Definition
The difference in an aircraft's total moment caused by adding, removing, or relocating weight. Because moment equals weight multiplied by arm (the distance from the reference datum), any change in the load or its position produces a corresponding change in the aircraft's total moment, which in turn shifts the center of gravity.
Plain English
How much the aircraft's balance figure goes up or down when you move, add, or take off weight. Move a bag forward and the moment changes one way; move it aft and it changes the other.
Context Anchor
Used in weight-and-balance work when calculating how moving passengers, baggage, fuel, or equipment changes the aircraft’s balance.
Derivation
Moment comes from the Latin momentum, meaning movement or turning effect. In weight and balance, a moment is the turning effect a weight produces around the datum. A moment change is simply how much that turning effect increases or decreases when something on the aircraft is moved, added, or removed.
Why Pilots Care
An unchecked moment change can move the center of gravity outside its limits, reducing stability or making the aircraft difficult or unsafe to control.
Analogy
Think of a seesaw. Moving the same person farther from the middle changes the balancing effect more than moving that person only a little.
Intuition Check
Moment does not mean a short amount of time here. In this context, it means the balancing effect of weight at a distance, and moment change is how much that balancing effect changes.
Example Sentence 1
Moving the 50-pound toolbox from the aft baggage area to the forward compartment produced a moment change that shifted the center of gravity forward by two inches.
Example Sentence 2
The fuel pump moving 20 gallons from the right tank to the left produced a moment change of 1,200 pound-inches.