The total moment of an airplane is the weight multiplied by the distance from the datum to the point of application (the arm). What is this distance commonly called?

• A. Arm length
• B. Moment arm
• C. CG distance
• D. Datum distance

Answer: (B)

The distance from the datum to where the weight acts is called the moment arm. The moment (or torque) produced by a force is the force multiplied by this distance, which is the lever that converts linear force into a rotational effect about the datum. In aircraft weight and balance, this distance is what you multiply by the weight to get the moment. While the word arm is sometimes used, the standard term for this distance in the moment calculation is moment arm. Arm length isn’t the common term here, CG distance refers to the location of the center of gravity itself relative to the datum, and datum distance isn’t a standard term.