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I am reading my company's FCOM and I saw there the following information.
APU Bleed + Electrical Load: Max Alt 10000 ft.
APU Bleed: Max Alt 17000 ft.
APU Electrical Load: Max Alt 41000 ft.
If APU can supply bleed air up to 17000 ft, why supplying electrical load at the same time is not possible?
Does bleed air usage on the APU affect the generation of electrical power?
Thank you all for your answers.
It's a function of the relative demand each service puts on the APU. You have to remember that bleed is power consumed; you're just taking it in a different form.
As a turboshaft jet engine, the primary limitation on total power is the amount of heat the turbine can take. When you take bleed from the compressor, the turbine is forced to generate a lot of torque to drive the compressor but it's not getting the mass flow downstream through the burner can and turbine that it would get without any bleed (it's like you were cranking a fan by hand to cool yourself, but some joker put a piece of cardboard between you and the fan to divert most of the air - you're getting some air but you'd be pretty hot and bothered and you just can't do the total work you could do if you were getting all the air).
Since one of its primary functions is to provide hot air to the cabin as opposed to making thrust, a much higher proportion of its mass flow is bled off an APU's compressor compared to a main engine, and is a significant portion of the engine's energy potential. The generator, on the other hand, puts much less demand on the engine.
The result is for the same shaft HP available to drive the generator, the turbine temperatures are much higher when bleed is on, basically because it's doing the job of providing shaft HP and providing compressed air. Hence the 10000 ft limitation when doing both at the same time.
The other altitude limits are based on the effect each service individually has on the APU's temperature limits. The compressed air service, the bleed, uses most of the APU's power potential and maxes out at 17000 ft, with the generator a distant second and when it's the only service the APU has the HP available to run the generator all the way to 41000 ft.
