon - single drilled. * Hexagon - tri-wing recess (3 winged Phillips style screwdriver slot). * Countersunk 100" - Phillips, tri-wing torque-set, and hi torque. * Internal wrenching (Allen key). * 1 2 point (double hexagon). * Cheese head (fillister). * Pan head with various drive recesses.

Coding

'The coding includes type of head; diameter; length (total length of screws - grip length for bolts); material and type of plating. The diameter coding is similar to AN and MS parts for most bolts and screws. For the remainder they are coded in numerical order.

'I'hreads may be UNC, IJNF, UNJC, UNJF or American National

Page 235: Materials and Hardware

The c,ode includes letters to indicate:

* Plating type. "ype of locking - drilled shank etc. * Type of recess:

T - - Torque set. H - -- Hi 'Torclue. P or R - - Phillips

A Material type: CR, C or E = Corrosion resistant steel. v - - Titanium.

Add 'DH' to part number to designate drilled head. Add 'AA' to part number to designate drilled shank.

NAS 144 A BH . 25

Fig. 9 NAS 144 t o NAS 158 INTERNAL WRENCHING BOLT

EXAMPLE: NAS 144ADH - 25

NAS""4 - % inch diameter bolt. A - Drilled shank. DH - Drilled head. 25 - 251 1 6 t h ~ inch long (1.6" long).

Page 236: Materials and Hardware

LOCKING AND RETAINING DEVICES

British Civil Airworthiness Requirements (BCAH) prescribes that an approved means of lockirlg must be provided on all connecting elements in the primary structure, fluid systems, controls or other mechanical systems essential to the safe operation of the aircraft.

I,ocking devices may be incorporated in the conlponent itself (self-locking nut) or may be a separate item (spring washer). Some locking devices may be used once only (locking wire) others may be used many times (locking plates). If in doubt about which one to use and whether it can be used a second (or third time) consult the AMM. Below are listed details of most of the locking methods.

Split Pix-I s

Manufactured from mild corrosion resistant steel or nickel alloy steel and u s in conjunction with drilled bolts and slotted or castellated nuts. The steel pin is fitted in a slot in the nut and passes through a hole drilled in the bolt. 'The pin is secured by bending the legs as shown in figure 10. Either method is acceptable in locking slotted or castellated nuts. Used the pin once only. Snip off legs if they are too long Drilling of bolt shanks is not permitted.

CASTLE NUT SLOTTED NUT

Fig. 10 SPLIT PIN USE

Spring Washers

This washer has either a single or double coil spring and is fitted beneath the nut, When the nu t is tightened and the spring compressed, friction is set u p between the faces (flanks) of the screw thread which is sufficient to prevent the nu t or bolt turning. Provided the spring washer retains its springiness and the edges of the single spring washer remain sharp then it may be re-used, but some manufacturers recommend they be changed every time anyway - particularly on joints that are dismantled infrequently.

The single washer has a sharp edge on the top and botlonl ends. One edge protrudes into the base of the nut slightly and the other protrudes into the component slightly. This further enhances the locking effect.

Page 237: Materials and Hardware

EDGES DIG INTO COMPONENT DOUBLE SPRING

WASHER

/--- SINGLE SPRING WASHER

Fig. 11 SINGLE & DOUBLE SPRING WASHER

Tab Washer

This is a metal washer with two or more tabs and is suitable for use with r i plain nut, one tab is bent against one of the flats of the nut and the other over a r ~ edge or into a small hole in the component. Unless the tab washer is of the multiple type, i t should be used once only.

Multiple types have more than one tab so a new tab can be used each time the washer is used.

LEG BENT INTO HOLE IN COMPONENT AND OTHERS BENT ONTO THE OFTHENUT FLAT OR FLATS \@

Fig. 12 SINGLE TAB WASHER

Shake Proof Washers

These washers are manufactured from spring steel and can be used in place of ;pring washers, either the internal or external diameters are serrated and the resulting legs are set at a n angle so as to 'bite' into the pressure faces of nut and component when the nu t is tightened. Used once only.

LEGS SET AT A EXTERNAL SMALL ANGLE WITH

HARP EDGES

/ INTERNAL

Fig. 13 SHAKE PROOF WASHERS

Page 238: Materials and Hardware

Locknuts

Locknuts are plain nuts which are tightened against ordinary plain nuts or against coxnponents into which male threaded items are fitted. Used marly times.

The first nut 1s tightened down in the normal way and the lock-nut is tightened down onto the first nut. When tightening the lock-nut the first nut should he held firmly by the use of a spanner.

Fig. 14 LOCKNUT

Locking Plate

A locking plate is usually manufactured from steel. After torque loading/correctly tightening the nut the plate is placed over the nut and locked with a gnlb screw. The gnlb screw is locked by the use of a spring washer. Carl be used repeatedly provided it renlsns a good fit around the nut or bolt.

Fig. 15 LOCKING PLATE

Wire Locking

The wire used for wire locking is normally 22 SWG (Standard Wire Gauge) corrosion resistance wire -- or that specified in the manual. When wire locking the following points must be considered:

a. No unsuppcrted length should exceed 3" (76mm). b. No untwisted length should be greater than 78" (10mm). c. The twisted wire should extend through the locking point by %"

(13mrn).

Page 239: Materials and Hardware

d. Angles of approach to be not less than 45" to the rotational =IS.

e. After removal of surplus wire, the twisted ends should be bent over to prevent injury and catching on clothing, etc.

f. The lay of the wire must always be such a s to resist any tendency of the locked part to become loose. Be careful to ascertain whether the thread is a left-hand or a right-hand thread. Right hand thxeads tighten clockwise, a left-hand thread is tightened by turnjrig ant1 - clockwise.

Examples of the use of locking wire are shown in figures 16 and 17 (single wire techniques). Figure 18 shows the twisted pair technique being used on pipeline unions.

Fig. 16 SINGLE WIRE LOCKING

LOCKNUTS TWISTED

/ \ ENDS STOWED /

READ SAFETY \

RH THREAD INSPECTION HOLE

Fig. 17 WIRE LOCKING A TENSION ROD TYPE TURNBUCKLE

TWISTED PAIR LOCKING ONE UNION NUT AGAINST THE OTHER

Fig. 18 WIRE LOCKING PIPE UNIONS

Page 240: Materials and Hardware

Cjrclips and Locking Kings

Manufactured from spring steel hardened and tempered. May be internal or external. An internal circlip is fitted into a bore and springs outwards to fit in a groove. An external circlip fits over a shaft and springs inwards into a groove. An example is shown below where the circlip retains the centre part of the universal or Hardy Spicer joint,

After fitting the circlip or locking ring an inspection must be made to ensure that the circlip is correctly bedded in its slot or groove.

HARDY SPICER TYPE UNIVERSAL JOINT

Fig. 19 PLATE & WIRE TYPE CIRCLIPS

Stiff Nuts

A stiff nut is self-locking. I t is designed so that when assembled to a stud or LA, the friction between the screw threads is so great that the nut is held securely in position. The friction is produced at one end of the nut by a built-in locking device which can either be a nylon insert, an elliptical collar (Kaylock) or de-pitching the last few threads (Philidas, Aerotight).

Note. When assembled, the end of the bolt must protrude from the end of the stiff nut by at least one complete thread.

Depending on application may be used more than once provided the nut cannot be txrned down by hand. Some rnanufact~~rers recoxnmend they are replaced every time when used - particularly if joints are critical, or difficult to get at, or are not often disturbed.

Page 241: Materials and Hardware

THREADS

OBDlE PHILIDAS AEROTlGMT KEYLOCK SINGLE ANCHOR ODDlE

DOUBLE

NYLOC CAPPED NYLOC ANCHOR FLOATING ANCHOR STRIPNUTS

NYLOC

Fig. 20 STIFF NUTS & ANCHOR NUTS - GENERAL

Peerling and 'Centre-Popping9

This method of locking should not be used unless specified in the manual. The bolt end is deformed to provide the locking which makes dismantling more difficult. The bolt must extend 1 '/z threads through the nu t before peening. A new bolt and nut is fitted each time. The peening is carried out using a hammer and a centre punch deforming the end of the bolt into the sides of the nut , or by 'centre- popping' the interface between the nut and the bolt. For some operations the bolt might need to be supported from the other side using a large mass such a heavy hammer.

Some countersunk screws are peened by using a chisel sharpened to a screwdriver point. The surrounding metal is forced into the screwdriver slot of the screw.

PEENING

@ CENTRE POPPING

SCREW-DRIVER SLOT \

Fig. 2 1 PEENING & 6CEIVTRE-POPPING'

Page 242: Materials and Hardware

Taper and Parallel Pins

Taper pins with a taper of 1 In 48 are used 0x3 tubular and solid circular sections, for securing control levers to torque shafts and fork ends to control rods etc. Made from steel or light alloy, they are classified by the small end diameter and. length.

Some are bifurcated, the legs are spread far locking, and others are solid and peened to lock. Others may be fitted wit.h a screw thread so locking is carried out using a nut and peening.

Locking Il sing Adhesives

Compoiients such as instruments, valves, switches etc may be locked with Shellac, Loctite, Araldite or similar materials. The advantage of using adhesive as a fastening method is the ability of an adhesive to fill the joint area keeping ou air and rnoisture and it i s also convenient. The manufacturer's recommendat ,s must be followed as regards the use of these methods.

LOCKING DEVICES - HOW OFTEN USED

DEVICE

Locking Wire Split Pin Tab Washers Circlips

Locking Plate Spring Washers Sh akeproof Washers

NIJMBER OF TIMES LJSED/APPLTCATION

Once Once Once (unless multi-tab type) Once (wire type) More than once (plate type) More than once More than once Once

SPECIAL FASTENERS

There is a wide range of these types of fasteners. A few have been selected for this book to demonstrate the range available.

Jo-Bolts may be classified as blind rivets. The complete item consists of a threaded bolt with a roundhead, a rivet shaped nut and a sleeve, assembled as illustrated in figure 23. Rotation of the bolt forces the sleeve up the tapered nut shank, clarriping the materials to be joined and a t a predetermined load the bolt shears just inside the nu t head, leaving, virtually, a solid steel rivet in the hole.

Page 243: Materials and Hardware

Jo-Bolts are manufactured with hexagon or. 100" countersunk heads, in either stainless or alloy steel and have a shear strength aln~ost equal to a bolt of equivalent size and material The bolts are pre-lubricated and must not be washed in solvent, since this would alter the gripping strength at which t l ~ e bolt shank breaks.

The tools used for placing Jo-Bolts are in two concentric parts, the outer part holding the nut and the inner part gripping the bolt shank. Different adapters are be fitted to the tool to accommodate the different size hexagon heads, or cruciform slots of c:ountersunk bolts.

BOLT /

BOLT LOCATED IN HOLE. TOOL GRIPS AND ROTATES HEXAGON HEADED NUT BOLT PULLING SLEEVE HELD IN TOOL. OVER NUT END

WHEN FORMING COMPLETE BOLT STEM BREAKS

DRAWING FROM CAP 562

Fig. 22 FORMING A JO-BOLT

Jo-Bolts have now been replaced by Jo-Locs. These are similar in operatiori to Jo- Bolts but with a higher specification.

Rivnuts

Rivnuts are a form of blind rivet which is used as an anchor nut , the internal bore being threaded to receive a bolt or screw. Rivnuts have either flat or countersunk heads. The countersunk head types are open ended and may or may not have a locating key, but the flat head types all have a locating key and are supplied with either a closed or open end.

Marks on the head indicate length in accordance with a manufacturer's code. Rivnuts are installed with a special tool fitted with a thrradetl mandrel; the mandrel is screwed into the rivnut and when the gun is operated the mandrel is pullecl in and the Rivnut expands as shown, locking ~t i n the hole.

Page 244: Materials and Hardware

PLATES TO

INSERTED

MANDREL UNSCREWED AFTER RIVNUT FORMED

I RIVNUT t CLOSED

DRAWING FROM CAP 562

Fig. 23 FORMING A RIVNUT

Fii-lex Fastener

This fastener is designed for joining composite panels. The countersunk hear. IS

an included angle of 1 30°. The forming of the fastener is shown in figure 25.

2. TURN UNTIL HEAD FULLYSEATEDgHEX SHEARS OFF. THREADCOLLARONTO

NER CLOCKWISE

THE FASTENER INTO THE STRUCTURE COUNTERGLOCKWISE USING A SOCKET ON THE CT DRIVING HEX.

3. WRENCHING FLATS ON COLLAR WILL TORQUE OFF AT PRE-DETERMINED

WRENCHING TORQUE. FLATS

Fig. 24 FORMING A HI-LEX FASTENER

Page 245: Materials and Hardware

Hi-lite Fastener

These are supplied in a variety of forms (figures 25 and 26). Figure 26 shows the forming process of the HPL type. Head shapes are pan head and 100" countersunk.

PARTS TO BE JOINED

INTERNAL THREAD MATES WITH THREADEDMANDREL

REUSABLE THREADED MANDREL

Fig. 25 HI-LITE HPL FASTENER

BASIC INSTALLATION

OFF AT PRE-DETERMINED TORQUE. INSTALLATION COMPLETE.

Fig. 26 FORMING A HI.

INSTALLATION ON SLOPE

PIN AS BEFORE

HI-LITE SELF ALIGNING COLLAR

LLAR WRENCHING HEX SHEAKS OFF AS BEFORE & INSTALLATION

4 *EITE HPL FASTENER

Page 246: Materials and Hardware

SCREW THREAD INSERTS

Screw thread inserts (or wire inserts) are used:

a. In soft rrlaterials to allow frequei~t assembly and dis-assembly of components with minimum thread wear to the component itself.

b. To increase the effective thread diameter and allow higher torques to be used.

c. In schemes to repair a damaged thread.

Prior to installation the insert is shorter in length and larger- in diameter than when in stalled.

In most cases the tools and inserts come in kit form with full instructions supplied.

Good motor skills are required to fitjremove inserts successively.

NOTCH PARTLY SECTION INSTALLED INSERT

TANG

DRAWING FROM CAP 562

Fig. 27 WIRE THREAD INSERT

Identification

British thread form inserts (BA, BSW, BSF & BSP) can be identified by yellow paint on the tang. Unified thread forms have no colour identification. More precise designation is achieved by code number systems issued by SBAC and other standards printed on the packet.

Installation

Since the internal and external threads on a thread insert have the same number of threads per inch and the internal thread is designed to be of standard size, then a special tap is required to cut the threads into which the insert is fitted. These taps and checking gauges are provided by the insert manufacturer.

Page 247: Materials and Hardware

In general the installation procedure is as follows:

1. Refer to manufacturers information supplied with the Insert ktt

2. Drill correct size hole.

3. Tap the hole (taps supplied in the kit).

4. Gauge the tapped hole (gauges supplied in the kit).

5. insertion of the insert (tools and inserts supplied in the h t )

6 . Removal of the insert tang.

7. Inspection and checking of work.

The hole for the insert should be drilled to the diameter and depth specified in tables supplied by the insert manufa-cturer. Care should be taken to ensure that the hole is drilled in the correct location and square to the surface and that all swal-f is removed before tapping.

Thread Tapping

'The thread should be tapped with the special tap, a straight-fluted tap being used for hand tapping and a spiral-fluted tap for machine tapping where this is possible. Normal workshop practices should be used for tapping, with special emphasis on cutting the thread coaxidly with the hole. Lubricant should be used according to the type of metal being cut, eg a light mineral oil is generally recommended for tapping light alloys. (Remember, when tapping to turn the tap nalf turn forward and a quarter turn back so as the break up the swarf ancl provide a smoother thread.)

Thread Gauging

After the insert thread has been cut it should be cleaned of all swarf and foreign matter. The thread should then be checked with a special CO/NO GO plug gauge provided to ensure that the thread is satisfactory. Any thread imperfections indicated by tightness of the GO gauge should be removed by furl her use of the original tap or, if this is ineffective, by use of a new tap.

Page 248: Materials and Hardware

Fitting the Insert

The insert should be screwed into the tapped hole by use of either an inserting key or an inserting tool of the pre-wind type, depending 01-1 which is recommended for the particular insert. A different sized key or tool is provided for each size of insert.

MANDREL HANDLE

DRAWING FROM CAP 562 Fig. 28 INSERTING KEY

NOZZLE HANDLE

MANDREL

DRAWING FROM CAP 562

Fig. 29 PRE-WIND INSERTING TOOL

The inserting key is used by sliding the insert onto it so that the tang is engaged in the driving slot at its forward end; the assembly should then be applied to the tapped hale, compressing the insert downwards with the thurr, and forefinger of one hand whjle turning the key with the other hand; no downward pressure should be applied on the key. The insert will wind into the thread and should be installed so that the outer end of the insert is at least half a pitch below the surface of the corriponent.

When a pre-wind tool is used the insert should be placed in the chamber with the tang towards the nozzle and the mandrel pushed fonvard through the insert to engage the tang in the slot. The mandrel should be rotated clockwise and pushed gently forward to engage the insert coil in the nozzle threads, rotation being continued until the insert is about to emerge from the outer end of the nozzle. The tool should then be placed squarely over the tapped hole and the handle rotated to transfer the insert from the tool into the tapped hole. No forward pressure should be used.

Page 249: Materials and Hardware

Removal of the Tang

It is not always necessary to remove the tang of a wire thread insert, but rernoval may be specified in some cases for screw clearance or product appearance. both in blind holes and in through holes A tang in a through hole is removed b y use of the inserting key used a s a punch, with the tang outside the engaglng slot, or by use of a special punch.

A sharp blow with a hammer on the key or punch will fracture the wire at the notch were the tang joins the coil. To remove the tang from an insert fitted in a blind hole, long round-nosed pliers are required; the tang should be bent backwards and forwards through the insert bore until it fractures at the notch and can be removed.

TANG

DRAWING FROM CAP 562

Fig. 30 TANG BREAK-OFF TOOL

Removal of Inserts

Under rlorrnal circumstances, particularly when fitting instructions have been carefully carried out, the removal of inserts should be unnecessary. However, if an insert has to be removed because of bad fitting, darnage or wear, this can be done by bending the top coil inwards to form a rough tang and unscrewing the insert with the insertion tool or a pair of pliers. Some manufacturers recommend the use of a tapered left-hand tap of appropriate size, which grips the top coils internally and unwinds the insert when rotated. Other manufactures provjcle a range of extractor tools, which are fitted with hardened and tempered blades. The blade will bite into the inner surface of the insert, which can then be unscrewed. After removal of a n insert, the threads in the hole should be carefully examined for damage before fitting a new insert.

Page 250: Materials and Hardware

HANDLE PRESS DOWN &

INSERT

INSERT

DRAWING FROM CAP 562

Fig. 31 AN EXTRACTOR TOOL

STUDS

These are supplied it? various thread forms and sizes to meet particular requirements. In general they can be grouped into four main categories.

Standard or Plain Stud

By far the most widely used type of stud. The diameter of the unthreaded portion is the same as the major diameter of the screw thread at both ends. This type of thread is called a minus thread - common to almost all bolts and screws. (Take note - the following drawings on studs use British Standard BS8888 symbols f ~ r the screw thread).

SCREWTHREAD

Fig. 32 STANDARD STUD

Page 251: Materials and Hardware

Waisted Stud

Used where reduction of weight without loss of strength is ~xnportant. The d~amet er of the plain (un-threaded) portion is reduced to I he minor diameter of the threaded (or less) thus reducing the weight of the stud without ampairing its effective strength.

Fig. 33 WAISTED STUD

Xepped Stud

'I'his type provides a stronger anchorage than the plain stud if the stud is used in soft or weak material. One thread is larger than the other. These can also be used as replacements for plain studs when the tapped stud hole becomes damaged and has to be re--drilled and re-tapped with a larger thread (iaw the SRM).

Fig. 34 STEPPED STUD

Shouldered Stud

This type is used where maximum rigidity of assembly is of prime impol-tance. The stud is machined to form a projecting shoulder between the two threaded portions. This shoulder seats firmly on the surface of the component and gives additional resistance to lateral stresses.

Fig. 35 SHOULDERED STUD

Page 252: Materials and Hardware

Studs may be fitted by the use of:

A. A spanner. Some studs are supplied with flats on the plain shank. -k rrwo nuts. The two nuts are locked together on the protruding thread

and the top nut is used to tighten the stud into the hole. J; A stud box. ~r A stud removal and inserticrn tool.

Stud Removal

If complete, the stud can be removed by:

i; A spanner - if provided with flats. * Two nuts (as above) but using the bottom nut to turn the stud out. k A stud removal and insertion tool.

If broken above the surface:

* File two flats on the stud and remove with a. spanner.

