What bleed air sources supply the PACKs?

• A. External bleed air only
• B. APU bleed air only
• C. Engine bleed air only
• D. External bleed air, APU bleed air, and Engine bleed air (all of the above)

Answer: (D)

Pack air is cooled and conditioned using bleed air, and the system is designed to be flexible about where that bleed air comes from. It can be supplied from engine bleed air when the engines are running and high demand exists, from APU bleed air when the engines aren’t providing enough or aren’t available, and from external ground air when the aircraft is on the ground connected to ground-conditioning equipment. This multi-source arrangement ensures the packs can deliver cabin conditioning in every phase of operation—during flight with engines or APU providing air, or on the ground with the appropriate sources available. Relying on only one source would restrict operation (for example, no engine bleed on the ground with engines off, or no option on the ground without external air), so the best answer is that all three bleed sources can supply the PACKs.