At what altitude is the APU capable of starting?

The Auxiliary Power Unit (APU) is designed to provide electrical power and pneumatic pressure for engine starts and other essential systems while the aircraft is on the ground or in the air. The capability of the APU to start at altitude is crucial for operations where ground services may not be available or when the aircraft is operating at high elevations.

The correct altitude at which the APU can start is 30,000 feet. This altitude is typically specified because it represents a limit within which the APU can effectively deliver the required performance to start the main engines and provide power to onboard systems. At altitudes higher than this limit, the performance of the APU may diminish due to reduced air density, resulting in insufficient thrust or pneumatic pressure needed for proper operation.

Understanding this limit is important for flight planning and ensuring the aircraft can operate efficiently at various altitudes. Therefore, the APU's starting capability at 30,000 feet allows pilots to manage power needs effectively during high-altitude operations.