RB211s starting

[Swedish Steve]

We operated the original -1s and 50s, and the APU was so efficient that we ran an air conditioning pack at the same time as we started the engine (apparently to avoid a pressure surge when the starter motor cut off).

Really depends on the OAT. The APU was OK in the cold, but not so good in the desert. With an older APU, and 40 deg C OAT I used to close the aft isolation valve, and turn off the B and C EDPs, and start nbr 2 first. Then, at about 40 pc N3, open the isolation valve and turn a pack on to stop the APU surging at starter cut out. You needed someone on the F/E panel who understood the aircraft.

Because of the design of the APU, it runs most efficiently in min mode with a pack on. Close the APU bleed valve and the APU surge valve opens and all the load compressor air is vented overboard and you can hear the APU struggling.