A number of terms are used in reference to airplane weight and balance.
These terms are described below:
Weight Terms
Unusable Fuel: fuel remaining in the fuel tanks after the engine stops from fuel starvation
Usable Fuel: the total fuel tank capacity minus unusable fuel
Standard Empty Weight: Includes the airframe, engines, full operating fluids (such as engine oil), unusable fuel, and all permanently installed equipment
Basic Empty Weight: standard empty weight plus optional equipment
Payload: the weight of pilots, passengers, cargo, and baggage
Useful Load: maximum allowable gross weight minus basic empty weight. It is the weight of the pilots, passengers, baggage, usable fuel, and drainable oil.
Maximum Takeoff Weight: the structural weight limitation for takeoff
Maximum Landing Weight: the structural weight limitation for the airplane upon landing, which may be less than the maximum takeoff weight because of fuel burn during flight.
Maximum Ramp Weight: the weight limitation for taxi, which may be greater than the maximum takeoff weight because of fuel burned during taxi.
Maximum Zero Fuel Weight: the weight limitation for the airplane when there is no usable fuel on board. Also commonly referred to as the maximum wing bending weight.
The downward force created by the weight of fuel in wing fuel tanks counters the upward force on the wings created by lift. By creating a weight limitation for the airplane when there is no fuel on board, manufacturers can ensure that the upward forces exerted by lift on the wings will not exceed the structural bending limitations of the wings and their attachment points.
CG Calculation Terms
Reference Datum Plane: an imaginary vertical plane from which all moment arm measurements are made. The datum plane is fixed and set by the manufacturer. It is set in a place that is permanently fixed on the airplane, a place that cannot move with if equipment is added or removed or the airplane modified. The engine firewall is often used as a reference datum. Manufacturers also commonly set the reference datum at a point out in front of the airplane, which assures all stations are behind the datum and all arms are positive. For example, a manufacturer might list the datum as being 150 inches forward of the engine firewall.
Station or Arm: the location of an item, in reference to the reference datum. An item located at a point in the airplane which is aft of the datum will have a positive arm number, while an item forward of the datum will have a negative station. For example, an item at station 26 is 26 inches aft of the datum plane. An item at station -7 is 7 inches forward of the datum plane.
Moment: Moment is weight times arm. (MOMENT = WEIGHT x ARM)
Example: If an item at station 10 weighs 5 pounds, then it's moment is...
Moment = Weight x Arm = 10 inches x 5 pounds = 50 pound-inches
More on Moment
Here is an example to help better understand moments. Imagine a board laid across a point, which supports the board only at its mid point, like a see-saw. If five pounds is positioned 10 inches to the right of the mid point, how much weight will be required to bring the board back into balance? Where must this weight be positioned?
The idea of moment allows these questions to be answered by allowing the distance to be factored in with the weight. Five pounds could be placed 10 inches to the left of the mid point. However, if we only had one pound to place on the board, placing that one pound 50 inches to the left of the mid point would have the same effect.
By finding the moment, the weight-distance combinations available become apparent. Five pounds times 10 inches is 50 pound-inches on the right side. Any combination of weight and distance that results in 50 pound-inches on the left side will balance the board. One pound times 50 inches works just fine. Fifty pounds placed one inch to the left, or 10 pounds placed five inches to the left, would both do the trick, as well.
Moments Applied to Airplanes
The occupant of a passenger seat located very near the airplane's center of gravity will have a minimal effect on the CG location. The further away something is from the CG, the greater ability they have to affect the CG location.
The tail is placed far behind the airplane in order to give the horizontal and vertical stabilizers more ability to effect its flight path. The further away from the CG the tail is located, the smaller the forces they must exert to have the same effect on the airplane. The ailerons are located on the outer portions of the wing, to give them better ability to induce a roll. The calculation of moments allows the pilot to determine if the airplane is safely balanced before flight.