Definition
An engine cylinder arrangement in which the cylinders are positioned one behind the other in a single straight row along the crankshaft. In-line engines are typically narrow and long, and may be mounted upright or inverted in the airframe.
Plain English
A type of piston engine where all the cylinders sit in a single straight line, one after the other, instead of being spread out in a circle or split into two rows.
Context Anchor
Seen in engine-layout discussions when the handbook describes how an aircraft engine’s cylinders are arranged.
Derivation
From 'in' plus 'line' — meaning literally 'arranged along a single line.' The term describes the geometry: cylinders sitting in a row, like beads on a string, rather than fanned out or paired off.
Why Pilots Care
Engine layout affects the aircraft's nose shape, weight distribution, cooling characteristics, and forward visibility. In-line engines allow a slim cowling but can be harder to cool evenly because rear cylinders sit behind the front ones in the airflow.
Intuition Check
Do not read in-line as meaning the engine is installed in line with the airplane’s path. Here it means the cylinders are arranged in a single straight row.
Example Sentence 1
Early training aircraft often used small in-line engines because the narrow profile gave the pilot a clear view over the nose.
Example Sentence 2
Early fighters often used in-line engines mounted ahead of the cockpit.