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AIRCRAFT WEIGHT & BALANCE
Transcript of AIRCRAFT WEIGHT & BALANCE
Weight and balance is of such vital importance that each mechanic or repairman maintaining an aircraft must be fully aware of his or her responsibility to provide the pilot with current and accurate information for the actual weight
of the aircraft and the location of the center of gravity. The pilot in command has the responsibility to know the weight of the load, CG, maximum allowable weight, and
CG limits of the aircraft.
The plumb bob could also be dropped from the center of the axle on the main landing gear, and a chalk mark made on the floor. With a tape measure, the distance between the two chalk marks could be determined, and the arm for the main landing gear would be known.
The maximum allowable weight for an aircraft is determined by design considerations.
Maximum operational weight may be less than the maximum allowable weight due to such considerations as high-density altitude or high-drag field conditions caused by wet grass or water on the runway.
The maximum operational weight may also be limited by the departure or arrival airport’s runway length.
To begin, assemble all the necessary equipment, such as:
1. Scales, hoisting equipment, jacks, and leveling
2. Blocks, chocks, or sandbags for holding the
airplane on the scales.
3. Straightedge, spirit level, plumb bobs, chalk line,
and a measuring tape.
4. Applicable Aircraft Specifications and weight and
balance computation forms
Two types of scales are typically used to weigh aircraft:
1. Those that operate MECHANICALLY with balance weights or springs.
2. Those that operate electronically with what are called LOAD CELLS.
5. Unless otherwise noted in the aircraft specification, the oil system should be completely drained through the normal drain ports.
6. Reservoirs or tanks containing hydraulic fluid, anti-icing fluid and other liquids which are considered part of the empty weight should be filled to capacity.
7. Weigh the aircraft in a level position.
1. The aircraft should be placed in a closed hangar to avoid vibrations or lift forces which would otherwise be caused by air flowing over the lifting surfaces.
2. The aircraft should be dry and cleaned inside and out.
3. The aircraft equipment should be checked against the equipment list section of the weight and balance record.
4. Fuel tanks should be drained in accordance with the manufacturer’s instructions.
One important preflight consideration is the distribution of the load in the aircraft. Loading the aircraft so the gross weight is less than the maximum allowable is not enough.
This weight must be distributed to keep the CG within the limits specified.
If the CG is too far forward, a heavy passenger can be moved to one of the rear seats or baggage can be shifted from a forward baggage compartment to a rear compartment.
If the CG is too far aft, passenger weight or baggage can be shifted forward.
The fuel load should be balanced laterally: the pilot should pay special attention to the POH or AFM regarding the operation of the fuel system, in order to keep the aircraft balanced in flight.
As an aircraft ages, its weight usually increases due to trash and dirt collecting in hard-to-reach locations, and moisture absorbed in the cabin insulation.
This growth in weight is normally small, but it can only be determined by accurately weighing the aircraft.
Changes of fixed equipment may have a major effect upon the weight of the aircraft. Many aircraft are overloaded by the installation of extra radios or instruments.
The replacement of older, heavy electronic equipment with newer, lighter types results in a weight reduction.
This weight change, however helpful, will probably cause the CG to shift and this must be computed and annotated in the weight and balance record.
If the newly calculated EWCG should happen to fall outside the range, it will be necessary to perform adverse loading check.
Adverse loading checks are a deliberate attempt to load an aircraft in a manner that will create the most critical balance condition and still remain within the design CG limits of the aircraft.
It is sometimes possible to install fixed ballast in order for the aircraft to again operate within the normal CG range.
STABILITY AND BALANCE CONTROL
STABILITY is the inherent quality of an aircraft to correct for conditions that may disturb its equilibrium, and to return to or to continue on the original ﬂightpath.
STATIC STABILITY refers to the initial tendency, or direction of movement, back to equilibrium. In aviation, it refers to the aircraft’s initial response when disturbed from a given AOA, slip, or bank.
• Positive static stability—the initial tendency of the aircraft to return to the original state of equilibrium after being disturbed.
• Neutral static stability—the initial tendency of the aircraft to remain in a new condition after its equilibrium has been disturbed.
• Negative static stability—the initial tendency of the aircraft to continue away from the original state of equilibrium after being disturbed.
STABILITY AND BALANCE CONTROL
Balance control refers to the location of the CG of an aircraft. This is of primary importance to aircraft stability, which determines safety in flight.
The CG is the point at which the total weight of the aircraft is assumed to be concentrated, and the CG must be located within specific limits for safe flight.
Both lateral and longitudinal balance are important, but the prime concern is longitudinal balance.
An airplane is designed to have stability that allows it to be trimmed so it will maintain straight and level flight with hands off the controls.
Longitudinal stability is maintained by ensuring the CG is slightly ahead of the center of lift.
This produces a fixed nose-down force independent of the airspeed.
This is balanced by a variable nose-up force, which is produced by a downward aerodynamic force on the horizontal tail surfaces that varies directly with the airspeed.
