Definition
A piston aircraft engine in which the cylinders are arranged in two horizontal banks on opposite sides of a central crankshaft, with each cylinder firing directly across from another. This flat, low-profile layout is the most common configuration in light general aviation aircraft.
Plain English
An engine where the cylinders lie flat and point sideways in two rows facing each other, with the crankshaft running down the middle between them. It is the typical engine you find under the cowling of small training and personal aircraft.
Context Anchor
Seen in aircraft systems descriptions, engine familiarization, and instructor explanations of common light-airplane engine layouts.
Derivation
Opposed comes from the Latin opponere, meaning 'to place against.' The cylinders are literally placed against each other across the crankshaft, which is exactly what the name describes.
Why Pilots Care
Opposed engines run smoothly, fit neatly into a low-profile cowling for good forward visibility and aerodynamics, and are simpler to cool in flight. Knowing the layout helps when reading engine diagrams, troubleshooting, or discussing maintenance with a mechanic.
Intuition Check
Do not read opposed as meaning the engine is against something or working in opposition to the airplane. Here it describes the physical layout: cylinders are placed opposite each other on either side of the engine.
Example Sentence 1
Most training aircraft are powered by a four-cylinder opposed type engine mounted horizontally in the nose.
Example Sentence 2
Preflight checks include verifying oil level on the opposed type engine before the first flight of the day.