APU Engine
APU Earlier aircraft used the piston engine to drive their APUs, it soon realize that gas turbine engine was safer, lighter and more reliable. The Gas turbine engine APU uses the same fuel as the main engines use, so the problem of fuel storage and fuel management systems is minimized. The type of gas turbine engine normally used to drive APUs is a turbo shaft engine with a free turbine. The free turbine, which is designed to run at a constant speed, drives the accessories via a gearbox. As it running at a constant speed, it ensures that the APUs generator maintains a constant frequency without the need for an additional constant speed drive unit Some aircraft use air bled from the APUs compressor to power the aircraft's pneumatic system, but it is also common for the free turbine to drive a separate load compressor to supply bleed air The APU is normally located in the tail of the aircraft where its operation will not harm personnel working. |
APU operations in Flight
APU of modern aircraft can be operated in the air, thus providing a back-up source of power to the aircraft systems in the event of an engine failure. This ability of the APU to operate while airborne has become more important with the introduction of twin engine aircraft which are now flying long haul routes under extended twin regulations. While other aircraft's APUs may or may not have the ability to be started while the aircraft is in flight, ETOPS compliant aircraft APUs must be capable of being started at all altitudes. If APUs malfunction, the aircraft cannot be released for ETOPs flight and must fly a longer route. |
APU Control and Operation
These two apu control panels shown in the diagram across illustrate that the pilot has very little control. On older aircraft apu panels incorporate load meters, EGT gauges and indicator lights, which warn of low oil quantity, low oil pressure, high oil temperature and over speed. The APUs turbo shaft engine is started by an electric starter motor powered from the aircraft battery, but some later model have a dedicated auxiliary power unit battery to preserve the aircraft batteries. When the APU is started, the R.P.M and EGT must be allowed to stabilise before it is used to power the aircraft's systems. If the APU EGT reaches a predetermined figure, then the bleed air valve will begin to close, reducing the ability of the APU to supply the pneumatic system. It is normal for the APU to be operated for a cooling period of at least one minute with no pneumatic, prior to shut down. |
System Operation
The APU will shut down for the following reasons;
The APU will shut down for the following reasons;
- Fire
- Low oil Pressure
- High oil pressure
- Overspeed
APU Fire Protection
The APU unit only has one fire bottle which is stored at a pressure of approx 800 P.S.I
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