If a rising air current should cause the nose to pitch up, the airplane will slow down and the downward force on the tail will decrease. The weight concentrated at the CG will pull the nose back down. If the nose should drop in flight, the airspeed will increase and the increased downward tail load will bring the nose back up to level flight.
If the CG is too far aft, it will be too near the center of lift and the airplane will be unstable, and difficult to recover from a stall.
If the unstable airplane should ever enter a spin, the spin could become flat and recovery would be difficult or impossible.
If the CG is too far forward, the downward tail load will have to be increased to maintain level flight.
This increased tail load has the same effect as carrying additional weight;
The aircraft will have to fly at a higher angle of attack, and drag will increase.
A more serious problem caused by the CG being too far forward is the lack of sufficient elevator authority. At slow takeoff speeds, the elevator might not produce enough nose-up force to rotate and on landing there may not be enough elevator force to flare the airplane.
Both takeoff and landing runs will be lengthened if the CG is too far forward.
The lateral balance can be upset by uneven fuel loading or burnoff.
The position of the lateral CG is not normally computed for an airplane, but the pilot must be aware of the adverse effects that will result from a laterally unbalanced condition.
Sweptwing airplanes are more critical due to fuel imbalance because as the fuel is used from the outboard tanks, the CG shifts forward, and as it is used from the inboard tanks, the CG shifts aft.
Before an aircraft can be weighed and reliable readings obtained, it must be in a level flight attitude. One method that can be used to check for a level condition is to use a spirit level, sometimes thought of as a carpenter’s level, by placing it on or against a specified place on the aircraft. Spirit levels consist of a vial full of liquid, except for a small air bubble. When the air bubble is centered between the two black lines, a level condition is indicated.
In Figure 4-16, a spirit level is being used on a Mooney M20 to check for a flight level attitude. By looking in the Type Certificate Data Sheet, it is determined that the leveling means is two screws on the left side of the airplane fuselage, in line with the trailing edge of the wing.
A plumb bob is a heavy metal object, cylinder or cone shape, with a sharp point at one end and a string attached to the other end. If the string is attached to a given point on an aircraft, and the plumb bob is allowed to hang down so the tip just touches the ground, the point where the tip touches will be perpendicular to where the string is attached. An example of the use of a plumb bob would be measuring the distance from an aircraft’s datum to the center of the main landing gear axle. If the leading edge of the wing was the datum, a plumb bob could be dropped from the leading edge and a chalk mark made on the hangar floor.
When an aircraft is weighed with full fuel in the tanks, the weight of the fuel must be accounted for by mathematically subtracting it from the scale readings. To subtract it, its weight, arm, and moment must be known. Although the standard weight for aviation gasoline is 6.0 lb/gal and jet fuel is 6.7 lb/gal, these values are not exact for all conditions. On a hot day versus a cold day, these values can vary dramatically. On a hot summer day in the state of Florida, aviation gasoline checked with a hydrometer typically weighs between 5.85 and 5.9 lb/gal. If 100 gallons of fuel were involved in a calculation, using the actual weight versus the standard weight would make a difference of 10 to 15 lb.
When an aircraft is weighed with fuel in the tanks, the weight of fuel per gallon should be checked with a hydrometer. A hydrometer 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 number values represent units of pounds per gallon. When placed in a flask with fuel in it, the glass tube floats at a level dependent on the density of the fuel. Where the fuel intersects the markings on the side of the tube indicates the pounds per gallon.
When 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.
Never weigh an aircraft with the fuel tanks partially full, because it will be impossible to determine exactly how much fuel to account for.
1. Weigh the aircraft with absolutely no fuel in the aircraft tanks or fuel lines. If an aircraft is weighed in this condition, the technician can mathematically add the proper amount of residual fuel to the aircraft, and account for its arm and moment. The proper amount of fuel can be determined by looking at the Aircraft Specifications or Type Certificate Data Sheet.
2. Weigh the aircraft with only residual fuel in the tanks and lines.
3. Weigh the aircraft with the fuel tanks completely full. If an aircraft is weighed in this condition, the technician can mathematically subtract the weight of usable fuel, and account for its arm and moment. A hydrometer can be used to determine the weight of each gallon of fuel, and the Aircraft Specifications or TCDS can be used to identify the fuel capacity. If an aircraft is to be weighed with load cells attached to jacks, the technician should check to make sure it is permissible to jack the aircraft with the fuel tanks full. It is possible that this may not be allowed because of stresses that would be placed on the aircraft.
For aircraft certified since 1978, full engine oil is typically included in an aircraft’s empty weight. This can be confirmed by looking at the Type 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. If the Aircraft Specifications or Type Certificate Data Sheet specifies that only residual oil is part of empty weight, this can be accommodated by one of 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 that would leave only the residual amount. The standard weight for lubricating oil is 7.5 lb/gal (1.875 pounds per quart (lb/qt)), so if 7 qt of oil needed to be removed, the technician would subtract 13.125 lb at the appropriate arm.