What is the maximum altitude for APU operation when using engine start assist?

The maximum altitude for Auxiliary Power Unit (APU) operation when using engine start assist is indeed 21,000 feet. This altitude limit is established to ensure efficient performance and reliability of the APU systems during the critical phase of engine start. At altitudes higher than 21,000 feet, the APU may not be able to provide sufficient power for starting the engines due to decreased air density and changes in operational dynamics.

Operating beyond this altitude could risk the stability and functionality of the APU, which is crucial for starting the engines and providing power while on the ground. Hence, adhering to this altitude limitation is essential for safe and effective aircraft operation.