What is the role of the starter motor in the APU system?

The starter motor in the APU (Auxiliary Power Unit) system plays a crucial role in initiating the combustion process that powers the APU. Specifically, its main function is to provide the necessary torque to start the combustion engine. This torque enables the APU to reach the required operational speed where it can stabilize and begin generating power for other systems.

By engaging and turning the APU's engine, the starter motor ensures that the fuel-air mixture within the combustion chamber ignites properly, leading to the production of energy needed for aircraft systems when the main engines are not running. Understanding this function is essential for maintaining and troubleshooting the APU, as effective operation of the starter motor directly impacts the APU's ability to provide backup power and support during ground operations.