Auxiliary Power Unit

Operating range
Aircraft APUs generally produce 115 V alternating current (AC) at 400 Hz (rather than 50/60 Hz in mains supply), to run the electrical systems of the aircraft; others can produce 28 V direct current (DC).
The APU can be used in flight if the main engines stop, and it will maintain cabin pressure and provide electricity for the aircraft’s systems. However some APU can’t be started above 25,000ft and can’t used above 37,000-41,000ft.

Advantages:
The APU can be run while the aircraft is at the gate so the aircraft can have power with out running the engines.
On most aircraft the APU is located in the tail of the aircraft so the exhaust can’t harm ground crew
Disadvantages:
The APU is very important to the start up of the aircraft, if the APU fails the aircraft can’t be started
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