The pilot should be familiar with terms used in working the problems related to weight and balance. The following list of terms and their definitions is well standardized, and knowledge of these terms will aid the pilot to better understand weight and balance calculations of any airplane. Terms defined by the General Aviation Manufacturers Association as an industry standard are marked in the titles with GAMA.
- Arm (moment arm)—is the horizontal distance in inches from the reference datum line to the center of gravity of an item. The algebraic sign is plus (+) if measured aft of the datum, and minus (–) if measured forward of the datum.
- Basic empty weight (GAMA)—includes the standard empty weight plus optional and special equipment that has been installed.
- Center of gravity (CG)—is the point about which an airplane would balance if it were possible to suspend it at that point. It is the mass center of the airplane, or the theoretical point at which the entire weight of the airplane is assumed to be concentrated. It may be expressed in inches from the reference datum, or in percent of mean aerodynamic chord (MAC).
- Center-of-gravity limits—is the specified forward and aft points within which the CG must be located during flight. These limits are indicated on pertinent airplane specifications.
- Center-of-gravity range—is the distance between the forward and aft CG limits indicated on pertinent airplane specifications.
- Datum (reference datum)—is an imaginary vertical plane or line from which all measurements of arm are taken. The manufacturer establishes the datum. Once the datum has been selected, all moment arms and the location of CG range are measured from this point.
- Delta—is a Greek letter expressed by the symbol �to indicate a change of values. As an example, �CG indicates a change (or movement) of the CG.
- Floor load limit—is the maximum weight the floor can sustain per square inch/foot as provided by the manufacturer.
- Fuel load—is the expendable part of the load of the airplane. It includes only usable fuel, not fuel required to fill the lines or that which remains trapped in the tank sumps.
- Licensed empty weight—is the empty weight that consists of the airframe, engine(s), unusable fuel, and undrainable oil plus standard and optional equipment as specified in the equipment list. Some manufacturers used this term prior to GAMA standardization.
- Maximum landing weight—is the greatest weight that an airplane normally is allowed to have at landing.
- Maximum ramp weight—is the total weight of a loaded aircraft, and includes all fuel. It is greater than the takeoff weight due to the fuel that will be burned during the taxi and runup operations. Ramp weight may also be referred to as taxi weight.
- Maximum takeoff weight—is the maximum allowable weight for takeoff.
- Maximum weight—is the maximum authorized weight of the aircraft and all of its equipment as specified in the Type Certificate Data Sheets (TCDS) for the aircraft.
- Maximum zero fuel weight (GAMA)—is the maximum weight, exclusive of usable fuel.
- Mean aerodynamic chord (MAC)—is the average distance from the leading edge to the trailing edge of the wing.
- Moment—is the product of the weight of an item multiplied by its arm. Moments are expressed in pound-inches (lb-in). Total moment is the weight of the airplane multiplied by the distance between the datum and the CG.
- Moment index (or index)—is a moment divided by a constant such as 100, 1,000, or 10,000. The purpose of using a moment index is to simplify weight and balance computations of airplanes where heavy items and long arms result in large, unmanageable numbers.
- Payload (GAMA)—is the weight of occupants, cargo, and baggage.
- Standard empty weight (GAMA)—consists of the airframe, engines, and all items of operating equipment that have fixed locations and are permanently installed in the airplane; including fixed ballast, hydraulic fluid, unusable fuel, and full engine oil.
- Standard weights—have been established for numerous items involved in weight and balance computations. These weights should not be used if actual weights are available. Some of the standard weights are:
- Gasoline . . . . . . . . . . . . . . . . . . . . . 6 lb/US gal
- Jet A, Jet A-1 . . . . . . . . . . . . . . . . 6.8 lb/US gal
- Jet B . . . . . . . . . . . . . . . . . . . . . . . 6.5 lb/US gal
- Oil . . . . . . . . . . . . . . . . . . . . . . . . 7.5 lb/US gal
- Water . . . . . . . . . . . . . . . . . . . . . 8.35 lb/US gal
- Station—is a location in the airplane that is identified by a number designating its distance in inches from the datum. The datum is, therefore, identified as station zero. An item located at station +50 would have an arm of 50 inches.
- Useful load—is the weight of the pilot, copilot, passengers, baggage, usable fuel, and drainable oil. It is the basic empty weight subtracted from the maximum allowable gross weight. This term applies to general aviation aircraft only.
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