An Auxiliary Power Unit, or APU, is a small turbine engine usually installed in the tail of a passenger jet. It gives the aircraft power when the main engines are off, especially at the gate or on a remote ramp. This matters because a modern aircraft needs electricity, air conditioning, cockpit displays, lights, and hydraulic support before it can fly.
The APU helps an airplane operate independently without relying only on airport ground equipment.
Inside the APU, air enters an intake, fuel is burned in a compact combustor, and hot gas spins a turbine. The turbine drives a generator for electrical power and can also provide compressed bleed air for cabin climate control and main engine starting. During engine start, bleed air from the APU spins an air starter on a main engine until that engine can run on its own.
Once the main engines are running, the APU is often shut down to save fuel and reduce noise.
Key Facts
- The APU is a small gas turbine, usually mounted in the rear fuselage near the tail.
- APU output commonly includes electrical power and pneumatic bleed air.
- Electrical power relation: P = VI, where P is power, V is voltage, and I is current.
- Energy used over time: E = Pt, where E is energy, P is power, and t is time.
- Bleed air from the APU can start main engines by spinning an air starter before fuel ignition.
- The APU can run when the main engines are off, but it still burns fuel and produces exhaust, heat, and noise.
Vocabulary
- Auxiliary Power Unit
- A small onboard turbine engine that supplies electrical power and compressed air when the main engines are not providing them.
- Bleed Air
- Compressed air taken from a turbine compressor and used for systems such as air conditioning, pressurization, and engine starting.
- Turbine
- A rotating engine part that extracts energy from hot, fast-moving gas to drive a shaft.
- Generator
- A device that converts mechanical rotation into electrical energy for aircraft systems.
- Air Starter
- A motor driven by compressed air that spins a jet engine fast enough for it to begin running on its own.
Common Mistakes to Avoid
- Thinking the APU provides thrust for flight. The APU is not used to push the airplane forward because its job is to supply power and compressed air, not propulsion.
- Assuming the APU only works on the ground. It is mainly used on the ground, but many aircraft can also use it in flight for backup electrical or pneumatic power when needed.
- Confusing APU bleed air with cabin oxygen. Bleed air is compressed air for aircraft systems, while passenger oxygen systems are separate safety equipment used during emergencies.
- Forgetting that the APU burns fuel. Even though it is smaller than the main engines, it still consumes fuel and creates exhaust, so crews shut it down when it is no longer needed.
Practice Questions
- 1 An APU generator supplies 115 V at 350 A. Use P = VI to calculate the electrical power in watts and kilowatts.
- 2 An APU produces 90 kW of electrical power for 25 minutes. Use E = Pt to calculate the energy supplied in kilowatt-hours.
- 3 A jet is parked at a gate with no external ground power available. Explain why the crew might start the APU before starting the main engines, and identify two aircraft systems it can support.