Definition
An imaginary line extending forward from the propeller or jet engine along the direction in which the engine produces thrust. It represents the path along which engine force acts on the airframe, and its position relative to the aircraft's center of gravity affects pitch behavior when power is added or reduced.
Plain English
It is the invisible line showing the direction the engine is pushing or pulling the airplane. Where this line sits compared to the airplane's balance point changes how the nose moves up or down when you change power.
Context Anchor
Seen in flight control discussions when explaining why power changes can make the nose want to rise or lower, and why the elevator may be needed to hold the desired pitch attitude.
Derivation
A compound of 'thrust' (the forward force produced by the engine) and 'line' (a straight path). Together it names the straight path along which engine thrust acts.
Why Pilots Care
A thrustline offset from the center of gravity produces pitching moments that must be countered with elevator input, especially during power changes.
Grounding Statement
Picture the engine pushing along an invisible straight path through the airplane; if that push is not lined up with the airplane’s balance point, it can try to rotate the nose.
Intuition Check
The thrustline is not a painted line on the airplane and it is not the path the airplane flies. It is an imaginary line showing the direction of the engine’s push.
Example Sentence 1
Because the thrustline is above the center of gravity on this design, applying full power produces a slight nose-down tendency.
Example Sentence 2
The designer positioned the engine to keep the thrustline aligned with the longitudinal axis and minimize pitch trim changes.