The Auxiliary Power Unit (or APU) is typically present in medium-and-large sized commercial airplanes that require power to kickstart electrical systems and devices. The primary purpose of an aircraft APU is to provide power to start the main engines. Turbine engines must be accelerated to a high rotational speed to provide sufficient air compression for self-sustaining operation. The APU is a flight critical system requiring routine and heavy maintenance checks to ensure commercial aircraft operators meet dispatch reliability goals.
Anatomy of an Aircraft Auxiliary Power Unit
The APU comprises a small turbine with a starter (mostly electric). The power unit takes fuel from the aircraft’s fuel tank and can start with the battery or external power. APU’s air intakes feature motorized doors that swing open when the unit starts. During taxiing, the APU and its doors shut down, concealing its presence. When the APU restarts, it builds a power unit inside the plane and supplies compressed air to the cabin. Compressed air is supplied to the hydraulics as well as the air conditioning packs of the cabin. You have undoubtedly experienced high temperatures in the cabin awaiting takeoff and can thank high failures in the APU for that.
The APU does not output any thrust, unlike other engines in the aircraft. It helps deliver air pressure and also generates power, which is then transferred to the generator. If you find this similar to the function of a turbine engine, know that the APU is considered to be a turbine/turboshaft engine itself.
This is how the flight operates with an APU:
- After starting both the battery and the APU, the aircraft is powered by the generator in the tail. It lights up the cockpit, starts the systems on the cockpit, light, water pumps in toilets, infotainment tablets and AC vents.
- Aircraft requires compressed air to start the engine. The APU can provide this compressed or bleed air for a smooth ignition.
- The power supply changes once the engines are successfully in operation. Next, all supply lines are diverted to the engines. APU is then ‘switched off’ and rested in dormant mode.
- In case of an emergency where one engine fails, the APU quickly comes to the rescue and provides power. It supplies energy to the hydraulics and compressed air to control other parts of the aircraft. The pilot can also land the aircraft in this state.
- In normal landing situations, the pilot switches on the APU after touchdown to bring all systems into check.
- Starting the APU also helps engines shut down quickly before the aircraft reaches the gate.
APUs are found in a wide variety of Aircraft
APUs can be found in today’s civil and military jets (medium and large size), military fighter jets, and some turboprop aircraft. Small jets do not carry the auxiliary power unit to avoid the extra weight as it can impact the airplane’s load.
APUs are consistently identified by aircraft operators as a top degrader and No Fault Found (NFF) component. Numerous OEM service bulletins are issued every year and the APU is regularly discussed at annual AMC events. The AMC event pairs commercial aircraft carriers with the component OEMs to solve persistent maintenance issues across the fleet.
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