beban pesawat · dipl.-ing h. bona p. fitrikananda | beban pesawat 2013 25 plumb bob • a plumb...

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Beban PesawatDipl.-Ing H. Bona P. Fitrikananda| 2013

Weighingand

Determining EWCGBeban Pesawat / Aircraft Loads

2013DIPL.-ING H. BONA P. FITRIKANANDA | BEBAN PESAWAT 2

Introduction


current and accurateinformation for the actual

weight of the aircraft and thelocation of the center of

gravity

The pilot in command has the responsibility to know theweight of the load, CG, maximum allowable weight, and CG

limits of the aircraft.

Requirements

The weight and balance report must include an equipment listshowing weights and moment arms of all required andoptional items of equipment included in the certificatedempty weight.

When an aircraft has undergone extensive repair or majoralteration, it should be reweighed and a new weight andbalance record started.

The A&P mechanic or the repairman responsible for the workmust provide the pilot with current and accurate aircraftweight information and where the new EWCG is located.


Equipment for Weighing


Scales

Spirit Level

Plumb Bob

Hydrometer

Others: chalk and tape measure



Scales• Two types of scales are typically used to weigh aircraft:

- operate mechanically with balance weights or springs,- operate electronically with what are called load cells.



Scales• The balance weight type of mechanical scale, known as a beam

scale. A sliding weight is then moved along the bar until the bar iscentered between a top and bottom stop.

• A mechanical scale based on springs is like the typical bathroomscale. When weight is applied to the scale, a spring compresses,which causes a wheel that displays the weight to rotate.



Scales• Electronic scales that utilize load cells come in two varieties:

⁻ the platform type and⁻ the type that mounts to the top of a jack.



Scales• Some aircraft are weighed with mechanical

scales of the low-profile type similar to thoseshown in Figure 4-1

Figure 4-1



ScalesFigure 4-2Low profile platformscales are used to weighsome aircraft. One scaleis placed under eachwheel. (Photo courtesyGeneral ElectrodynamicsCorp.)



Scales• Large aircraft, including

heavy transports, areweighed by rolling themonto weighing platformswith electronic weighingcells that accuratelymeasure the force appliedby the weight of theaircraft.

Figure 4-3



Scales



Scales

• The platform type of electronic scale sits on the ground, with the tireof the airplane sitting on top of the platform. Built into the platform isan electronic load cell, which senses the weight being applied to it andgenerates a corresponding electrical signal. Inside the load cell there isan electronic grid that experiences a proportional change in electricalresistance as the weight being applied to it increases. An electricalcable runs from the platform scale to a display unit, which interpretsthe resistance change of the load cell and equates it to a specificnumber of pounds. A digital readout on the display typically shows theweight.



ScalesFigure 4-3Mooney M20being weighedwith portableelectronicplatform scales.



ScalesFigure 4-4Display unit showing nosewheel weightfor Mooney M20.



Scales

• The second type of electronic scale utilizes aload cell that attaches to the top of a jack.The top of the load cell has a concave shapethat matches up with the jack pad on theaircraft, with the load cell absorbing all theweight of the aircraft at each jacking point.Each load cell has an electrical cableattached to it, which connects to the displayunit that shows the weight being absorbedby each load cell.



ScalesFigure 4-5Airplane on jacks with load cells in use.



ScalesFigure 4-6Airplane on jacks with load cells in use.



ScalesFigure 4-7Helicopter on jacks with load cells in use.



Scales

• An important advantage of weighing an aircraft this way is that itallows the technician to level the aircraft. An aircraft needs to be in aflight level attitude when it is weighed. If an aircraft is sitting on floorscales, the only way to level the aircraft might be to deflate tires andlanding gear struts. When an aircraft is weighed using load cells onjacks, leveling the aircraft is easy by simply adjusting the height withthe jacks.



Scales

• Mechanical scales should be protected when they are not in use, andthey must be periodically checked for accuracy by measuring a knownweight. Electronic scales normally have a built-in calibration thatallows them to be accurately zeroed before any load is applied.



Spirit Level

• Before an aircraft can be weighed and reliable readings obtained, itmust be in a level flight attitude. One method that can be used tocheck for a level condition is to use a spirit level, sometimes thought ofas a carpenter’s level, by placing it on or against a specified place onthe aircraft. Spirit levels consist of a vial full of liquid, except for a smallair bubble. When the air bubble is centered between the two blacklines, a level condition is indicated.



