All pilots need to be familiar with the terms related to aircraft weight and balance.
Some of the more common terms are:
Arm (moment arm)  the horizontal distance in inches from the reference
datum to the item. The algebraic sign is plus ( + ) If measured aft of the datum and
minus (  ) if measured forward of the datum;
Center of gravity (CG)  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 be concentrated;
Center of gravity limits  the specified forward and aft points beyond
which the CG must not be located during flight. The CG moment envelope is contained
in the aircraft flight manual and FAA Aircraft Specifications or Type Certificate Data
Sheets;
Center of gravity range  the distance between the forward and aft CG limits;
Datum line  an imaginary vertical plane or line from which all measurements
of arm are taken. The datum is established by the manufacturer. After the datum is
selected, all moment arms and the CG range must be computed with reference to that
point;
Fuel load  the expendable part of the aircraft load. Fuel load includes
only usable fuel and not the fuel required to fill the lines or that which remains
trapped in the tank sumps;
Moment  the product of the weight of an item multiplied by its arm.
Moments are expressed in inch pound (in.lb.);
Total moment  the weight of the aircraft multiplied by the distance
between the datum and the CG;
Moment index  the moment divided by a constant such as 100, 1,000, or
10,000. The purpose of using a moment index is to simplify computations of weight
and balance on large aircraft where heavy items and long arms result in large
unmanageable numbers. It is simply a matter of reduction to the least common
denominator;
Mean aerodynamic chord (MAC)  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 tile same aerodynamic
characteristics of the actual wing. Center of gravity is usually located at or near
the forward 25 percent of the chord;
Station  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 zero and the station and arm are usually identical;
Useful load  the weight of the pilot, copilot, passengers, baggage,
usable fuel, and drainable oil;
Empty weight  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, unusable (residual)
fuel, and undrainable (residual) oil.
Reminders:
A simple and fundamental weight check should always be made before flight to assure that
the aircraft useful load is not exceeded. If there is the slightest doubt about the
loading, it will be advisable to calculate it by using actual weights and moment
arms to determine that the aircraft is loaded within safe limits.
