While using APU bleed air to start both engines, the operating engine's thrust must not exceed what percentage of N2?

When starting both engines using APU bleed air, it is crucial to limit the thrust of the operating engine to ensure that the APU (Auxiliary Power Unit) can provide adequate bleed air pressure for the engine start process. The specified limit of 70% N2 is established because exceeding this threshold can reduce the APU's capability to maintain the necessary pressure and airflow for successful engine starting.

Typically, the N2 percentage reflects the rotational speed of the high-pressure compressor section of the engine, which is imperative for creating the required conditions for starting. Keeping the operating engine's thrust at or below 70% N2 ensures that there is sufficient bleed air flow without overloading the APU, thus allowing for a smooth and reliable engine start. This limitation is pivotal for maintaining safety and operational efficiency during engine starts.