Spirit Level

• In Figure 4-8, a spirit level is beingused on a Mooney M20 to check fora flight level attitude. By looking inthe Type Certificate Data Sheet, it isdetermined that the leveling meansis two screws on the left side of theairplane fuselage, in line with thetrailing edge of the wing.

Figure 4-8Spirit level being used on a Mooney M20



Plumb Bob

• A plumb bob is a heavy metal object, cylinder or cone shape, with asharp point at one end and a string attached to the other end. If thestring is attached to a given point on an aircraft, and the plumb bob isallowed to hang down so the tip just touches the ground, the pointwhere the tip touches will be perpendicular to where the string isattached.



Plumb Bob

Figure 4-9Plumb bob dropped from a wingleading edge.



Hydrometer

• When an aircraft is weighed with fuel in the tanks, the weightof fuel per gallon should be checked with a hydrometer. Ahydrometer consists of a weighted glass tube which is sealed,with a graduated set of markings on the side of the tube.

• The graduated markings and their corresponding numbervalues represent units of pounds per gallon. When placed in aflask with fuel in it, the glass tube floats at a level dependenton the density of the fuel. Where the fuel intersects themarkings on the side of the tube indicates the pounds pergallon.

Preparing an Aircraft for Weighing

Weighing an aircraft is a very important and exacting phase ofaircraft maintenance, and must be carried out with accuracyand good workmanship. Thoughtful preparation saves timeand prevents mistakes.



To begin, assemble all the necessary equipment, such as:

1. Scales, hoisting equipment, jacks, and leveling equipment.

2. Blocks, chocks, or sandbags for holding the airplane on thescales.

3. Straightedge, spirit level, plumb bobs, chalk line, and ameasuring tape.

4. Applicable Aircraft Specifications and weight and balancecomputation forms.



Weigh Clean Aircraft Inside HangarThe aircraft should be weighed inside a hangar where wind cannot

blow over the surface and cause fluctuating or false scale readings.The aircraft should be clean inside and out, with special attention

paid to the bilge area to be sure no water or debris is trapped there,and the outside of the aircraft should be as free as possible of allmud and dirt.



Equipment ListAll of the required equipment must be properly installed, and there should

be no equipment installed that is not included in the equipment list. If suchequipment is installed, the weight and balance record must be correctedto indicate it.

BallastAll required permanent ballast must be properly secured in place and all

temporary ballast must be removed.



FuelWhen weighing an aircraft to determine its empty weight, only the weight

of residual (unusable) fuel should be included.To ensure that only residual fuel is accounted for, the aircraft should be

weighed in one of the following three conditions.1. Weigh the aircraft with absolutely no fuel in the aircraft tanks or fuel lines.

2. Weigh the aircraft with only residual fuel in the tanks and lines.

3. Weigh the aircraft with the fuel tanks completely full.



FuelNever weigh an aircraft with the fuel tanks partially full, because it will be

impossible to determine exactly how much fuel to account for.Drain fuel from the tanks in the manner specified by the aircraft

manufacturer. If there are no specific instructions, drain the fuel until thefuel quantity gauges read empty when the aircraft is in level-flightattitude. Any fuel remaining in the system is considered residual, orunusable fuel and is part of the aircraft empty weight.



Fuel If it is not feasible to drain the fuel, the tanks can be topped off to be sure

of the quantity they contain and the aircraft weighed with full fuel.After weighing is complete, the weight of the fuel and its moment are

subtracted from those of the aircraft as weighed. To correct the emptyweight for the residual fuel, add its weight and moment.



Oil SystemThe empty weight for aircraft certificated under the CAR, part 3

does not include the engine lubricating oil. The oil must either bedrained before the aircraft is weighed, or its weight must besubtracted from the scale readings to determine the empty weight.For aircraft certified since 1978, full engine oil is typically included in

an aircraft’s empty weight. This can be confirmed by looking at theType Certificate Data Sheet.If full oil is to be included, the oil level needs to be checked and the

oil system serviced if it is less than full.



Oil System If the Aircraft Specifications or Type Certificate Data Sheet specifies that

only residual oil is part of empty weight, this can be accommodated by oneof the following two methods.

1) Drain the engine oil system to the point that only residual oil remains.

2) Check the engine oil quantity, and mathematically subtract the weight of the oil thatwould leave only the residual amount. The standard weight for lubricating oil is 7.5lb/gal (1.875 pounds per quart (lb/qt)), so if 7 qt of oil needed to be removed, thetechnician would subtract 13.125 lb at the appropriate arm.