If broken below the surface you can (with difficulty):

~r Drill the stud centrally using a drill aboul. l/z the diameter of the stud. Drive a square steel tang (locally manufactured) firmly into the hole and remove the stud by turning on the tang,

* Use an 'Easyout'. Drill the stud using a drill (the size is specified on the Easyout). Insert the Easyout (which has a knuckle type tapered left hand thread). Remove the stud using a spanner on the tang of the Easyout. Easyouts are supplied in boxed sets.

* For larger studs. Drill and tap with a left hand thread. Screw in a left hand threaded bolt and continue to turn (in an anti-clockwise direction) to remove the stud.

* Drill the stud out using a drill to the crest diameter of the tappec hole. Re-tap the hole. (None of the above processes are easy and this one is even more difficult).

Stud Box

A stud box is a tool used for inserting studs and corlsists of a deep hexagonal nut with an ordinary bolt fitted at one end. The stud is entered into the stud box, the bolt is then tightened down onto i t , the stud box hexagon is then turned with a spanner until the stud is fully screwed home.

Page 253: Materials and Hardware

COPPER DISC

w

Fig. 36 STUD BOX

Stud Removal and Insertion Tool

This can be used to fit and remove a stud.

The stttd is passed through the hole in the end plate until i ts plair, part is positioned within the hole in the cage, the locating screw is then. adjusted so that the stud can not enter further into the hole. On rotating the tool body, the cam followers are pressed tightly on to the stud plah shank and the stud can then be screwed lri (or out).

CAM FOLLOWERS IN CAGE

END PLATE A @ Fig. 37 STUB REMOVAL & INSERTION TOOL

Page 254: Materials and Hardware

These are supplied in sets providing a range of sizes. They ax-e made of steel with a tapered left hand knuckle thread at one end and a square drive at the other. When turned anti-clockwise into a drilled hole in the end of a broken stud the tapered thread %ites' into the hole and provides a positive drive. When drilling into the end of a broken stud it is important not to drill the hole too large. This would cause the Easyout to swell the stud ill the hole as it is inserted and the stud would be impossible to screw out.

LEFT HAND 'KNUCKLE' THREAD

SQUARE END FOR WRENCHING

\

Fig. 38 'EASYOUT'

KEYS AND KEYWAYS

These are used where rotary power is to be trarlsrnitted from (or to) a shaft (or hub) and a drive-wheel. The key is a solid piece of metal of rectangular or square cross-section, fitted into a rriatched recess which is formed between the shaft and the drive-wheel. Several different types are available.

Tapered Keys

These are made with a standard taper of 1 in 100 on the thickness - the tapering face of the key matching the taper of the recess or keyway farmed in the bore -f the wheel. The ability of the key to resist axial movement between the hub an,, the shaft depends on the fit of the key in the keyway. Careful fitting is essential. 'he following types of taper keys are in common use:

Hollow Saddle Key - One side of this key is curved to suit the radius of the shaft when driven into position, its taper provides a friction grip between hub and shaft that is capable of taking a moderate load only. There is no keyway on the shaft.

Flat Saddle Key - This form of taper key is rectangular or square in cross-section and it bears on a flat formed on the shaft. It provides a more positive grip between shaft and hub than js achieved by the hollow saddle key, but still cannot t,&e very high loads.

Page 255: Materials and Hardware

WHEEL KEY I KEYWAY

Fig. 39 HOLLOW SADDLE KEY

Fig. 40 FLAT SADDLE KEY

Plain Taper and Gib-Headed Keys - These forms of taper key fit into keyways which are formed partly in the shaft and partly in the hub. They are capable of transmitting greater power than either of the saddle types. The gib-headed key provides for easier removal.

Fig. 4 1 GIB-HEADED KEY

Feather Keys

Keys of this type are used in circumstances where it is required to allow axial movement between shaft and wheel -- for example, a feather key rnight be useti if it is necessary for a pulley or gearwheel to move along a shaft while still Ijejrrg driven. The hub keyway is cut to allow for side and top clearance round thcl key, so permitting a sliding fit to the key in the keyway.

Page 256: Materials and Hardware

Fig. 42 FEATHER KEY

W oodruff Key

'This key is made in the form of a segment of a parallel--sided disc. It fits into a ke-yway of similar shape, which is formed partly in the shaft and partly in the wheel. The cavity in the shaft conforms closely to the rounded portion of the kGi, while an axial groove, of uniform rectangular cross-section, is cut in the whet o a depth which permits a push fit between hub and key. Woodruff keys may be fitted to parallel or tapered shafts.

Fig. 43 WOODRUFF KEY

RIVETS AND RIVETING

Riveting is a semi-permanent form of joining material (metal, composites etc) together and may be divided into three categories:

1. Solid rivets. 2. Blind rivets. 3. Special rivets/fasteners/blind bolting.

Page 257: Materials and Hardware

Solid Rivets

These rivets need access to both s ~ d e s of the material being joined during the forming process. They have a good strengthlweight ratio but require skill to form. They are water and airtight and are less expensive than other types of rivet. They are strong in shear but not so strong in tension.

All solid rivets are supplied from the manufacturer with the head pre-formed. Both British and American rivets are identified by head or shank end markings except where a material is easily identified by its weigh! or natural c:olour. Certain British rivets are coloured all over for ease of identification.

Blind Rivets

These require access to one side of the material only. They are more expensive md require special equipment to form. Some are not water or airtight and some are weaker than solid rivets. They require less skill to form.

Special Rivets/Fasteners/ Blind Bolting

There is a wide range of special fasteners and many are a cross between a rive1 and a nut and bolt assembly. Most can be used in the 'blind mode'. Usually more expensive than blind rivets. Generally stronger in tension and shear and all require special tools to form.

SOLID RIVETS - BRITISH

Standards for these are set by the Society of British Aerospace Companies (AS series) and the British Standards Institute (SP series). Rivets are identified by a Standard Number and a Part Number. The Standard Number identifies the head shape, material and finish, while the Part Number indicates the length ant1 diameter of the shank.

DIAMETER - thirty-seconds of a n inch or millimetres x 4 0 . LENGTH - sixteenths of an inch or millimetres.

eg:

AS 162 408. AS 162 indicates A1 alloy L58 90" countersunk head, anodic finish, colour green. 408 indicates %" diameter and %" long

SP142-40-16. SP142 indicates A1 alloy L86 100" countersunk head, anodic finish, colour violet. 40-1 6 indicates 4mm diameter and I Grnrn long

Page 258: Materials and Hardware

SP inch size rivets are made in a range from 1 / 16" to 3/8" diameter and from 1/8" to 3" long. The SP series has superseded the AS series.

SNAP MUSHROOM COUNTERSUNK COUNTERSUNK UNIVERSAL HEAD HEAD HEAD TRUNCATED HEAD

RADIUSED HEAD

. . .. - -. - --

1 O o O 100"

-- - - - - - INCH SIZES METRIC SIZES

L = LENGTH D = DIAMETER

Fig. 44 SP INCH & METRIC RIVETS

AS countersunk heads include 90" and 120" with SP countersunk heads IOQu. Flat countersunk and raised countersunk heads are available. The raised countersunk head denotes close tolerance.

SOLID RIVETS - AMERICAN

The code used for American rivets is similar to that used for British rivets and illustrated as an example is MS20470 AD 5- 12, which has the following meaning:

a. MS signifies Military Standard. This standard has superseded the old AN (Army-Navy) standard.

b. 204'90 is a code for the head shape and basic material (aluminium universal head in this instance).

c. AD is a code for the rivet material (2 1 17 aluminium alloy in this instance).

d. 5 is the diameter in thirty-seconds of an inch. e. 12 is the length in sixteenths of an inch.

British rivets can be used in place of American rivets if the correct material is used and the rivet is at least of equal strength,

'Table 1 shows some of the rivets available in the SP range. Table 2 shows some of the SP metric range and table 3 shows some of the MS American range.

Page 259: Materials and Hardware

British Material Material Head Finish Iderlt~fication Standard Specification Type Mark

aluminium aluminium alloy alumiriium alloy steel aluminium aluminium dloy aluminium alloy aluminium alloy monel metal monel metal

L 36 L 37* L 86 BS 1109 L 36 L 37" L 58 L 86 DTD 204 DTD 204

100" csk 100" csk 100" csk snap snap snap snap snap snap 100" csk

black anodic natural violet anodic cadn~iurn black ariodic natural green anodic violet anodic natural natural

TABLE 1 SOME S P SERIES RIVETS

British Material Material Head F'ini s h Identification Standard Specification Type Mark

SP 142 aluminium alloy L 86 100" csk violet anodic indented dot SP 157 aluminium alloy L 86 universal violet modic indented dot SP 158 monel metal DTD 204 universal natural two indented dots SP 160 aluminium alloy L 58 universal green anodic raised cross SP 162 aluminium alloy L 37* universal natural raised broken line

and centre point

TABLE 2 SOME S P METRIC SIZE RIVETS

Rivet and Material Material Head Identification Material Code Specification Type mark on head

aluminium aluminium alloy aluminium alloy aluminium alloy corrosion resistant steel monel metal copper aluminium alloy aluminium alloy aluminium alloy carbon steel

QQ--N-28 1 QQ-W- 34 1 5056 2024" 20 17" QQ-S- 633

100" c:sk 100" csk 100" csk 100" csk 100" csk

100" csk 100" csk universal universal universal universal

Nil Dimple Raised double dash Raised dot Recessed dash

Nil Nil Raised cross Raised double dash Raised dot Recessed tr ia l ~gle

Not(,. For MS 20613 rivets P indicates cadmium plated and Z indicates zinc plated

TABLE 3 SOME AMERICAN RIVETS IN GENERAL USE

Page 260: Materials and Hardware

Notes 1. * Require heat treatmerit before use (all tables). 2. In table 3 the MS number (eg 20426) indicates the head type and the

letters jeg DD) are the materid code. 3. Some rivets can be heat treated then stored in a refrigerator to retard

age hardening. These are sometimes called "ice box" rivets - eg material specifications 2024 and 20 17.

UNIVERSAL ROUND BRAZIER COUNTERSUNK FLAT HEAD HEAD HEAD HEAD 100" HEAD

L = LENGTH D = DIAMETER

Fig. 45 AMERICAN MS RIVETS

Temper Codes

Some rivets are supplied and used "as received" - in other words there is no requirement for f ~ r t h e r heat treatment before use. Some rivets require solution treatment (normally using an electrically heated oven) before forming so as to allcw the rivet to achieve its maximum strength due to age hardening. American rivets usually have a temper designation -- eg T4 = solution heat treated, F = as fabricated etc.

NAS Rivet Codes

Some drawings - particularly American drawings - will have the rivet specification laid out in a format similar to that shown in figure 47. The symbol standard j 1

NAS (National Aerospace Standard) and has four quadrants called: North Wec' (NW); North East (NE); South East (SE) and South West (SW).

RIVET MATERIAL 8 TYPE OF HEAD

/ s l o e

~.

NORTH WEST QUADRANT

COUNTERSINK LENGTH NOT

NORTH EASl QUADRANT

SOUTH WEST QUADRANT

SHOWN

SOUTH EAST I QUADRANT

Fig. 46 NAS RIVET CODE STANDARD

Page 261: Materials and Hardware

The NW code specifies the rivet material and type of head. The N E code specifies the shank diameter and whether the head is near side or far slde. The SE (:ode is for length which is not shown, and the SW code is blarrk for protruding head rivets and C for countersunk rivets.

The NAS rivet code is printed on the drawing at the end of each row of r~vets.

SE1,F:CTION OF RIVETS

When carrying out a repair it is most important to select the correct rivet It must be the correct size, shape of head and rnaterial. Check the specific repair drawing in the SRM (Structure Repair Manual) or check the repair specification for the type of rivet to use. When ordering the rivets from stores it is important to check the correct rivet specification by reference to the stores specification label on the packet of rivets.

The shear strength of rivets used is not the only factor which determines the strength of a riveted joint. Generally, if' the thickness of the sheets is less than half the diameter of the rivets used, failure of the joint will depend on the bearing stress rather than on the shear stress of the rivets.

In the absence of specific instructions 3/32 inch rivets should be used for 24 and 22 swg (standard wire gauge - UK) material, 1/23 inch rivets for 20 and 18 swg and 5/32 for 16 swg.

If rivets of reduced diameter have to be substituted during repair work, the total number of rlvets must be increased to provide equivalent cross-sectional area. Where 22 swg and thinner material is used and there are no specific instructions regarding repair after a rivet failure, the substitution of mushroom head rivets for snap head rivets could be considered.

When British rivets have to be used in American-built aircraft, rivets of the material with the nearest equivalent shear strength to the material of the original American rivets should be used. If the available British rivets have lower shear strengths than the American rivets either the total number of rivets should be increased or rivets of larger diameter should be used to make the strength of the joint in bearing and shear not less than it was originally. However, an increase in the size of the rivets does not necessarily increase the strength of a join; ~f the rivet sizes are increased beyond a certain limit, a reduction in strength will result.

NOTE. In all circumstances where the SRM cannot be adhered to, permisszon will be required -from the chief engineer of the company, or failing that, the munznfoch~rer of the uirc,rc@ to carry out any "on-stnndurd' work.

Page 262: Materials and Hardware

Countersinking

When counters1;lnk rivets are to be used, there are two methods of accommodating the rivet head f o ensure a flush fit. Cut-countersinking is employed where sheet thickness is greater than the depth of the rivet head, but for thinner sheets dimpling is necessary. Where sheets of different thicknesses are joined together it may be found that both methods are used, the thin outer sheet being dimpled into a courltersurlk thick inner sheet.

Cut-Coun tersinking

Table 4 shows the minimum sheet thickness which may be countersuslk for particular rivet diameters and is applicable where 100° or 120" countersunk head rivets are used.

Where special rivets are used the aircraft manufacturer may specify a differel minimum sheet thickness and when oversize rivets are being fitted it may be recommended that the rivet heads are milled in preference to further countersinking.

Rivet diameter (inch) 118 5/32 3/16 114 Minimum sheet thickness (swg) 18 16 14 12

TABLE 4 MINIMUM SHEET THICKNESS FOR CUT COUNTERSINKING

Special countersinking tools should be used for cut-countersinking. The tools should have a centralising spigot and an adjustable depth stop which will limit the depth of cut. The rivet head should always be slightly proud of the work t 3re riveting and ideally flush with t.he metal after. This can be set by trial countersinking and riveting on scrap material prior to carrying out the task ol, ihe aircraft.

Aircraft manufacturers usually specify a tolerance on head protrusion after riveting and this is usually of the order of 0.005 inch above the skin surface. The rivet head should not be below the skin surface.

Dimpling

This is a process for indentlrlg thin sheet material (not normally thicker than 16 swg) around a drilled hole to accommodate a countersunk rivet. If correctly performed, dimpling has a beneficial effect on the strength of the joint, but the method of dimpling must be related to the ductility of the material to prevent overstressing and cracking.

Page 263: Materials and Hardware

To ensure correct seating, countersunk headed rivets slaould always be installed in dimples or countersunk holes of the same angle as the rivet head. R~vel-s with countersunk heads of 70" or 82" i ~ ~ c l u d e d angle are after1 used in pos~tions where sealing is of primary importance, such as in integral fuel tanks.

When 1 hese rivets require replacement care is necessary to ensure that rivets with the correct angle heads are selected.

Heat Treatment of Rivets

Rivets can only be heat treated when specified in the rivet specification and should only be SOLUTION-TREATED. Temperatures and methods of cooling are specified in the rivet specification.

QUESTlON Where would you find the published rivet specification? (2 mins)

ANSWER Any good technical (or sometimes non technical library). The rivet specification will be printed on the packet, and armed with this knowledge, the actual specification can be found in the technical library. Here it will say exactly what heat treatments (if any) can be carried out. It will also give a great deal of other technical data about the rivet - the metal composition - what form it is supplied in (wire for rivets) etc.

The best way of heating rivets is in a thermostatically controlled electrically heated oven (sometimes a salt bath is used) - with rivets placed in a wire cage.

The following is an example of the solution treatment of a rivet. Far a specific case you must consult the rivet specification.

Heat the rivet (in a wire basket if there are several) to a temperature of 4952SiCfor a period (soaking time) of 1 5 minutes.

Remove the rivets and quench in cool water. Wash thoroughly if heated in a sult bath. The rivets will commence to 'age-harden' (get stronger and harder) but. can be used within 2 hours of treatm.ent. Some rivets must be used within 20 minutes (consult the speczjication).

Age hardening may be delayed by refrigeration eg, if the rivets are placed in a fridge at -20 T immediately after treatment they can be kept up to 150 hours hefore they must be used - or re-heat treated.

If used they must be used within 2 hours of removal from cold storage. If the riz1et.s are not used within the prese:ribed time they can be re-treated to a mcucimurn o f .3 times.

Page 264: Materials and Hardware

BLIND RIVETING SYSTEMS

These are riveting systems that require access to one side of the material only. To fit solid rivets (described above) two people are required -- one to operate the riveting hammer and the other to hold the reaction block. With blind riveting only one person is needed.

They can be used in place of solid rivets only when stated in the AMM.

There is a wide range of blind riveting systems used and each has its own special advantages and disadvantages. They are identified on the packet by the manufacturer's name and stores part numbers. They all require special tools (supplied by the rivet manufacturer) and procedures to fit but the general. procedure is as follows:

1. Drill the correct size hole (clearance hole). 2. Check total thickness of materials to be joined (Grip range). 3 . Select correct size of rivet. The length is related to the grip range of

the rivet as stated in the manufacturer's literature. 4. Select correct forming tool and load rivetlrivet mandrel. 5. Form Pivet in hole. 6. Remove tool and inspect rivet for correct forrrling.

Tucker Pop Rivets

Each rivet is supplied complete with mandrel (which does not look too unlike an ordinary wood nail). The rivet can be formed using hand operated lazy tongs or cranked pliers, or may be formed using power tools.

The operation of closing a Tucker Pop rivet is as follows:

(a) Select the correct diameter and length (grip range) of rivet. (b) Ensure forming tool has correct size jaws and head fitted. (c) Insert the mandrel into the jaws of the chuck.

(d) Insert the rivet into the hole. Hold the chuck containing the rivet firmly against the material and square to the surface being riveted and operate the tool. The head of the mandrel will pull inta the rivet tail forming the rivet and then break off.

The mandrel can be of two types, break-head or break-stem. The break-stem type has a waisted shank and breaks below the head, thus the broken head portion is trapped within the rivet. This type is used where it is impossible to retrieve the broken off head of the mandrel. A sealed type rivet is supplied for use in pressure cabin construction.

The head of the break-head type breaks off and falls out.

Page 265: Materials and Hardware

Note. Broken mandrel sterns, swarf, rivet heads and shanks, etc which we discarded during the repair operation, must be cleaned up using a vacuum cleaner after all work has been completed.

The rivet will not be as strong as a solid rivet or the more sophisticated bhnd rivets, but it is cheap and easy to use. Sealants may be used to weather--proof the rivet .

BREAK STEM

WlTH BREAK HEAD TYPE HEAD BREAKS AND FALLS AWAY.

RIVET INSERTED WITH BREAX STEM INTO HOLE TOOL PULLS MANDREL TYPE STEM BREAKS

THROUGH RlVET AND HEAD STAYS IN FORMING RIVET HEAD THE RIVET.

DRAWING FROM CAP 562

Fig. 47 FORMING A TUCKER POP RIVET

SHORT BREAK

RlVET INSERTED IN HOLE TOOL PULLS MANDREL a LONG BREAK OR

FORMS RIVET HEAD BREAK STEM TYPE LEAVING HEAD IN RIVET SHEARS OUTSIDE RlVET

DRAWING FROM CAP 562

Fig. 48 SEALED TUCKER POP RIVET

Chobert Rivets

Supplied in snap or countersunk forni. These rivets are similar to 'I'ucker Pop rivets, but have a tapered internal hole and are not supplied with a rnandrt.1. ln Chobert riveting the head of the steel mandrel js pulled through the rivet a ~ l d is not broken off.

Page 266: Materials and Hardware

The rivets are closed by a special riveting tool; a magazine type of riveting tool is available which carries a number of rivets on the mandrel, thus avoiding time in threading rivets individrlally after each closing. This tool can close many rivets with just the one loading. (See the book in this series on hand power tools).

STEEL MANDREL

\ RIVET SEALING PIN DRIVEN IN FROM MANUFACTURERS' HEAD SIDE

MANUFACTURERS' HEAD

/ MANDREL PULLS THROUGH RIVETING TOQL RIVET EXPANDING SHANK,

FORMING HEAD & PARALLEL INTERNAL HOLE.

