Auxiliary Power Unit

Question 1

The APU

Answer 1

Often a critical element of the certification for ETOPS.
Started by a small electric starter motor powered from aircrafts main batteries.
Usually in rear fuselage so that its exhaust presents no danger, crew and ground staff.

Question 2

APU inlet

Answer 2

Inlets are of two general types:

A permanently open APU inlet in the form of an NACA duct.
An inlet door which opens with APU activation.

Exhausts are usually permanently open to atmosphere.

Question 3

APU purpouse

Answer 3

Generates pressurised air an electrical power to allow an aircraft to operate independently of ground support equipment.

Can provide back-up electrical power in the event of in-flight engine failure.

Can provide high pressure air to start an engine.

Provides air for cabin pressurisation and conditioning on the ground.

Question 4

APU operation

Answer 4

Apart from starting and shut down, the APU operation is almost entirley automatic.

Internal sensors monitor the unit and shut it down in the event of a number of indications.

Some aircraft have an external control panel to allow ground crew to start and run the unit from the outside.

Question 5

APU auto shut down

Answer 5

Fire warning
Excessive EGT
Low oil pressure
High oil temperature
Airspeed/altitude limits exceeded
APU overspeed
High bleed air temperature.

Question 6

APU schematic

Answer 6

Gearbox, Load compressor, Air inlet (second off engine compressor), combustor and turbine, Exhaust.