The memory items from the QRH are as follows:
1 ENGINE START switches (both) . . . . . . . . . . .FLT
2 Engine start levers (both) . . . . . . . . . . . CUTOFF
3 When EGT decreases:
Engine start levers (both) . . . . . . .IDLE detent
4 If EGT reaches 950°C or there is no increase in EGT within 30 seconds:
Engine start lever (affected engine) . . . . Confirm . . . . . CUTOFF, then IDLE detent
If EGT again reaches 950°C or there is no increase in EGT within 30 seconds, repeat as needed. - - - - - - - - - - - - - - - - - - - - - - -
5 At or above FL270, set airspeed to 275 knots. Below FL270, set airspeed to 300 knots.
Note: Engines may accelerate to idle very slowly, especially at high altitudes or in heavy precipitation. If N2 is steadily increasing and EGT stays within limits, do not interrupt the start.
Note: Do not wait for successful engine start(s) before starting the APU
One would want to start the APU ASAP as you are also on standby power assuming the engine speed has dropped low enough to lose the IDG, but APU start attempts aren't recommended above 25,000 feet.
Does anyone have any practical experience starting the APU above that?