DRAWING FROM CAP 562

Fig. 49 FORMING A CHOBERT RIVET

Chobert rivet.s can have the same strength a s solid rivets and the general forming process is a s follows:

1. Check grip range and size of rivet. 2. Check the steel mandrel that it has not worn beyond limits (GO NOT-

GO gauge). 3. Thread rivet/s on the mandrel - tail first. 4. Insert mandrel into the jaws of the forming tool. 5. Place rivet in hole and operate tool. The mandrel will be pulled

through forming the rivet as shown. 6. To seal the rivet and/or to increase the strength, tap sealing pin ir'?

hole using a hammer,

Avdel Rivets

These rivets are similar to Chobert rivets, but each is fitted with its own stem (mandrel). The stem is pulled into the body to close the rivet and a t a predetermined load, breaks proud of the manufactured head, leaving part of the stem inside the body in the form of a plug. Excess stem material may be nipped off and milled (in American books called Shaving) flush with the rivet head when required, eg on external surfaces, but stainless steel and titanium rivet stems break flush with the rivet head at the maximum grip range limit and milling is not necessary. The action of closing an Avdel rivet is shown.

Page 267: Materials and Hardware

MANDREL TOOL PULLS MANDREL THROUGH RIVET WHICH WHICH FORMS HEAD

RIVET PLACED IN HOLE

EXCESS MANDREL. SNIPPED OFF, MILLED FLUSH B PIN TESTED

TOOL

DRAWING FROM CAP 562

Fig. 50 FORMING AN AVDEL RIVET

?vdel rivets are lubricated by the mariufact~rer to facilitate forming and on no account should the rivets be cleaned in solverlt before use, or re-lubricated 'The lubricants used are specially prepared for each type to obtain consistent results.

The shear strength of Avdel rivets is similar to that of solid rivets.

To check that the mandrel is a firrn fit in the rivet after milling a spring-loaded pin tester is used -- if the mandrel pushes out, the rivet must be drilled out and a new one fitted.

The MHC Avdel rivet is a later version which locks itself into the hole, and breaks flush with the surface so no milling is required.

Fig. 5 1 MBC AVDEL RIVET

Page 268: Materials and Hardware

RIVETING TOOL

- , RIVET -

LOCKING RlNG

/

THE MANDREL IS PLACED IN THE NOSE OF THE TOOL 8 THE RlVET IS INSERTED INTO THE HOLE. A DIFFERNENT NOSE IS REQUIRED FOR EACH RlVET SIZE.

THE TOOL IS OPERATED, PULLING CONTINUES LOCKING RING HOLDS MANDREL THE JAWS ENGAGE THE UNTIL THE RIVET IS FULLY IN PLACE WITHIN THE RIVET. MANDREL WHICH IS FORMED 8 THE MANDREL PULLED THROUGH THE BREAKS. THIS ACTION RIVET FORMING THE ALSO FORMS THE HEAD. LOCKING RING ON THE

MANDREL

Fig. 52 FORMING AN MBC AVDEL RIVET

Cherry Rivets

These are rivets of American manufacturer a n d are very similar to Avdel rivets. C~;lrir:g the final stages of forming a locking collzr, located in a recess in the rivet head, is forced into a groove in the stem and prevents the stem from further movement. The action of closing a cherry rivet is shown.

After forming the stem protrudes slightly beyond the rivet head and this excess, plus part of the locking collar, may be milled off to provide a flush finish.

SERRATED STEM WITH BREAK NOTCH 8 LOCKING COLLAR

INTEGRAL DRIVING ANVIL TO FORM LOCKING

LOCKING COLLAR 8 ENSURES COLLAR RECESS / FLUSH STEM BREAK

FASTENERSLEEVEOR RlVET

DEFORMING LOCKING COLLAR -DEFORMS INTO LOCKING RINGS IN STEM 8 LOCKING RECESS IN RlVET

Fig. 53 THE CHERRYMAX RIVET

Page 269: Materials and Hardware

PRESSURE OF ANVIL COLD

STEM IS PULLED THROUGH RIVET FORMS LOCKING COLLAR INTO STEM KEY SYSTEM SECURING STEM IN RIVET.

RIVET PLACED IN HOLE HEAD' 'OLE a CLAMP

STEM BFWKES FLUSH 8 REMAINING RIVET COMES COMPLETE

PLATES TOGETHER STEM & ANVIL ARE REMOVED

WITH STEM AND ANVIL

Fig. 54 FORMING A CHERRYMAX RIVET

STEM\m / SHEAR GROOVE

LOCKING

LOCKING COLLAR

LOCKING COLLAR LOCKS STEM TO SLEEVE & STEM BREAKS OFF

STEM EXPANDS THE SLEEVE, FORMS THE

STEM PULLED HEAD 8 PUSHES THROUGH SLEEVE COLLAR INTO GROOVE

DRAWING FROM CAP 562

Fig. 55 FORMING A CHERRYLOCK RIVET

Cherry rivets are installed using hand or power operated tools and it is important that the tools are fitted with the correct type of head for the particular size or type )f rivet. Either the aircraft or tool manufacturer normally supplies details.

Cherry rivets are identified by a four-figure number followed by a figure indicating the diameter in thirty-seconds of an inch and a further figure indicating the maximum grip in sixteenths of an inch. A s a n example, CR 2 162-3-6 refers to a Cherry rivet in aluminium alloy, with a countersunk head and standard stem, 3/32 inch diameter and a maximum grip of 3/13 inch.

Page 270: Materials and Hardware

RIGID PIPES

Used for the moverrlerit of gasses and fluids within the aircraft and used in systems such as:

* Hydraulic systems * Pneumatic systems * Fuel systems. k Oxygen systems. * Anti-icing systems. A Domestic water and waste systems. X Air conditioning systems.

Pipelines may be made of:

x Alunllrlium 01- aluminium alloy - for low pressure systems. j, Steel. + Stainless steel. * Copper. * Tungum - a copper alloy. k Brass.

When replacing a pipe it is important to replace it with a plpe made of the same material, diameter, length, shape and gauge. It is also important, of course, that the end fittings are identical with the old ones.

UNION ADAPTER NUT OR CONE ADAPTOR UNION NUT OR OUTERSLEEVE

COLLAR

FLARED E

COMPONENT TO COMPONENT COUPLINGS I INNER SLEEVE

,OUTER SLEEVE

FLARED PIPE FL.ARED PIPE

/

[IRAWING FROM CAP 562

Fig. 56 TYPICAL HIGH PRESSURE FLAWED COUPLINGS

Page 271: Materials and Hardware

Somc pipes can be bade up' at user unit level while others have to he ob!;l~ned fron~ stores using the appropriate stores part number and reference numt)rr

The aircraft manufacturer should design the pipes and their layout in sr4r.h a way that mis-connection is impossible, either by having different length pipes 111 the same run so that unions, or connections, of one pipe do not occur at the snlrle place as a pipeline next to it; or by having pipelirles (and unions) of d~ffercnt diameters so mis-connection is impossible. 'This is riot always done, howevex, and it is important that all systems are put through a complete functional test after any pipeline replacement/disconnection/reconnection.

Flared Couplings - Many high pressure couplings have this method of assembly using a flared pipe, adapter nipple, collar, outer and inner sleeve. Nipples now in use have a parallel extension (called a skirt) to ensure that they are correctly aligned in the pipeline - and this should always be inserted into the flared pipe, which has the collar and outer sleeve fitted. Used for high pressure pipes.

Flareless Couplings - Also used for high pressure couplings. A 'preset' of the correct size is placed over the unflared pipe end. The pipe is pushed fully home into its union adapter and the union nut is tightened to a specific torque v r ii I ue. This causes the 'preset' to bend inwards and form a leak proof compressio~~ joint with the pipe. It is important that the correct torque value is used. Used for high pressure pipes.

UNION NUT \ - ,SLEEVE

'PIPE

PIL ORRECT PRESET

UNDERTIGHTENED PRESET OVERTIGHTENED , PRESET

DRAWING FKOM CAP 562

Fig. 57 FLARELESS PIPE COUPLING

Page 272: Materials and Hardware

Compressed Rubber Couplirzgs -- lJses a compressed rubber gland and used for low pressure systems such as F'itot static systems. The pipe end must be hard against the shoulder of the recess in the union adapter- before any attempt is made to tighten the union nut.

Prior to fitting the pipe, the pipe ends should be suitably protected against the corrosive action of the rubber. Copper pipes should be tinned, whilst stainless steel and aluminium alloy pipes should be protected with a varnish such as RSX17. All sharp edges should be removed from the pipe ends.

UNION UNION NUT ELASTOMERIC SEAL

DRAWING FROM CAP 562

Fig. 58 LOW PRESSURE RUBBER COUPLING

Hose Clips - llsed for low pressure connections of hoses to metal pipeline attachments. Any hose clips used must be of an approved type and must fit correctly in relation to the pipe ends or beading. A clearance of between 0.25in and 0.50in (6 to 13mm) should be allowed between the ends of the pipes so that the ends will not make contact should flexure of the pipe occur.

If a new pipe connection proves difficult to fit, it may be lubricated with the fluid used in the particular system, but for some types of pipe, hot water immersion in accordance with the manufacturer's instructions is recommended.

Couplings are affected by expansion, contraction, vibration and heat and sho- '-1. be inspected regularly for deterioration and freedom from oil and grease. When connectors are to be removed from pipe ends, it is essential that levering with a screwdriver or similar tool be avoided, since this could damage the pipe.

Brazed Nipple Couplings - A conical nipple is brazed or silver soldered to the end of one pipe and a union sleeve is brazed onto the other pipe. A union nut screws onto the union sleeve to hold both pipes firmly together. Used on high pressure sys terns.

Self-Sealing Couplings - May-be of the screw or bayonet type and allows for quick release and assembly of the joint without fluid loss or air inlet.

Page 273: Materials and Hardware

When making or b r e a l n g the joint of a self-sealing coupltng, care must t ~ t x taken to avoid turning between the two halves, otherwise the seating for the valve in the unlon half-coupling may damage the seal jn the fixed half -corlpling.

UNION SLEEVE UNION NUT

BRAZED ON PIPE NIPPLE B W E D ON PIPE

\ UNFLARED PIPE YYU UNFLA~ED PIPE

DRAWING FROM CAP 562

Fig. 59 BRAZED NIPPLE COUPLING

Note. This does not apply to Avimo type self-sealing couplings, which are connected by a bayonet pin and socket arrangement. I t is therefore necessary to rotate this coupling to make or break the joint.

HOSE CLIP OR VALVE 1 OPERATING

Fig. 60 TYPICAL SCREW-ON SELF SEALING COUPLING

When a self-sealing coupling is disconnected, blanks should be fitted to both halves.

A leaking half-coupling should be replaced.

Banjo llnions - these consist of an inlet union screw, inlet union and two bonded seals or bonded washers. They are designed to allow a pipe connectiorl to he made to a cornponent a t right angles to the component. The inlet union screw is drilled internally to allow for fluid passage as is the inlet union. The bonded washers are madc up of a metal plain washer to which is bonded an elastorneric seal. This is usually bonded to the inside diameter of the washer and is designed to h r thicker than tl l t t trlc'tal washer.

Page 274: Materials and Hardware

A s the union screw is tightened down so the elastomeric seaJ will be squeezed and provide a seal. The union screw is usually torque loaded.

INLET UNION SCREW Screwed into component INLET UNION

BONDED SEALS OR BONDED WASHER SE

ELASTOMERIC COMPONE

down of the union screw

Fig. 6 1 BANJO UNION

Vee Flange Couplings -- a rnethod of joining used or1 larger diameter pipes such as fuel pipes and low pressure pneumatic pipes. The ends of the pipes to be joined have a Vee section brazed onto them. The pipes and the two Vee sections are butted together and retained by a Vee Flange Coupling. I t is important that the abutment faces of the two Vee flange sections are clean, absolutely flat, undamaged, not strained and parallel.

This method has three main advantages over the method whereby the flanges are held together by bolts (or studs) and nuts, namely:

(a) Even distribution of stress around the flanges. (b) Ease and speed of removal/installation.

(c) The assembly has a good strengthlweight ratio and is more compact.

Examples of usage of Vee flange couplings are to:

k Attach the cases of driven comporlents to engine gearboxes. ,+ Join components such as valves to ducts.

Join sections of ducting/pipework in air-conclitioning/fuel systems.

Page 275: Materials and Hardware

BOTTOM HALF CLAMP

Fig. 62 DOUBLE BOLTED VEE CLAMP

Fig. 63 SINGLE BOLTED VEE CLAMP

Such diverse uses will demand a variety of designs and materials; therefor? the appropriate AMM must be referred to prior to working on any Vee flange coupling.

Note. The torque loading of the clamp bolt(s) on Vee flange couplings js crltical and thv clamp halves need to be torque tightened slightly then tapped with soft faced mallet to distribute any tension. The effect of this will reduce the torclue load~ng of the clamp bolt, which then requjres re torqueing. This procedure is continued until the final correct torque value is reached.

Page 276: Materials and Hardware

Fuel Delivery Pipes (1,P)

Not nornially required to handle pressures over 50psj (345kPa) and are usually made from alrlminiurn alloy with the dianletrr being large enough to cope with the high flow rates and typically are about 2%" (64ma1) in diameter.

COUPLING BODY COUPLING NlJT

FUEL PIPE

O-RING SEAL

O-RING SEAL

COUPLING BODY

COUPLING NUT

Fig. 64 TYPICAL LP RIGID PIPE COUPLING

RINGS

FLEXIBLE HALF COUPLING FLEXIBLE FULL COUPLING

Fig. 65 TYPICAL LP FLEXIBLE PIPE COUPLINGS

Page 277: Materials and Hardware

Pipes are thin walled and need care when flandling.

Pipes sectlons are fitted with various connectors. These need lo be fuel tight us~rlg seals capable of withstanding any flexing that may occur (wings tend to flex considerable during flight and the pipes themselves are not able to take much flexing). Also, all pipe-work and coupljngs must be electrically bonded because of the fluid flow inducing static build-up.

Figure 64 shows a typical example of a rigid coupling and figure 65 shows examples of flexible couplings. There are many types in use and reference should always be made to the AMM for type and fitting instructions In general:

* Ensure the correct seals are fitted and in the correct way. * Ensure pipes are un-damaged particularly around the seal mating

surfaces. * Torque load correctly. * Ensure correct bonding. " Carry out leak checks after assembly. This may need an engine run. * Some airlines require a duplicate inspection on fuel feed pipelines.

Pipes in Pressurised and Fire Risk Areas

Where pipes have to pass through pressurised areas (rare) or fire risk areas additional precautions are taken to ensure that any leakage does not get outside the immediate vicinity or to any engine hot sections. These features can include such things as scuppers and channels to direct the spilt fluid overboard. (Engines have a fire-proof bulkhead by regulation). Any couplings near the engine would be enclosed and provided with an overboard drain,

Pipes also run to the back of the aircraft for tail mounted engines, tail mounted fuel tanks (aircraft trimming and fuel transfer) or a n APU. Here flexible pipes are -1sed shrouded by a 'normal' aluminium alloy fuel pipe. Any leakage from the llexible pipe is transferred into the shroud. The shroud is ported to a drain mast and any fuel accumulation is drained overboard. A standard 'ramp' check would include checking the mast for any fuel. If any is present the leak must be found and rectified.

Pipe Installation

Before pipes are fitted into aircraft they should be inspected for damage, cleanliness and corrosion. If damage to the pipe is suspected, the pipes shormld he pressllrc tested and the roundness of the bore checked.

Checks should be made that the pipes are of the specified type and should have approved certificates identified to the pipe (EASA form 1).

Page 278: Materials and Hardware

Prior to assembly, all pipes must bc blown through with clean dry air and, where applirable, flushed out wlth clc.;in filtered fluid of the type to be used in the particular system in which t%-re pjpe is to be installed.

APU SHROUD

APU SHROUD

CENTRE TANK SHROUD DRAIN LlNE CONNECTION

DRAIN MAST OUTLET APU FUEL SUPPL.Y LINE CONNECTION (LOOKING FORWARD)

Fig. 66 TYPICAL FUSELAGE PIPELINES

For oxygen systems, a final approved degreasing process must be used to ensure cleanliness, since oil or grease in contact with oxygen under pressure would cause an explosion.

If the pipe is not to be installed immediately, its ends must be blanked using ~,,e correct blanks. Plugs and caps conforming to standards appropriate to the system pipeline should be used. In instances where standard blanks cannot be fitted, it must be ensured that the blank is so made that it is impossible for it to be left in position when the pipe is connected.

Note. 'The rise of rag, tape or paper for blanking off purposes is not allowed.

When installing pipes, they should not be allowed to come into contact with materials which might cause galvanic corrosion. Some small aluminium alloy pipes are treated internally and externally with varnish. Pipes so treated must not bt. used in fuel, oil, pneumatic and oxygen systems, or i n any system where peeling varnish may cause malfi~nctloning of the system.

Page 279: Materials and Hardware

Supporting Pipes

Pipes must be supported in accordar~ce with the AMM,

Multiple pipe clamps may be used. These are often made of fibre, al-ilurn~n~~~rr~, moulded rubber and other ~rlaterials. The two halves of the clamps are ~ lsua l ly joined together by bolts, whlch also serve to secure the clamp to the aircraft structure It is important to ensure that the clamps are of the correct sizr t o preverit damage to the pipe.

Where packing is required between the pipes and the clamps, the rnater~al used should be in accordance tvitl.1 the AMM. Typical materials are cork sheet, tinned copper gauze and various types of tape, but leather must never be used, since this will cause corrosion.

Some pipe clamps are self-bonding (electrical bonding), but wher-e this is riot possible, metal gauze or a cork-based material having copper strands intenvoven, can bc used.

Where single pipes require support, standard clips such as 'P' clips can be used.

Clearance between pipes and structure should be a t least 0 . lin (2.5nlr-n) arid adequate clearance should be provided between pipes and moving parts, eg landing gear bays [tyres may 'grow9 when rotating by a s much a s %in (51 mni) in diameter and lin (25mm) in width when rotating fast].

Connecting Pipes

Before connecting the pipe union nuts, a check should be made to ensure that the pipe end is of the correct type and size, that is clean and undamaged.

Two spanners must always be used when tightening (or disconnecting) a pipe coupling; one to hold the sleeve or adapter and one to turn the union nut. Over- tightening of couplings must be avoided. Special tightening techniques and torque's, when specified, must be used.

If lubrication of the threads is specified, it is essential that only the correct lubricant is used and that it does not enter the bore of the pipe.

For oxygen systems, the following lubricants are suitable: DTL) 900/404%, which provides a dry self-lubricating film of graphite and which should be applied to a thoroughly degreased surface and allowed to air dry before being put into scrvice; and DrJ'D 900/4286 which acts as a sealant a s well a s a lubricant and has a grease- like corlsistency; and YI'FE tape.

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Pipes with Standard Couplings

When connecting pipes having standard types of threaded couplings, eg those corr~plying with the A G S series, the following points should be checked:

(a) That flared pipe ends are free from cracks, distortion or other damage.

(b) That union nuts are free to be withdrawn over their entire length, that they are not impeded by bends or other obstructions and that they rot at e freely.

(c) That all loose parts such as adapter nipples, rubber glands, washers, etc are fitted to the coupling, are of the correct type and are correctly located.

(d) That the pipe end aligns correctly with its mating part. Pipe ends must never be forced into position, since this may induce considerable stress into the pipe and the coupling.

(ej Thai the pipes are never drawn together by their union nuts, sincc this imposes a strain on the flaring which may cause deformation or other damage.

Once a standard coupling has been bedded in initially, less torque will be required on subsequent reassembly to make a leak proof joint However, should a leak occur, the coupling must not be over tightened in an attempt to stop the leak, but must be disconnected and the cause of the leak ascertained.

Adapter nipples with skirts have replaced those without skirts but it is important to check that the nipple sits correctly before assembly of the union. (It has been known for un-skirted nipples to rotate in the assembly prior to tightening thus causing a weak joint and one that is not pressure proof).

FLEXIBLE HOSE ASSEMBLIES

Fitted in systems where there is some movement between components - for example: in brake pipelines where there is movement between the retractable landing gear and the airframe and, further down the line, where there is rnovement within the shock-absorber and where there is movement between the wheel bogie and the shock-absorber. In some cases, however, swivel unions may be fitted instead (eg on the torque links).

From a designer's point of view, the following points should be borne in mind when selecting a hose assembly for a particular purpose.

k Maximum system pressure. * Maximum system fluid temperature, particularly soak temperatures

after system shut down when fluid temperatures could increase by a s much as 20°C.

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* Compatibility of hose materlal and ~t s end fittings with the syst ern fluid and external environnlerltal conditions. This includes fl uxtf s to be used in other systems where they may come into c-ontact wrth a particular hose installatnor1

The general construction of a hose xncludes the following features:

X A flexible ~mpermeable inrler linixig compatible with thc fluid llsed in the system. Has little strength.

* A flexible support structure to the lining which usually contairls reinforcing. Has the strength to resist the forces set u p by the system pressure.

