FAA Powerplant Written Practice Test

What mechanism transfers the energy of the starter motor to the engine flywheel in a typical direct cranking starter for large reciprocating aircraft engines?

1. Direct drive system

2. Gear reduction system

3. Ratcheting mechanism

4. Synchro system

The correct answer is: Gear reduction system

The gear reduction system is the correct answer because it plays a crucial role in connecting the starter motor to the engine flywheel in aviation applications. In large reciprocating aircraft engines, the starter motor operates at a much higher RPM (revolutions per minute) than what is necessary for the engine's operation to initiate combustion. To effectively utilize the high speed of the starter motor, a gear reduction mechanism is employed. This system reduces the motor’s output RPM while simultaneously increasing torque, providing the necessary force to turn the heavier engine flywheel. This is essential in ensuring that the engine starts smoothly and without stressing the starter motor beyond its capacity. Other mechanisms such as direct drive could not provide the necessary torque conversion due to the mismatch of speeds and torques required. A ratcheting mechanism is primarily used for one-way movement and would not continuously engage the flywheel for a sustained engine start. The synchro system is generally limited to applications involving feedback or control systems rather than direct mechanical engagement for starting engines.