Hi all and Happy New Year!!!

I have a question relating to APU shutdown procedures on Airbus A320. During normal APU shutdown sequences, I noticed that after around 1m20s, the FAULT legend on APU master switch will flash.

I wonder if the FAULT legend comes on to indicate APU intake flap is closed or any other reasons.


1 Answer 1


If it's the flap closing, you should be able to verify that from the APU ECAM page.

Based on the time frame you mentioned, most likely it's the simulated overspeed, which is part of the shutdown sequence and is basically the signal that closes the fuel solenoid (which you can also verify from the APU page).

APU flap closing comes later. For more on the shutdown sequence, see: Why would smoke enter the cabin if the batteries are switched off before the APU flap closes?

The cool-down cycle lets the APU operate for 60 seconds at a low stabilized temperature. (...) Overspeed test occurs when the cool-down cycle is complete. The ECB supplies a signal to simulate an overspeed condition, that causes the fuel solenoid to close, and starts the APU shutdown. [emphasis added]

Source: APU training manual

  • $\begingroup$ Thanks for your answer. However, I cannot verify if the flap is closing as the power is supplied by battery only. Therefore, the ECAM doesn't show any info. $\endgroup$ Jan 3, 2020 at 10:33
  • $\begingroup$ @dingdingdong: Perhaps when ground power is available then. $\endgroup$
    – user14897
    Jan 3, 2020 at 12:51

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