What is the maximum altitude for single pack operation?

• A. 28,000 ft
• B. 31,000 ft
• C. 33,000 ft
• D. 35,000 ft

Answer: (B)

Working with one environmental control system pack at a time tests the capacity of the ECS to provide adequate conditioned air and maintain cabin pressure with reduced capability. When two packs are available, they share the load and can support higher altitude operations. But with only one pack, the mass flow and cooling/heating capacity are reduced, so the system must cap the altitude to ensure cabin temperature stays within limits and air supply remains sufficient for all zones. That limit is typically around 31,000 feet, which is why the maximum altitude for single-pack operation is 31,000 ft. Going higher with just one pack would risk inadequate conditioning and potential cabin pressure/temperature control issues. The other options are either below this practical limit or represent altitudes that would require two packs to sustain safely.