What is the maximum altitude for electrical operation of the APU?

The maximum altitude for electrical operation of the Auxiliary Power Unit (APU) is set at 33,000 feet. This altitude is critical as the APU is responsible for providing electrical power and air conditioning to the aircraft when the engines are not running or during ground operations. At altitudes above this threshold, the performance of the APU may degrade, potentially leading to insufficient electrical power supply or other operational concerns.

The choice of 33,000 feet is informed by various engineering parameters, including atmospheric pressure and density at high altitudes, which influence the APU's ability to function properly. Ensuring that the APU operates within this altitude limit helps maintain safety and efficiency in all operational scenarios.