•  CG limits—the specified forward and aft points within  which the CG must be located during flight. These  limits are indicated on pertinent aircraft specifications.  •  CG range—the distance between the forward and aft  CG limits indicated on pertinent aircraft specifications.  •  Datum (reference datum)—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 CG range are measured from this  point.  •  Delta—a Greek letter expressed by the symbol  r  to  indicate a change of values. As an example,  r CG  indicates a change (or movement) of the CG.  •  Floor load limit—the maximum weight the floor  can sustain per square inch/foot as provided by the  manufacturer.  •  Fuel load—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.  •  Licensed empty weight—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—the greatest weight that  an aircraft is normally allowed to have at landing.  •  Maximum ramp weight—the total weight of a loaded  aircraft including all fuel. It is greater than the takeoff  weight due to the fuel that will be burned during the  taxi and run-up operations. Ramp weight may also be  referred to as taxi weight.  •  Maximum takeoff weight—the maximum allowable  weight for takeoff.  •  Maximum weight—the maximum authorized weight  of the aircraft and all of its equipment as specified in  the TCDS for the aircraft.  •  Maximum zero fuel weight (GAMA)—the maximum  weight, exclusive of usable fuel.  •  Mean aerodynamic chord (MAC)—the average  distance from the leading edge to the trailing edge of  the wing.  •  Moment—the product of the weight of an item  multiplied by its arm. Moments are expressed in  pound-inches (in-lb). Total moment is the weight of  the airplane multiplied by the distance between the  datum and the CG.  •  Useful load—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 (GA) aircraft only.  •  Station—a location in the aircraft 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.  •  Standard weights—established weights 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:  •  Standard empty weight (GAMA)—aircraft weight  that consists of the airframe, engines, and all items of  operating equipment that have fixed locations and are  permanently installed in the aircraft, including fixed  ballast, hydraulic fluid, unusable fuel, and full engine  oil.  •  Payload (GAMA)—the weight of occupants, cargo,  and baggage.  •  Moment index (or index)—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 aircraft where heavy items  and long arms result in large, unmanageable numbers.  It is imperative that all pilots understand the basic principles  of weight and balance determination. The following methods  of computation can be applied to any object or vehicle for  which weight and balance information is essential.  By determining the weight of the empty aircraft and adding  the weight of everything loaded on the aircraft, a total weight  can be determined—a simple concept. A greater problem,  particularly if the basic principles of weight and balance are  not understood, is distributing this weight in such a manner  that the entire mass of the loaded aircraft is balanced around  a point (CG) that must be located within specified limits.  The point at which an aircraft balances can be determined by  locating the CG, which is, as stated in the definitions of terms,  Principles of Weight and Balance Computations  10-5  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