Definition
A propeller or engine arrangement in which the line along which thrust acts passes below the aircraft's center of gravity. Because thrust is offset below the CG, an increase in power produces a nose-up pitching moment, while a reduction in power produces a nose-down pitching moment.
Plain English
The engine's pulling or pushing force acts along an imaginary line that sits lower than the aircraft's balance point. When you add power, the nose tends to rise. When you reduce power, the nose tends to drop.
Context Anchor
Seen in longitudinal stability discussions, especially when explaining why some airplanes pitch up or down when power is changed.
Derivation
"Thrust line" simply names the imaginary straight line along which the engine's thrust acts. "Low" describes its position relative to the center of gravity. The term is descriptive: a thrust line that sits low on the airframe relative to the CG.
Why Pilots Care
Power changes can alter pitch attitude unexpectedly, requiring pilot compensation to maintain level flight.
Analogy
If you push a box forward from below its center, the box tends to tip upward. A low thrust line works in a similar way on an airplane.
Intuition Check
Low does not mean low power or low speed here. It means the engine’s forward push acts below the airplane’s balance point.
Example Sentence 1
Because the trainer has a low thrust line, the student noticed the nose pitched up noticeably each time he advanced the throttle.
Example Sentence 2
Designers used a low thrust line to improve stability during power changes in cruise.