Which sources can supply air to the air conditioning packs?

• A. External air
• B. APU air
• C. Bleed air from the engines
• D. External air, APU air or bleed air from the engines, depending on the situation

Answer: (D)

The air conditioning packs are designed to be fed from multiple possible sources so cabin conditioning can happen in different phases of flight and on the ground. Bleed air from the engines is the primary source when the engines are running, delivering the hot, high‑pressure air that the packs then cool and condition for cabin use. When the engines aren’t providing bleed air—such as on the ground or in certain flight situations—the APU can supply bleed air to the packs, allowing the cabin to be conditioned without the main engines running. In some aircraft configurations, external air can also be used to feed or support the packs, for example during ground operations or when bleed air isn’t available, with the system automatically managing the source that’s appropriate. Because the setup depends on the phase of flight and available power sources, all three—external air, APU air, or engine bleed air—can supply the packs depending on the situation.