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.
Understanding Aviation: The Auxiliary Power Unit
An APU is a complete engine system, not just a source of spare power. It has its own fuel supply path, lubrication system, starter motor, electronic controller, fire detection equipment, and exhaust outlet. A crew member commands a start from the cockpit, but the controller performs much of the sequence.
It checks conditions such as battery power, fuel pressure, rotor speed, and exhaust temperature. The starter first turns the rotating compressor. Fuel enters only when enough airflow is present.
Igniters light the fuel, then the engine accelerates to a stable speed. This automatic control prevents a poor start that could overheat parts or leave unburned fuel in the exhaust.
The electrical side shows why power management matters. A generator changes the APU shaft rotation into electricity for buses, which are the main pathways that distribute electrical energy around the aircraft. Different buses feed equipment such as cockpit instruments, cabin lighting, pumps, galley equipment, and charging systems.
Each item takes a share of the available electrical power. Power equals voltage times current. This means a system can draw more power by drawing more current at a given voltage.
Over a long ground delay, energy use builds up because energy equals power times time. Crews and aircraft computers must avoid overloading a generator. They can shed nonessential loads if demand becomes too high, keeping vital systems supplied.
Compressed air requires careful handling because it is hot and under pressure. Before it reaches the cabin, air conditioning packs cool it, control its pressure, and mix it with recirculated cabin air. The same pneumatic system may operate equipment such as wing anti ice systems on some aircraft.
Air taken from an APU or engine is called bleed air because it is tapped from the compressor section before the air reaches the combustor. Removing too much air reduces the engine's operating margin.
For this reason, valves, sensors, and computers regulate where the air goes. When a main engine is being started, the aircraft often limits other pneumatic demands so the starter receives enough airflow.
Students can connect the APU to ideas from energy transfer and reliability. The chemical energy in jet fuel becomes heat, moving gas, shaft rotation, electricity, and useful compressed air. None of these transfers is perfectly efficient.
Some energy leaves as heat and sound, while friction causes further losses. An APU is useful because it gives the aircraft another way to support essential functions if airport equipment is unavailable. It is not a replacement for the main engines in flight.
Pay attention to the difference between electrical power and pneumatic power, since both come from the same small turbine but travel through different systems. Also notice that aircraft design involves tradeoffs. Carrying an APU adds mass, fuel use, maintenance needs, and tail heat protection, yet it improves independence on the ground and supports safe, controlled engine starts.
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.