Definition
On a multi-engine airplane, a configuration in which the two engines' propellers turn in opposite directions — one clockwise and the other counter-clockwise as viewed from behind. This arrangement is designed so that the asymmetric thrust effects (such as the descending propeller blade producing more thrust than the ascending blade) cancel each other across the airplane, eliminating the concept of a 'critical engine.'
Plain English
The two propellers spin in opposite directions, like mirror images of each other. This balances out the side-to-side forces the engines would otherwise create, so losing either engine has roughly the same effect on the airplane.
Context Anchor
Seen in multiengine training, especially when studying engine-out control and the factors used to determine minimum control speed.
Derivation
From 'counter-' (Latin contra, meaning 'against' or 'opposite') and 'rotating' (Latin rotare, 'to turn'). Literally 'turning against' — here, two things turning in opposite directions relative to each other.
Why Pilots Care
Eliminates the critical-engine penalty, lowers VMC, and makes engine-out control requirements identical regardless of which engine fails.
Grounding Statement
Picture standing behind a twin-engine airplane and seeing one propeller turn clockwise while the other turns counterclockwise.
Intuition Check
Counter-rotating does not mean an engine is turning backward or being reversed in flight. It means the two propellers are built to spin in opposite directions during normal operation.
Example Sentence 1
Because the Seminole has counter-rotating propellers, it has no critical engine.
Example Sentence 2
Because the props were counter-rotating, loss of the left engine produced the same yaw as loss of the right engine.