what is the role of AUXILIARY POWER UNITS?

When a modern aircraft’s engines are not operating, there are two available sources of power to operate its other systems. They are the aircraft’s battery and the auxiliary power unit. Because of the limited capacity of the batteries, the amount of power they supply is insufficient to provide all but the very basic needs. Operation of an aircraft’s air-conditioning system, for example, would require significantly more power than could be supplied by the batteries.

The APUs are gas-turbine engines, using the aircraft’s own fuel supply, which provides the power to run the attached generators. In addition, the APU is typically large enough to provide sufficient pneumatic power to start the aircraft’s engines. The presence of an APU eliminates the need for ground power units (GPUs).

The Boeing 747 auxiliary power unit is manufactured by the AiResearch Division of Garret Corporation. This unit is capable of producing approximately 660 lb/min airflow to the pneumatic systems and 90 kVA from the two attached generators to the aircraft’s electrical system. The APU has a separate battery for starting and is protected by its own fire protection system.


APU Operation and Control

Operating controls and indicating systems are typically separated by the functions they serve. The gas turbine operations and the control switch for the air bleed valve are found on the same panel; see Figure below. The APU operations panel usually includes controls for the APU start and stop, fire-protection test and indicating, and the fire extinguisher manual discharge discussed earlier in this chapter. Additionally, controls for the fuel valve and APU inlet doors may be found on the panel along with the typical engine operation indicators.

APU control and indicating panel. (Boeing Commercial Aircraft Co.)

The APU inlet door controls access to the gas-turbine compressor inlet and provides cooling air for the APU’s accessories. This door may be powered by an electric motor or may use the suction caused by the operation of the gas turbine to open a spring-loaded door.

Airflow and Bleed Air

The amount of airflow and its pressure are dependent upon the ambient temperature and the load on (power being drawn from) the APU. The greater the load and/or ambient temperature the lower the airflow and pressure.
The air-bleed valve should be closed during start-up and not operated until the APU is at approximately 95 percent power. When the APU bleed valve, also referred to as the “load-control valve,” is opened by placing its switch in the cockpit in the OPEN position, airflow is supplied to the aircraft’s pneumatic power-distribution system.

APU control and indication
Cross-section schematic of an APU. (Boeing Aircraft Co.)

Accessory cooling is provided from inlet air through a shutoff valve, which is closed unless there is a specified amount of pressure being discharged by the APU’s compressor.

APU system components
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