Definition
The perpendicular distance from a pivot point or reference axis to the line of action of a force. In aircraft performance and weight-and-balance, it is the distance from a reference (such as the center of gravity, an axis of rotation, or a datum) to the point where a force is applied, used to calculate the turning effect (moment) that force produces.
Plain English
How far away a force is pushing or pulling from the point something turns around. The longer this distance, the more turning effect the same force produces.
Context Anchor
Seen in multi-engine airplane discussions of engine failure, especially when explaining why one operating engine can turn the airplane’s nose toward the failed engine.
Derivation
From the mechanical idea of a lever arm. 'Moment' comes from Latin momentum, meaning 'movement' or 'turning influence,' and 'arm' refers to the length of the lever. Together it describes the lever length that converts a force into a turning effect.
Why Pilots Care
A longer moment arm increases the yaw created by asymmetric thrust, raising the speed at which the pilot can still maintain directional control.
Analogy
Think of a wrench on a bolt. The same hand-force at the end of a long wrench turns the bolt much harder than at the middle. The length of the wrench from your hand to the bolt is the moment arm.
Intuition Check
Moment does not mean a short period of time here, and arm does not mean a body part. Together, moment arm means the distance that gives a force leverage around a turning point.
Example Sentence 1
Because the operating engine sits well out on the wing, its thrust acts through a long moment arm, producing a strong yawing tendency toward the dead engine.
Example Sentence 2
Reducing the moment arm by banking slightly toward the operating engine helps lower VMC.