Learn what APU means in Microsoft Flight Simulator, how to start it, connect generator and bleed air, and fix common APU problems.
In Microsoft Flight Simulator, APU means Auxiliary Power Unit: a small turbine, usually in an airliner's tail, that supplies electrical power and pneumatic bleed air while the main engines are off. Start it, wait for an AVAIL or stable RPM indication, connect its generator and bleed air, then start the engines.
What does the APU actually power?
In most jet airliners, the APU can power the aircraft's electrical buses and provide compressed bleed air for air conditioning and engine starting. It does not produce useful propulsion.
The exact capability depends on the aircraft. Some modern types start their engines electrically rather than pneumatically, while light aircraft and many smaller turboprops have no APU at all. The aircraft model matters more here than whether you use Microsoft Flight Simulator 2020 or 2024.
If the whole cockpit is unpowered, our general cold-and-dark starting sequence explains where battery, external power and the APU fit into the wider procedure.
How do you start and use the APU in Microsoft Flight Simulator?
Use the aircraft's overhead or systems panel and follow its checklist, because switch names and electrical logic differ between types.
- Secure the aircraft. Set the parking brake and turn on the required battery or batteries. Confirm that fuel is available; some aircraft manage APU fuel feed automatically, while others require a particular pump or tank configuration.
- Arm the APU. Select the APU master switch, if fitted. This may open the intake door and prepare the fuel and fire-protection systems.
- Command the start. Press START or move the APU selector to START. A spring-loaded selector may return to ON by itself; that does not mean the start has failed.
- Wait for a ready indication. Look for
AVAIL,READY, stable APU RPM or the relevant generator indication. Do not connect loads merely because you can hear the turbine. - Connect electrical power. Select the APU generator or generators as required. Some aircraft connect them automatically when external power is absent. Verify that the electrical display shows the APU feeding the buses.
- Turn on APU bleed air when required. Conventional pneumatic starters need APU bleed pressure. Configure the packs and isolation valves according to the aircraft checklist; turning every pack off is not correct for every type.
- Start the engines. Use the normal engine-start selector and fuel controls. Watch for starter rotation, rising core RPM, fuel flow and a controlled exhaust-temperature increase.
- Transfer the loads. Once the engine generators are online, disconnect APU electrical and pneumatic loads in the prescribed order. Restore the normal pack configuration, observe any specified unloaded cool-down period and shut down the APU.
The type-specific differences can be substantial. Our Boeing 747 cold-and-dark procedure covers its APU generators and bleed configuration, while the A320neo V2 starting flow shows the Airbus MASTER SW, START and AVAIL logic.
Should you use the APU or external power?
Use external power for extended gate preparation and start the APU before the ground supply is disconnected or pushback begins.
| Task | Best source | Reason |
|---|---|---|
| Cockpit preparation at a serviced stand | External power | Supplies electricity without running the APU or burning its fuel. |
| Cabin air without ground air equipment | APU | Its bleed air can run the air-conditioning packs on conventional aircraft. |
| Normal pneumatic engine start | APU | Provides the compressed air needed to rotate the starter. |
| Electrical or pneumatic backup | Aircraft-dependent | APU availability and altitude limits differ by type. |
A ground power unit normally supplies electricity, not the bleed air needed by a conventional pneumatic starter. A separate ground-air source may be used, but many MSFS aircraft and airports do not model that complete service.
Why is the APU not working?
Most APU problems in Microsoft Flight Simulator come from a missing power, fuel, generator or bleed-air step rather than a simulator fault.
| Symptom | Likely cause and fix |
|---|---|
| Nothing happens when START is selected | Check the battery, APU master, available fuel and fire handle. Give the intake door time to open if the aircraft models it. |
| The APU runs, but the cockpit remains on battery | The APU generator is not connected or its bus has not accepted the load. Check generator switches, bus ties and the electrical display. |
| AVAIL appears, but an engine will not rotate | Check APU BLEED, isolation valves, pack configuration and the engine-start selector. An electrical-start aircraft follows different logic. |
| The APU shuts down after starting | Look for inadequate fuel feed, an overload, overheat or an enabled system failure. Correct the cause before attempting another start. |
| The switch moves but no system responds | The aircraft may use simplified or unmodelled APU systems. Assistance features or duplicate hardware bindings can also reverse a cockpit switch immediately. |
A mistake we see constantly is connecting the generator but forgetting APU bleed, or doing the reverse. Electrical power can light the cockpit without providing starter air, and a running APU does not necessarily feed either system until the appropriate connection is made.
Can you leave the APU running?
You can leave the APU running through pushback and engine start, but it is normally shut down after the engines and their generators are stable.
Some aircraft permit APU use in flight as an electrical or pneumatic backup, subject to operating limits. Do not assume it will provide bleed air at every altitude. Shutting it down too early is the more common MSFS error: if external power has gone and the engine generators are not yet online, the cockpit may lose power.