What does the Airbus A320 APU do, and how is it used?

The Airbus A320’s auxiliary power unit (APU) is a small gas turbine in the tail that supplies electrical power and pneumatic bleed air when the main engines cannot. Pilots use it mainly for ground electricity, air conditioning and engine starting, then shut it down after engine generators and bleed air take over.

What does the A320 APU power?

The APU provides two separate outputs: electricity from its generator and compressed air from its bleed system. In real-world aviation and detailed A320 simulators, those outputs must be selected and monitored separately.

• Electrical power: The APU generator can supply the aircraft’s electrical network before engine start, during a turnaround or as a backup in flight. It also allows the electrical system to support battery charging.
• Pneumatic bleed air: APU bleed air runs the air-conditioning packs and supplies the pneumatic starter used to rotate each main engine.

The APU does not propel the aircraft and does not directly pressurise the hydraulic systems. Its electricity can power electrically driven equipment, but that is an indirect effect. Our guide to A320 electrical sources and automatic bus transfer explains where the APU generator sits alongside external power, batteries and engine generators.

How do you start and use the APU on an Airbus A320?

Start the APU from the overhead panel, wait for the green AVAIL indication, and then select only the electrical or pneumatic output that the aircraft needs.

1. Establish battery power. Put the batteries in their normal operating state. External power may remain connected while the APU starts.
2. Select APU MASTER SW ON. This powers the APU control system and commands the air-intake flap to open.
3. Press APU START. The automatic controller manages the starter, fuel and ignition. Monitor the APU indications on ECAM rather than repeatedly pressing the switch.
4. Wait for APU AVAIL. This confirms that the APU has reached a stable operating condition. It does not mean that bleed air is already selected.
5. Use the required output. The APU generator pushbutton is normally left in its standard ON configuration and connects automatically when appropriate. Select APU BLEED ON when air conditioning or pneumatic engine starting is required.
6. Start the engines. With APU bleed available, place the engine mode selector in the start position and operate each engine master as required. A correctly modelled A320 automatically closes the pack valves while an engine starter is using bleed pressure.
7. Transfer the loads and stop the APU. Once the engines are stable, confirm that their generators and bleed systems have taken over. Select APU bleed off and then the master switch off in accordance with the applicable checklist.

The APU may continue running briefly after the master switch is turned off while its automatic cooling and shutdown logic operates. Aircraft variants, operator procedures and simulator add-ons differ in how visibly they model this period, so an immediate stop should not be expected.

For the surrounding cockpit sequence, see where these actions fit into the complete A320 normal operating flow. MSFS users flying that particular add-on can also follow our worked Fenix A320 cold-and-dark start sequence.

When should you use external power instead of the APU?

Use external power at the stand when it is available and only electrical power is required; use the APU when the aircraft needs its own electricity, bleed air or both.

A separate ground-air unit can provide pneumatic pressure, but the A320’s EXT PWR connection itself supplies electricity only. On arrival without external power, start the APU and wait for AVAIL before shutting down the engines; otherwise, the aircraft may briefly lose its normal AC supply.

Can the A320 APU be used in flight?

The A320 APU can be started and operated in flight within its certified operating envelope. It may provide backup electrical power, pneumatic assistance or support an engine restart after a failure.

Generator and bleed-air availability can have different altitude limits, and those limits vary with the installed APU and aircraft configuration. In a real abnormal situation, crews follow ECAM and the approved procedure rather than treating the APU as an unrestricted substitute for an engine. Simulator add-ons may model only part of this logic.

Why is the APU available but providing no power or bleed air?

An APU AVAIL indication confirms that the unit is running; it does not prove that its generator is connected or its bleed valve is open.

• No electrical power: Check that the APU generator is not selected off or showing a fault. External power can retain priority while it remains selected, so the lack of an APU generator connection may be normal.
• No cabin air: Confirm that APU BLEED is on and that no bleed fault is displayed. Pack operation also depends on the air-conditioning controls.
• Engine does not crank: Check for APU bleed pressure, the engine mode selector and the relevant engine master. Merely reaching AVAIL does not open the starter valve.
• APU will not start: Verify battery power, available fuel and that the APU fire control has not been activated. Conflicting hardware bindings can also move an overhead switch back immediately in a simulator.
• APU does not stop immediately: Allow the automatic cooling sequence to finish. Cycling the master switch usually delays diagnosis rather than helping.

A mistake we see constantly is shutting the APU down after the first engine starts. Unless a crossbleed start or ground-air procedure is being used, keep the APU and its bleed supply available until both engines are running and their normal electrical and pneumatic sources have taken over.