Miscallenous FluidUnless otherwise noted in the Aircraft Specifications or manufacturer’s

instructions, hydraulic reservoirs and systems should be filled, drinking andwashing water reservoirs and lavatory tanks should be drained, andconstant speed drive oil tanks should be filled.

Flight ControlsThe position of such items as spoilers, slats, flaps, and helicopter rotor

systems is an important factor when weighing an aircraft. Always refer tothe manufacturer’s instructions for the proper position of these items.



Other ConsiderationsInspect the aircraft to see that all items included in the

certificated empty weight are installed in the proper location.Remove items that are not regularly carried in flight. Also look

in the baggage compartments to make sure they are empty.Replace all inspection plates, oil and fuel tank caps, junction

box covers, cowling, doors, emergency exits, and other partsthat have been removed. All doors, windows, and slidingcanopies should be in their normal flight position.Remove excessive dirt, oil, grease, and moisture from the

aircraft.



Jacking the AircraftAircraft are often weighed by rolling them onto ramps in which load cells

are embedded. This eliminates the problems associated with jacking theaircraft off the ground. However, many aircraft are weighed by jacking theaircraft up and then lowering them onto scales or load cells.

Extra care must be used when raising an aircraft on jacks for weighing. Ifthe aircraft has spring steel landing gear and it is jacked at the wheel, thelanding gear will slide inward as the weight is taken off of the tire, and caremust be taken to prevent the jack from tipping over.



Jacking the AircraftFor some aircraft, stress panels or plates must be installed before they are

raised with wing jacks, to distribute the weight over the jack pad. Be sureto follow the recommendations of the aircraft manufacturer in detailanytime an aircraft is jacked.

When using two wing jacks, take special care to raise them simultaneously,keeping the aircraft so it will not slip off the jacks. As the jacks are raised,keep the safety collars screwed down against the jack cylinder to preventthe aircraft from tilting if one of the jacks should lose hydraulic pressure.



Leveling the AircraftWhen an aircraft is weighed, it must be in its level flight attitude so that all

of the components will be at their correct distance from the datum. Thisattitude is determined by information in the TCDS.

Some aircraft require a plumb line to be dropped from a specified locationso that the point of the weight, the bob, hangs directly above anidentifiable point. Others specify that a spirit level be placed across twoleveling lugs, often special screws on the outside of the fuselage.



Leveling the AircraftOther aircraft call for a spirit level to be placed on the upper door sill.Lateral level is not specified for all light aircraft, but provisions are

normally made on helicopters for determining both longitudinal andlateral level. This may be done by built-in leveling indicators, or by aplumb bob that shows the conditions of both longitudinal and laterallevel.The actual adjustments to level the aircraft using load cells are made

with the jacks. When weighing from the wheels, leveling is normallydone by adjusting the air pressure in the nose wheel shock strut.



Weighing PointsWhen an aircraft is being weighed, the arms must be known for the points

where the weight of the aircraft is being transferred to the scales. If a tricycle gear small airplane has its three wheels sitting on floor scales,

the weight transfer to each scale happens through the center of the axlefor each wheel.

If an airplane is weighed while it is on jacks, the weight transfer happensthrough the center of the jack pad.



Weighing PointsFor a helicopter with skids for landing gear, determining the arm for the

weighing points can be difficult if the skids are sitting directly on floorscales. The problem is that the skid is in contact with the entire top portionof the scale, and it is impossible to know exactly where the center ofweight transfer is occurring.

In such a case, place a piece of pipe between the skid and the scale, andthe center of the pipe will now be the known point of weight transfer.



Weighing PointsThe arm for each of the weighing points is the distance from the center of

the weight transfer point to the aircraft’s datum. If the arms are not known, based on previous weighing of the aircraft or

some other source of data, they must be measured when the aircraft isweighed. This involves dropping a plumb bob from the center of eachweighing point and from the aircraft datum, and putting a chalk mark onthe hangar floor representing each point.



Weighing PointsThe perpendicular distance between the datum and each of the weighing

points can then be measured. In Figure 4-10, the distance from the nosewheel centerline to the datum is

being measured on a Cessna 310 airplane. Notice the chalk lines on thehangar floor, which came as a result of dropping a plumb bob from thenosewheel axle centerline and from the datum. The nosewheel sitting onan electronic scale can be seen in the background.



Weighing Points


Figure 4-10Measuring the nosewheel armon a Cessna 310.

