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of an aircraft:
• The total weight of the aircraft must be no greater than
the maximum gross weight allowed by the FAA for the
particular make and model of the aircraft.
• The center of gravity, or the point at which all of the
weight of the aircraft is considered to be concentrated,
must be maintained within the allowable range for the
operational weight of the aircraft.
Aircraft Arms, Weights, and Moments
The term arm, usually measured in inches, refers to the
distance between the center of gravity of an item or object
and the reference datum. Arms ahead of, or to the left of
the datum are negative (–), and those behind, or to the right
of the datum are positive (+). When the datum is ahead of
the aircraft, all of the arms are positive and computational
errors are minimized.
Weight is normally measured in pounds. When weight is
removed from an aircraft, it is negative (–), and when added,
it is positive (+).
There are a number of weights that must be considered in
aircraft weight and balance. The following are terms for
various weights as used by the General Aviation Manufacturers
Association (GAMA).
• The standard empty weight is the weight of the
airframe, engines and all items of operating weight that
have fixed locations and are permanently installed in the
aircraft. This weight must be recorded in the aircraft
weight and balance records. The basic empty weight
includes the standard empty weight plus any optional
equipment that has been installed.
• Maximum allowable gross weight is the maximum weight
authorized for the aircraft and all of its contents as
specified in the Type Certificate Data Sheets (TCDS) or
Aircraft Specifications for the aircraft.
• Maximum landing weight is the greatest weight that an
aircraft normally is allowed to have when it lands.
• Maximum takeoff weight is the maximum allowable
weight at the start of the takeoff run.
• Maximum ramp weight is the total weight of a loaded
aircraft, and includes all fuel. It is greater than the
takeoff weight due to the fuel that will be burned during
the taxi and runup operations. Ramp weight is also
called taxi weight.
The manufacturer establishes the allowable gross weight and
the range allowed for the CG, as measured in inches from a
reference plane called the datum. In large aircraft, this range
is measured in percentage of the mean aerodynamic chord
(MAC), the leading edge of which is located a specified
distance from the datum.
The datum may be located anywhere the manufacturer
chooses; it is often the leading edge of the wing or some
specific distance from an easily identified location. One
popular location for the datum is a specified distance forward
of the aircraft, measured in inches from some point such as
the leading edge of the wing or the engine firewall.
Arm (GAMA): The horizontal
distance from the reference datum to
the center of gravity (CG) of an item.
Standard empty weight (GAMA):
Weight of a standard airplane
including unusable fuel, full
operating fluids and full oil.
Basic empty weight (GAMA):
Standard empty weight plus optional
equipment.
Maximum landing weight
(GAMA): Maximum weight
approved for the landing touchdown.
Maximum takeoff weight (GAMA):
Maximum weight approved for the
start of the takeoff run.
Maximum ramp weight (GAMA):
Maximum weight approved for
ground maneuver. (It includes weight
of start, taxi, and runup fuel.)
2– 2
The datum of some helicopters is the center of the rotor mast,
but this location causes some arms to be positive and others
negative. To simplify weight and balance computations, most
modern helicopters, like airplanes, have the datum located at
the nose of the aircraft or a specified distance ahead of it.
A moment is a force that tries to cause rotation, and is the
product of the arm, in inches, and the weight, in pounds.
Moments are generally expressed in pound-inches (lb-in) and
may be either positive or negative. Figure 2-1 shows the way
the algebraic sign of a moment is derived. Positive moments
cause an airplane to nose up, while negative moments cause
it to nose down.
Consider these facts about the lever in Figure 2-2: The 100-
pound weight A is located 50 inches to the left of the fulcrum
(the datum, in this instance), and it has a moment of
100°?–50 = –5,000 lb-in. The 200-pound weight B is located
25 inches to the right of the fulcrum, and its moment is 200°
+25 = +5,000 lb-in. The sum of the moments is –5,000
　
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