Definition
The horizontal distance from the reference datum to the airplane's center of gravity, expressed in inches. The CG arm is found by dividing the total moment by the total weight of the airplane.
Plain English
It is how far the airplane's balance point sits from a fixed starting line on the airplane, measured in inches. You find it by dividing the airplane's total moment by its total weight.
Context Anchor
Seen in weight-and-balance calculations, loading worksheets, and the airplane’s operating handbook.
Derivation
Arm' comes from the idea of a lever arm — the distance from a pivot point at which a force acts. In weight and balance, the 'arm' is the distance from the reference datum (the pivot reference) to where a weight sits. The CG arm is simply the arm of the airplane's overall balance point.
Why Pilots Care
It determines whether the airplane is loaded within approved limits, directly affecting stability, control feel, and stall recovery.
Analogy
Think of balancing a loaded board on a support. The CG arm is the distance from a chosen starting mark on the board to the spot where the board balances.
Intuition Check
Arm does not mean a physical arm or airplane part here. It means a measured distance from a fixed reference point to the airplane’s balance point.
Example Sentence 1
After loading the passengers and baggage, the pilot calculated a CG arm of 88.5 inches, which fell within the approved range.
Example Sentence 2
After moving baggage aft, the new CG arm increased, requiring a forward ballast adjustment.