If engine bleed air is unavailable, which alternative sources may supply air to the packs?

• A. External air only
• B. APU air only
• C. External air or APU air
• D. Bleed air from engines

Answer: (C)

When the engine bleed system is not available, the packs can still receive conditioned air from two alternative sources: external air from outside the aircraft or air produced by the APU. External air is used on the ground or when ram air is acceptable in flight, while the APU can supply pneumatic air when engines aren’t providing bleed. Because either option can feed the packs, the best choice is that external air or APU air may supply the packs.