Hi all,
With the Classic B747, which I believe is the same, systems wise, as the -400 -:
Firstly, prior to pushback, the FE pressurizes the #4 hydraulic system with an electrically driven pump, for the aircraft brakes to work.
He also pressurizes the #1 hydraulic system with the Air Driven Pump (ADP), to power the body gear steering during the push.
Which also acts as a back-up brakes system for #4, (FE selectable), should it fail!
Even though we do start 4,3,2 &1, I've been trying to get our people to change to 1,2,3 &4, because:
#1 has the ADP running, using bleed air from the APU, which could be better used to wind up the engines a little better, especially with some of the, slightly used, APU's. Once #1 is running, the EDP output will 'auto-off' the ADP, thereby providing better bleed air pressure for the other three!
I think, in QF Classics, after #1 is started, with now no ADP running, they turn on one aircon pack during the starting of the others, but they seem to have better quality APU's than most!
Cheers