Definition
An imaginary line passing through the center of the propeller hub (or jet engine) along the direction in which the engine produces thrust. It defines the axis along which the propulsive force acts on the airplane.
Plain English
It is the straight line showing which way the engine is pushing or pulling the airplane. If you drew an arrow out from the middle of the propeller in the direction it is pulling, that arrow is the thrust line.
Context Anchor
You will see this term in airplane handling and design discussions, especially when explaining how power changes can affect the airplane’s nose position.
Derivation
Thrust means a pushing force, and line means a straight path or reference. Together, thrust line means the straight path of the engine’s push, even though the line itself is usually imaginary.
Why Pilots Care
An offset or angled thrust line creates pitching or yawing moments that affect trim, climb performance, and handling characteristics.
Analogy
Think of pushing a shopping cart. If you push straight through the middle, it goes straight ahead. If you push from above, below, or off to one side, the cart tends to tilt or turn as well as move forward.
Intuition Check
Do not picture the thrust line as a painted line on the airplane. It is an imaginary line showing the direction the engine’s push acts.
Example Sentence 1
Because the thrust line is mounted above the center of gravity on this design, adding power causes a slight nose-down pitching tendency.
Example Sentence 2
The designer angled the thrust line slightly downward to reduce the effects of P-factor during climb.