Definition
A shift in the aircraft's pitching moment that tends to rotate the nose upward about the lateral axis. In ground effect, the reduction in downwash at the horizontal tail decreases the tail's download, producing a nose-up change in moment that the pilot must counter to maintain the desired pitch attitude.
Plain English
A change in the forces acting on the airplane that pushes the nose up. Something has shifted the balance of the aircraft so that, without pilot input, the nose would rise.
Context Anchor
Seen in ground effect discussions, especially during takeoff, landing, and flight very close to the runway.
Derivation
A 'moment' in physics is a turning effect — a force acting at a distance from a pivot point. The aircraft pivots about its center of gravity, so any force that tries to rotate it nose-up creates a nose-up moment. Knowing 'moment' means 'turning effect' makes the phrase straightforward: it is a change in the turning effect that pitches the nose up.
Why Pilots Care
The nose-up pitching tendency can cause the aircraft to balloon upward or require unexpected forward control pressure near the runway.
Analogy
It is like a seesaw getting a little more force on the side that lifts one end. The airplane is not just moving upward; it is tending to rotate nose-up.
Grounding Statement
Just above the runway, the changed airflow in ground effect can make the airplane want to raise its nose.
Intuition Check
Moment does not mean a short amount of time here. It means a turning tendency, and nose-up tells you which way the aircraft tends to rotate.
Example Sentence 1
As the airplane entered ground effect on short final, the reduced downwash on the tail produced a nose-up change in moment that the pilot trimmed out.
Example Sentence 2
During the landing flare the nose-up change in moment can lift the nose higher than intended if the pilot does not anticipate it.