A End fittings, usually metal, secured to the flexible part of the )lose which allows the hose to be secured to components etc. When secured will provide a leak-proof joint and has provision for tightening (usually a hexagon uriiora nut) and loclting.

Correct Methods of Fitting a Flexible Pipe

Hose assemblies for use in high-pressure fluid systems are usually suppliecl by the manufacturers complete with end fittings which, in most cases, cannot be dismantled or repaired in any way. However, there are some types of hose assernblies on which the end fittings may be changed if necessary.

The hose lining is made of a material to withstand the pressure, ternperature and to be compatible with the fluid in the system.

The hose is strengthened by high tensile steel wire braiding or fabric reinforcement.

Hose assemblies are generally designed either for specific functions or for a limited range of functions and it is essential lo ensure that only the hose specified in the Illustrated Parts Catalogue (IPC) is fitted.

A material which is widely used is polytetrafluoroethylene (PTFE). This material is chemically inert, is unaffected by synthetic oils and fluids and operates at high temperatures and normally has a n unlimited shelf life. PTFE hose is, howcver, more susceptible to damage from careless handling than rubber hose and care is required cluring handling.

Hose assemblies fitted in high temperature areas (eg near engines, brakes etc) rnay be protected by Gre protective coverings.

Elose assemblies often go through a great deal of flexing. They may also have a natural ageing process. I t is therefore: import ant that lives a s stated in thc AMM are not exceeded.

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CORRECT INCORRECT

DO NOT BEND OR TWIST HOSE. ALLOW ENOUGH SLACK IN THE HOSE TO PROVIDE FOR CHANGES IN LENGTH DUE TO COMPONENT MOVEMENT AND PRESSURE CHANGES. LENGTH CAN CHANGE FROM +2% TO -4% DUE TO PRESSURE CHANGE. METAL END FITTINGS ARE NOT PART OF THE FLEXIBLE PORTION. THE USE OF ELBOWS B ADAPTERS CAN MAKE FOR EASIER FITTING 8 THE REMOVAL OF STRAIN FROM THE ASSEMBLY. KEEP AL.L BEND RADII AS LARGE AS POSSIBLE. AVOID INVERTED "U" BENDS IF POSSIBLE. ALWAYS FIT HOSES IN ACCORDANCE WITH THE AMM.

Fig. 67 FITTING HOSES

LENGTH - - LENGTH I-- -

Fig. 68 LENGTH OF HOSE ASSEMBLIES

The end fittings on a hose assembly are rnade from steel or light alloy and are designed to exert a grip on both the tube and wire braids so as to resist high-- pressure, twisting and vibration loads. They also provide an electrical bond.

'The length of a hose assembly with straight end fittirlgs is taken as the distance hetween the extremities of' the two nipples. In the case of an elbowed end fitting, t h e length is taken from the centre of the elbow bore.

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Construction of High. Pressure Hose Asseriiblies

A typical high-pressure hose assembly colisists of an inner linlrlg cuverecl by one or two closely woven wire braids, either rnc,ulded in, or sandwicl-red be-tw~en, the synthetic rubber of the lining or woven on thy surface of the tube.

UNION NUT \ REINFORCEMENT

/ NIPPLE SLEEVE

(SWAGED ON) \

HOSE OUTER COVER

DRAWING FROM CAP 562

Fig. 69 HIGH PRESSUlU3 HOSE ASSEMBLY

The whole assembly may be enclosed by an outer cover, the purpose of which is to provide protectjon for t.he inner parts of the hose, to resist abrasion and the effects of weather and external fluids and chemicals, and, in some cases, lo provide a degree of fire resistance.

Low Pressure Hose Assemblies

These are thin-walled and textile-reinforced. They are used for Pjtot--static instrument lines especially where they pass between the structure and instrument panels mounted on anti-vibration mountings. The rubber or canvas spiral-corrugated hose having a spiral steel spring embedded in the corrugations, is often used for systems where there are negative pressures.

With low-pressure hose it is important to ensure that bends are not too acute, since this may result in kinking of the hose at the bend. Where sharp bends cannot be avoided an internal support coil may be fitted.

RE--USABLE END FITTINGS

Us~mlly consist of a socket, nipple and union nut. When the nipple is screwed ~ n t o the hose (and socket), the taper in the nipple causes the hose to be c1ampc.d firmly bctween nipple and socket - so forming a seal. This is known as a cornpresslon seal b11t othvr methods of assembly may be used.

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Make sure that when a hose is selected it conforms to the specification a s laid down in the AMM and that it is given a visual examination for any srgns of damage. When cutting the host. it is important to cut it to the correct length usxng a fine-toothed hacksaw blade (r-ernove any debns)

NIPPLE UNION SOCKET \ NUT

DKAWING FROM CAP 562

Fig. 70 TYPICAL RE-USABLE END PPILY'ING

Actual assembly of the hose and socket is carried out by holding the socket finnly in a vice and screwing the hose into the socket until it bottoms. (Some manufacturers recommend that, after screwing the hose fully into the socket, it should be unscrewed a quarter turn to allow for expansion when the nipple is inserted.)

After assembly the hose should be marked with a grease pencil, paint or tape, at the point where it enters the socket, in order to provide a means of checking that the hose is not forced out of the socket during the subsequent insertion of the nipple. The nipple is then screwed into the socket (and hose), torque loaded and locked (usually with locking wire).

The assembly should have a metal identification tag attached and be pressurc tested (see module 7).

PIPE LlNE IL3ENTIFICATIC)N (BS M 2 3 )

All pipes are marked with date of manufacture, drawing or part. number, inspection stamp, test stamp and name of manufacturer. These markings can be stencilled on the external surface of the hose or stamped on a rnetal tag or band (soldered/brazed in a loop to the pipe). The date can be a colour code woven into the cotton brand.

Flexible hose assemblies are marked along their- length with one or rrlore continuous thin lines to indicate any twist on installation.

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Rl F U E L

WARNING SYMBOL

DIRECTION OF FLOW

NUMBERS ARE BRITISH STANDARDS COLOUR SERIAL NUMBERS: 1 - BLUE 2 - GREEN 3 -YELLOW 4 - BROWN 5 - ORANGE 6 - RED 7 - GREY

Fig. 7 1 COLOUR/ SYMBOL PIPELINE IDENTLFICATION

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SYSTEM IUEN'I'IFICATION

Systems js)isten~ papes may be identified by tape or identiiication labels attached to each section of pipe Systems irr use include:

* Manufacturer's own system. * The colour/syrnbol system. This uses words, colours and synlbols to

indicate the contents/ system of the pipe. k The ATAlOO code system. Based on the ATAI OC) chapter numbering

system of the A M M and will indicate:

(a) The system -- by a symbol. (b) The component to which the pipe is fitted. (c) The subsystem to which the pipe is fitted. (d) Whether it is suction -- pressure, etc.

'I'he ATA 1 00 Code System

The numbering system (figures 72 and 73) may take the following form (starting from the end of the pipe):

1 st Par1 , , ~ . "....,.. *.. . Pipe end identlficatioxl number.

211" Part o . . ~ . , ~ . . . , . . ~ . . . . System symbol.

3rc' Part * . e o . = . . c . . . ~ e . . , ~ ATA chapter number.

4 t h Part e . o . . . . . . . . ~ . . . . = . Component key number.

Sth Part c . . s s . e . . . . . m . , . . ~ Component. port or connection code.

6 t h Part e s . . . ~ e . . " s e . e ~ . , . Pipe function and subsystem code.

7 t h Part . * . . - . G o " . . . " . . . . = Flow direction -- if applicable.

PlPE END IDENT A'TA CHAPTER COMPONENT PORT OR NUMBER I: 1 OR 2 NUYBER A CONNECTION

INTERNA~IONAL \ SYSTEM CODE COMPONENT KEY eg HYDRAULIC NUMBER

,FLOW DIRECTION

PlPE FUNCTION 8 SUB SYSTEM CODE

Fig. 72 ATAIQO PIPE CODING

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RESERVOIR COMPONENT KEY NUMBER 3100 LANDING GEAR SELECTOR p = PRESSURE LINE

KEY NUMBER 3004

END 4 POWER GREEN SYSTEM LANDING GEAR CONNECTION L)

/zixGiq pKq

Fig. 73 EXAMPLES

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CONTENTS

Page

Aircraft electrical cablcs 1 Re quiremerlts 1 lnstallation 4 Types of cable 6 Identification 7 Cable types 13 Crimping 17

Tools 18 Procedure 20

Plugs & sockets 25 Mechanical Flexible Remote Control Systerns 29

Control Cables 29 Terms used 32 Cable specific a t ' ~ o n s 32 End fittings 34 System cornponerits 35

Specialised remote control sys terns 44 Teleflex controls 45 Bowden controls 50 Flexbal! controls 53

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AIRCRAFT E1,ECTRICAL. CABLES

In the early days thy cables used in aircraft were rrianufactured to a sirn~lar standard to those used in the automobile ~ndustry . It was soon learrlt thd t these cables didn't stand up to the severe climatic and e~~vironrnental cond~tions encountered during aircraft operation and therefore had to tw designed specifically for alrcraft use. A variety of' cable types have been developed, the choice of cable for a particular functjon will be governec-l 1)y its purpose and locatron.

Requirements

These are laid down in HCAR's section D, K and G (old system), now EASA25 (large aeroplanes), EASA27 and 29 (helicopters) etc.

Reliability is of prime consideration for aircraft cables since the perforrrlarice and safety of an aircraft and its occupants is usually dependant on electrically operated systems. Care, therefore, must be exercised during the manufacture of cable looms and circuits and these must be thoroughly tested on conlpletion. Listed below are a number of qualities which an aircraft cable should possess.

Minimum Weight and Dimensions. A large aircraft may require many miles of --

electrical wiring and even small reductions in the size and mass of a cable will result in a considerable weight saving, therefore allowing a n increased payload.

Resistant to Fluids. The likelihood of a n aircraft cable encountering a variety of aircraft fluids is high. It is therefore important that aircraft cables are able to withstand the effects of: water, engine oils, hydraulic oils, fuels, solvents, etc.

Non-inflammability. Wiring is necessary in high fire risk areas such as engine nacelles, and APU bays. Such wiring should not cause any fire to spread and for this reason the protective covering should be of self extinguishing material. There has been doubt about Capton wiring in this respect - although it is still in use.

During flight many cables will experience a large temperature range arld must remain flexible within this range with the insulation remaining in tact.

Resistance to Abrasion. An aircraft cable must possess a number of 'physical' --

qualities and in particular must have high resistance to abrasion (iritl~lct.tl by aircraft vibration). Cables should also be physically strong and easily ~vorka ble.

Electrical --- Recluirerne~B. The conducting element must have a low resistivity c - o efficient with a low volts drop per unit length and the irisulation r n u t /lave a s11f f~cic.ntIy high resistar~c-e valur, to cope with t hc maxirnllm appl~rcl vol I ugc..

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Current Rating

The normal current rating of a cable can be defined as: "The amount of current ~t will carry without sustaisring a temperature rise sufficient to cause the value of the insulation resistance to deteriorate to an unacceptable level or without exceeding a specified voltage drop per unit length". Earl~er cables either had the current rating stamped on the outer sheath or had a colour identification related to the current ratlnlg.

However, because a cable's current carrying capacity is influenced by a number of factors other than electrical load current, it is nowadays the practicct of cable manufacturers to use a classification based on the American Wire Gauge (AWG).

Modern aircraft cables have a wire gauge number stamped 011 the outside. The electrical systems designer will take into account the factors listed below before choosing a cable for a particular job:

* The electrical loading of the cable. * The amount of heat generated by neighbouring cables (cables in a

bundle or loom for example). * The number of cables in the loom. * The ambient temperature of the surrounding air (its loc t' a ion on

the aircraft - near an engine for example). * Whether the cable is er~closed or in free air. k The t hermal conductivj ty of the cable.

Deterioration

Aircraft cables are designed to provide the best possible combination of resistance to deterioration caused by extremes of temperature, mechanical damage and contamination by fluids, and in general, are suitable for installation without additional mechanical protection.

Working conditions and environment, however, may necessitate the provision of extra protection (additional support, conduits etc) in those places where the cables are exposed to the possibilities of local damage or conditions which colild cause deterioration.

Iicceipt Storage and Handling of Cables

Prior to delivery, cable e r ~ d s are sealed to prevent ingress of moisture. The cnt)lcs are supplied on dr i~rns s ~ ~ i t a b l y labelled and protected to prevent clamage during transit anci storage.

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Smaller slzes of cable may sometimes by supplied in wrapped coils V ~ s u a I exammation of cables on receipt, by nature of the packing, is often restr icdted to the olrter turns . Such a n examination is of little value in checking for taults In the cable, therefore, if the condition of the packing, as received, gives rlse to doubt regarding the soundness of the cable, ~t should be returned to thf. manufacturer.

Note. Check the cable part nurnber/batch number and confirm its identification against its documentation/stores release certificate (EASA form 11.

Cables should be stored in a clean, well-ventilated store They should not be stort:cl near chemicals, solvents or oils and, if necessary, protection sho~tlcil be provided against accidental damage. Loose coils, whether wrapped or not, must not btr stored so that a heavy weight is irnposed on them, since thls rn:Ijr cause unacceptable distortion of the insulation or damage to the protective coverings.

The ends of cables in store should be sealed against the ingress of moisture by the use of waterproof tape or sealing compound.

It is important that cables are handled carefully a t all times.

When taking long lengths of cable from a drum or reel, the cable should not be allowed to come in contact with rough or dirty surfaces. Preferably the drum or reel should be mounted so that it can rotate freely.

Care should be taken to remove the twist out of each turn of cable drawri from loose coils, otherwise kinking, with consequent damage to the cable, may occur.

Before being made u p the cable length should be inspected for any signs of damage or deterioration and given a continuity and insulation check.

Made-up Cabling

Cable looms and cable runs made-up on the bench should be inspected before installation in the aircraft to check the following:

(a) That all cables, fittings, etc, arc of the correct type, have bi:t.r~ obtained from a n approved source, have been satisfactorily tc-sted before making u p and have not deteriorated in storage or been damaged in handling

(b) That all connectors and cable loon~s conform to the relevant AMM, Wiring Diagram Manual or Modification Drawing in respect o f terminations, length, anglc- of o~ltlet s and orientation of c.0111 :~c.t assemblies, identificat~on, a n d protectioln of connections

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( c ) That all crimped joints and soldered joints have been made In

accordance with the relevant AMM, Wir~ng Diagram Mailual or Modification Ilrawing, are clean and sound, and that insr~lating materials have not been damaged in any way.

(dl That cable loorrl bind~ng and strapping is secure.

( e ) Carry out continuity, resistance and insulation tests.

(fj Cables should be identified using the correct aircr-aft wiring code iaw the wiring diagram. Identification marking may be carried out by printing on sleeves and attaching sleeves at the end of each cable run or the cable may be printed on a t regular intervals along its length. If direct cable marking uses a heat marking system then the cable must be inspected to check that the insulation has not been damaged and a n insulation check carried out. Many looming shops have special machines that will automatically mark the cable along its length at regular intervals with the identificatior. a t the same time carrying out insulation tests etc.

Installation of Cabling in Aircraft

Guidance on the factors requiring special attention during the installation is given in the following paragraphs - but always check the AMM.

Contamination. To prevent moisture from running along the cables and seeping into the associated equipment, the cables should be so routed as to run downwards away from the equipment. Where this is not possible, the cable should incorporate a descending loop immediately before the connection to the equipment.

Where conduits, tubes or ducts are used, they should be installed in such way that any moisture accumulating in them will be able to drain safely away. Cables which are routed through such fittings should be capable of withstanding any such moisture.

Interference. Interfering magnetic fields may be set up by electrical equipment, -- electrical currents in the cabling, or the aircraft structure, and also by magnetic materials. Cables are required, therefore, to be instalIed so a s to reduce electrical interference to a minimum and to avoid interaction between the different electrical services.

Note. Requirements for the avoidance of compass and radio interference are g ~ v e n in Chapter J4-1 of British Civil Airworthiness Requirements. (Now EASA 23 - light aircraft, 25 -- large aeroplanes, 27 8r, 29 -- helicopters)

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Protec.tion -- of cabling. The cables are required to be protected from abrasion, mech,inical strain and excessive tieat and a g a i n ~ t the deleterious ef'fect:i of fuel, oil and other aircraft fluids, water in either liquid or vapour form and thc. weather. Where aircraft skin temperatures could be a problem (nn hot c-Irniates) cables should be routed away from the skln of the aircraft. The cables should not bt: run near the hot parts of engines, APUs, exhausts, heat exchar1gi.r~ etc unless a cooled-air space or heat barrier is provided.

Where, cables are routed through metal fittings or bulkheads etc, the edges of the holes through which they pass must be radiised and smoothed and fitted with an insulated bush or sleeve. Cables which are drawn through holcs or tubes must be a n easy fit requiring only a moderate, steady pull, care bcing taken to keep the cables parallel to one another and to avoid the formation of kinks (which may cause fracture).

C o n d ~ ~ i t s , ducts and trunking used for carrying cables should have srnooth internal surfaces.

Cables being fitted through pressure bungs should be fitted into the correct size holes for the size of cable, to ensure efficient sealing. Only the recommended cable threading tool should be used for this purpose to avoid damaging the bung.

Support of Cabling. The cabling must be adequately supported througt-lout ~ t s length, and a sufficient number of cable clamps must be provided for ear.h run of cable to ensure that the unsupported lengths will not vibrate unduly Bends in cabie groups should not be less than eight times the outside diameter of the cable group. However, a t terminal blocks, where the cable is suitably supported a t each end of the bend, a minimum radius of three times the outside diarneter of the cable, or cable bundle, is normally acceptable.

Cables must be fitted and clamped so that no tension will be applied in any circunlstances and so that loops or slackness will not occur in any position where the cables might be caught and strained by normal movement of persons or controls in the aircraft.

Where it is necessary for cables to flex, eg connections to retractable landing gear, the amount and disposition of slack must be strictly controlled so that the cal~le is not stressed in the extended position, and that the slack will not be fouletl, chafed, kinked or caught on any projection during movement in either dire(: tic )n.

Cab1t.s should normally be supported independently of, and with m~axltrrurn pract~cable separation from, all fluid and gas carrying pipelines. To prevc.nt contan~ination or saturation of the cables in the event of leakage, cables should be rorlced above rather than below licpid carrying pipelines.

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Cable Types

The pages a t the back of this section give information on various types of cables to be found on aircraft. You would not be required to remember the details but you sl-iould understand the inforrrlation that is given. Cables and equipment should meet the requrrements laid down in BCARs and ,JARS to provide electric shock protection to personnel as well as heat protection - if equipment gets hot during normal operation.

Airframe Cables. Used for runs throughout the airframe.

Interconnecting Cables. 'This is used for the interconnection of equipment within racks, therefore their insulation is thinner than normal airframe cabling. They are lighter and more flexible.

Equipment Wire. Sometimes known as 'wire' it is used within equipment and is therefore flexible and suitable for soldering. It is not designed as interconnecting wiring though some aircraft manufacturers do use it for tlr, in protected parts of the airframe.

Fire Resistant Cables. This type of cable is required to retain a defined level of resistance in certain fire or overheat conditions. The cable is classed as Fire Resistant if able to withstand 1100°C for 5 minutes, and Fire Proof if able to withstand the same temperature for 15 minutes (EASA 25 &, 1 - if close to the outside of a firewall should not suffer damage if firewall heated to 1 100°C for 15 minutes).

Firepro~f Cables. These are required to operate for 15 minutes in a designated zone defined in BCARs and JAR 1 and are used in designated fire zones.

Conducting elements on electrical cables are sometimes plated to improve their ant-corrosive properties. The plating on copper conductors will normally determine the maximum continuous working temperature, eg

135°C Tin plating 200°C Silver plating 260°C Nickel plating/ cladding

Cable Maintenance

The requirements, laid down by the CAA for the installation of electrical cables, are laid down in BCAKs section J and EASA 23, 25, 27, 29.

Only the cables a s specified in the AMM, or approved equivalents, should be used. This will ensure that the cables will be capable of taking the voltages (during operatior1 and testing) and the maximum ulrrent in the most ;idverse conditions, without da~xmge to the cables. For more information on maintenance see rnodule 'i in this series.

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CABLE IDENTIFICATION

Cables have two identifications, one is carried out by the nlanufacture o f the cable and the other is carried out by the aircraft manufacturer - to co~rlplv with the w~ring diagrams.

Cable Manufacturer's Identification

Each rnanufacturer will stamp it's identification (:ode on the cable at regular intervals along its length. This is done automatically either by an ink printing process or a heated die process. It rnay i n c l ~ ~ d e :

(a) The cable size. (b) The manufacturers name.

