What happens when the APU control switch is placed to RUN?

When the APU control switch is placed to RUN, the APU inlet door will open. This mechanism is crucial for the operation of the Auxiliary Power Unit (APU), as it enables the unit to start properly by allowing sufficient airflow to enter the engine. Opening the inlet door helps to ensure that the APU receives the necessary air for combustion, which is essential for generating electrical power and bleed air for various aircraft systems.

The functioning of the APU is closely related to its inlet door because the door's operation directly affects the engine's ability to function efficiently. This makes understanding the role of the inlet door vital for properly managing APU operations and ensuring safety and efficiency during aircraft ground operations.