Above what altitude is APU bleed air extraction not permitted?

The correct answer is based on operational limitations regarding the use of the Auxiliary Power Unit (APU) bleed air at higher altitudes. Specifically, APU bleed air extraction is not permitted above 25,000 feet due to the reduced efficiency and potential operational issues associated with utilizing bleed air at such altitudes.

At elevations greater than this specified altitude, the atmospheric conditions can lead to lower air density and other factors that might negatively impact the performance of the APU when supplying bleed air. Consequently, this limitation is established to ensure the safe and efficient operation of the aircraft systems that rely on APU bleed air, such as environmental controls and engine starting procedures.

Understanding this limitation is crucial for pilots and crew members to ensure they operate within safe parameters, utilize the APU effectively, and maintain compliance with aircraft operational manuals.