How Does the APU (Auxiliary Power Unit) Work When Main Engines Are Off?

Have you ever boarded an aircraft and noticed that the cabin lights, air conditioning, and cockpit systems were already working even though the main engines were still off? The answer lies in a powerful onboard system called the Auxiliary Power Unit (APU).

Fact: The APU is often called the aircraft’s “third engine”, even though it does not produce thrust.

What Is an APU?

The Auxiliary Power Unit is a small gas turbine engine usually installed in the tail section of an aircraft. Its job is to provide:

• Electrical power
• Compressed air (bleed air)
• Backup support for aircraft systems

Unlike the main engines, the APU does not propel the aircraft forward.

Main Purpose: Allows the aircraft to operate without external ground support.

Where Is the APU Located?

Most commercial aircraft place the APU inside the tail cone.

• Keeps it isolated from passengers
• Provides proper airflow and exhaust routing
• Improves safety and maintenance access

Interesting Detail: The small exhaust opening at the rear of many airliners belongs to the APU exhaust system.

How Does the APU Start?

The APU usually starts using the aircraft’s battery power.

The startup sequence includes:

• Opening the APU air inlet door
• Activating the starter motor
• Fuel injection and ignition
• Turbine acceleration

Once running, the APU becomes self-sustaining.

Think: Similar to starting a small jet engine inside the aircraft tail.

Core Components of an APU

1. Power Section

This section contains the combustion chamber and turbine, where fuel burns to generate rotational power.

2. Load Compressor

Produces compressed bleed air used for engine starting and air conditioning.

3. Gearbox Section

Transfers turbine power to:

• Electrical generators
• Fuel pumps
• Cooling systems

Engineering Insight: The APU acts like a compact self-contained power station.

How Does the APU Generate Electricity?

The turbine spins an electrical generator to produce aircraft power.

Where:

• P = Power
• T = Torque
• ω = Angular Velocity

This electricity powers:

• Cockpit avionics
• Cabin lighting
• Galley systems
• Air conditioning

Important: Aircraft APUs commonly generate 115V AC at 400 Hz.

How Does the APU Start the Main Engines?

The APU provides high-pressure bleed air to spin the main engine compressors during startup.

Once the engine reaches sufficient RPM:

• Fuel is injected
• Ignition begins
• Main engine becomes self-sustaining

Critical Role: Without the APU, many aircraft would require external air-start equipment.

Why APUs Are Important on the Ground

• Provides cabin cooling before boarding
• Keeps avionics active
• Reduces dependence on airport equipment
• Saves fuel compared to running main engines

Operating the APU is far more efficient than running massive turbofan engines at the gate.

Passenger Benefit: Cabin lights and air conditioning work before engine startup.

Can the APU Work During Flight?

Yes, some APUs can operate in flight as:

• Emergency electrical backup
• Backup source for bleed air
• Support during engine generator failure

However, APUs are usually turned off during normal flight to save fuel.

Safety Feature: APUs provide additional redundancy during emergencies.

Why APUs Produce That Distinct Sound

The high-pitched sound heard near parked aircraft comes from:

• Small high-speed turbine rotation
• Exhaust gas flow
• Compressor operation

Fun Fact: Aviation enthusiasts can often identify aircraft types by their APU sound.

Limitations of the APU

• Consumes fuel
• Creates airport noise
• Requires maintenance
• Lower power than main engines

Environmental Concern: Airports increasingly use ground power units (GPU) to reduce APU usage.

Future of APU Technology

• Hybrid-electric APUs
• Hydrogen-powered systems
• Lower-emission turbines
• More efficient generators

Future Trend: Aircraft power systems are moving toward greener and quieter APUs.

Conclusion

The Auxiliary Power Unit (APU) is one of the most essential systems in modern aviation. Even when the main engines are off, it keeps the aircraft alive by supplying electricity, compressed air, and system support.

From engine startup to cabin comfort, the APU ensures smooth and efficient aircraft operations both on the ground and in emergencies.