Plain English Meaning

An aircraft engine where the cylinders lie flat on their sides, sticking out left and right from a crankshaft running down the middle. The pistons on each side push toward and away from each other as the engine runs.

Full Definition

A reciprocating piston engine in which the cylinders are arranged in two horizontal rows on opposite sides of a central crankshaft, with each pair of opposing pistons moving toward and away from the crankshaft in unison. This layout is the most common configuration in light general aviation aircraft and typically uses an even number of cylinders (four, six, or eight) cooled by ram airflow.

Context Anchor

Seen in aircraft engine descriptions, preflight discussions, and the reciprocating engine section of the Pilot’s of .