Safety Considerations

Special precautions must be taken when raising an aircrafton jacks.

1. Stress plates must be installed under the jack pads if themanufacturer specifies them.

2. If anyone is required to be in the aircraft while it is being jacked,there must be no movement.

3. The jacks must be straight under the jack pads before beginningto raise the aircraft.

4. All jacks must be raised simultaneously and the safety devicesare against the jack cylinder to prevent the aircraft tipping if anyjack should lose pressure. Not all jacks have screw down collars,some use drop pins or friction locks.


Example Weighing


Determining the Center of Gravity

Determine the CG by adding the weight and moment of eachweighing point to determine the total weight and totalmoment. Then divide the total moment by the total weight todetermine the CG relative to the datum.

As an example of locating the CG with respect to the datum,which in this case is the firewall, consider the tricycle landinggear airplane in next Figures 4-11 and Tabelle.




Figure 4-10


When the airplane is on the scales with the parking brakes off,place chocks around the wheels to keep the airplane fromrolling.

Subtract the weight of the chocks, called tare weight, fromthe scale reading to determine the net weight at eachweighing point.

Multiply each net weight by its arm to determine its moment,and then determine the total weight and total moment.



The CG is determined by dividing the total moment by thetotal weight.

The airplane in Figures 4-11 and Tabelle has a net weight of2,006 pounds, and its CG is 32.8 inches behind the datum.



Plotted on a graph, the CG limits would look like what isshown in Figure. When graphically plotted, the CG limits formwhat is known as the CG envelope.


Empty-Weight Center of Gravity Formulas

A chart like the one in last Figure helps visualize the weights,arms, and moments when solving an EWCG problem, but it isquicker to determine the EWCG by using formulas and anelectronic calculator.

There are four possible conditions and their formulas thatrelate the location of CG to the datum. Notice that theformula for each condition first determines the moment ofthe nose (F x L)/W wheel or tail (R x L)/W wheel and thendivides it by the total weight of the airplane. The arm thusdetermined is then added to or subtracted from the distancebetween the main wheels and the datum, distance D.



With: distance (D)

weight of the nose wheel (F)

distance between main wheels and nose wheel (L)

total weight of the airplane (W)

Empty Weight Center of Gravity Formulas for each conditionis in next slides.



Nose wheel airplanes with datum forward of the main wheels.

Nose wheel airplanes with the datum aft of the main wheels.

Tail wheel airplanes with the datum forward of the mainwheels.

Tail wheel airplanes with the datum aft of the main wheels.



Datum Forward of the Airplane - Nose Wheel Landing Gear

The datum of the airplane in Figure 4-11 is 100 inches forwardof the leading edge of the wing root, or 128 inches forward ofthe main-wheel weighing points. This is distance (D).

The weight of the nose wheel (F) is 340 pounds, and thedistance between main wheels and nose wheel (L) is 78inches.

The total weight of the airplane (W) is 2,006 pounds.





Figure 4-11

Determine the CG byusing this formula:



The CG is 114.8 inches aft of the datum. This is 13.2 inchesforward of the main-wheel weighing points which proves thelocation of the datum has no effect on the location of the CGso long as all measurements are made from the samelocation.



Datum Aft of the Main Wheels - Nose Wheel Landing Gear

The datum of some aircraft may be located aft of the mainwheels. The airplane in this example is the same one justdiscussed, but the datum is at the intersection of the trailingedge of the wing with the fuselage.

The distance (D) between the datum of the airplane in Figure4-12 and the main-wheel weighing points is 75 inches, theweight of the nose wheel (F) is 340 pounds, and the distancebetween main wheels and nose wheel (L) is 78 inches. Thetotal net weight of the airplane (W) is 2,006 pounds.



Datum Aft of the Main Wheels - Nose Wheel Landing Gear


Figure 4-12

The location of the CG maybe determined by using thisformula:


Datum Aft of the Main Wheels - Nose Wheel Landing Gear The CG location is a negative value, which means it is 88.2 inches

forward of the datum. This places it 13.2 inches forward of the mainwheels, exactly the same location as it was when it was measured fromother datum locations.

Location of Datum

It makes no difference where the datum is located as long as allmeasurements are made from the same location.



Datum Forward of the Main Wheels- Tail Wheel Landing Gear

Locating the CG of a tail wheel airplane is done in the sameway as locating it for a nose wheel airplane except theformulas use (R x L)/W rather than (F x L)/W

The distance (D) between the datum of the airplane in Figure4-13 and the main-gear weighing points is 7.5 inches, theweight of the tail wheel (R) is 67 pounds, and the distance (L)between the main-wheel and the tail wheel weighing points is222 inches.

