I talking about 737 NG
APU can be started at all level, the only problem is in case of no AC power available ( loss of both EDG), the fuel boost pumps are not powered.
The Fuel supply to the APU will be by feeding line only which reduce the chance of a succesful start.
And you are only on battery power, any unsuccessful start will affect your battery endurance by 7 min i heard
So for me any reference not to start APU above FL 250 is only applicable to this specific scenario and has to do with fuel boost pumps not powered anymore
Last edited by tony montana; 10th Jul 2012 at 07:49.