(c) The manufacturers code, cable name etc

For example:

Minyvin GBx XX X 22 (1) (2 ) (3) (4) (5)

(1) Manufacturer's name of the cable. (2) Country of origin. (3) Manufacturer's cable code. (4) Year of manufacture. (5) Cable size.

These details should be checked against the stores release documents to ensure they are the same.

Tnstallation Identification

Besides the identification of the cable by the cable manufacturer there is a requirement to identify the cable in the aircraft installation. During aircraft manufacture a cable is installed (suitably routed, supported and connected crimped etc). Prior to assembly the cable is marked with a code that identifies it and relates it to the aircraft wiring diagram.

The code - made up of a series of letters and numbers may be printed on sleeves which are placed on the cable ends prior to being nrade up - or more likely printed along the cable length itself.

The printing may be carried out by a srnall heated hand operated macl-line. It is ribbon fed and prior to cable marking is set up with the correct nurnheri and letters (cable code). These are found by reference to the appropriate airrr-;ift wiring diagram.

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The cable may be marked by being put through an automatic identification anti testing machine -- once set up this will pull the cable through and print the code on the cable at the required intervals. It will test the cable for continuity and insulation and any cable faults found will cause the machine to stop and give a n aural warning. I t will stop automatically a t the end of the cable run. At any rate it is important that the cable is coded a t both ends and at any point where it passes through bulkheads, seals, etc.

Always visually check the cable insulation for darnage after heat identing as the ident may have penetrated the insulation and exposed the conducting core. (Fires have been caused by this, so it is important to check carefully and reject the cable if found). This is why the automatic identing machines carry out a n insulation test at the same time as the identing procedure.

The code will identify such things as:

(a) Cable size. (b) Circuit. (c) Circuit function. (d) Cable number.

The code may be devised by the aircraft manufacturer or may be based on the ATA 100 specification system. An example of this is shown below.

I E F G B 22 NMSV

(1) (2) (3) (4) (5) (6)

1. Unit number, used where components have identical circuits. 2. Circuit function letter and circuit designation letter which indicate

circuit function and the associated system. In figure one there is only one letter - the letter L indicating a lighting circuit.

3 . Cable number, allocated to differentiate between cables which not have a common terminal in the same circuit. Generally, contacts of switches, relays, etc, are not classified as common terminals. Beginning with the number one, a different number is given to each cable.

4. Cable segment letter, which identifies the segment of cable between two terminals or connections, and differentiates between segments of the circuit when the same cable number is used throughout. Segments are lettered in alphabetical sequence, excluding the letter I and 0. A different letter is used for each of the cable segments having a common terminal or connection.

5. Cable L 9' me. 6. Suffix data, used to indicate the type of cable and to identify its

connection fullction. For example code NMS V indicates Nyvin Metsheath (a BICC cable) ungrounded cable in a single-phase system.

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Tho rec:ommendation is that the cable is cotletl at regular intervals along it's lengt b and it is most important that it corresponds to the appropriate 31 r c . r aft wiring diagram.

Figure 1 shows a n actual aircraft example of a wiring diagram. It is two ( dbles (in a bundle) associated with ice formation spot-lights. The drawing shows the cable code (which should be printed om the cable) io urliquely identify t hii t cable on the aircraft. Note the change of the third digit a s the cable run progresses towards the lamp.

I POWER SUPPLY JUNCTON CONTROL PANEL CONTROL BOX LEFT I

Fig. 1 AIRCRAFT WIRING DIAGRAM - EXAMPLE

When replacing a cable it is important to:

(a) Fit the correct replacement cable. (b) Correctly route and support the cable. (c) Ensure its correct identification along its length. (d) Employ the correct terminations. (e) After replacement carry out appropriate electrical tests followeci

by a functional test.

For certain electrical systems, cables are required to perform a more spccialised function than that of the cables alreatly referred to. Some ex:irnples of what are generally termed 'special purpose cables' are described bc.10~~

Page 300: Materials and Hardware

Ignition Cables (Ignition f-Iarnesses)

'These are used for the t ransrn~ssion of high tension voltages (high voltages) in bolkr piston and turblnr engine ignition systerr~s. They are usually of the single cor"i- stranded type with a high level of insulatlorl, and screened by metal braided sheathing to prevent interference.

The number of cables requlred for a system correspond to the number of spark plugs or igniter plugs as appropriate, and they are generally made u p into a complete ignition cable harness. Depending on the type of engine installation, the cables may be enclosed in a metal conduit, which also forms part of the harness, or they may be routed without conduit.

Cables are connected to the relevant system components by special end fittings comprising either small springs or contact caps secured to the cable conductor, insulation, and a threaded coupling assembly.

The harness is supported in P clips or similar with rubber packing to help prevent vibration damage.

'. Fig. 2 TYPICAL WIRING HARNESS - R R TRENT

CABLE

RUBBEWTEFLON LINER

Fig. 3 TYPICAL 'P' CLIP INSTA%%ATI[BN

Page 301: Materials and Hardware

'I'hermocouple Cables

These cables are used for connection of cylinder head temperature ~ntl~c.!ors and turbine engine exhaust gas temperature (egt) tndicators to their r~spective thermocouple sensing elements.

The conducting materials are normally the same a s those in the tf-lerrnc,c.ouple sensnrig element, for example, iron and constantan or copper anti c-onsrantan for cylinder head thermocouples, and chrornel (an alloy of chrom~urrl anti nickel) and alumel (an alloy of aluminium and nickel) for egt therxnocoti1)les.

SMALL I j'

PROBE

ALUMEL

/\\ , / CHROMEL

- - FWD

Fig. 4 EGT LEADS - G E 9 0 ENGINE

In the case of cylinder head temperature indicating systems, only one thermocouple sensing element is used and the cables between it and a flrewall connec:tor are normally asbestos covered.

For egt measurement a number of thermocouples are required to be rad~ally disposed around the jet pipe in the gas stream. The cables are usually arranged in the form of a harness tailored to suit a specific engine installation.

The insulating material of the harness cables is either silicone rubber or PTFE impregnated fibreglass. The cables terminate a t an engine or firewall junction box from which cables extend to the flight deck indicator. The insulating material of extension cables is normally of the polyvinyl type, since they are subject to lower ambient temperatures than the engine harness.

In solrre applications extension cables are encased in silicorle paste w i t h ~ n a metal braided flexl ble conduit.

Page 302: Materials and Hardware

Co-axial Cables (Figures 5 a n d 6)

Co-axial (co-ax) cables contain two or more separate conducting eleme~its - one inner and one outer. The irirler conductor may be solid or stranded copper wire, and may be plain, tinned, silver-plated or even gold-plated in some appllcatlons, depending on the degree of conductivity required.

The outer conductor is made in the form of a circle usually of fine wire braid inslllated from and surroundirig the inner core. The insulation (dielectric) between the two is usually polyethylene or Teflon.

COPPER WIRE BRAID SCREEN

DIELECTRIC . SEPARATOR

WEATHTER PROOF OUTER INSULATION

INNER CONDUCTOR SIGNAL CARRIER

Fig. 5 CROSS SECTION OF CB-AXIAL CABLE

0 1 1 ter coverings or jackets serve to weatherproof the cables and protect them from fluids, mechanical and electrical damage. The materials used for the coverings are manufactured to suit operations under varying environmental corid~ Lions.

Co-axial cables are used for the transmission of low power signals, with the signal line (the inner conductor) protected from unwanted signals (noise) by the outer wire braid. The outer braid provides a shielded against electrostatic and magnetic fields.

Any electrostatic field does not extend passed the outer braid and the field? due to current flow in the inner and outer conductors cancel each other. Also, since co-axial cables do not radiate any fields, then likewise they will not pick up any energy, or be influenced by other strong fields.

('0-axial cables are used on radio equipment, for the connection of antennae to receivers1 transmitters, and capacitance type fuel quantity i~ldicating systems for the interconnection of talnk units to amplifiers.

The construction of a typical co-axial cable and end fittings are shown. For details of how end fittings are attached the reader is referred to the appropriate book in module 7, but in general the outer wire braid is cut back arid folded onto the inner adapter and the inner conductor is left protruding.

Page 303: Materials and Hardware

COPPER BRAID INNER ADAPTER Turned back to Slides under wire braid, up fit over inner to screw thread, and adapter \ screws into plug assembly

/

/ DIELECTRIC

INNER SEPARATOR

CONDUCTOR \ \ SOLDER HOLES

COUPLING RING

/

INNER PLUG ASSEMB1.Y

Fig. 6 CO-AX CABLE END FITTINGS

The sub-assembly is screwed to the adapter thereby clamping the outer conductor firmly between the two components.

In some cases the outer conductor may also be soldered to the sub-assembly through solder holes. Soldering a contact on to the inner conductor a ~ l d screu~ing the coupling ring on to the sub- assembly completes the assembly.

CAE31,E TYPES

The following pages give technical data on a selection of cables made by UlCC. You would not be required to remember the details but you should read and understand the information.

You should note the performance rating of the cables, the properties and the identification. You should note the current ratings and how they are affect-ed by being 'bunched' (bundled or fitted as part of a loom), and the reasons w11y.

The performance data table is given for one type of cable only -- as an exaniple. Each cable will have its own data table.

blank

Page 304: Materials and Hardware

1 lght we~ght flexible arrframe wiring cable suppl~ed in single or multrcored versions screened or sheathed to spec~ficat~on BS2G222

PVC GLASS BRAID NYLON

TINNED COPPER OR SILVER PLATED COPPER

Performance. Voltage rating 300V at 1600Hz rms (250V for size 24). Temperature range -75°C to t 105°C (-30°C for flexible installations). Properties. Resistant to abrasion, fuels, hydraulic fluids, ester based oils, de-icing fluids, fire extinguishants, cleaning solvents, fungus and mildew. Resistant to flame and readily printable for ident purposes. Identification. Supplied in reels labelled and packaged ready for transportation. Cable is printed with cable code, country of origin, manufacturer, date code, size code and specification code. Colour white. w. Tinned copper conductor range 22 to 12. Silver plated copper conductor range 24 only. Current Ratinqs. The current ratings given in table 1 are based on conductor temperature rise of 40°C iri

an ambient temperature of 65°C. If the ambient temperature (t°C) is continuously above 65°C the rat1 , must be multiplied by a factor K where:

Similar to Minyvin above but has a voltage range up to 600V, a size range from 22 to 0000 and meets the requirements of specification RSG177.

PVC GLASS BRAID

\ NYLONOR NYLON BRAID & LACQUER

EFGLAS

Similar to Nyvin except that its temperature range is -70°C to +260°C. Has nickel plated copper conductor arid is flexible throughout temperature range. Meets specifications BSG222, AIR4524 (GROUP 250.-280).

PTFEIGLASSIPTFE

NICKEL PLATED COPPER

Page 305: Materials and Hardware

MAXIMUM RATINGS FOR MiNYVlN CABLES BUNCHED IN FREE AIR

Rating conditions: A = Continuous B = 5 minute rating C; = 1 minute rating Uninyvirlal = aluminium cored.

* = May apply to a srnaller number of cables as specified,

Cable No Rating Max rating in amperes (cables in bundle)

Uninyviri Uninyvinal condition I 3 7 12

Page 306: Materials and Hardware

Slmllar to Nyvln except that ~ t s normal temperature range IS -55°C to +190"C with ar! ultlmate life of 5 rn~nutes at 1100'C for the operation of essenttal c~rcu~ts Colour orange and speclflcatlons rneets BSG189 ~nterchangeable wlth MIL VJ-5777

SILICON RUBBER

\ NlCKEL PLATED COPPER

GLASS BRAID POLYURETHANE VARNISH

FEPSIL.

Similar to Tersil. Colour green and produced to specif~cation BSG208 interchangeable with MIL-W-8777.

SILICON RUBBER FEP

. NICKEL PLATED GLASS BRAID COPPER

Similar to Fepsil with a service life of 50,000 hours at 150°C and a temperature range of -65°C to .+150aC. No smoke emission at up to 300°C. Single cable white. Multi cores have different colours.

FPA 150

/ POLYMIDE TAPE ~ ~ ~ ~ ~ ~ L A T E D POLYURETHANE lNSULATION

\ CONDUCTOR

KPA 1 50MS

FEP SILVER PLATED SHEATH COPPER SCREEN

THERMOCOUPLE CABLES

Temperature range -65°C to + 260°C. 10,000 hours life at the top temperature. Positwe, nickel chromium, with white insulation. Negative, nickel aluminium with green insulation. Sizes - 20 sheath colour green and 22 sheath colour green with white stripes.

+ve NICKEL CHROMIUM POLYMIDUPTFE

Page 307: Materials and Hardware

A crimped connection is one in which a cable corlductor is secured by compression to a termination so that the metals of both are held togetht:r In

close contact. A typical crimp termination has two principal sections, c-r-lrnping barrel and tongue, together with, in some types, a pre-~nsula ted copper sleeve which mates with the crimping barrel a t one end arltl is formed, during the crimplng process, so as to grip the cable insulatior~ at the other in ordcr to give a measure of support.

The barrel i s designed to fit closely around the cable conductor so that after pressure has been applied a large number of points of contact are made 'The pressure is applied with a hand or hydraulically operated crlmping tool fitted with a die or dies, shaped to give a particular cross-sectional form to thc. completed joint.

The precise form of the crimp is determined by such factors as the size and construction of the conductor, the materials, and the dimensions of the termination. It is, therefore, most important that only the correct type of die and crimping tool, should be used, depending on the termination and that the necessary calibration checks have been made to the tool.

Conductor protrusion ""'" (""m) \ @

Number of dots '4 / \ I / I indicate correct tool ---P CORRECT used for the job DISTANCE

INSULATION CRIMP ) / 1 1 1 c m w

Fig. 7 TYPICAL CRIMP TERMINATION

There is a considerable range of terminations (and crimping tools) available, many of which are colour coded and suitable for use only with specific types of aircraft cable/crimp ends. It is, therefore, important that the appropriate manufacturer's instructions regarding the use of cables and termin a t ' ions are fo1lowc.d.

Only aluminiuni or bimetal (AICu) terrninations should be uscd to terrriit l i l t e aluminium cables and the cable should be stripped irnmediatcly prior to

rnakiny the joint.

Page 308: Materials and Hardware

'The barrel o f some aluminlurn tc-rminatlons may contain a qtiantit-y of inhibiting compound, others not so filled require that inhibiting compound l ~ e applied before crimping takes place. Sorne specifications also req~lire additlorla1 sealing after crimping. The compound will also minirriise later oxidation of the corripleted connection by excluding moisture and air.

TOOLS

These inclttde: wise cutters, sidc cutters, cable strippers and crimping pliers/crimping tools. They all come in a variety of shapes and sizes and what follows is a general description of the AMP crimping method.

The special tool used for crimping AMP terminals has several features to ensure a good crimped joint. These include:

1. Crimp ratchet. 2. Locator. 3. Insulation adjusting pins. 4. Colour and dot coding.

The crimp ratchet is common to most crimping tools. It ensures the bottoming of the die jaws before the jaws can be opened again. This rrieans that once the crimp has been started it must be ftllly corripleted (the handles closed to their fullest extent) before the tool will release and be removed frorn the cable/crimp.

This rrleans that a crimp cannot be half completed.

The locator holds the terminal. in the correct position in the die jaws and allows the conductor strands to protrude 0.8 rnm from the terminal barrel when the wire is fully inserted. Most tools have some form of location device.

The insulation adjusting pins allow for small variations in wire size and ensures o p t i m ~ ~ m mechanical strength of the joint by crimping both the insulation and the conductor. The die head has three levels of adjustment:

1. Tight. r\ L. Medium. 3. Loose.

Colour and Dot coding. The "dot" coding system is needed to identify the terminals which have been crimped in the correct AMP hand tool. If a red terrnirial is crimped in a red handled tool, a single dot impression will be left on the insulation at the barrel end.

Page 309: Materials and Hardware

Fig. 8 AMP CRIMPING TOOL

Cable Strippers

Used to cu t the insulation away from the conductor and come in a variety of sizes and designs. These may have separate locations on the same tool for differcnt sizes of cable or there may be one cutting part that is ad.justable for a particular size of cable.

It is most important that the strippers are set-up first by using them on a spare piece of cable, adjusting the cutltechnique so as to make a clean cu t of the insulation without doing any damage to the conductor. This spare piece of cable (the same size and type as the actual cable to be worked on) can also be used for the practice crimp. After the practice crimp is completed the joint is inspected to check for security of attachment with no damage or splitting. If all is well then the settings/technique can be employed on the actual cable to be crirnpcd. Before carrying out crimping of termination, the following slrlould be verified:

(4 Correct size and type of wire for the job (AMM). (t)) Correct size and type of terminal with suitable size crimp t)ar.rel to

accommodate wires and if necessary, the insulation.

( c ) Correct crimping tool and associated dies, selected to be compatible with type of terminal and wire size.

(d) Correct tool being used. Note that the ratchet and pawl harict type tools will only release on completion of crimping c.ycle.

(eJ Correct ant-oxidant - if required

Page 310: Materials and Hardware

Preparation of Wire

1 Us~ng approved stripp~ng tool, remove specific length of insulation 2 Inspect stripped end for severed or damaged conductor strands. If

any fc)urid damaged or severed cut cable back anti start again. 3. Insert all conductor st-rands into barrel. 4 . Ensure that no insulating materials enter.

Conductor strands must be laying together to allow for 100% insertion. If the lay of the strands is disturbed they should be re-imposed with a light twisting action of the fingers. Excessive twisting should be avoided a s this increases the conductor diameter.

Preparation of Tool

1. Insert insulation adjustment pins into the N o 3 position. 2. Locate terminal in crimping jaws. 3. Insert conductor into barrel and insulation into the insulation grip

portion of the terminal. 4. Close handles until crimp ratchet releases. 5. Remove terminal and check insulation support as follows. Bend

the wire back and forth once, terminal sleeve should retain grip on wire insulation.

6. If wire pulls out set insulation adjustment pins in next tighter position (No 2).

7 . Repeat Items 3 to 5.

Crimping AMP Terminals - Example

1. Select the appropriate terminal for the size of wire being terminated and to suit the stud size of the terminal fitting.

2. Select a tool by reference to the colour of the terminal. Check wire size range stamped on tool face.

3. Inspect the tool for serviceability and adjust the insulation crimping adjustment pins.

4. Insert the terminal into the jaws so that the barrel rests against the locator.

5. Squeeze handles until terminal is lightly gripped by the jaws. 6. Insert prepared wire end into terminal barrel ensuring that all

conductor strands enter. When fully inserted the conductor should extend beyond the barrel by approximately 0.8 mm.

7 Hold wire in position and crimp by squeezing h;~ndles until ratchet releases.

8. Remove compleled crimped joint and inspect fol- dot code impression.

Page 311: Materials and Hardware

On completion of crimp check:

( i ) Correctness of form and locatiox~ of crimp. (ii) Adequate insertion of conductor strands in barrel (iii) If insulation support is provided, check correctness of Corrn and

location of insular crimp. (IV) Check any codification by crimp dies is correct in detail ant1

position. b7) Check joint for freedom from fracture, rough or sharp edges a n d

'flash'.

(vi) Carry out a millivolt drop test.

PRE-INSULATED B + 0.8mm COPPER SLEEVE

CRIMPING BARREL & TONGUE

B = Barrel length. C = Insulation grip. Stripping length = barrel length + 0.8mm

Fig. 9 TYPICAL CRIMP TERMINATIONS

Crimping In-line or Butt Splices

1 Select the required Butt Splice and a tool of the sarne colo~lr coding.

2. Adjust the insulation crimping ad~ustment pins as detailed above. 3. Insert butt splice into crimping jaws until properly located. 4. Squeeze handles until butt splice is lightly gripped. 5. Insert prepared wire into terminal barrel. When inserted the

conductors should be visible in the inspection window. 6. Hold wire in position and complete crimping operation. 7. Inspect for correct formation of completed crimp. 8. Insert other end of butt splice into jaws until properly lo(-at(-ci. 9 . Complete crimping operation by repeatirig Items 4 , 5, 6 ancl ' I . 1 0 . Carry out a millivolt drop test.

Each l~arrel of a connector must carry only one cable uriless specifically perrnit ted by the air worthiness authority.

Page 312: Materials and Hardware

In-line crimps must be fitted either horizontal or positioned so that any ingress of fluid is impossible. Protective sleeves, additional to the crimp insulation, will not be provided to prevent ingress of fluid - particularly important in exposed positions such as wheel wells.

Care must be exercised to ensure that in-line crimps are only used in positions where the operating temperatures do not exceed the specified limits. Specific approval must be obtained from the airworthiness authority before incorporating in-line crimps in the following:

(1) Screened cable. (2) Coaxial cable. (3 ) Multicore cable. (4) Cables in excess of size 10 (35 amp).

(5) Thermocouple cables. (6) High voltage cables, ie above 250V rms (eg igniter- ht leads, aerial

feeders).

