APU engines usually consist of a centrifugal compressor and a radial turbine however, axial compressors and turbines may be added or used in their own right. Centrifugal compressors are used because of their high compression and small size and when combined with a radial type turbine make the APU very compact. These components are also very robust and require less maintenance than axial flow components. Use is also made of reverse flow combustion chambers that again makes the overall size smaller.
In most cases there is a design compromise made between the ideal APU for an aircraft i.e. its ability to provide air and electricity throughout the operational envelope of the aircraft, and it weight and size. It is usual therefore to find that air and electricity are limited to various altitudes dependant upon the parameter required.
APU systems are very basic and the APU will shut down if a problem is sensed. Most APU's will shut down for the following faults:
Low Oil Pressure
110% Honeywell 108% Sundstrand
Loss of Speed Signal
Electronically monitored APU's need this signal to control the APU.
Los of EGT Signal
Some APU's shutdown if they drop below 90%.
(Some will try and relight at 95%)
Electronic Unit Failure
Loss of Control
The APU may also shut down on the ground (not in flight) for the following faults:
May cause a warning horn in the u/c bay to sound
Generator Drive Low Oil Pressure or High Oil Temperature