The student of weight and balance needs to be familiar with terms used in publications related to many aspects of the subject. These terms are fairly well standardized; however, terms related to general aviation aircraft do not always apply to air carrier aircraft. Where there is a difference, the following definitions will indicate to which type of aircraft the term applies.

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

2. Center of gravity (c.g.) - is 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 he concentrated. It may be expressed in percent of MAC (mean aerodynamic chord) or in inches from the reference datum.

3. Center of gravity limits - are the specified forward and aft or lateral points beyond which the c.g. must not be located during takeoff, flight or landing. These limits are indicated on pertinent FAA aircraft type certificate data sheets, specifications, or weight and balance records, and meet the requirements of Federal Aviation Regulations.

4. Center of gravity range - is the distance between the forward and aft c.g. limits indicated on pertinent aircraft specifications.

5. Datum (reference datum) - is an imaginary vertical plane or line from which all measurements of arm are taken. The datum is established by the manufacturer. Once the datum has been selected, all moment arms and the location of permissible c.g. range must be taken with reference to that point.

6. Delta - is a Greek letter expressed by the symbol . It is used in weight and balance calculations, as well as in other forms of mathematics, to indicate a change of values. As an example, delta c.g. indicates a change (or movement) of the c.g.

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

{See Figure 8}

8. LEMAC - is the leading edge of the mean aerodynamic chord.

9. Moment - is the product of the weight of an item multiplied by its arm. Moments are expressed in pound-inches (lb-in) or inch-pounds. Total moment is the weight of the aircraft multiplied by the distance between the datum and the c.g.

10. 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 large aircraft where heavy items and long arms result in large, unmanageable numbers.

11. Mean aerodynamic chord (MAC) - is 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 the same aerodynamic characteristics as the actual wing.

12. Reduction factor - is the constant which when divided into a moment results in an index. Reduction factors of 100, 1,000, or 10,000 are used to simplify weight and balance calculation processes.

13. Standard weights - have been established for numerous items involved in weight and balance computations. These weights are not to be used in lieu of available actual weights. Standard passenger weights should not be used in computing the weight and balance of charter flights and other special services involving the carriage of special groups; e.g., athletic groups, small foreign nationals, etc. Some of the standard weights are:

14. Station - is 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 station zero. The station and arm are usually identical. An item located at station +50 would have an arm of 50 inches.

15. Useful load - is the weight of the pilot, copilot, passengers, baggage, usable fuel, and drainable oil. It is the empty weight subtracted from the maximum allowable takeoff weight. This term applies to general aviation aircraft only.

16. Weight, basic operating - is the weight of the aircraft, including the crew, ready for flight but without payload and fuel. This term is only applicable to transport aircraft.

17. Weight, empty - consists of 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, and undrainable (residual) fuel and oil. When oil is used for propeller feathering, such oil is included as residual oil.

18. Weight, maximum landing - is the maximum weight at which the aircraft may normally be landed. The maximum landing weight may be limited to a lesser weight when runway length or atmospheric conditions are adverse.

19. Weight, maximum takeoff - is the maximum allowable weight at the start of the takeoff run. Some aircraft are approved for loading to a greater weight (ramp or taxi) only to allow for fuel burnoff during ground operation. The takeoff weight for a particular flight may be limited to a lesser weight when runway length, atmospheric conditions, or other variables are adverse.

20. Weight, maximum allowable zero fuel - is the maximum weight authorized for the aircraft not including fuel load. Zero fuel weight for each particular flight is the operating weight plus the payload.

21. Weight, ramp or taxi - is the maximum takeoff gross weight plus fuel to be burned during taxi and runup.

NOTE - The weights above are used for illustration only. The actual values will vary for each aircraft and each flight.