Originally Posted by
Musician
What is the first item in the APU auto-start sequence, then?
I'm surprised it's not the opening of the inlet, is there an outlet that needs to be opened first? Fuel to be pumped?
It may depend on the controller. I know the
747-400 needs to see the door open before the starter even engages. There is a 13 degree switch on the door which I believe is used for that purpose. On the ground, the door opens to 45 degrees. In the air, the door moves to 15 degrees if the APU is kept running for takeoff.
On the
777, the door must also be open (and rpms below 12% before the start sequence begins). The DC fuel pump is also activated with the switch moved from OFF to ON.