As you know, there are two push buttons (MASTER SW and START) related to the APU in the Airbus A320 family.

Why there are two pushbuttons? I mean is there any condition in which users switch only one pushbutton on?

  • 3
    $\begingroup$ Do you fly the A320? Curious because you said “we”, and I would presume a pilot have a copy of the manual… Anyway, an electrical master switch is different than a push to crank switch. Just like your car ignition has a few different positions. $\endgroup$ Oct 22, 2022 at 14:25
  • 1
    $\begingroup$ Please read my question anain and try to understand properly. Is there any situation, which APU master is ON but start is OFF or vice versa. If you use APU both pb's are on and if you dont use both are off. We use our cars ignition on different positions according to our requirements. Indeed most modern cars have only ONE pb for ignition. That pb do not have seperate positions. $\endgroup$
    – Udemir
    Oct 22, 2022 at 15:17
  • 2
    $\begingroup$ @Udemir Only pushing the START button will not do anything according to the FCOM. Only pushing MASTER SW is possible and will boot the APU computer, open the intake flap and possibly start the fuel pump. I'm not sure if this ever done in normal operations though. $\endgroup$
    – Bianfable
    Oct 22, 2022 at 15:26
  • 2
    $\begingroup$ I am trying to understand your question, which is I asked you to clarify whether you have access to the flight manual or not. I could probably figure out the answer myself now that Bianfable has provided a link to it, but your choice of words like "as you know" and "we" reads like you are a 320 pilot asking fellow 320 pilots. Presumably this is not true, because it would be taught in ground school, and/or can read about it in the manual, but I've been wrong about these things before... $\endgroup$ Oct 22, 2022 at 15:49
  • 4
    $\begingroup$ Thank you, it is a design philosophy question, not a "what does this button do?" question. These are important distinctions that affect how people answer. I appreciate you clearing that up. $\endgroup$ Oct 22, 2022 at 16:26

1 Answer 1


Are there cases that the APU MASTER SW is used but not the Start PB?

Yes: diagnosis, emergency, and maintenance.


Turning on the APU master switch without pressing the start button triggers the following actions:

  • APU is electrically powered and the ECB performs a self-test
  • APU air intake flap opens
  • APU fuel valve opens
  • APU fuel pump starts (if needed)
  • APU page appears on the ECAM

If something is not working, you can try to see in the ECAM page what is happening without actually starting the APU.

Emergency procedure

QRH - Removal of smoke

The procedure does not require to actually start the APU.


Many maintenance procedures require to power-up the APU but not start the APU. One case is also in the FCOM:

enter image description here


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .