Definition
A reciprocating aircraft engine layout in which the cylinders are arranged one behind another in a single straight row along the crankshaft. This produces a long, narrow engine with a small frontal area, which reduces aerodynamic drag but generally results in a higher weight-to-horsepower ratio than other arrangements.
Plain English
An engine where all the cylinders sit in a single line, one after the other, along the crankshaft. The shape is long and slim rather than wide.
Context Anchor
Seen when comparing basic aircraft engine layouts in reciprocating engine discussions.
Derivation
"In-line" simply means "in a line." The cylinders are placed in a single straight row, which is exactly what the name describes.
Why Pilots Care
The in-line layout's narrow profile gives good forward visibility and low drag, but its length and weight make it less common in modern light aircraft, where horizontally opposed engines dominate. Recognizing the layout helps when reading older aircraft documentation or flying vintage types.
Intuition Check
“In-line” does not mean the engine is part of a fuel line or oil line. Here it describes how the cylinders are arranged: one row.
Example Sentence 1
The trainer's manual noted that its older in-line engine had been replaced years ago with a horizontally opposed design for better cooling and lower weight.
Example Sentence 2
Maintenance manuals for older aircraft often cover cooling requirements specific to in-line engines.