When should the APU be used for power and when should it be shut down after start?

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Multiple Choice

When should the APU be used for power and when should it be shut down after start?

Explanation:
The main idea is when the aircraft can rely on external power versus needing the APU. The APU provides electrical power and bleed air on the ground or when no external power is available, so you use it to power systems and air conditioning during preflight and startup. If external power is available, you connect it and shut the APU down to save fuel, reduce wear, and rely on the external source for power. After the engines are started and their generators can supply the needed electrical load, the APU is typically shut down unless you still need it for environmental control or bleed air per SOP. Starting the APU after landing isn’t how power needs are met during the start sequence, and keeping it running for the entire flight or never using it aren’t correct practices.

The main idea is when the aircraft can rely on external power versus needing the APU. The APU provides electrical power and bleed air on the ground or when no external power is available, so you use it to power systems and air conditioning during preflight and startup. If external power is available, you connect it and shut the APU down to save fuel, reduce wear, and rely on the external source for power. After the engines are started and their generators can supply the needed electrical load, the APU is typically shut down unless you still need it for environmental control or bleed air per SOP. Starting the APU after landing isn’t how power needs are met during the start sequence, and keeping it running for the entire flight or never using it aren’t correct practices.

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