What are the functions of the APU BLEED switch when set to AUTO?

When the APU BLEED switch is set to AUTO, the APU control system takes over the management of the bleed air system. This means the APU controller will automatically open or close the bleed valve as needed, based on the operational requirements of the aircraft's systems that require bleed air. This automated management allows for optimal performance and safety, as the APU can adjust the bleed air flow without manual intervention.

In the AUTO setting, the APU controller assesses various factors such as engine start demands or air conditioning system needs and adjusts the bleed air supply accordingly. This contributes to enhanced efficiency and reliability during flight operations.

When considering the other options, while the engine or APU can close or open the bleed valve, this action does not occur independently when the switch is set to AUTO. Instead, these actions are dictated by the APU controller, reinforcing the functionality of the automatic setting.