In modern aviation, efficiency and reliability are critical. One of the lesser-known yet vital components that contribute to both is the Auxiliary Power Unit (APU). Though small compared to the main engines, the APU plays a crucial role in ensuring aircraft systems function smoothly on the ground and during certain phases of flight.
What is an APU?
The Auxiliary Power Unit (APU) is a small gas turbine engine installed in an aircraft, typically located in the tail section (empennage). Its primary function is to provide electrical power and compressed air when the main engines are not operating. Essentially, the APU allows the aircraft to be self-sufficient while parked at the gate or during engine startup.
Functions of the APU
1. Electrical Power Generation
The APU drives an electrical generator that supplies power to the aircraft’s electrical systems. This includes lighting, avionics, cabin systems, and environmental control units.
2. Pneumatic Power Supply
The APU provides bleed air—high-pressure air—used for:
Starting the main engines.
Operating the air conditioning and pressurization systems while on the ground.
3. Backup Power Source
In the event of main engine or generator failure during flight, the APU can supply limited electrical power to essential systems, enhancing aircraft safety and redundancy.
How the APU Works
The APU operates similarly to a small jet engine:
1. Air Intake: Ambient air enters the APU through an intake duct.
2. Compression: The air is compressed and mixed with fuel in the combustion chamber.
3. Combustion: The fuel-air mixture ignites, producing high-temperature, high-pressure gases.
4. Turbine Expansion: The gases drive a turbine connected to the compressor and a gearbox.
5. Power Output: The gearbox powers an electric generator and a pneumatic supply system.
The exhaust gases are released through a small exhaust port in the tailcone.
Location
The APU is usually mounted in the aft fuselage, specifically in the tail cone area. This location minimizes noise and heat effects on passengers while allowing efficient air intake and exhaust routing.
Starting the APU
The APU is typically started using:
Aircraft batteries, or
External ground power (GPU) supplied at the airport gate.
Once started, the APU can power the aircraft’s systems and start the main engines using bleed air.
Advantages of Using an APU
Reduces dependency on ground support equipment
Enables quick and independent engine start-up
Maintains cabin comfort during ground operations
Provides emergency backup power in flight
Limitations
While the APU is highly useful, it has certain limitations:
It consumes fuel, typically Jet-A, even when the aircraft is on the ground.
It is not designed to provide thrust or replace the main engines.
Some airports restrict APU use to minimize noise and emissions.
Maintenance and Safety
Regular maintenance of the APU is vital to ensure reliability. Key areas include:
Inspection of fuel and oil systems
Monitoring temperature and vibration levels
Checking for leaks, corrosion, or damage
Aircraft manufacturers and regulatory authorities set strict inspection intervals and operational limits for APUs.
Conclusion
The Auxiliary Power Unit may be small, but its role in aircraft operations is indispensable. From powering essential systems on the ground to providing emergency energy in flight, the APU enhances both operational efficiency and safety. As aviation technology continues to evolve, APUs are also becoming more fuel-efficient and environmentally friendly, supporting the industry’s push toward greener skies.