(7) Cables used in fire-resistant circuits (fire detector and extingui er circuits within the protective zone).

(8) Types of cable, totally enclosed in conduits or ducts, which cannot readily be visually inspected.

Maximum allowable millivolt drop usually 5mV for 10 amps

CALIBRATED TEST LEAD

INSULATION

INSULATED SUPPORT BLOCK

Fig. 10 MILLIVOLT DROP TEST ON CRIMPED TERMINATIONS

Kepair schemes are restricted to the following:

(a) The minimum distance between joints in any one cable must be 2ft.

(b) Not more than two joints are to be made in any 1 Of t length of cable.

(c) Multiplicity of joints in cables must be avoided, if possible, and in no case must the number exceed the following:

(i) Runs up to L o f t - 3 joints.

(i i) Runs 11p to 200ft - S joints.

( ~ i i ) Kunri over- 200ft - 8 joints.

Page 313: Materials and Hardware

On 11-1st allation, wherever possible, observe the following:

(a) All joints mus t be accessible for visual inspection.

( 1 ) Joints must be positiorled so that they do not touch one another or touch duct cable-retaining straps and other fixtures which may set u p 'tracking' paths.

j c ) Joints must, if possible, be positioned on the outside of the loorns unless special fixing attachments are preferable; all fixing attachments, such as corrugated wrapping strip, must be approved.

(cl) If it is impracticable to accommodate a stagger of joints along a cable run, positive separation, eg using insulation or cable c l~ps , must be carried out.

Erma I-land-operated Hydraulic Crimping Machine

For large size cables various hydraulic crimping machines are available, described here is the Erma Crimping Machine.

This machine is supplied a s a kit containing eight sets of dies for cable size from AWG 6 to AWG 0000, and a n Allen key used for fitting the dies to the machine. The crimp formed is a regular hexagon shape and h a s two code letters impressed on it by the dies during crimping. These code letters are HG, HH - l-1N (for cable sizes AWG 6, 4 - 0000) and are the same as those marked on the cable lugs by the manufacturer.

Preparation of Machine

The machine operating handles should be screwed into position and the code letters stamped on the dies checked for size, If the dies are to be changed carry out the following procedure:

(4 Select the two matched dies bearing the correct code letters for the size of cable in use. Check that the lugs to be used have the same code letters marked on the terminal palm.

(1)) Remove the upper die adapter by sliding it from the dovetailed head of the tool. This leaves the slotted head of the tool open to allow the lower die to be fitted to the ram. Insert the spigot on the upper die into the hole in the die adapter until it is held in position by a spring-loaded steel ball.

Close the hydraulic valve by turning the knob clocktvisc. Pump the handles a few times to move the ram forwards and show the hexagon socket screws which hold the lower die. Slacken thr-se screws using the Allen key provided. Fit the lower d ~ e into the, ram so that the screws fit into the recesses on cither side of t h t r l l c h

Page 314: Materials and Hardware

Tighten the screws to hold the die, ensuring that they are below the surface of the ram body. Open the hydraulic valve to retract the ram

(4 Slide the upper die adapter, corriplete with die, illto the dovetailed grooves until i t is located centrally by a spring-loaded steel ball.

\ FIXED HANDLE

RAh HEXAGON ~ ~ Y D R A L J ~ I C CONTROL SOCKET VALVE SCREW

Fig. 11 ERMA HYDRAULIC CRIMPING MACHINE

Operation

Check that the two-letter code on the cable lugs and on both dies is correct for the size of the cable to be terminated.

(a) Close the hydraulic valve. Place the lug centrally between the ales and pump the handles until the lug is lightly gripped.

(b) Strip the cable insulation so that when it is inserted in the lug the insulation lies flush against the end of the barrel and the conductor projects slightly from the other end.

(c) Insert the conductor into the barrel of the lug arld pump the machine handle until the dies are fully closed. Clperate the handle until the safety valve operates with an audible click and pressure on the pump handle reduces.

((j Open the hydraulic valve to allow the ram to retract. The crimped termination can then be removed from the machine and inspected.

Page 315: Materials and Hardware

PLUGS A N D SOCKETS

Most wires are terminated in a pin or contact which is fitted ~ r l t n a plug /socket along with many other pins/coritacts.

On older aircraft wires/cables were soldered ~ n t o small 'cups' at the end of each pinlcontact. In rnost modern systems the method of conncc.tion 1s by crimping. After the pinlcontact is crimped onto its wire it is Insvrted in to the correct hole within the plug/socket.

To prevent damage and the entry of debris protective caps should be f~ttcd to plugs/sockets at all times when disconnect from the aircraft and no work is being carried out on them.

Care should be taken when handling and connecting miniature and sub- miniature connectors. Both plugs and sockets should be checked for any signs of dirt, bent pins or physical damage to the shells before attempting to connect. If connectors will not mate, check for the reason, and rectify or renew. On no account should force by used to effect mating. Rent pins should be removed and new ones crimped in position.

SHELL ATTACHMENT

SHFLL SHELL FLANGE \ \ SCREWTHREAD

AUG~MENT / \

ALIGNMENT / MOULDING GROOVE LUG SOCKET

CONTACTS

Fig. 12 TYPICAL PLUG & SOCKET

Lubrication

Some ranges of plugs and sockets require the engaging threads to be lubricated with a suitable lubricant to ensure that they can readily be disconncctcd.

Fitmcnt/ Removal of Wired Pins/ Contacts

Each pin/contact is crimped to its respective wire then fitted in to the plug/socket. To ensure that the wire is corirlected to the correct pin/c:oilt;~ct and the pinlcontact is fitted into the correct hole in the plug sorket tllcx followi~~g location procedure is followetl:

Page 316: Materials and Hardware

i Each wire is identified by a unique aircraft wiring diagrarr~ rium txr.

2. When fitting the crimped pin/socket it is located irito a numbered hole in accordance with the wiring diagram.

3 When the plug is screwed into the socket there is a locator lug/groove so it can only be orientated one way.

POLARISING OR PINISOCKET LOCATOR LUG/ CONTACT NUMBERS

Fig. 13 PIN/SOCKET LOCATION IDENTIFICATION

There are two basic types of pin/contact retention used in plug and socket connectors in aircraft, one where the contacts are released for removal from the rear and orie were removal is from the front. The correct insertion/removal tool is used in each case.

Front release. The contact is removed by pushing from the front of the connector and removing the contact frorn the rear.

Rear release. The extraction tool enters the connector from the rear of the connector and the contact is also removed from the rear.

Multiway connectors, termlnal junctions, inline single wire connectors, switches, motors, indicators, instruments and other electrical components may now be terminated by a rear release system.

Contacts crimped with a standard crimping tool are inserted and removed using a single fail-safe plastic toc;l for each size of contact.

INSERTION END REMOVAL END

7--- (WHITE)

--.-.-I

Fig. 14 REMOVAL AND INSERTION TOOL

Page 317: Materials and Hardware

'The Hvllermann Deu tsch 4601 450 Series Connectors, t errninal junctior~ modules and custom-made component termination modules can be used All terminations are inserted and removed by a single expendable plastic. tool which is fail-safe in tha t mis-handling will result in damage to the tool rather than to the connector or termination modules.

In figure 15 the spring clips snap in behind the shoulder of the contact '['he removal tool displaces the clips sufficiently to allow the contact to be withdrawn.

PLUG BODY

\ cY'Mp RETENTION CLlP

RETENTION CLIP /

Fig. 15 PIN RETENTION - FRONT RELEASE

RETENTION CLIP

RETENTION CLlP

Fig. 16 PIN RETENTION - REAR RELEASE

Contact Insertion - Typical Procedure

1 . Remove the backshell or other accessory from the rear of the connector and thread on to the cable loom.

Snap the coloured end of the appropriate insertion/removal tool on to the wire. When inserting the wire into the tool, use the thumb and not the thumb nail as this could damage the insulation Position the tool on the contact shoulder, except in the case of size 22 contacts, in which case the tool should be positioned on thc back of the crimp bucket.

Holding the connector with the rear insert facing you, slowly push the contact straight into the connector. A positive stop will \)e felt when the contact is locked in by the retention clip.

Page 318: Materials and Hardware

ICSEZTIBN CORRECT Too, INCORRECT

+ / b SIZE 22 ONLY

U

ALL OTHER SIZES

INCORRECT

CORRECT

Fig. 17 INSERTION OF PIN INTO SOCKET

Cont.act Removal

1. The removal procedure is virtually the reverse of the insertion procedure.

2. Holding the connector with the rear insert facing you, snap the white end of the appropriate insertion/removal tool over the wire to be removed.

3. Slowly slide the tool along the wire into the conrlector, until a positive stop is felt. The retention clip will now be unlocked.

4. Press the wire against the serrations of the central section of the tool and withdraw both wire and tool together.

A s you can see, tc) release the contact, you must put the extr-actiorr tool over the front of the contact and down between the contact arid clip to release the clip from behind the front shoulder.

Page 319: Materials and Hardware

'Thls neth hod ha s wide usage. Some of the connectors you are likely to I lsc w ~ t h thls ft,ature are Amphenol 246 and 48 serles, Bendlx FYI' SE, Cannon FKF', KPSE, Flight FH, FC Hellerman~l Deutsch SLPT, DS, Cinch C0909, P y l c National KPL/FPK, ZZ and the AMP/AM series of rack and panel conrltxc;tors

In the case of the rear release, the extraction tool enters from the rear of the connector between the contact and the clip to release the contact. The contact is then pulled out through the rear whilst still in the tool.

MECHANICAL FLEXIBLE REMOTE CONTROL SYSTEMS

CONTROL CABLES

Aircraft control cables are generally fabricated from high tensile carbon steel or corrosion-resistant steel wire and may be of either a flexible or non-flex~ble type construction.

Cables are used in tension only and used for the control of primary and secondary flying control systems, engine controls and the operation of certain valves and equipment.

AILERON \ 1 SERVO TAB

BRAN?

AUTOPILOT

TURNBUCKLES ERCONNECTING BALANCE CABLES

DISCONNECT

TURNBUCKLES

TURNBUCKL

Fig. 18 TYPICAL A I R C M m CABLE CONTROL RUM - BAe 1146

Page 320: Materials and Hardware

Where a single cable is used tht. 'return' of the system would be by the use of a spring at the oe~tput end o f t h e cable system. Where corltrol is required in both directions - as in flying control systems for example - the cable is formed Into a complete loop with pulleys or quadrants or something s~rrlrlar at both ends.

The cat~lt: itself will almost certainly not be continuous, but be connected in various lengths by turnbuckles, cable connectors, quadra ri t s etc.

Figure 18 shows a control cable run to operate the servo tabs of the BAe 146 ailerons. I t is typical in that the system uses cable tension regulators (to keep correct tensions), pulleys (to allow a change in cable direction) and seals (where the cable passes through the pressure hull).

Legal Requirements for Flying Corltrol Cables

Cables used on aileron, elevator and rudder must not be smaller than 0.125 (3.17mm) diameter. Tensions must be kept reasonably constant.

Pulleys must be fitted with guards to prevent cable displacement or fouling.

A cable must not change direction more than 3" after passing through a fairlead. Specified parts of the cable system must have access for inspection.

Advantages of Cable Systerns

The advantages of a cable system over a rod system include a weight reduction and a cost saving. Though, of course, the cable system is heavier than an electronic/light data transmission system such a s fly-by-wire.

The structure will change its length as the temperature changes and since t' coefficient of A1 alloy (a = 23 x 10-6) is nearly twice that of steel (a -15 x 10 the cable tensions will vary considerably.

QUES'I'ION What would happen to the tension of a steel cable system within an A1 alloy structure when the aircraft increases altitude -- assuming there are no automatic cable tension adjusters fitted? (5 mins)

ANSWER A s the aircraft clirribs the ambient temperature drops (down to -56°C at 36,000ft) so the structure will get shortcr-. The steel cables will also shorter1 (assuming they experience the same temperature drop, which is probably not likely) but will not shorten by the same arnount - so the cable tensions will decrease.

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'To overcome this problem either very high terlslons are used on the g r o ~ l n d when setting u p (as was the case on older aircraft) or use is madc of automatic cable tension regulators. Most aircraft now use Cable Tension Regulators whic.11 provide a nearly constant tension at all times a t all altitudes. This means lower rigged tensions on the ground with a saving in weight and wear

Components in the system would include

Push/ pull rods. Pulleys. Automatic cable tensioning devices. Fairleads. Seals - for pressure cabins. Cable adjusters - turnbuckles. Cable connectors. Chains and sprockets. Torque tubes. Bellcrank levers.

A s well as input levers/mechanisms and output devices to include levers, valves. powered flying control units etc.

D NON-FLEXIBLE 1 x 7 DIAMETER

NON-FLEXIBLE I X I 9

STRAND

FLEX'BLE DIAMETER EXTRA-FLEXIBLE

7 x 7

Fig. 19 TYPICAL CABLE CONFIGURATIONS

PITCH -

CABLE - STRAND WIRE

Fig. 20 CABLE MAKE-UP

Page 322: Materials and Hardware

'I'erms used a~rcl-aft control cables (figllrc 19).

Wire. A h ~ g h tens~le steel small diameter- (about I rnm or less) wire

K~ng wire. The centre wlre in a strand around which all the other wires are laid.

Strand. A group of wires twisted together helically forms a strand. Several --

strands are laid together to make u p a complete ~:able.

Core strand. The central strand of a cable around which the remaining strands are helically wound.

Preformed cable. A cable in which the wires and strands are shaped prior to -- wound onto the complete cable. These tend to unravel less tllan un-preformed cable whcn cut. Un-preformed cable will unravel quickly when cut (using bolt cutters) so the cable is bound with cord either side of the cut prior to cutting.

Cable diameter. The diameter of the cable measured across its greatest thickness.

Lay or Twist. The helical form taken by the wires and strands in the cable. A cable is said to have a right-hand lay if the wires and strands twist in the same direction as on a right-handed screw thread (most screw threads). If twisted the other way it is said to have a left- handed lay.

Pitch. The axial distance a strand or wire travels in one complete twist about the axis of the cable or strand respectivel.y. Similar to pitch on a screw thread.

Cable Specifications

Cable sizes and strength data are given in tables with sizes ranging from 1 / 3 2 n d of a n inch (0.03 1 in) (0.79mm) to 1/2 a n inch (0.5in) (12.7mm) diamt r.

Tables 1, 2 and 3 give some examples. Most cables used on British built aircraft conform to British Standards BS W9, W 1 1, W 12, and W 13 or American specification MIL,-W-83420.

Non preformed cables are used, but on systems that not so important. Preformed cables have the following advantages:

* More flexible. x Easier to hand splice. J; Resistant to kinking. * Does not unravel when cut - for splicing etc. A Strands, when broken, tend to lie flal and not stick up - this may

cause jamming of the controls when passing through fairleads and around pulleys and is also a hazard to personnel.

Page 323: Materials and Hardware

The construction of the cable is deterxriined by the number of wires n7hlc\h go to make up each strand and the number of strands that go to make up the, cable. For cxample, a cable designated 7 x 19 is made u p of 7 strands each having 19 wires For the more common types of cable (those a t the bottorn of figure 19) each wire is laid around a king wlre in layers and the strands are lad ;irc~und a core strand.

- l x 7 a n d 1 x 1 9 -. -- - - -- - - 7 x 7, 7 x 19 and 6 x 19 1_1WK? on-fexlble,--7l Corrosion Flexible, -lFlexihg, c o ~ rosion

-- - ~~ c'arbon resist= Carbon MIL - W - 6940

Lbs 100 ft 100 ft 100 ft

54 10.20 L__-_- _ 6,300 10.20 1 6,300 / 8.60 / 5,000

Nite 100 ft l= 30.49m-llb force = 4.448N 1 inch =

resist lng . . - / MIL - (: - 5424

"pZt I Breaking Strength

I per 1 I,G

TABLE 1 MIL SPEC CABLE DATA - A SELECTION

Pre-formed cable is usually made of galvanised carbon steel (BS W 9 , W 12 and MIL-W--83420 composition A), or corrosion resistant steel (BS W 1 1, W 1 3 and M I L W-83420 composition B) and is impregnated with lubricant during m:inufacture (reduces internal friction and wear). MIL specifications also exist that provide for a series of nylon covered cables.

Check the Aircraft Maintenance Manual (AMM) and the Illustrated Parts Catalogue (IPC) for your aircraft for the actual cable used for any particular system.

Minimum breaking Load fcwt force)

Construction --. Diarrie BS W9

~ ~

0.065 0.08 0.12

7 x 19 0.15 7 x 19 0 . 1 6 7 x 19 0.18 7 x 19 0.2 1 7 x 19 --

. -.

Note 1 cwt force = 498.176N

er (in) BSW11 - - -- - ---

0.065 0.08 0.12 0.15 0.16 0 18

TABLE 2 BS CABLE DATA - A SELECTION

Page 324: Materials and Hardware

Nominal Diameter

Construction I Minirnum breaking load I

-. - . ---

. . . . .. . .. .. . . . .. ... - Carbon steel 7 - E ~ w - 8 3 4 2 0 (A)~

BS W12 lbs

-. -- --- - - - -- 7 7 I 480 7 x 7 920 7 x 19 2000

4200 1 7000 7 x 19 . --

CR steel MIL-W-84320 (B)

lbs

TABLE 3 CABLE CLASSIFICATION BY DIAMETER - A SELECTION

On some aircraft, control cables are enclosed in an aluminium tube or cladd' -g and are called Lockclad cables. Used for straight runs and supplied a s a complete assembly from the manufacturer.

Cable End Fittings

'To allow the cable to be attached to a component the end of the cable terminates in either a spliced end fitting, a sleeved crimped end fitting, or a swaged end fitting.

A spliced eiicl fitting normally takes the form of the cable being placed around a thimble or similar fitting and the individual strands of the cable woven or spliced back through the cable (see module 7 for more details).

OVAL SLEEVE CRIMPED

CABLE TENSION REGULATOR

SPLICED CABLE

SWAGED END AROUND A BOBlN

INSPECTION HOLE

SPLIT PIN HOLE

Fig. 2 f CABLE END FITTINGS

Page 325: Materials and Hardware

A s tlle individual s trands are spliced back through the cable the nurnbcr of full tucks are reduced after the third and forth row of tucks to gradually retluce the diarntlter of the splice. The total number of tucks is 5.

Splicing is a very difficult task and should only be undertaken by propvrlv trained and skilled personnel. This is not allowed to be carried out at user unit level for some aircraft systems and the person qualified to carry out the control cable splicing should submit test pieces for destructive testing at regular intervals.

With :I crimped joint, a sleeve or ferrule is put on thc cable and the cable is placecl around a thimble and back into the slecve. A special crimping or swaging tool is used to crimp (compress) the sleeve to hold the two parts of the cable securely. The sleeve has to be a special size (as does the thimble) to match the cable and the finished crimp is checked with a GO/NOT GO gauge.

Is a rrquirement that this joining method is not used on certain control systems on aircraft.

Swaging is carried out by placing the cable in the end fitting and the end fitting squeezed (swaged) in a special tool using special dies. It is similar to electrical cable crimping but it may take several swaging operations to complete the job with the finished swage being checked with a GO/NOT GO gauge.

SYSTEM COMPONENTS

Control Stops

Usually adjustable by the engineer to obtain the correct range of movement and may be fitted to both ends of the control system run. Secondary control stops are fitted a t the cockpit or flight deck end of the system while primary control stops are fitted a t the output end -- the flying control surface for a flying control system.

JLL ROD

PIVOT

BELLCRANK

ADJUSTABLE STOPS

CABLE

Fig. 22 CONTROL STOPS

Page 326: Materials and Hardware

On some aircraft the range of rnovernent of the system is changed autoniatically during flight (sorne flying control systems for example - as thy aircraft 's speed changes so the range changes - fitted to the BAe 146 for example to reduce the range of movement of the control surface a s the aircraft speed in(-reases)

Stops will be fitted so a s to control the range of movement of a component SU(:~I

a s a bell crank lever - and hence the range of movement of the whole system. They \will have provision for lockilzg once adjustment is completed. Locking can be by locknut, locking wire, locking plate, split pin/cotter pin etc.

PUSH PULL ROD /

Fig. 23 CHAIN & SPROCKET DETAIL

Chains and Sprockets

Cables may go around a pulley or be connected to pulley end fittings. The cable may terminate a t a chain fitting - usually swaged into a turnbuckle type end fitting - and t.he chain passed around a sprocket. This provides a positive d e to the sprocket.

Chains may be of the 'non-reversible type', which means that they are so designed that they cannot be put on the sprocket the wrong way round.

Cable Support.

Cables can be supported by pulleys and special quadrants where they can change - angular direction. Where little 01- no change in direction is required various types of fairleads are used.

