Aircraft Fuel Systems

Business and Commercial Aviation Magazine

“An aircraft's fuel system has a more profound effect on aircraft performance than any other airframe system. Without fuel, the mission inevitably comes to an abrupt stop and, unless the flight crew is very, very lucky, the ensuing forced landing will cause severe or catastrophic aircraft damage.” ~ Fred George, 6/20/06 http://www.avweek.com/avnow/news/channel_bca_story.jsp?id=news/fuel0606.xml

Introduction

The aircraft fuel system consists of the fuel storage area and the components within that storage area, the system ends at the fuel low pressure cock.

The fuel system is required to supply the main aircraft engines with sufficient fuel at a given pressure under all operating conditions.

It also supplies fuel to the aircraft’s “auxiliary power unit”.

Fuel types

There are two types of fuel currently used in aviation: AVGAS — Aviation gasoline for use in

conventional piston engines with ignition systems.

Avtur — Aviation kerosene for use in gas-turbine engines and the new diesel engines that are being developed and licensed for aircraft use.

AVIATION GASOLINE — AVGAS

Aviation gasoline having a low density (compared to kerosene) is volatile and has a high vapour npressure. Gasoline easily vaporises (evaporates) at sea level on an ISA standard day.

As the ambient pressure decreases, the volatility of the fuel increases and is said to be boiling off (e.g. at 10 000 ft, the ambient pressure has dropped by approximately 31% of the sea level value).

AVIATION KEROSENE — AVTUR Aviation turbine fuel was developed from

paraffin for use in gas-turbine engines. This fuel has a greater density than AVGAS and a lower vapour pressure (about 0.14 psi), so it is less

The fuel has to be chemically stable. However, if it is stored over a long period in conditions where oxidation can occur, soluble gums and insoluble particulates form which clog filter units and pipesolatile.

Fuel Table

Definition

Flash point – the temperature at which the fuel will readily ignite.

Freezing point – the temperature at which the fuel will start to solidify, crystals will form within the liquid.

S.G. (specific gravity) – the ratio of the density of the fuel compared to the density of pure water. Fuel is lighter than water so the figure will be a number less than one.

FSII (fuel system icing inhibitor) – a fuel additive used to reduce the freezing point of the fuel.

Volatility is the measure of a fuel’s tendency to change state from a liquid to a vapour.

FUEL CONTAMINATION

DISSOLVED WATER :Water that is absorbed into a fuel from the water vapour in the atmosphere has been broken down into minute particles. When passing into the fuel, the particles are supported by the fuel due to their size.

FREE WATER : Dissolved water that has precipitated out as the fuel cools (saturation point), or water that has entered the fuel on mass, due to its greater mass, collects at the bottom of the tank.

FUNGAL CONTAMINATION : An airborne fungus called cladisporium resinae can contaminate fuel tanks. The fungus exists in the fuel/water interface living in the fuel and deriving food from the hydrocarbon fuel.

WATER SEDIMENT CHECKS

WATER SEDIMENT CHECKS

Clear refers to checking the fluid for traces of sediment, particulates, and other solid matter.

Bright refers to checking the fluid for traces of free water and dissolved water, which can also be termed entrained wate

Typical Fuel Tanks

Rigid Flexible Integral

Rule of Thumb for max. fuel volume: 85% for wing tanks and 92% for fuselage tanks, measured to the external skin surface (exception: bladder tanks, 77% and 83%, respectively)

RIGID TANKS

Externally Mounted Rigid Fuel Tank

RIGID TANKS

TYPICAL INTERNAL RIGID FUEL TANK

RIGID TANKS The rigid fuel tank and the extra strengthening required adds to the

overall mass of the aircraft and reduces the useful load of the aircraft.

As they are built as a separate structure, if they are large, they have to be built up in situ as the aircraft is manufactured, making maintenance replacement awkward.

For practical and economic purposes, rigid tanks can only be fitted where there is sufficient space available for a large uncomplicated shape to be manufactured.

They have to be tied to the aircraft’s structure. This requires the surrounding structure to be strengthened to support the added mass and cater for the acceleration/deacceleration loads imposed by the fuel tank and its contents.

FLEXIBLE TANKS

Flexible Fuel Tank

FLEXIBLE TANKS

Flexible Fuel Tank

FLEXIBLE TANKS

There is no guarantee that the bottom of the tank is flat. It can be rucked, causing ridges that trap water.

The fuel from a leaking flexible tank can run down the internal structure before showing on the exterior of the aircraft.

The areas in which they are fitted have to be lined with tape to prevent any sharp edges from puncturing them.

They have to be clipped or tied to the surrounding structure to preventing them from collapsing as the fuel is used.

Once used with fuel, the tank must not be allowed to dry out as it can split and leak.

Over a period of time, these tanks can become porous, so they have a finite life.

