What is the APU maximum altitude for bleed air use in the CRJ 550/700/900?

The maximum altitude for bleed air use from the Auxiliary Power Unit (APU) in the CRJ 550/700/900 is indeed 25,000 feet. This specific limit is important because it defines the operational ceiling at which the APU can effectively provide bleed air for cabin pressurization, air conditioning, and engine start, among other functions. Operating above this altitude can compromise the APU's performance and result in inadequate air supply for the necessary aircraft systems. Thus, understanding this altitude limit is crucial for ensuring safe operations and compliance with performance specifications outlined in the aircraft's operating manual.