APU Limitations

[tsrp]

Why is there a Maximum Altitude limitation while the APU is running?


Thanks

tsrp

[411A]

On the L1011, it is an APU generator cooling issue.
Max KW reduced as well, while airbourne.

[spannersatKL]

APU Surge Margin and basicaly keeping the fire alight!

[Chrome]

There is no maximum altitude limitation running the APU on the A320 in a normal 2 engine operation. But at levels greater than FL300 or in greater ISA conditions, APU generator load reduces. There are however limits to APU battery-assisted starts and running the APU for bleed operation.

It has something to do with the pressure, density and temperature at higher levels. Someone else help, I am too hungry now to think