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AIRCRAFT ST STRUCTURE

UBBING STRIP

SPLIT FAIRLEAD SLJPPORT CLIP

Fig. 24 FAIRLEADS

Fairleads

Fairleads (figure 24) are usually made of composite material and must not be lubricated - unless, of course the AMM says otherwise. They may be split which aids replacement without disconnecting the cable. The split fairlead shown in the bottom right hand corner of figure 24 is fitted in two halves and movecl fonvard into its support clip. The supporting clip is held in place by a brackct attached to the structure.

Pulleys (figure 25)

Made from composite, plastic or metal and are used to support the cable and also to give a change of direction to the cable run. Guard pins are fitted to retain the cable on the pulley should tensions become too low (accidentally) and some pulleys have debris guards to keep out unwanted small items which might foul the pulley/ cable.

Quadrants

Not too unlike pulleys in that they support the cable in groves, however the cable run usually terminates at a quadrant. Can bc used to support a cahle run and join one cable to another (figure 26 left-hand picture) or to transfer cable movement to push/pull rod movernent (figure 26 right-hand plc t~rrc) .

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Turn buckles

These vary in des~gn (figures 27, 28 and 29), but in general may be of the Barrel type or thr Trrlsion Rod type. They all have a left-hand thread at one end and a right hand thread at the other to allow tension adjustment.

When the centre part is rotated - holding the two cable ends to prevent them rotating - both threads will screw in or out depending on which way the centre part is ro ta t rd Thr cable tension will then either increase or decrease. i t is important that, after adjustment and prior to wire locking, that the threads are in safely (enough are engaged to ensure that they are strong enough to take the tension loads).

SUPPORT BRACKET

BOLT

Fig. 25 PULLEYS

For the barrel type turnbuckle safety means that. all the threads must be buried in the barrel.

QUADRANT QUADRANT

CABLE 2

TORQUE TUBE

Fig. 26 CABLE QUAISRBPJTS

Page 329: Materials and Hardware

For the tension rod type the threads must be screwed deep enough i r ~ r o the fork rnds so that a piece of locking wire will not pass through thr insptl(.tirin hole The wire should be the same size a s the inspection hole and shoul(t not come out the other side.

LOCKING WlRE \

BARREL SHACKLE PIN FOR \ THlMBLElBOBBlN

\ \ ATTACH ME NT 1,

FORK END LH THREAD RH THREAD

Fig. 27 BARREL TYPE TURNBUCKLE

To fit the locking clips to the locking clip barrel type turnbuckle first ensure that the threads are in safety and align the indicator notch with the barrel grove. Each clip is then fed into its locking groove with the other end snapped into place in the centre hole in the barrel. To remove the clip it is first cut using a pair of wire cutters.

Rernernber to discard the old clips safely in the metal recycle bin.

SWAGED LOCKING CUT HERE TO REMOVE CABLE END GFKlOVE LOCKING CLIP

RH THREAD

GROOVE INDICATES LH THREAD

LOCKING CLlP SLOT INDICATOR NOTCH INSPECTION HOLE

Fig. 28 BARREL TYPE TURNBUCKLE - LOCKING CLIP TYPE

FORK END RH THREAD LH THREAD

\ \ TENS'0N 1 /

INSPECTION HOLE \ NUT LOCKING WlRE

Fig. 29 TENSION ROD TYPE TURNBUCKLE

Page 330: Materials and Hardware

Cable Connectors

'Thesv are fitted to some cable systems at positions where the cables need to bfa disc-onnected frequently for maintenance pixrposes

Each half of the connector may be keyed in such a way that it can only be fitted hack to its mating half (Murphy proof) and is used where several cables run close together and all wlth connectors a t the same airfra~ne location. Each keyed pair are unique a t that location.

The connectors allow for quick cable disconnect and re-connect without the possibility o f connecting two wrong cables together. They usually do not provide for any cable terlsioxl adjustment - but some do.

The two halves may be locked together using a circlip like device with a lock pin pushed through and secured with locking wire. On other systems the two- keyed halves are held together using a sleeve, which is placed onto the cablc before the two-keyed ends are joined then slid into place. It is held in plact y locking clips.

OCKCLAD FEMALE QUICK DISCONNECT TERMINAL

LOCKCLAD OCKCLAD MALE QUICK DISCONNECT TERMINAL

CABLE QUICK DISCONNECT PIN ASSEMBLY

/ LOCK-WIRE

ASSEMBLED CABLE CONNECTION

Fig. 30 NOM-KEYED CABLE CONNECTOR

Page 331: Materials and Hardware

ADJUSTABLE KEYED CONNECTOR

Fig. 31 KEYED CABLE CONNECTORS

Cable Tension Regulators

'The majority of modern aircraft use cable-operated systems for their flying controls. This is due, in a large part, to the development of efficient Cable Tension Regulators.

Fig. 32 QUADRANT TYPE CABLE TENSION REGULATOR

Page 332: Materials and Hardware

Cable tension regulators are mechanical devlces and can be rnade 111 rnany configuratior~s, for exaniple, quadrants, beli crank levers, pulleys etc. Some systems simply have a spring loaded pulley to maintain tension, but for clescript~ve purposes we wlll consider the quadrant type cable tension regulator

The unit conslsts of a pair of spring-loaded quadrants with a pointer scale for recording the cable tensions. The swaged ends of the cable are inserted through slots in the recessed ends of the V grooved quadrants and the cable ends are secured at the cable anchorages.

When the cables are tightened equally (as with the fuselage getting longer as the aircraft descends) the quadrants rotatc about the centre shaft and the linlts pull the cross-head frecly along the locking shaft, compressillg the springs and, in effect tensioning the cables.

The springs react against the cross-head and when the cables slacken (with 1

increase in altitude), push the cross-head back along the shaft, thus tlghtt ng the cables and maintaining them a t the correct tension.

TENSIONER SPRINGS

\ TO PFCU

CABLES SLACKEN EQUALLY Fuselage temperature reducing No pilot input Crosshead moves freely with springs maintaining cable tension

QUADRANTS

-CABLES TIGHTEN EQUALLY Fuselage temperature increasing No pilot input Crosshead moves freely with springs maintaining cable tension

----a CONTROL INPUT APPLIED Pilot input (at any time) Crosshead locks on locking shaft Quadrants lock and unit behaves as a pulley

Fig. 33 CABLE TENSION REGULATOR - OPERATION

Page 333: Materials and Hardware

Whelr a control load is applied by the pilot only one ql~adrant will tend to rriove (the orie on the tension side). The link will tend to xrlove, tilting the cross-head on its locking shaft (by a very small amount) and locklng it to the shaft, preventing movement of one quadrant relative to the other with the whole systern now acts as a pulley.

Both quadrants are, therefore, locked together and operate as a solid pulley until the control load is released.

Each tension regulator incorporates a scale and pointer, which provides a visual tension indication of the cable tension. When rigging a regulated cable system therefore, a tensiometer is not required, the cables being tensioned until the correct reading is obtained on the regulator scale. The correct reading depends on the ambient temperature and must be obtained from a special graph providetl for each regulator in the aircraft.

Pressure Bulkheads

On pressurised aircraft where cable control runs pass through the pressure bulkhead special seals are provided to help minimise pressure loss. They must allow freedom of cable movement, be self-aligning, require little or no maintenance and provide a good air seal.

Several types are available - 2 are shown here.

BELLOWS SECURED TO BUKHEAD

BELLOWS

CABLE

\

AIR TIGHT JOINT PRESSURE BULKHEAD

Fig. 34 BELLQWS TYPE SEAL

Figurc 34 shows a self-aligning seal Made of an elastorrleric material, part of which moves with the control cable. 'This arrangement is used wit 11 control systerlls using twin cables (one up and one down). As the cabin pressure acting on thc bellows causes a load on the c:ontrol cable, which must he balancc~d by :in t:c]iial and opposite load - on the other cable. 'rends to increase the s in t~c frictio~l in the system, bu t is self-aligning and provides a complete seal.

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Fig. 35 GLAND TYPE SEAL

Alternative methods of sealing include friction type seals. Several types are available with some relying on packing rings of silicon rubber composite or similar to provide the airtight joint. Others use an elastorneric material suc a s that shown in figure 35. Remember, they should be kept clean and not lubricated. This is a more popular type of seal, though they do allow some air seepage and wear is a problem. Also they are not self-aligning.

SPECIALISED MECHANICAL REMOTE CONTROL SYSTEMS

These employ a push/pull or pull only cable type system, which is housed within a conduit (sleeve), which may be rigid or flexible. The 'cable' is of a special design. The controls are usually manually operated to allow flight crew/pilot to operate such remote services as:

* Trim tabs. * Fuel cocks. -k Brake control valves. -k Engine controls. k Cabin air durnp valves. -h Flying control trirn systems. k Selector valves for hydraulic, pneurnatic and emergency services.

They can bc used by ground crew for the operation of remote valves such a s toilet drain valves etc, which are operated from outside the aircraft.

They may also be used to give a n indication of the position o f the landing gear (back u p system on some aircraft) and the position of flaps - though usually on older/ smaller aircraft.

Page 335: Materials and Hardware

Types of Cable Systerns

(:ABLE SYSTEMS _. -- - _---- ---

-1

,/-/-- -----I-- TENSJON ONLY 'I'ENSION it. (:OMPRESSION

/ WITHOU'T CONDUIT Example - flying control systelns. If direction change is required a pulley, quatirant, or similar is used. T~vo cahles are used to glve pull 111 both directions.

CURVEU \ \

WI'TtI CONIIUIT Usually a flexible coiiduit n ~ ~ t h a single cable and return being achieved by the use of a return spring. Example parking brakc lever

RICIDJSEMI RIGlU, CON1)UIT Will take both a push and pull input For tllc operation of such things a s control valves etc no spring return 1s provlcled

The above shows the general categorisation of flexible control systems, but there may be variations that do not fit exactly into the general scheme a s shown..

The a ~ n o u n t of load the system is designed to take will determine what type of system is used. Trim tabs usually use a light weight high tensile steel cable and pulley system with chains and sprockets, push/pull rods, torque tubes etc. Flying control systems have similar cables and components though they are normally designed to take heavier loads.

Fuel cocks, brake and engine controls may use Teleflex and Bowden type controls. Cabin pressure dump valves may use a system not too unlike the trim tab system. Teleflex and Bowden systems are of the cablelconduit type where the cable moves back and forth within a tube-like structure called a conduit. The cables are usually of high tensile steel while the conduit may be made of aluminium alloy, steel, copper alloy or even a polymer material.

The cable is fitted a t both ends to suitable end fittings and comparatively light loads can be transmitted by the cable - enough to operate selector valves for example. The conduit i s also attached a t both ends to prevent i t from moving and to allow for the correct operation of the system.

THE 'TELEFLEX CONTROL SYSTEM

This uses a lightly loaded cable system moving inside a fixed rigid corltlilit that will transmit both a tensile (pull) load and a corripressjve (push) load. 'I'fiis

means, for example, that a lever in the flight deck can be used to inptlt ; I load in either direction to operate a remote device such as a l-lydraulic- selectur valve, engine throttle etc.

Page 336: Materials and Hardware

There 1s no s p r ~ n g return : d s 111 t h e case of Bou7den Controls for example The systrrn uses wheel unrts where the helix winding of the cable engages with a toothed wheel and a s the cable moves back and forth so the wheel is rotated Rotation is limited by the arnoulit of linear travel of the cable, which is up to about 4 in<-hes (1 02rnm).

Sliding end fittings (with a swivel joint) rnay be used in place of a wheel unit where a linrar movement is required.

The conduit must be supported a t regular intervals and may have quick release break units fitted for ease of dismantling.

Figure 36 shows a system set up with a s many comporlents as possible a s a demonstration of what the system can so.

The c;ontrol cable starts a t the single entry unit and is continuous to the 180" unit where it will rnove in and out of the spent travel tube. Each of the whec units (single entry, straight lead, junction box, 90" double entry and 180" L It) house a toothed wheel which engages with the helix winding of the cable.

From the junction box a second cable engages with the toothed wheel to transmit the movement to the sliding end fitting.

90°DOUBLE JUNCTION BOX ENTRY UNIT

ROTARY MOVEMENT NOT EXCEEDING 90'

CLAMP BLOCK

NIPPLE TYPE CONNECTOR

FITTING

180° DOUBLE SPENT TRAVEL BREAK CONNECTOR ENTRY UNIT TUBE

Fig. 36 GENERAL LAYOUT O F A TELEFLEX CONTROL SYSTEM & COMPONENTS

Page 337: Materials and Hardware

'I'lrest. may be of various designs but shown I n figure 3'7 is a number 2 arid a rrurnber 380 type cable (See manufacturer's Irterature fbr further types! They havt: helix windings of opposite hand, are not interchangeable, each hav~ng (he11 own fittings.

COMPRESSION SPACER WINDING WINDINGS

\ /

/

TENSION WlRE 'HELIX

pTEK3-1 WINDING

SPACER WINDING t HELIX WINDING

TENSION WlRE

Fig. 37 TYPES QF CABLE

The cable will take reasonably light tensile and compressive loads with the core cable taking the tensile load and the compression windings taking the compressive load (the type 2 suitable for higher compressive loads). The helix winding is designed to be threaded into a n end fitting.

Made of aluminium alloy, steel or tungum (a copper alloy). The conduit should be supported every 3ft (0.9m) but clamp supports should not be fitted where the conduit curves.

Clarrlp Blocks

Fitted on straight sections to support the conduit.

Connectors

Uscd to connect one section of conduit to another. There are several tyjxs:

k Nipple type -- similar to flare-end hydraulic pige-line cormect ~ o n s but without the olive. Clamp type - this clamps the two conciuits together a s a butt joint.

Page 338: Materials and Hardware

* Quick break tvpe -- these allow for the disconnection of the system for cornponerlt rernoval etc and the re-assembly of the joint without having to set--11p the system again. The cable joining fittings consist of mac;t-lined rods with interlocking slotted ends attached Lo the end of each c-able.

Wheel Units

'I'hese consist of a hous~ng in which a 'threaded' wheel engages with the helix winding of the cable. They allow for conversion of linear movt:rnent to rotary ~novement and vice-versa.

7'hel-e are several types inclutling the:

k Single entry type. * Straight lead type. * Junction box type. * 90" and 180" types.

The cable enterslleaves the unit via a conduit connector and in the case of the single entry unit the cable must have a minimum engagement (at its extreme end of travel) as laid down by the equipment manufacturer/AMM.

TEETH -rc4

CABLE. 11 &?

ANGLE

Fig. 38 SINGLE ENTRY WHEEL UNIT

Sliding End Fittings

These are used where the linear movement of the cable is not converted to rotary movement. A sliding end fitting is attached after a swivel joint and the assernbly is used to move levers etc.

Page 339: Materials and Hardware

End Fittings

Fitted to the end of the push/pull rod, which is connected to the lever .lrm of a slidin? end fitting or to a n arm f1ttt.d to the rotating shaft of a control utilt.

Some push/pull rods will have an end fitting a t both ends. They are adjustable for lcrlgth and have ball-end or ball and socket connect~ons

INSPECTION HOLE -

\ SCREWED END SCREWED END I3AL.L END

FITTING SOCKET END FITTING

Fig. 39 END FITTINGS

When adjustment is required it is important that the correct range of move~nent is achieved and that the fitting is in safety (checked by not beillg able to pass a piece of wire the same diameter a s the hole through the inspection hole). The unit should be locked after final adjustment either using the lock-nut, or a tab washer, or locking wire (as per the AMM of course).

Figurt: 40 shows how the cable is screwed into a screwed-end fitting, which is also screwed into the outer sleeve locking the slider tube, cable and complete end fitting together. When the cable is caused to move it will move slider tube and end-fitting together. Note - the slider tube is passed through the o~r ter sleeve and over the conduit first with the belled end resting inside the taper of the o~ lter sleeve.

INSPECTION SCREWED END HO

MINIMUM THREAD SPLIT FOR CABLE TAPERED

END Cable screwed into screwed end to minimum depth at inspection hole.

THREAD TO SUIT Outer sleeve locks slider tube and cable

THREAD IN FITTING as it is tighted onto tapered split end.

\ Lock-nut tightened onto outer sleeve.

Fig. $0 CONNECTION OF GABLE TO END FITTING

Page 340: Materials and Hardware

Split C:ollvt Type End Fitting

These are fitted direct to the cable for the operatio11 of slidlng erid fittings.

THE BOWDEN CABLE CON'TMOL SYSTEM

T l ~ e system is used for lightly loaded controls (selector valve operation, parking brake operating cable etc) and relies on the cable working in tension only, with return being by a spring usually fitted a t the componerit end.

The flexible conduit is fixed at both ends, which means that the cable system can be routed around bends (so long as they are not too sharp).

Cable

Made of non-corrodible high tensile steel wire not too unlike cables fitted t~ flying control systems - though much smaller.

Conduit

The conduit consists of a close coiled wire designed to keep the cable system stiff and takes mainly compressive loads. This is covered with cotton braiding followed by a waterproof polymer coating. To give support a t the ends and to prevent fraying, metal end -caps are fitted. On some installations rigid metal conduit is used on straight runs.

STEEL WlRE FLEXIBLE CONDUIT

/ SWAGED END FITTING

/ BALL END FITTING

Fig. 41 BOWDEN CABLE - GENERAL ARRANGEMENT

METAL END CAP \

MATERIAL BRAIDING \

COILED COMPRESSION WlRE

\ WATER PROOFING

Fig. 42 BOWDEN CABLE - END DETAIL

Page 341: Materials and Hardware

These may be various types of soldered nipples or swaged end fitt~rigs 'The swaged end fittings may be threaded, eye end or any design su~tnble f o r the conlpor~ent to which ~t is to be attached.

Nipples are made of brass and soldered onto the cable end. To fit therrl t h e conduit and cable is made u p to the correct length (the cable end is tinned to prevent unravelling) and the metal end-caps are fitted over the cable :in(j onto the conduit.

'The nlpple recess is tinned, the cable is thcn passed through the nipple so that the end shows level with the top surface of the recessed end of the nipplc. The strands of the cable are then unravelled as far a s possible within the r-ccess and the recess filled with molten solder. When the solder hardens the nipple is firmly attached to the cable.

SPHERICAL TYPE TRUNNION TYPE PLAIN TYPE

Fig. 43 NIPPLES

In some cases the cable may be swaged into the nipple using a special nipple and swaging machine.

End Fjttings

These are usually levers and handles. They may be fitted with adjustable stops so that the range of movement can be set to those specified in the AMM. To fit the cable to a n end fitting the AMM must be consulted, but in general terms the following applies to systems that employ nipple type connections to Imth ends:

Adjust both end fittings to glve the greatest range of moven~t>nt to each.

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O n those conduits that are adjustable for length, adjust thlern to their shortest length. (Some conduits have a turnbuckle type adjuster part way down thelr length, which wlll adjust the length of the conduit but not the cable The cable passes straight through the adjuster). T h ~ s means that there is more slack in the system in this condition than wotlld otherwise be the case. I t will allow easier fitting of the nipples. Align the cable so that the nipple will pass into the fitting hole and the cable will pass through the cable slot (cable rotated to 90" to its normal position). Move the control cable through 90" so that the control cable is now laylng in its correct orientation with the metal end fitting of the conduit resting on the fixed part of the end fitting. Carry out the same procedure at the other end of the systeln. This may require a higher level of motor skills because there is less slack in the cable system because the other end has taken up some of the free play between the cable and the conduit. Adjust the conduit length adjuster to take u p the slack in the conduit, which means increasing its length. Make sure the adjuster is in safety and correctly locked. Ensure that both conduit metal end-caps are firmly in place at their respective ends - input end and component end. Check for correct sense of movement, eg if it is a throttle system, pushing the throttle forward increases engine power. Adjust the stops at the input end and the component end to give the correct range of movement (check the AMM). It is usual to adjust the stops at the input end so that they control the range of movement - but check the AMM. Check for free movement. Check the lay of the cable assembly. Ensure all adjusters are in safety and correctly locked. Carry out a full functional check. Record all the work done and sign.

Figure 44 shows a typical use of a Rowden control. The nipple is firmly located in its recess in the brake handle and the conduit is firmly located in the adjustable end fitting. When the brake lever is pulled it will pivot and pull on the Rowden corltrol cable. This will give a pull output a t the other end to operate a brake lever on the brake control valve. When the lever is released a return spring at the brake control valve end will pull the cable to release the brakes arid return the hand brake lever to the upright position.

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PARKING LEVER

With plenty o f slack in the cable the nipple is fed into the hole in the lever with the cable passing through the slot. The nipple is then rotated t o the vertical through a vertical slot to lay as shown. The end fitting is adjusted to remove all slack from the cable.

