The maximum altitude for APU bleed air is which value?

• A. 28,000 ft
• B. 30,000 ft
• C. 20,000 ft
• D. 25,000 ft

Answer: (D)

APU bleed air is used to feed the environmental control system and for engine starts when engines aren’t providing bleed. The APU’s compressor can’t reliably deliver the required bleed air flow and pressure above about 25,000 feet, where the air is too thin to maintain the needed conditions. So the system is limited to 25,000 feet to ensure the packs and cabin pressurization receive stable air. At higher altitudes, other sources (engine bleed or electric packs) are used instead.