At what altitude is the APU maximum start altitude?

The maximum start altitude for the Auxiliary Power Unit (APU) in the ERJ 170/175 is set at 30,000 feet. This altitude limit is critical because it ensures that the APU has sufficient air density and performance capabilities to start and function effectively. Beyond this altitude, the APU may not receive enough airflow to start properly, which could lead to failure in initiating the APU or inadequate power generation for electrical systems and bleed air systems during flight.

Understanding these limitations is essential for flight crews to maintain operational safety and efficiency, especially during high-altitude operations where a reliable power source is necessary for aircraft systems.