 G-2 Centroid. The distance in inches aft of the datum of the  center of a compartment or a fuel tank for weight and balance  purposes. Permanent ballast (fixed ballast).  A weight permanently  installed in an aircraft to bring its CG into allowable limits.  Permanent ballast is part of the aircraft empty weight. Temporary ballast.  Weights that can be carried in a cargo  compartment of an aircraft to move the location of CG for a  specifi fl ght condition. Temporary ballast must be removed  when the aircraft is weighed. 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 Basic empty weight.  (GAMA) Standard empty weight plus  optional equipment. Basic operating index.  The moment of the airplane at its  basic operating weight divided by the appropriate reduction  factor. 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. Basic operating weight (BOW).  The empty weight of the  aircraft plus the weight of the required crew, their baggage,  and other standard items, such as meals and potable water. 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. Bilge area.  The lowest part of an aircraft structure in which  water and contaminants collect. Chord.  A straight line distance across a wing from leading  edge to trailing edge. 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.  Lateralmoments thatcausetheaircraftto rotateclockwiseare  positive (+) , and those that cause it to rotate counterclockwise  are negative (–). 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. 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. 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. 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 Delta (Δ).  The Greek symbol “Δ” means a change in  something. ΔCG means a change in the center of gravity  location. 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 cord) or in inches  from the reference datum. Centerof lift.  The location along thechord line of an airfoilat  which all the lift forces produced by the airfoil are considered  to be concentrated. 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.