At what altitude can the APU operate with just bleed air?

The Auxiliary Power Unit (APU) is designed to provide power and bleed air for the aircraft while on the ground and during certain phases of flight. The APU's operational capacity regarding altitude is influenced by its ability to supply sufficient bleed air for both air conditioning and engine start requirements.

The correct altitude at which the APU can operate with just bleed air is 17,000 ft. At this altitude, the APU can effectively supply bleed air without external assistance, ensuring that the aircraft's systems remain operational. This is a critical altitude for flight safety and operational efficiency, as above this level, the capability of the APU diminishes due to reduced atmospheric pressure, which can affect performance.

In summary, 17,000 ft represents a significant threshold for APU operation, balancing the needs for bleed air with the limitations imposed by altitude.