What is an aircraft APU, and what does it do?

An aircraft APU, or auxiliary power unit, is a small onboard powerplant—usually a gas turbine—that supplies electricity and, on many airliners, compressed air. It powers systems on the ground, helps start the main engines and may provide limited backup in flight, but it normally produces no thrust.

What does APU stand for in aviation?

In Aviation & Real-World Flying, APU stands for auxiliary power unit. It is auxiliary because it supports the aircraft's main power sources; it is not necessarily reserved for emergencies.

An APU is common on airliners, larger business aircraft and some military aircraft. Many light aeroplanes do not need one because their batteries and engine-driven systems meet their electrical and starting requirements.

Is an APU an engine?

Yes, most aircraft APUs are small gas-turbine engines, so the informal phrase “APU engine” is understandable. The crucial distinction is that an APU is a power engine, not a propulsion engine: it drives a generator and may supply compressed air, but it does not normally turn a propeller or produce useful thrust.

The unit generally burns the same aviation turbine fuel carried for the main engines. It has its own intake, exhaust, control system, fire detection and, on transport aircraft, fire-extinguishing provision.

What does an aircraft APU do?

An aircraft APU normally provides electrical power, pneumatic power or both, depending on the aircraft's design.

The APU does not usually cool the cabin by itself. On a conventional airliner, it feeds compressed air to the environmental-control packs, which regulate its pressure and temperature. On a more-electric design, APU-generated electricity may power electric compressors instead.

What is APU air?

“APU air” usually means hot, compressed bleed air taken from the APU compressor. It can operate the air-conditioning packs or turn an engine's air-turbine starter, but it is not untreated air blown straight into the cabin.

Valve positions, pack demand and aircraft limitations determine where that air goes. Our guide to engine and APU bleed-air operation explains the pneumatic system in more detail.

How does an auxiliary power unit work?

An auxiliary power unit works by using a small turbine to drive its compressor, accessory gearbox and electrical generator.

1. The start is commanded. The APU controller opens or confirms the intake arrangement and uses a battery-powered or externally powered starter to rotate the turbine.
2. Fuel and ignition are introduced. The controller monitors speed and temperature while combustion accelerates the turbine.
3. The APU becomes self-sustaining. The starter disengages after the turbine can continue running under its own power.
4. Availability is confirmed. Once speed, temperature and other parameters are within limits, the cockpit displays an indication such as APU AVAIL or shows stable APU speed.
5. The required output is selected. The crew connects the APU generator, opens the APU bleed valve or does both. Some aircraft automate part of this process.

A controller also protects the APU against conditions such as excessive temperature, overspeed and certain system faults. Exact protections and shutdown logic differ by aircraft.

Most transport-aircraft APUs sit in a fire-resistant compartment in the tail cone, although other locations are possible. The intake and hot exhaust remain hazards to people and equipment on the ground even though the unit is much smaller than a main engine.

Does the APU start the main engines?

On most conventional jet airliners, the APU starts a main engine indirectly by supplying compressed air to that engine's air-turbine starter.

When the engine start valve opens, APU bleed air turns the engine compressor. The aircraft then introduces ignition and fuel at the specified point in the start sequence. Once the engine is self-sustaining, its starter valve closes.

Some more-electric aircraft use APU-generated electricity and starter-generators instead of pneumatic starting air. An engine may also be started using an external air-start unit or, where approved, crossbleed air from another running engine.

Can an aircraft fly with the APU running?

Many transport aircraft can fly with the APU running, provided it is operated within the type's approved altitude and configuration limits.

Crews may use it in flight for extra electrical redundancy, after the loss of an engine-driven generator or to support an engine restart. Electrical generation and bleed-air availability can have different altitude limits, so a running APU may not provide every function available on the ground.

The APU does not make the aircraft fly or add meaningful propulsion. Once the main engines are operating normally, their generators and pneumatic systems usually carry the aircraft's loads, allowing the APU to be shut down.

An aircraft may also be permitted to operate with an inoperative APU under its approved minimum equipment provisions. That decision depends on the route, aircraft condition, operating rules and availability of external power or starting air; it is not a blanket rule for every flight.

Should crews use the APU or ground power?

Crews normally choose ground services when suitable equipment is available and use the APU when the aircraft needs an independent onboard source or is preparing for engine start.

A ground power unit does not normally provide pneumatic air, so connecting one does not by itself supply an air-turbine starter. Our comparison of aircraft ground power and onboard power covers connection, capacity and turnaround use.

Using ground equipment can reduce fuel burn, noise, heat and exhaust at the stand. Airport restrictions and operator procedures may also limit how long an APU can run.

What does APU mean in MSFS, and how do you use it?

In Microsoft Flight Simulator 2020 and Microsoft Flight Simulator 2024, APU means the same auxiliary power unit used on the real aircraft, although the depth of its simulation depends on the aircraft model.

A typical cold-and-dark sequence is:

1. Establish starting power. Switch on the aircraft battery or connect external power as required by the aircraft.
2. Arm or switch on the APU system. This may be labelled APU MASTER, APU or something similar.
3. Command the start. Select START and allow the simulated turbine to accelerate.
4. Wait for availability. Look for APU AVAIL, stable APU speed or the equivalent aircraft-specific indication.
5. Connect electrical power. Select the APU generator if the model does not connect it automatically, then confirm that the aircraft buses are powered.
6. Select APU bleed when needed. For a pneumatic engine start, open the APU bleed valve and configure packs, isolation valves and engine-start controls according to that aircraft's checklist.
7. Transfer the loads after starting. Confirm that the engine generators and normal bleed sources are operating before shutting down the APU.

Airbus, Boeing, business-jet and add-on cockpits do not use identical controls. Some simplified default aircraft automate the sequence, omit parts of the pneumatic system or have non-functional APU switches. For aircraft-specific interpretation, use our practical MSFS APU start and connection guide.

Why is the APU running but nothing works?

An operating APU may be available without its generator or bleed-air output being connected. Treating APU AVAIL as confirmation that every system is online is one of the most common cold-and-dark mistakes we see in flight simulation.

• The cockpit remains dark: check the APU generator switch, electrical bus connection and external-power priority.
• The engine does not rotate: check APU BLEED, pneumatic pressure, isolation or crossbleed valves, packs and the engine start selector.
• The APU will not start: confirm battery power, the APU master or control switch, fuel availability and that the fire control has not been activated.
• The APU starts and shuts down: a protection trip, interrupted fuel supply, incorrect switch sequence or aircraft add-on failure state may be responsible.
• No APU controls respond: the selected aircraft may not have an APU or may not simulate it beyond a generic command.

Use the aircraft's own checklist before assuming the simulator is faulty. APU electrical power and APU bleed air are separate outputs, and having one does not prove that the other is selected.

When should the APU be shut down?

The APU is normally shut down after the main engines have taken over the required electrical and pneumatic loads, but the exact timing belongs to the aircraft and operator procedure.

Before shutdown, crews commonly remove bleed demand and confirm that engine generators are online. Some APUs require a cool-down period after carrying pneumatic load; some aircraft manage that delay automatically. The APU may instead be kept running for take-off or landing when a checklist, equipment defect or operating policy requires the extra power source.

Our guide to choosing when to start and stop an aircraft APU covers the operational timing without assuming one procedure fits every type.