Definition
The horizontal distance, in inches, from the aircraft's reference datum to the center of gravity of an item. Arm is expressed as a positive value if the item is aft of the datum and a negative value if forward of it. It is one of the two values used in weight and balance calculations, the other being weight, with their product giving the moment.
Plain English
The distance between a fixed starting point on the aircraft (the datum) and where a piece of weight sits. It is measured in inches, and it tells you how far that weight is from the reference point used for balance calculations.
Context Anchor
Seen in aircraft weight-and-balance calculations, loading charts, equipment lists, and maintenance records.
Derivation
From mechanics, where 'arm' is the lever arm — the distance from a pivot point to where a force is applied. The longer the arm, the greater the leverage, which is why distance from the datum matters as much as the weight itself.
Why Pilots Care
Accurate arm values keep the center of gravity within safe limits so the aircraft handles properly.
Analogy
Think of a seesaw. A small child sitting far from the center can balance a larger child sitting close to the center. The 'arm' is how far each one sits from the pivot — and it matters just as much as how heavy they are.
Intuition Check
Arm does not mean a body part or a bracket here. It means a measured distance from the aircraft's fixed reference line.
Example Sentence 1
The pilot's seat has an arm of plus 37 inches, meaning it sits 37 inches aft of the datum.
Example Sentence 2
Adding the battery's weight and arm to the chart updated the total moment for the flight.