Bilge area. The lowest part of an aircraft structure in which water and contaminants collect.

Butt (or buttock) line zero. A line through the symmetrical center of an aircraft from nose to tail. It serves as the datum for measuring the arms used to determine the lateral CG. Lateral moments that cause the aircraft to rotate clockwise are positive (+), and those that cause it to rotate counterclockwise are negative (–).

Calendar month. A time period used by the FAA for certification and currency purposes. A calendar month extends from a given day until midnight of the last day of that month.

Civil Air Regulations (CAR). The predecessor to the Federal Aviation Regulations.

CAM. The acronym for the manuals containing the certification rules under the Civil Air Regulations.

Center of gravity (CG). (GAMA) The point at which an airplane would balance if suspended. Its distance from the reference datum is determined by dividing the total moment by the total weight of the airplane. 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. It may be expressed in percent of MAC (mean aerodynamic chord) or in inches from the reference datum.

Center of lift. The location along the chord line of an airfoil at which all the lift forces produced by the airfoil are considered to be concentrated.

Centroid. The distance in inches aft of the datum of the center of a compartment or a fuel tank for weight and balance purposes.

CG arm. (GAMA) The arm obtained by adding the airplane’s individual moments and dividing the sum by the total weight.

CG limits. (GAMA) The extreme CG locations within which the aircraft must be operated at a given weight. These limits are indicated on pertinent FAA aircraft type certificate data sheets, specifications, or weight and balance records.

CG limits envelope. An enclosed area on a graph of the airplane loaded weight and the CG location. If lines drawn from the weight and CG cross within this envelope, the airplane is properly loaded.

CG moment envelope. An enclosed area on a graph of the airplane loaded weight and loaded moment. If lines drawn from the weight and loaded moment cross within this envelope, the airplane is properly loaded.

Chord. A straight line distance across a wing from leading edge to trailing edge.

Curtailment. An operator created and FAA-approved operational loading envelope that is more restrictive than the manufacturer’s CG envelope. It ensures that the aircraft will be operated within limits during all phases of flight. Curtailment typically accounts for, but is not limited to, in-flight movement of passengers and crew, service equipment, cargo variation, seating variation, etc.

Datum. An imaginary vertical plane or line from which all measurements of arms are taken. The datum is established by the manufacturer. Once the datum has been selected, all moment arms and the location of CG range are measured from this point.

Delta (Δ). The Greek symbol “Δ” means a change in something. ΔCG means a change in the center of gravity location.

Dynamic load. The actual weight of the aircraft multiplied by the load factor, or the increase in weight caused by acceleration.

Empty weight. The weight of the airframe, engines, all permanently installed equipment, and unusable fuel. Depending upon the part of the federal regulations under which the aircraft was certificated, either the undrainable oil or full reservoir of oil is included.

Empty weight center of gravity (EWCG). This is the center of gravity of the aircraft in the empty condition, containing only the items specified in the aircraft empty weight. This CG is an essential part of the weight and balance record of the aircraft.

Empty weight center of gravity range. The distance between the allowable forward and aft empty-weight CG limits.