Definition
A multi-engine airplane in which the engines are mounted along the longitudinal axis (front and rear of the fuselage) so that their thrust lines pass through, or very close to, the aircraft's center line. Because thrust is aligned with the center line, the loss of one engine produces little or no asymmetric yawing moment. The Cessna 337 Skymaster is the classic example.
Plain English
A twin-engine airplane with one engine at the front and one at the back, instead of one on each wing. Because both engines push along the middle of the airplane, losing one doesn't try to swing the nose sideways the way it would in a normal twin.
Context Anchor
Seen in multiengine airplane descriptions, pilot certification limits, and maintenance discussions about engine location and engine-out handling.
Derivation
Center line refers to the imaginary line running nose-to-tail through the middle of the airplane. Thrust is the forward force from the engines. So the term literally describes engines whose pushing force lines up with the airplane's center, rather than being offset out on the wings.
Why Pilots Care
Engine failure produces no yaw or roll moment, greatly simplifying control compared with conventional wing-mounted twins.
Intuition Check
Do not read “center-line” as the runway centerline. Here it means the airplane’s own lengthwise middle line, where the engine push is lined up with the body of the airplane.
Example Sentence 1
Because the Cessna 337 is a center-line thrust airplane, losing the rear engine on takeoff doesn't produce the strong yaw a pilot would feel in a conventional twin.
Example Sentence 2
Center-line thrust airplanes are often used for multi-engine training because they remove the need to manage asymmetric thrust after an engine failure.