Definition
A one-way (overrunning) clutch used in helicopter and turbine-engine drive systems that transmits torque in one direction of rotation and freewheels in the other. It contains shaped cams or wedges (sprags) between an inner and outer race that lock the races together when driven, but release and allow free rotation when the driven side turns faster than the driving side.
Plain English
A mechanical coupling that drives a shaft in one direction only. When the engine is pulling the load, it locks and transmits power. When the load tries to spin faster than the engine, it disconnects and lets the load coast freely.
Context Anchor
Seen in powerplant maintenance, especially in starter drive systems where the starter must turn the engine, but the running engine must not turn the starter.
Derivation
From the British engineering term 'sprag,' originally a wooden prop or wedge used to stop a wheel from rolling backwards. The shaped cams inside the clutch act like tiny wedges that jam to lock rotation in one direction.
Why Pilots Care
It automatically disconnects a failed engine from the rotor, allowing safe autorotation and landing.
Analogy
Like the freewheel on a bicycle: when you pedal, the wheel drives forward; when you stop pedaling, the wheel keeps turning on its own without forcing the pedals around.
Intuition Check
Do not think of this as a clutch pedal that the pilot operates. Here, the clutch works automatically inside the mechanism and only allows drive in one rotational direction.
Example Sentence 1
When the engine failed, the sprag clutch released the rotor, allowing the pilot to enter autorotation.
Example Sentence 2
The technician checked the sprag clutch for scoring on the inner race during the 100-hour inspection.