Definition
The vertical movement of the horizon bar on the attitude indicator above or below the miniature aircraft, showing the airplane's pitch attitude relative to the natural horizon. Each bar width of displacement typically represents a specific change in pitch, allowing the pilot to read pitch attitude in precise increments.
Plain English
It is how far the horizon line on the attitude indicator moves up or down compared to the little airplane symbol. That distance tells the pilot how much the nose is pitched up or down.
Context Anchor
Seen while using the attitude indicator to hold or correct straight-and-level flight.
Derivation
Displacement comes from the Latin dis- (apart) and placere (to place) -- literally 'moved out of place.' Here it means the horizon bar has shifted from its centered position, and that shift is what the pilot reads.
Why Pilots Care
Allows precise pitch control when outside visual references are unavailable.
Grounding Statement
If the horizon bar is no longer in its normal relationship with the miniature aircraft, the attitude indicator is showing a change in the airplane’s nose position.
Intuition Check
Do not read this as the real outside horizon moving. The instrument’s horizon bar is moving on the display to show the aircraft’s attitude.
Example Sentence 1
A small horizon bar displacement above the miniature aircraft indicated a slight nose-down pitch, so the pilot applied gentle back pressure to return to level flight.
Example Sentence 2
The pilot applied forward pressure until the horizon bar displacement returned to center.