Weight and Balance
A review of fundamentals
By Kevin E. Bricker
Weight influences more aircraft performance variables than any other single parameter. These include the typical items such as takeoff and landing characteristics, acceleration capability, maximum velocity and rate of climb. While it should be obvious that an overloaded and/or improperly balanced aircraft requires more power, has a greater fuel consumption rate, and its stability and control characteristics will be seriously affected; the effects of changes of equipment and the addition of paint don't often register as having a large impact on the performance of the aircraft.
Weighings - why, how, and when?
The aircraft basic weight and associated center of gravity must be accurately known at all times to ensure a safe aircraft operation. Non-military aircraft are weighed per specified operator's procedures or for prescribed schedules or instructions.
According to 43.13-1B, the removal or addition of equipment results in changes to the center of gravity (CG). The empty weight of the aircraft, and the permissible useful load, are affected accordingly. You must investigate the effects of these changes as the aircraft flight characteristics may be adversely affected. Information upon which to base the record of weight and balance changes to the aircraft may be obtained from the pertinent Aircraft Specifications, Type Certificate Data Sheet (TCDS), prescribed aircraft operating limitations, aircraft flight manual, aircraft weight and balance report, and maintenance manual.
Also, removal or addition of minor items of equipment such as nuts, bolts, rivets, washers, and similar standard parts of negligible weight on fixed-wing aircraft do not require a weight and balance check. (Rotorcraft in general, are more critical with respect to controlling changes in the CG position.) The procedures and instructions in that particular model's maintenance or flight manual should be followed.
All aircraft, regardless of type, are weighed in the longitudinal and lateral level conditions using any of the approved scale type procedures. When weighing on scales, the procedure used assumes that the lateral level condition is 0.0 degrees when the tires have been properly serviced, and the OLEO struts (main gear) evenly extended. The weighing facility floor should not exceed 0.25 inches of slope per foot. The lateral level condition is controlled by jack extension when using load cell (top-of-jack) type equipment.
As aircraft weighings are required, weighing with calibrated scales is the only sure method of obtaining an accurate basic weight and center of gravity of the aircraft.
A variety of scale types and equipment may be used for weighing to include: electronic top-of-jack load cells, portable platform scales, or stationary pit scales.
Organizations that operate, maintain, or modify aircraft, are responsible for having all weighing equipment under their jurisdiction calibrated periodically and certified by a government inspector of weights and measures, or by commercial scale officials. If the operator of such an organization does not have any scales, they have an option of hiring an outside company who specializes in weight and balance, and the calculation of the center of gravity for aircraft.
The entire aircraft weight may be considered to be concentrated at the center of gravity. Therefore, the moment of the aircraft about the datum is the weight of the aircraft times the horizontal distance between the C.G. and the datum. Example:
If the weight of this airplane is 2000 lbs. and the arm from the datum to the center of gravity is 16 inches, the moment of the aircraft about the datum is 2000 x 16 or 32,000 in. lbs.
It includes information for A&P mechanics to determine the weight and CG changes caused by repairs or alterations.
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