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.
Plain English
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.
Context Anchor
Seen in aircraft engine descriptions, preflight discussions, and the reciprocating engine section of the Pilot’s Handbook of Aeronautical Knowledge.
Derivation
‘Horizontally-opposed’ describes the geometry: the cylinders lie horizontally (flat, not upright) and are opposed (facing each other across the crankshaft). The term comes from engineering shorthand for engine layouts — compare ‘inline’ (cylinders in a row) and ‘radial’ (cylinders arranged in a circle).
Why Pilots Care
This is the engine layout sitting in front of most training aircraft. Knowing the configuration helps you understand why the cowling is wide and flat, how cooling air flows across the cylinders, and why even cylinder-head temperature monitoring matters.
Analogy
Picture two boxers on opposite sides of a punching bag, throwing simultaneous jabs at the bag in the middle. Each pair of pistons works the same way against the central crankshaft.
Intuition Check
“Opposed” does not mean the engine is fighting itself. It means the cylinders are positioned across from each other on opposite sides of the engine.
Example Sentence 1
The Cessna 172 is powered by a four-cylinder horizontally-opposed engine, which is why the cowling is wide and relatively flat.
Example Sentence 2
The horizontally-opposed engine provides better cooling and a lower profile compared to other reciprocating designs.