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Aviation Learning Center Document Weight and Balance-P-8740-05
Author: Federal Aviation Administration Date: 1995
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Weight and Balance Terms
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All pilots need to be familiar with the terms related to aircraft weight and balance. Some of the more common terms are:

Arm (moment arm) - the horizontal distance in inches from the reference datum to the item. The algebraic sign is plus ( + ) If measured aft of the datum and minus ( - ) if measured forward of the datum;

Center of gravity (CG) - the point about which an aircraft would balance if it were possible to suspend it at that point. It is the mass center of the aircraft or the theoretical point at which the entire weight of the aircraft is assumed to be concentrated;

Center of gravity limits - the specified forward and aft points beyond which the CG must not be located during flight. The CG moment envelope is contained in the aircraft flight manual and FAA Aircraft Specifications or Type Certificate Data Sheets;

Center of gravity range - the distance between the forward and aft CG limits;

Datum line - an imaginary vertical plane or line from which all measurements of arm are taken. The datum is established by the manufacturer. After the datum is selected, all moment arms and the CG range must be computed with reference to that point;

Fuel load - the expendable part of the aircraft load. Fuel load includes only usable fuel and not the fuel required to fill the lines or that which remains trapped in the tank sumps;

Moment - the product of the weight of an item multiplied by its arm. Moments are expressed in inch pound (in.-lb.);

Total moment - the weight of the aircraft multiplied by the distance between the datum and the CG;

Moment index - the moment divided by a constant such as 100, 1,000, or 10,000. The purpose of using a moment index is to simplify computations of weight and balance on large aircraft where heavy items and long arms result in large unmanageable numbers. It is simply a matter of reduction to the least common denominator;

Mean aerodynamic chord (MAC) - the average distance from the leading edge to the trailing edge of the wing. The MAC is specified for the aircraft by determining the average chord of an imaginary wing which has tile same aerodynamic characteristics of the actual wing. Center of gravity is usually located at or near the forward 25 percent of the chord;

Station - a location in the aircraft which is identified by a number designating its distance in inches from the datum. The datum is therefore identified as zero and the station and arm are usually identical;

Useful load - the weight of the pilot, copilot, passengers, baggage, usable fuel, and drainable oil;

Empty weight - the airframe, engines, and all items of operating equipment that have fixed locations and are permanently installed in the aircraft. It includes optional and special equipment, fixed ballast, hydraulic fluid, unusable (residual) fuel, and undrainable (residual) oil.


A simple and fundamental weight check should always be made before flight to assure that the aircraft useful load is not exceeded. If there is the slightest doubt about the loading, it will be advisable to calculate it by using actual weights and moment arms to determine that the aircraft is loaded within safe limits.

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