Definition
A reciprocating piston engine arrangement in which the cylinders are mounted in two rows, lying flat and pointing directly away from each other on opposite sides of a central crankshaft. The pistons in each opposing pair move toward and away from the crankshaft simultaneously, which gives the engine a smooth, balanced running action and a low, flat profile that fits well in light aircraft cowlings.
Plain English
An aircraft engine where the cylinders lie flat and face each other in two rows, one row on each side of the crankshaft, instead of being arranged in a V or a circle.
Context Anchor
Seen in discussions of common piston aircraft engine types, especially on many training airplanes and light general aviation aircraft.
Derivation
Horizontally means lying flat, and opposed means facing each other. The name simply describes the layout: cylinders lying flat, with each cylinder directly across from another on the opposite side of the crankshaft.
Why Pilots Care
This layout keeps the engine profile low, improves airflow cooling, reduces vibration, and allows easier access during inspections.
Analogy
Picture a boxer throwing two punches at the same time, one with each fist, in opposite directions. Each pair of pistons works the same way, pushing outward from the crankshaft in unison. This is why these engines are sometimes called boxer engines.
Intuition Check
Opposed does not mean the pistons collide or fight each other. It means the cylinders are positioned across from each other on opposite sides of the engine.
Example Sentence 1
Most training aircraft are powered by a four- or six-cylinder horizontally opposed engine because the flat layout fits neatly under the cowling.
Example Sentence 2
Preflight checks include verifying that the horizontally opposed engine has no oil leaks on the lower cylinders.