Definition
The movement of the elevator control surface up or down from its neutral position when the pilot pushes or pulls the control yoke or stick. This deflection changes the airflow over the tail, which raises or lowers the aircraft's nose and adjusts pitch attitude.
Plain English
How far the elevator on the tail moves away from its resting position when you push or pull the controls. The bigger the push or pull, the more the elevator moves, and the more the nose pitches up or down.
Context Anchor
Seen in instrument flying when using the attitude indicator to set or correct pitch during straight-and-level flight.
Derivation
Displacement' comes from the Latin 'dis-' (apart) and 'placere' (to place) -- literally 'moved out of place.' In aviation it simply means how far a control surface has moved from its neutral, streamlined position.
Why Pilots Care
On instruments, pitch is controlled with very small elevator movements. Over-displacing the elevator causes the nose to chase up and down, leading to altitude excursions and a porpoising ride. Smooth, small displacements keep the aircraft stable.
Grounding Statement
A small forward or aft control movement changes the elevator’s position, and the attitude indicator shows the resulting nose movement.
Intuition Check
“Elevator” does not mean a lift inside a building here; it means the airplane control surface that helps move the nose up or down. “Displacement” does not mean how far the airplane has traveled; it means how far that control surface has moved from neutral.
Example Sentence 1
A small elevator displacement is usually all that's needed to correct a minor pitch deviation on the attitude indicator.
Example Sentence 2
The pilot reduced elevator displacement to prevent the airplane from climbing during level flight.