Bleed air from the APU is not permitted above what altitude?

• A. 15,000 ft
• B. 20,000 ft
• C. 25,000 ft
• D. 30,000 ft

Answer: (C)

Bleed air from the APU is limited by altitude because the APU’s compressor can’t maintain the required bleed pressure and temperature as ambient air becomes thinner. Once you’re at twenty-five thousand feet and higher, the APU can’t reliably supply enough bleed air to feed the environmental control system and any packs or starting needs. That’s why using APU bleed air above that altitude isn’t permitted; at those heights you rely on other sources (like engine bleed air) for pneumatic needs, and the APU is kept out of the bleed-loop to avoid insufficient or unstable air supply.