Before beginning a study of aircraft weighing procedure or attempting the actual weighing of an aircraft, it is necessary to become familiar with the weight and balance information in the applicable Aircraft Specification or Type Certificate Data Sheet.
The specification for Taylorcraft, model BC and BCS airplanes, illustrated in figure 3-6 has been reproduced in its entirety. A few of the items need explaining; the rest are self-explanatory.
The designation 2 PCLM is read "2 place, closed land monoplane" and indicates that the airplane seats two persons, has an enclosed cockpit, can be operated from the solid part of the earth's surface, and has only one wing. Two PCSM indicates that the airplane is a "2 place, closed sea monoplane." It should be noted that the CG range, EWCG range, and the maximum weight are different for the landplane and the seaplane. The location of the seats indicates a side-by-side arrangement. The datum and the leveling means are shown in the portion of the specification that is pertinent to all models. Since the datum and the leveling means are directly connected to weight and balance, they would be among the first items referred to in planning the weighing operation. Figure 3-6. A typical aircraft specification.
Although the location or arrangement of the landing gear is not shown in figure 3-6, this information is given in the Aircraft Specification or Type Certificate Data Sheets and the maintenance manual. The location of the wheels has important significance, since this can be used as a double check against actual measurements taken at the time of weighing.
Weighing an Aircraft
Weighing an aircraft is a very important and exacting phase of aircraft maintenance and must be carried out with accuracy and good workmanship. Thoughtful preparation saves time and 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 the scales.
3. Straightedge, spirit level, plumb bobs, chalk line, and a measuring
tape.
4. Applicable Aircraft Specifications and weight and balance computation
forms.
If possible, aircraft should be weighed in a closed building where there are no air currents to cause incorrect scale readings. An outside weighing is permissible if wind and moisture are negligible.
Prepare Aircraft For Weighing
Drain the fuel system until the quantity indication reads zero, or empty, with the aircraft in a level attitude. If any fuel is left in the tanks, the aircraft will weigh more, and all later calculations for useful load and balance will be affected. Only trapped or unusable fuel (residual fuel) is considered part of the aircraft empty weight. Fuel tank caps should be on the tanks or placed as close as possible to their correct locations, so that the weight distribution will be correct.
In special cases, the aircraft may be weighed with the fuel tanks full, provided a means of determining the exact weight of the fuel is available. Consult the aircraft manufacturer's instructions to determine whether a particular model aircraft should be weighed with full fuel or with the fuel drained.
If possible, drain all engine oil from the oil tanks. The system should be drained with all drain valves open. Under these conditions, the amount of oil remaining in the oil tank, lines, and engine is termed residual oil and is included in the empty weight. If impractical to drain, the oil tanks should be completely filled.
The position of such items as spoilers, slats, flaps, and helicopter rotor systems is an important factor when weighing an aircraft. Always refer to the manufacturer's instructions for the proper position of these items.
Unless otherwise noted in the Aircraft Specifications or manufacturer's instructions, hydraulic reservoirs and systems should be filled; drinking and washing water reservoirs and lavatory tanks should be drained; and constant speed drive oil tanks should be filled.
Inspect 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 parts that have been removed. All doors, windows, and sliding canopies should be in their normal flight position. Remove excessive dirt, oil, grease, and moisture from the aircraft.
Properly calibrate, zero, and use the weighing scales in accordance with the manufacturer's instructions.
Some aircraft are not weighed with the wheels on the scales, but are weighed with the scales placed either at the jacking points or at special weighing points. Regardless of what provisions are made for placing the aircraft on the scales or jacks, be careful to prevent it from falling or rolling off, thereby damaging the aircraft and equipment. When weighing an aircraft with the wheels placed on the scales, release the brakes to reduce the possibility of incorrect readings caused by side loads on the scales.
All aircraft have leveling points or lugs, and care must be taken to level the aircraft, especially along the longitudinal axis. With light, fixed wing airplanes, the lateral level is not as critical as it is with heavier airplanes. However, a reasonable effort should be made to level the light airplanes around the lateral axis. Accuracy in leveling all aircraft longitudinally cannot be overemphasized.
Measurements
The distance from the datum to the main weighing point centerline, and the distance from the main weighing point centerline to the tail (or nose) weighing point centerline must be known to determine the CG relative to the main weighing point and the datum.
An example of main weighing point to datum and main weighing point to tail weighing point is shown in figure 3-7. See figure 3-8 for an example of main weighing point to datum and main weighing point to nosewheel measurements.
These distances may be calculated using information from the Aircraft
Specifications or Type Certificate Data Sheets. However, it will often
be necessary to determine them by actual measurement.
| After the aircraft has been placed on the scales (figure 3-9) and leveled, hang plumb bobs from the datum, the main weighing point, and the tail or nose weighing point so that the points of the plumb bobs touch the floor. Make a chalk mark on the floor at the points of contact. If desired, a chalk line may be drawn connecting the chalk marks. This will make a clear pattern of the |
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After all weights and measurements are obtained and recorded, the aircraft may be removed from the scales. Weigh the tare and deduct its weight from the scale reading at each respective weighing point where tare is involved.
Balance Computation
To obtain gross weight and the CG location of the loaded airplane, first determine the empty weight and the EWCG location. When these are known, it is easy to compute the effect of fuel, crew, passengers, cargo, and expendable weight as they are added. This is done by adding all the weights and moments of these additional items and recalculating the CG for the loaded airplane.
The scale readings and measurements recorded on the sample form in figure 3-10 form the basis for the examples of computing the empty weight and the empty weight CG.
Empty Weight
The empty weight of the aircraft is determined by adding the net weight on each weighing point. The net weight is the actual scale reading, less the tare weight.
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This gives the aircraft weight as weighed.
Empty Weight CG
The CG location is found through the progressive use of two formulas. First calculate the total moments using the following formulas:
Moment = Arm x Weight
then divide the sum of the moments by the total weights involved:
Consequently, the CG, as weighed, is 57.67 in from the datum.
Since the aircraft was weighed with the oil tank full, it is necessary to remove the oil to obtain the empty weight and empty weight CG.
Again using the formula:
The EWCG is located 61.64 in aft of the datum.
The maximum allowable gross weight as shown in the Aircraft Specifications is 1,733 pounds. By subtracting the aircraft empty weight from this figure, the useful load is determined to be 450 pounds.