What action must be taken if using both engines started with APU bleed air?

When both engines are started using APU bleed air, the correct action is to limit the thrust of the operating engine. This limitation is critical because starting engines on APU bleed air places a significant demand on the pneumatic system. If both engines were to operate at full thrust during this time, it could exceed the available system pressure, potentially leading to a reduction in engine performance or even a failure to start one of the engines correctly.

By limiting the thrust, the airflow provided by the APU is managed effectively, ensuring that there is adequate pressure and flow to support both engines' needs as they spool up. This precaution helps maintain operational integrity and safety during the start sequence. It's necessary to adhere to this limitation to prevent any adverse effects on the aircraft systems and ensure a stable engine startup process.