Definition
An imaginary straight line extending forward from the engine through the propeller (or out the back of a jet engine) along the direction in which the engine produces thrust. Its position relative to the aircraft's center of gravity affects pitching tendencies when power is added or reduced.
Plain English
The line along which the engine pushes or pulls the aircraft. Where this line sits compared to the aircraft's balance point changes how the nose moves when you add or take away power.
Context Anchor
Seen in longitudinal stability discussions, especially when explaining why adding or reducing power can change pitch.
Derivation
“Thrust” means a push. “Line” means a straight path or reference. Here, the term means the straight path of the engine’s push, not a line painted on the airplane. “Power” in this phrase refers to engine power producing that push.
Why Pilots Care
If the line passes above or below the center of gravity, adding or reducing power creates a pitching moment that must be countered with elevator input to maintain stable flight.
Analogy
It is like pushing a shopping cart above or below its balance point. The cart still moves forward, but the push can also make it want to tip or rotate slightly.
Grounding Statement
If the engine’s push does not pass through the airplane’s balance point, changing power can also create a pitching effect.
Intuition Check
Do not picture a physical line on the airplane. The power or thrust line is an invisible reference showing the direction and location of the engine’s push.
Example Sentence 1
Because the thrust line on this airplane sits above the center of gravity, adding full power on a go-around tends to pitch the nose down.
Example Sentence 2
The pilot trims the elevator to counteract the pitching moment created when the thrust line and center of gravity are not aligned.