The total weight of the airplane (W) is 1,218 pounds.



Datum Forward of the Main Wheels- Tail Wheel Landing Gear


Figure 4-13


Datum Aft of the Main Wheels- Tail Wheel Landing Gear

The datum of the airplane in Figure 4-14 is located at theintersection of the wing root trailing edge and the fuselage.This places the arm of the main gear (D) at -80 inches. The netweight of the tail wheel (R) is 67 pounds, the distancebetween the main wheels and the tail wheel (L) is 222 inches,and the total net weight (W) of the airplane is 1,218 pounds.



Datum Aft of the Main Wheels- Tail Wheel Landing Gear

The CG is 67.8 inches forward of the datum, or 12.2 inches aftof the main-gear weighing points. The CG is in exactly thesame location relative to the main wheels, regardless ofwhere the datum is located.


Figure 4-14


Two Ways to Express CG Location

The location of the CG may be expressed in terms of inchesfrom a datum specified by the aircraft manufacturer, or

as a percentage of the MAC. The location of the leading edgeof the MAC, the leading edge mean aerodynamic cord(LEMAC), is a specified number of inches from the datum.



Location with Respect to the Mean Aerodynamic Chord

The aircraft mechanic or repairman is primarily concernedwith the location of the CG relative to the datum, anidentifiable physical location from which measurements canbe made. But because the physical chord of a wing that doesnot have a strictly rectangular plan form is difficult tomeasure, wings such as tapered wings express the allowableCG range in percentage of mean aerodynamic chord (MAC).




The allowable CG range is expressed in percentages of theMAC. The MAC, as seen in Figure 4-15, is the chord of animaginary airfoil that has all of the aerodynamiccharacteristics of the actual airfoil.

It can also be thought of as the chord drawn through thegeographic center of the plan area of the wing.





Figure 4-15



The relative positions of the CG and the aerodynamic centerof lift of the wing have critical effects on the flightcharacteristics of the aircraft.

Consequently, relating the CG location to the chord of thewing is convenient from a design and operations standpoint.Normally, an aircraft will have acceptable flightcharacteristics if the CG is located somewhere near the 25percent average chord point. This means the CG is locatedone-fourth of the total distance back from the leading edge ofthe wing section. Such a location will place the CG forward ofthe aerodynamic center for most airfoils.




In order to relate the percent MAC to the datum, all weightand balance information includes two items: the length ofMAC in inches and the location of the leading edge of MAC(LEMAC) in inches from the datum.

The weight and balance data of the airplane in Figure 4-16states that the MAC is from stations 144 to 206 and the CG islocated at station 161.





Figure 4-16



The location of the CG expressed in percentage of MAC isdetermined using this formula:

The CG of the airplane is located at 27.4% MAC.

It is sometimes necessary to determine the location of the CG ininches from the datum when its location in %MAC is known.




Determine the location of the CG in inches from the datum byusing this formula:




The CG of this airplane is located at station 160.9 inches aft ofthe datum. It is important for longitudinal stability that theCG be located ahead of the center of lift of a wing. Since thecenter of lift is expressed as a percentage of the MAC, thelocation of the CG is expressed in the same terms.


Example


Example

By analyzing the data identified for the airplane being weighed, thefollowing needed information is determined.• Because the airplane was weighed with the fuel tanks full, the full weight of the fuel

must be subtracted and the unusable fuel added back in. The weight of the fuelbeing subtracted is based on the pounds per gallon determined by the hydrometercheck (5.9 lb/gal).

• Because wheel chocks are used to keep the airplane from rolling off the scales, theirweight must be subtracted from the scale readings as tare weight.

• Because the main wheel centerline is 70" behind the datum, its arm is a +70".

• The arm for the nosewheel is the difference between the wheelbase (100") and thedistance from the datum to the main wheel centerline (70"). Therefore, the arm forthe nosewheel is −30".


Example

To calculate the airplane’s empty weight and empty weight center ofgravity, a six column chart is used.


Example

Based on the calculation shown in the chart, the center of gravity is at+50.1", which means it is 50.1" aft of the datum. This places the center ofgravity forward of the main landing gear, which must be the case for atricycle gear airplane. This number is the result of dividing the totalmoment of 66,698 in-lb by the total weight of 1,331.5 lb.


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