AOPA - Career Pilot

Turbine Aircraft Technology

Auxiliary Power Units

Jet engines' trusty sidekicks


The auxiliary power unit, or APU, found on many jet aircraft plays the role of reliable sidekick to the aircraft engines. Essentially a small, self-contained gas turbine engine, the APU can supply both electrical current and pneumatic (compressed) air, which is then used for starting engines, and it is also employed in air conditioning systems. The APU's generator can power most aircraft electrical buses, too. It is commonly used on the ground when the aircraft's engines are shut down and when external power supplies are unavailable. Besides reducing the number of operating cycles on the engines, the APU generally uses less fuel and is otherwise cheaper to operate than the aircraft engines.

Depending upon its de-sign, an APU may also be operated in flight but not as a source of thrust. On some twinjets, it is permissible to dispatch a flight with the APU generator substituting for an inoperative engine-driven generator. If APU pneumatic air is also available, it can be used for pressurizing the cabin. An engine failure is one occasion when the APU might be used in this role. The benefit to doing so is that engine bleed air would not have to be used for cabin pressurization, making more thrust available from the operating engine or engines.

Like the aircraft's main engines, an APU must be in-stalled behind a firewall. It also needs its own fire detection and extinguishing system. On most jets, the APU is installed in the far aft tail-cone section, so as to isolate it as much as possible from other systems in the event of an APU fire. Because the APU is often operated on the ground, dual fire-control panels are sometimes installed, one in the cockpit and one accessible from the aircraft's exterior. This allows an APU fire to be fought from either location. Since an APU is usually not closely monitored by the crew once it is started, most are designed to shut down automatically in case of a fire or loss of oil pressure.

Starting an APU is generally just a matter of placing the battery switch On and moving a single APU start switch. These simple steps initiate a complex automatic start sequence. The aircraft battery supplies the initial power to turn the APU compressor and open the air inlet door and fuel valves, and it also provides a source of ignition. Once ignition occurs and the APU has spooled up sufficiently the APU starter is disengaged and electrical power from the APU generator is available for use.

Like a jet engine, an APU must be operated within certain exhaust gas temperature limitations. The more electrical or pneumatic demand placed on an APU, the higher its EGT will rise. To prevent an APU from exceeding its maximum EGT under high-demand conditions, a load-control system automatically limits the amount of demand that the APU can satisfy. The system priority first allows the APU to supply maximum electrical load. It then reduces the pneumatic output from the APU as necessary, in order to keep the EGT within acceptable limits.

Like many sidekicks, the hard-working but underappreciated APU is taken for granted most of the time. When it is needed most, however, pilots quickly realize how sweet it is to have this trusty backup standing by.