Definition
The horizontal distance, measured in inches, from the aircraft's reference datum to the center of gravity of an item. The arm is used together with an item's weight to calculate its moment for weight and balance purposes.
Plain English
How far an item sits, measured in inches, from a fixed starting line on the aircraft. That distance is one of the two numbers (the other is weight) used to figure out how the item affects balance.
Context Anchor
Seen in aircraft weight-and-balance charts, loading forms, and baggage or fuel calculations.
Derivation
From the everyday sense of 'arm' as the lever between a pivot and a load — like the arm of a seesaw. A longer arm means a small weight has a bigger balancing effect. Aviation uses the word the same way: the arm is the lever distance from the reference datum.
Why Pilots Care
Accurate arm values are required to compute the center of gravity; errors can produce an unstable aircraft.
Analogy
Think of a seesaw. The same weight has a bigger balancing effect when it is farther from the middle. An aircraft arm works the same way in the loading calculation.
Intuition Check
Arm does not mean a body part here. It means a measured distance from the aircraft’s fixed reference point to where a weight acts.
Example Sentence 1
The baggage compartment has an arm of 95 inches, so 50 pounds of bags loaded there produces a moment of 4,750 inch-pounds.
Example Sentence 2
Placing the same weight at a longer arm moves the center of gravity farther aft.