FAA Powerplant Written Practice Test

What role do sprag clutches play in pneumatic starters?

1. To regulate air pressure

2. To automatically engage and disengage the turbine

3. To monitor starter temperature

4. To enhance starter efficiency

The correct answer is: To automatically engage and disengage the turbine

Sprag clutches serve a critical function in pneumatic starters by automatically engaging and disengaging the turbine during the starting process. When the pneumatic starter is activated, the sprag clutch allows the turbine to rotate and thus engage the engine. As the engine starts and reaches sufficient speed, the sprag clutch then disengages the starter from the turbine. This mechanism prevents the starter from being damaged by excessive speeds and allows for a smooth transition between the starter and the engine. Moreover, by ensuring that the starter only engages when necessary, sprag clutches help to improve reliability and efficiency in the starting system, which is essential for aircraft operations. Other answer options relate to functionalities that sprag clutches do not perform; regulating air pressure is managed by other components in the starter system, monitoring temperature is outside the scope of the clutch's function, and enhancing starter efficiency is a result of the clutch's ability to engage and disengage effectively, not its primary function.