INTEGRAL TANKS

Integral Tank made by sealing the joints and fastenings of wing's structure

INTEGRAL TANKS

INTEGRAL TANKS The system has the advantage of being lighter and making use of

structural bays that would otherwise be left as voids. The disadvantage, as with the flexible tank, is that the rubberised

sealant must not be allowed to dry out. Otherwise, it can crack and cause the tank to leak.

This weight reduction in turn allows either more fuel to be carried to extend the aircraft’s range or more payload in place of the extra fuel.

For modern turbine-powered air transport aircraft with thinner wing sections, the utilisation of the wet wing system allows fuel to be stored further toward the tips.

Increasing the weight at the tips, counters the upward bending action created by lift

ILLUSTRATION OF 3 TANK TYPES

FUEL TANK INSTALATION ON JET AIRLINER

Fuel distributed around center of gravity

Tank components

Baffles Check valves Temperature sensor Pumps Screens Suction valves Vent system Relief valve Maintenance panel Fuel drain Water drain

FUEL SYSTEM LAYOUT 2%

Contents measuring devices Float type gauge (On-Ground)

Contents measuring devices

Manual measuring

Magnetic Float Level Indicator

Contents measuring devices

Manual measuring

Dipstick and Tabs for Light Aircraft

Contents measuring devices

Float type gauge (Light Aircraft)

Simple Float Type Gauging System

Contents measuring devices

Float type gauge (Light Aircraft)

Simple Float Type Gauging System

Contents measuring devices

Contents measuring devices

Capacitance type fuel gauging systems

Simple Float Type Gauging System

Fuel Pump

Fuel Pump

LIGHT AIRCRAFT FUEL SYSTEM

Fuel Sytem Twin Light

Venting Suystem

AUTOMATIC TOP-OFF UNIT

Some (main) fuel tanks are fitted with an ‘Automatic top-off unit’

This unit consists of a float valve set at a pre-determined level that initiates feed from an auxiliary or subsidiary tank

The feed may be either a gravity or pump feed and will maintain the main tank at the pre-set level

Fuel Dumping Systems

Needed to meet landing weight limits of landing gear or runway length

System of fuel pumps and valves Usually ejected from wingtips Sometimes from aft-most point of fuselage Usually designed to allow the plane to go

from max take-off weight to max landing weight in 15 minutes or less.

777 Fuel Dumping http://www.aerospaceweb.org/question/planes/q0245b.shtml

http://www.centennialofflight.gov/essay/Evolution_of_Technology/refueling/Tech22G5.htm

In-Flight Refueling

Original motivation: endurance records Currently used only in the military sector Two main types: Boom and Receptacle Probe and Drogue

http://www.answers.com/topic/aerial-refueling

FUELLING ZONES Fuelling zones should be established before fuelling commences.

These zones should be regarded as extending at least 6 metres (20 feet) radially from the filling and venting points on the aircraft and fuelling equipment.

Within this zone, smoking, use of naked lights and operation of switches which are not of the approved pattern should be forbidden. A. Unless fuelling takes place in a designated a no smoking area ‘no

smoking’ signs should be displayed not less than15 metres (50 feet)f rom the fuelling equipment and a/c tank vents

B. APUs which have an exhaust discharge into the zone should, if required during fuelling,be started before filler caps are removed. If APU stops,it should not be restarted until refuelling has ceased.

C. GPUs should be located as far as practical from a/c should not be connected/disconnected during fuelling.4. Fir extinguishers should be located so as to be readily accessible and preferably be of the CO2and/or BCF types.

PRECAUTIONS PRIOR TO FUELLING The aircraft should be connected to an effective earthing point and to the

fuelling equipment. This is always achieved through the undercarriage. When overwing fuelling, the nozzle of the hose should be bonded to the

aircraft structurebefore removing the tank filler cap. When fuelling from hand-operated equipment, including pumping from cans

or drums, similar precautions should be taken to bond equipment, hoses, nozzles and containers.

If funnels are used, they too should be bonded to the nozzle or can and to the aircraft.

If a chamois leather filter is used, the funnel and all metal parts securing the leather should be included in the bonding circuit.

When pressure fuelling, the fuel tank pressure relief valves should,if possible, be checked for correct operation and the bonding lead on the nozzle should be connected to the receptacle, located adjacent to the fuelling point,before connecting the nozzle.

SPECIAL PRECAUTIONS

Aircraft should not be fuelled within30 metres (100 feet)of radar equipment, under test or in use, in aircraft or ground installations.

When any part of an aircraft landing gear appears overheated, the fire service should be called and fuelling should not take place until heat has dissipated.

Extreme caution should be exercised when fuelling during electrical storms. Fuelling should be suspended during severe electrical disturbances in the vicinity of the aerodrome.

The use of photographic flash bulbs and electronic flash equipment should not be permitted.

Any Questions?