PARKING BRAKE LEVER CATCH

CONTROL COLUMN

ADJUSTABLE

METAL END C

END

:AP

Fig. 44 BOWDEN CABLE CONNECTION TO PARKING BRAKE LEVER

FLEXRALL CONTROLS

A flexible control system fitted to some aircraft to provide control to take light tensile and compressive loads. Example - the tail rotor control system of the Eurocopter EC135 were it is used to transmit the control inputs from the pilot's yaw pedals to the yaw actuator a t the tail rotor.

The system is made u p of two outer stainless steel rails and stainless steel balls located either side of a stainless steel centre rail. The balls are spaced a t intervals and located within a stainless steel cage (can be YTFE). The centre rail slides back and forth between the balls to transmit both tensile and compressive loads.

PROTECTIVE

OUTER RAIL

Fig. 45 CROSS SECTION OF FLEXBALL SYSTEM

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- /

PVC COVER

Fig. 46 FLESBALL SYSTEM

'I'he assembly is housed in a semi flexible steel casing weather proofed by an outer PVC protective cover.

Lengths supplied up to 6Sft (1 9.81~1) long and 5 different sizes.

Moving end fittings are directly at.tached to the centre rail whilst the outer case is attached to the non-moving part of the component.

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Revision Questions

1. Exhaust systenrs are usually made from stainless steel which is susceptible to a) filiform corrosion b) intergranular corrosion C) surface corrosion

2. Fabric seams a re preferable a) spanwise to the line of flight b) parallel to the line of flight c) oblique to the line of flight

3. Drive planes on an epicyclic gear are a) at different angles to the plane b) at right angles to the plane c) around a common axis of the plane

4. 'The doubler used to support a scarfed patch plywood repair should be made from plywood of a minimum

a) 318 inch thick b) 114 inch thick c) 118 inch thick

5. Oxide on exposed silver plated wires is a) non corrosive b) an insulator c) a conductor

6. A clevis bolt in a control cable fork end would be loaded in a) tension b) both tension and shear c) shear

7. To check the interior of tubular members for corrosion attack

a) dye penetrant testing should be used b) ultra sonic testing is necessary c) any form of test is acceptable

8. Nickel coated cables temperature range is

a) 150 to 200°C b) 100 to 150°C c) 200 to 250°C

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9. Phosphating of steels is carried out by immersing the steel in ta a solution of a) nitric acid and sulphur

b) phosphoric acid and metal phosphates c) metal phosphates and sulphuric acid

10. Monel metal consists of approximately a) 66% Nickel and 33% Copper

b) 66% Copper and 33% Nickel

c) 66% Chroiniun~ and 33% Copper

1 I. Exfoliation corrosion is sometimes referred to as a) sub-surface corrosion b) filifonn corrosion c) layer corrosion

12. 'Turnbuckles are correctly fitted when a) both rods are seen to touch in the ~nspection hole

b) both rods enter the barrel by the same amount c) the inspection hole is blind or the required number of threads are showing

13. 'The length and time that a catalyzed resin will remain in a workable state i s called the

a) service life b) shelf life c) pot life

14. Fatigue failure may be defined as a) reduction in strength due to alternating loads

b) failure caused by stress in excess of the material U.T.S c) failure due to impact

15. Fabrics may be fitted to airframe structures by a) wood nails b) always riveting c) tying on with string

16. In the galvanic series, the most noble metal mill, if joined to another metal a) always be at the top of the table

b) allow the less noblc metal to corrode first

c ) corrode before the less noblc metal

17. The British system of heat treatment codes is a) a series of numbers b) numbers and letters c) a series if letters

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I$ . In an autoclave, to apply pressure

a) a vacuum bag is used b) weights are used c) clamps are used

19,A non-electrolytic chemical treatment for aluminium alloys to increase corrosiorr resistance and paint bonding qualities is called

a) alodizing b) anodizing c) dichromating

20. \Vhat is an advantage of a double flare on aluminium tubing? a) Ease of construction

b) It is less resistant to the shearing effect of torque c) It is more resistant to the shearing effect of torque

21. A washer having both twisted teeth and spring actions is a) AN936 shake-proof lock washer b) AN935 split-ring lock washer c) AN970 large-area flat washer

22. Composite inspections by means of acoustic emissions monitoring

a) pick up the 'noise' of corrosion or other deterioration taking place

b) analyse ultrasonic signals transmitted into the parts being inspected

c) create sonogram pictures af the areas being inspected

23, The markings on the head of a Dzus fastener identify the a) body diameter, type of head and length of the fastener

b) body type, head diameter and type of material

c) manufacturer and type of material

24- Bubbles are removed from a wet composite lay-up by a) application of pressure b) application of vacuum c) use of a roller

25. A splayed patch repair may be used on plywood damage which does not exceed

a) 15 times the skin thickness

b) 20 times the skin thickness c) 10 times the skin thickness

26. In an airtoclave what pressure would the vacuum alarm be set at? a) Operating pressure b) Lower than operating pressure c) Higher than operating pressure

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27. Arrstenitic stainless steels are a) magnet~c b) non-magnetic c) hardened by heat treatment

28. Adapter nipples are not required on 3) pipe to internally coned adapter

b) pipe to extenlally coned adapter c) pipe to pipe coupling

29. Which of these materials is the most cathodic? 3) Zinc b) 2024 aluminium alloy C) Stainless steel

30, What type of material would hydraulic pipes on an undercarriage leg or bay be n ~ a d e from?

a) 7075, H14 b) Stainless steel, annealed, 14H c) 1 100, 2024, in half hard state

3 1. Direct removal connector pins are fitted from the rear a) and removed from the rear

R) are fitted from the front but removed from the rear c) and removed from the front

32. If a material is found to be in the tertiary phase of creep the following procedure should be implemented:

a) The crack should be stop drill, condition monitoring should be applied

b) The component should under go dye penetrant process and condition monitored

C) The component should be replaced immediately

33. Cast iron is a) heavy and brittle b) very malleable c) tough

34. What is used for marking out steels? a) Copper sulphate b) Engineers blue c) Wax crayon

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35. Cobalt steel tested on the Brinell test would have a BHN number between a) 300 to 400 b) l00to 175 c) 600 to 700

36. What type of test involves using a weighted pendulum to strike a material until fracture?

a) Fatigue Testing b) Impact Resistance Test c) Hardness Test

37. The dope applied to an aircraft's fabric covering causes shrinkage a) on the first coat only b) on all coats c) on the last coat only

38. Which can be re-used'?

a) Shake proof washer, spring washer, locking plate

b) Tab washer, circlip, locking plate c) Locking plate, circlip, spring washer

39. Cable current ratings are based on a conductor temperature rise of 40°C and if the milximum design ambient temperature is continuously exceeded they should be

a) divide by the 'M' factor

b) ~nultiplied by the 'K' factor. c) halved

40. Tempering of hardened steel is carried out to a) retain core hardness, but soften the surface b) significantly reduce the brittleness without suffering a major drop in its

strength C) retain surface hardness, but soften the core

41. The pitch of a screw thread is a) 2 x crest to root b) crest to root c) crest to crest

42. Elow is the diameter of a cable measured? a) Diameter of one wire only b) Overall diameter

c) Diameter of one wire lnultiplied by the number of wires

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4.3. Titanium can be identified by placing it on a grinding wheel and looking for a) White Sparks b) Red Sparks C) Yellow Sparks

44. Which statement about Military Standard QMS) flareless fittings is correct?

a) MS flareless fittings should not be lubricated prior to assembly

b) MS flareless fittings must be tightened to a specific torque

c) During installatiorn, MS flareless fittings are norrnally tightened by turning the nut a specified amount after the sleeve and fitting sealing surface have ~nade contact, rather than being torqued

45. What two components of a three part polyester resin are dangerous to mix together directly?

a) Accelerator and free catalyst b) Accelerator and resin c) Catalyst and resin

46. When using a spring washer, the plain washer would be fitted a) between spring and part b) between head and spring c) under the nut

47. Fabric to be hand sewn must be doubled under a t the edge to a minimum distance of

a) 3/8 inch b) 112 inch c) 114 inch

48. Flexible hose used in aircraft systems is classified in size according to the a) inside diameter b) outside diameter c) wall thickness

49, The normal moisture content in the wood of a wooden aircraft structure is a) 20-30% b) 10-12% c) 0-2%

50. Adhesives containing phenol-formaldehyde, to cure, requires a) high temperature b) low temperature c) room temperature

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51. What is meant by the term Pitch Ratio? a) The distance between the hole and the edge of the material b) 'The area of contact between the two sheets of metal when joining by rivet\ c) The distance between two holes

52. Kccovering or repairing of an aircraft with a fabric other than the original fabric type is

a) a major modification and requires approval b) a minor modification, providing the fabric is the same strength as the

original c) prohibited

53. Fork cnd fittings on control rod ends should have a) anti vibration compound b) bolt heads fitted upwards c) 0.002 inch axial movement

54. What is the purpose of the guard, where a control chain goes around a sprocket? a) Stops the chain coming off if it goes slack b) Protects personnel when carrying out maintenance c) Prevents entry of foreign bodies

55. The effect of a lower temperature than ambient during the curing period of a resin, will cause the curing time to

a) remain unchanged b) decrease c) increase

56. Shrinkage of wood is a) negligible in the longitudinal direction b) greatest in the longitudinal direction c) negligible in the radial direction

57. Re-treatment of aluminium alloys can be performed by a) selenious acid treatment b) alocrom treatment c) brushing on phosphate treatment followed by paint

58. A worm drive creates a) a drive in 2 planes but transmits I direction only b) a drive in 1 plane but transmits both directions c ) a drive in 2 planes and transmits both directions

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59- During construction, sharp internal corners and inaccessible places should he avoided to reduce

a) crevice corrosion b) fretting corrosion

c) lil~form corrosion

60. Colour identification of an aluminium rivet is a) black b) violet c) green

6 1. Rivnuts were originally used for a) securing structural parts b) securing rubber de-icing boots c) securing cabin floorings

62. Aircraft fabric lacing cord is reinforced with a) wax

b) epoxy c) lanolin

63. What amperage is an 18 swg cable'?

a> 10 amp b) 1 amp c) 5 amp

64. An alumiriiunl alloy L37 rivet identification is a) e~nbossed b) D embossed C) 0 embossed

65. The vacuum connections on a fibreglass repair must be placed onto the a) peel P ~ Y b) top layer of glass fabric directly c) breather mat

66. An alliminium oxide layer on a conductor will do what when the temperature is increased?

a) Become thicker b) Remain the same c) Become thinner

67. The main reason why crimped joints are preferable to soldered joints is a) no flux is needed

b) the quality of crinlped joints wrll be constant

c) there is no heat required

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68. In a sheet metal store the following is marked on a sheet of aluminium alloy:

The following is true:

a) Sheet one has a shinier surface than sheet 2 b) Sheet two is of a thicker gauge than sheet 1

c) Sheet one is more ductile than sheet 2

69. 'The critical process of heat treatment is a) method of heating only b) temperature, method of heating and cooling c) temperature and method of heating only

78. /\ press fit requires a) some sort of driving force b) the shaft to be shrunk by cooling c) the hole to be expanded by heat

71. The oxide film on the surface of aluminium is a) nonporous b) hard and porous c) porous

72. 'The teeth of a gear would normally be a) tempered b) nitrided c) case hardened

73. A co-axial cable is better than a normal cable because a) it has less resistance b) weight for weight it can carry more signal c) there is an electrostatic field around it which helps to reduce the

electromagnetic field

74. Aircraft sheet plywood skins are a) covered in fabric b) sealed and doped c) sealed and varnished or painted

75. What material would be used where a high temperature application is required, e.g. a firemall?

a) Ceramic fibres b) CarbonJgraphite fibres c) Aramid (Kevlar) fibres

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70. Heavy corrosion deposits on clad aluminium alloys should be removed a) chemically by use of trichloroethylene

b) mechanically using a pneumatic vibrator C) chemically by use of phosphoric acid

77. When using a hot bonder to effect a composite repair, you use a) synthetic resin adhesives b) inorganic resin adhesives c) organic resin adhesives

78. lntergrannular corrosion in structural alunlinium alloy parts a) may be detected by the white, powdery deposit formed on the surface of

the metal b) cannot always be detected by surface indications

c) are not likely to occur in parts fabricated from heat-treated sheet aluminium

79. A spring should be inspected for correct a) width, length and strength b) length, strength and squareness c) width, strength and squareness

80. Recovering o r repairing of an aircraft with a fabric other than the original fabric type is

a) a major modification and requires approval

h) a minor modification, providing the fabric is the same strength as the original

c) prohibited

81. An AN steel bolt is identified by what marking on the head? a) 14E b) An 'x' c) A dash

82. What metal is suitable for riveting alloy steel? a) Monel metal b) Mild steel c) Aluminium alloy

83. What action should be taken on finding intergrannular corrosion? a) Replace complete component part b) De-corrode and reprotect c) Renew corroded area by patching

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84. 'rhe strength classification of fabric used in aircraft covering is based upon a) tensile strength b) shear strength c) bearing strength

85. Bolt holes through wooden structures should be

a) sealed with varnish and wet-assembled with the bolt before the varnish has dried

b) sealed, and the sealant allowed to dry before fitting the bolt

c) left unsealed and unvarnished inside the hole

86. Which of these is a common cause of corrosion? a) Spilled battery acid b) Water in he1 C) Untreated metal

87. What is Alumina? a) An alloy of aluminium b) Aluminium ore

c) A ceramic oxide of aluminium

88. Evcept where specified by the manufacturer, a wooden spar may be spliced a) at no point

b) at any point except under the wing attachment fittings c) at any point

89. In the anodic film inspection and sealing test, if a good seal has been accorilplished

a) the dye mark will rub off

b) the dye mark has no importance c) the dye mark will not rub off

90. If a sheet of aluminium alloy of 0.032 and 0.064 is to be joined together the rivet should be

a) 0.064 plus ID b) 0.032 plus 2D c) 0.096 plus 1.5 D

91. A self aligning bearing is a a) angular bearing b) precision bearing c) radial bearing

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92. How would you test a hydraulic hose? a) Pressure test 2.0 x working pressure b) Pressure test I .5 x working pressure

c) Pressure test 1.0 x working pressure

93. When buffing surface of Alun~inium Alloy, what material are you removing? a) Aluminium b) Alloy c) Oxide layer

04. A material's yield strength is the ability to a) resist deformation b) withstand a crushing fbrce c) resist side loads

95. Aircraft fabric covering is made from a) polyester b) silk c) nylon

96. A factor which determines the. minimum space between rivets is the

a) thickness of the material being riveted

b) diameter of the rivets being used

c) length of the rivets being used

97. Cable stops a re manufactured from a) magnesium alloy b) stainless steel c) copper

98. To assist the bending of plywood, a heated bending former must be heated to a temperature of

a) 100°C b) 150°C c) 300°C

99. Corrosion may be regarded as the destruction of metal by a) hydroelectric action b) electromechanical action c) electrochemical action

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100. l 'he stecpest slope permitted on the scarf of a scarfed plywood repair is a) 1 i n 4 b) I in 12 c) 1 in 20

101. Spherical roller bearings resist what loads?

a) Large radial but no thrust

b) Large thrust and moderate radial

c) Large radial and moderate thrust

102. 'l'he pin section of an AN/MS connector is normally installed on a) either side of a circuit (makes no difference)

b) the ground side of the circuit c) the power supply side of the circuit

103. Where would you find the inspection interval for chains? a) Overhaul manual b) Maintenance manual c) Maintenance schedule

104. \%'hat load a re spring hooks subjected to? a) Compressive b) Bending c) Tension

105. In a Telefles flexible control system, the Teleflex cable consists of a) multi strand steel wires and is used primarily as a single one way device

operated from a control lever

b) a flexible seven or nineteen strand steel cable used for the operation of manual flying controls

c) a high tensile steel wire with a right or left hand helix wire wound on to it. The system can operate in two directions

106. From the following list of metals, which is most cathodic? a) Magnesium b) Nickel c) Stainless steel

107. i\ pre-load indicating washer is correctly loaded when

a) the inner ring is gripped

b) the outer ring is gripped c) the inner ring rotates

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108. What a re the signs of fretting corrosion on steel? a) Dark staining around area b) Surface cracking as corrosion breaks through to surface of component c) Rust on surface

109. Plug pins are numbered a) from the outside in - clockwise

b) from the inside out - clockwise

c) from the inside out - anticlockwise

I 10. Galvanic corrosiorl is most likely to be most rapid and severe when a) the surface area of the anodic metal is smaller than the surface area of the

cathodic material

b) the surface area of the cathodic metal and the anodic material are approxi~nately the same

c) the surface area of the cathodic metal is smaller than the surface area of the anodic material

1 I I . Which of the following is the definition of cure time? a) The time taken for the mixed compound to reach a final rubbery state

b) The time required for the mixed compound to reach an initial rubbery state

c) The period after which the surface of the compound no longer exhibits adhesive properties

1 12. Pure aluminium is a) highly resistant to corrosion b) reasonably resistant to corrosion C) not resistant to corrosion

113. A faint line running across the grain of a wood spar generally indicates a) compression failure b) shear failure c) decay

114" Which methods can be used to inspect fibreglasslhoneycomb structures for cn trapped water?

a) Xray and back-lighting b) Acoustic cn~ission and Xray c) Acoustic emission and back-lighting

115, What is stress corrosion'? a) Corrosion in an area under cycIic loading b) Corrosion due to fretting c) Corrosion in an area under continuous loading

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i 16, Identification of British aluminium alloy rivets is with a) a colour and number stamped on the head

b) a part number on the head C) a letter and number code

1 IT. A wire thread insert tap is a) supplied in a fitting kit

b) slightly larger than the hole c) slightly smaller than the hole

118. i\ splayed patch repair may be used on plywood damage which does not exceed a) 10 times the skin thickness b) 20 times the skin thickness

c) 15 times the skin thickness

119. Jointing compound is used for what reason'? a) To bond the components together

b) To prevent dissimilar metal corrosion

c) To make the components easier to disassemble

120. " on a n electrical cable indicates a) control system b) emergency power c) AC power

121. An S-N cuwe is useful in the design evaluation process for testing a) shear force b) tension c) fatigue life

122. A crankshaft would be fitted with a a) spherical roller bearing b) cylindrical roller bearing c) taper roller bearing

123. Which of the following is a temporary protective measure? a) Sacrificial protection b) Chromating c) Paint finish

124. When a steel part is welded, corrosion occurs because

a) the strip has become anodic

b) it is affected by spatter

c) the paint has been removed

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125. What type loads cause the most rivet failures? a) Head b) Shear c) Bearing

126. \.Vhat does the 0 in 2024-'r3 mean? a) The percentage of impurities In the alloy

b) The alloy has been nlodified

c) 'I'he alloy has not been inodified

127.14 thermosetting adhesive a) can be re-formed when hot

b) undergoes a chemical transformation and creates an insoluble substance

c) will be resistant to heat

128. Grain size will effect the mechanical properties of metal. Which of the following is true'?

a) Materials with large grain size are more prone to creep

b) Small grain size is normally attributed to rapid cooling rates and will give less tensile strength

c) Large grain size is attributed to slow cooling rates and will give less tensile strength

129. 'The general rule for finding the proper rivet diameter is

a) two times the rivet length

b) three times the thickness of the rnaterial to be joined

c) three times the thickness of the thickest sheet

130. A material containing approximately 66% nickel and 33% copper is known as a) Invar b) Nimonic c) Monel metal

131, The impact testing technique is used on a material to test for a) toughness b) hardness c) shear strain

132. The difference between annealing and normalizing is a) both are heated below the LJC'I', cool in air to anneal, cool slowly to

normalize

b) both are heated above the UC'T, cool slowly to anneal, cool in air to normalize

c) both are heated above the UCT, cool in air to anneal, cool slowly to normali~e

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133. ('orrosion will spread more rapidly when metals are exposed to a) high temperatures b) dry climates c) cold climates

134. What action is taken to protect integral tanks from corrosion due tu micro- biological growth?

a) A biocidal additive is used in the fuel b) Rubber liners are installed in the tank c) The inside of the tank is coated with yellow chromate

135. R,laximum temperature of tin coated copper cable is a) 260°C b) 105°C c) 200°C

136. What is the indication of fretting corrosion on aluminium alloy? a) Black powder b) Brown powder c) White powder

137. Hand sewn stitch must be locked a t a minimum of

a) the end of the stitch only b) 20 stitch intervals c) 10 stitch intervals

138. Iligh speed steel relies heavily on the following metallic element for its ability lo cut other metals, even when it is heated to a dull red colour

a) Tungsten b) Vanadium c) Nickel

139. ?'he Alocrom 1200 process was designed to treat a) surfaces too large for dip treatment b) chromium plating c) small surfaces

140. 'J'he maximum permissible grain deviation in wood is

a) 1 :20 b) 1:15

c) 1:8

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