Definition
The fundamental weight and balance formula stating that the moment of any item loaded in an aircraft equals its weight (in pounds) multiplied by its arm (the horizontal distance in inches from the reference datum). Moment is expressed in pound-inches (lb-in) and represents the turning effect that weight produces about the datum. Summing all moments and dividing by total weight yields the center of gravity location.
Plain English
How much an item shifts the airplane's balance point depends on two things: how heavy it is, and how far it sits from the reference point. Multiply those two numbers together and you get the 'moment' — a single number that captures its balancing effect.
Context Anchor
Seen during weight-and-balance calculations before flight, especially when adding fuel, passengers, baggage, or equipment.
Derivation
Arm comes from the idea of a lever arm — the distance from a pivot point where force is applied. Moment comes from Latin 'momentum,' meaning movement or turning effect. Together they describe how a weight at a distance produces a turning force, the same principle as a seesaw.
Why Pilots Care
Accurate moment values are required to keep the center of gravity within certified limits, ensuring stable flight characteristics and safe handling.
Analogy
Think of a seesaw. A small child far from the center can balance a larger adult sitting close in. Weight times distance — that product is the moment.
Grounding Statement
A heavy bag near the tail affects the airplane's balance more than the same bag placed closer to the reference point.
Intuition Check
Moment does not mean a brief period of time here. In weight and balance, moment means the balancing effect of a weight at a measured distance from the aircraft's reference line.
Example Sentence 1
Using weight x arm = moment, the pilot calculated that 50 pounds of baggage at an arm of 95 inches produced a moment of 4,750 pound-inches.
Example Sentence 2
All individual moments are added together before dividing by total weight to locate the center of gravity.