When should I start and shut down an aircraft APU?

Start an aircraft APU shortly before its electrical power or bleed air is needed—normally before pushback and engine start, or during taxi-in before the engines are shut down if gate power is unavailable. Shut it down only after its loads have transferred to engine generators or ground power and any required cool-down is complete.

In aviation and real-world flying, the exact timing comes from the aircraft checklist, operating manual and company procedure. APU start limits, automatic load transfer and cool-down logic differ between aircraft. If the terminology is unfamiliar, our explanation of the APU’s electrical and pneumatic jobs covers what each system supplies.

When should you start the APU?

Start the APU early enough for it to reach its available indication before you need to connect electrical or pneumatic loads.

• Before departure with ground power: Leave the APU off while a GPU and ground-conditioned air meet the aircraft’s needs. Start it before pushback if it will supply bleed air for engine starting or replace the GPU.
• Before departure without ground services: Start it when electrical power, cabin conditioning or engine-start air is required, subject to ramp restrictions. A standard GPU supplies electricity but not the compressed air normally required by an air-turbine starter.
• During taxi-in: Start it early enough to stabilise before the engines are shut down when external power will not be connected immediately. Do not rely on a GPU that is merely expected to be available.
• In flight: Start or operate it only when the normal or abnormal checklist requires it and the aircraft is within the APU’s altitude and operating limits. Typical reasons include replacing a failed generator or supporting an approved operational procedure.

In a simulator, the same decision points apply. Our general cold-and-dark start flow shows where APU operation normally fits between ground power, pushback and engine start.

Can you shut down the APU as soon as the engines start?

No; wait until the final required engine is stabilised, its generator is online, the pneumatic system is correctly configured and the aircraft no longer depends on the APU.

Starter cut-out alone is not enough. Check that the electrical buses remain powered, engine bleed air is available where required, packs are operating in their normal configuration and no warning indicates an incomplete transfer. On arrival, keep the APU running until external power has been connected and accepted by the aircraft, not merely plugged in or showing available.

Why might the APU remain running after engine start?

The checklist may retain the APU for a specific operational reason rather than shutting it down immediately.

• APU bleed may be used for air conditioning or an approved take-off configuration.
• An engine generator, bleed system or pack may be unavailable under an approved dispatch procedure.
• The operating procedure may require a backup electrical source during a particular phase.
• Hot-weather, high-altitude or performance procedures may alter normal bleed use, where the aircraft manufacturer permits it.

These cases are type-specific. For an example of how the decision sits within a complete workflow, see our A320 departure and arrival sequence.

What should you check before APU shutdown?

Before a normal APU shutdown, verify that every load it was carrying has another valid source and follow the aircraft’s prescribed shutdown sequence.

1. Confirm replacement electrical power. Check that engine generators or external power are connected to the buses. An AVAIL indication does not always mean the source has been selected or accepted.
2. Confirm the pneumatic transfer. Ensure engine starting is complete and bleeds, packs and isolation valves are in their normal positions. The exact configuration varies by type.
3. Check for a continuing requirement. Do not shut down the APU if a checklist, dispatch restriction or abnormal condition requires it to remain available.
4. Unload and cool it as directed. Some aircraft require an unloaded running period after heavy bleed use; others manage the delay automatically. Do not invent a universal waiting time.
5. Monitor the shutdown. A short delay while the APU cools and spools down may be normal. Keep the required aircraft electrical supply on until the checklist permits complete power removal.

A fire warning, severe malfunction or other emergency overrides the normal cool-down procedure. Use the aircraft’s emergency shutdown or fire checklist immediately.

What APU timing mistakes cause problems?

Most APU timing problems come from starting too late or removing it before the replacement source is actually carrying the load.

• Starting immediately before pushback: The APU may not reach its available state before the GPU must be disconnected, delaying the push or leaving the aircraft on battery power.
• Shutting down after the first engine starts: The remaining engine may still need APU bleed air, and its generator may not yet support the intended electrical configuration.
• Confusing available with connected: External power or a generator can show available while the buses are still supplied by the APU.
• Interrupting an automatic cool-down: Removing battery or aircraft power too early can interfere with the modelled shutdown and inlet-door sequence.
• Leaving the APU running by habit: Unnecessary operation burns fuel, creates noise and may breach airport restrictions. Shut it down once its work is finished and the checklist allows it.