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

An aircraft auxiliary power unit (APU) is usually a small gas-turbine engine, often installed in the tail, that supplies electrical power and compressed air when the main engines are not running. It helps start the main engines, powers ground systems, supports cabin conditioning and can provide limited backup power in flight.

In aviation and real-world flying, the key distinction is that an APU is a power source, not a propulsion engine. Its exact capabilities depend on the aircraft: many conventional airliners use both electrical and pneumatic output, while some newer designs rely much more heavily on electricity.

What power does an aircraft APU provide?

An APU normally provides electricity and, on many airliners, compressed bleed air.

The APU does not usually cool the cabin directly. On a conventional bleed-air aircraft, it supplies compressed air to the environmental-control packs, which condition that air. On an electrically oriented design, the APU generator may instead power electric compressors.

The same distinction matters in simulation. Our explanation of how A320 electrical power is distributed shows why selecting the APU generator is separate from opening the APU bleed valve.

How does an aircraft APU work?

An APU works as a self-contained turbine whose compressor and turbine drive an accessory gearbox, electrical generator and, where fitted, a bleed-air supply.

1. The APU start is commanded. A starter, commonly powered by the aircraft battery or external electricity, begins turning the unit while its controller manages the intake, fuel and ignition.
2. The turbine accelerates. Once combustion is established and the APU becomes self-sustaining, the starter disengages.
3. The APU becomes available. After speed, temperature and other parameters stabilise, the crew can connect its generator and request bleed air. Some connections are automatic on newer aircraft.
4. Loads are removed before shutdown. The crew normally disconnects bleed and electrical demand, then follows the aircraft checklist. Any cool-down period is aircraft-specific and may be controlled automatically.

The APU generally burns the same fuel carried for the main engines, drawn through its own feed arrangement. Our overview of the fuels used by different aircraft explains why turbine APUs normally consume aviation kerosene rather than a separate fuel.

Does the APU start the main engines?

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

The start valve opens, pneumatic pressure turns the engine compressor, and fuel and ignition are introduced at the required point in the start sequence. Bleedless or more-electric aircraft may use APU-generated electricity to power starter-generators instead.

Why can an engine still fail to start with the APU running?

An operating APU does not guarantee that starting air or electrical power is reaching the engine. A mistake we see constantly in simulators is treating an APU AVAIL indication as proof that every required connection has been made.

• The APU generator may be connected while APU BLEED remains off.
• An isolation or crossbleed valve may be in the wrong position.
• The air-conditioning packs may not be configured as required for the start.
• The engine start selector, fuel control or ignition may not be set correctly.
• The APU may be running but carrying too much load or operating outside the modelled pneumatic limits.

For a model-specific example, the Fenix A320 cold-and-dark start sequence shows when APU availability, bleed air and engine start selections occur. If the turbine starts and then stops, use our checks for engines that will not start or stay running in MSFS.

Can an aircraft fly with the APU running?

Many transport aircraft can operate the APU in flight, but approved starting and operating altitude limits vary by type.

Crews may run it for additional electrical redundancy, after losing a main generator, or to support an airborne engine restart where the aircraft procedure permits. An APU cannot automatically replace every service from a running engine, and it may have less capacity at altitude.

An aircraft can often fly safely without an operational APU because its engine-driven generators provide power once the engines are running. Dispatch with an inoperative APU is governed by the aircraft's minimum equipment provisions, route requirements and the availability of external starting equipment. Many light aircraft have no APU at all.

Where is the APU located?

The APU is usually installed in a fire-resistant compartment in the tail cone of a transport aircraft, with its own air intake, exhaust, fire detection and extinguishing system.

Other locations are possible, so the tail position is not universal. Wherever it is mounted, the hot exhaust and intake create hazards for ground personnel and equipment; the unit is small, but it is still a running turbine engine.

Should crews use the APU or ground power?

External ground services are preferable at a suitably equipped stand when they can meet the aircraft's needs, while the APU provides independence where those services are unavailable or an engine start is approaching.

Airports and operators may restrict APU running to reduce fuel use, emissions and noise. Even though its fuel consumption is far below that of the main engines at high power, leaving it running throughout a long turnaround still carries a meaningful cost.