What is the maximum altitude at which a single air conditioning pack can operate?

• A. 29,000 ft
• B. 30,000 ft
• C. 31,000 ft
• D. 32,000 ft

Answer: (C)

The key idea is how the environmental control system supplies conditioned air with limited capacity when only one pack is available. Each air conditioning pack handles the cooling and delivery of conditioned air to the cabin, and it relies on ambient air cooling (via ram air) and its own heat-exchanger capacity. As you climb, the air becomes thinner and the pack has less effectively cooling air to work with, while the cabin still demands a certain amount of conditioned air to keep temperature and pressure within limits. A single pack has enough capacity up to about 31,000 feet to maintain comfortable cabin conditions and proper pressurization. Beyond that altitude, a single pack can no longer meet the cooling and airflow demands, so two packs are required to maintain acceptable cabin conditions. That is why 31,000 feet is the maximum altitude for single-pack operation.