Cabin Pressurisation & Differential Pressure

Pressurisation does not work by pumping the cabin up to a set pressure. The packs deliver an essentially constant flow of conditioned air, and cabin pressure is controlled entirely by how much of that air is allowed to escape through the outflow valve. Closing the valve traps more air and the cabin pressure rises; opening it lets more out and the cabin pressure falls.

The controller follows a cabin altitude schedule so that passengers experience a gentle rate of change, typically a few hundred feet per minute, while the aircraft climbs far faster. It also enforces the maximum differential pressure, which is a structural limit on the pressure hull, not a comfort limit. Once that limit is reached the cabin altitude must rise with the aircraft to protect the structure, which is why a higher cruising altitude gives a higher cabin altitude. Positive and negative pressure relief valves provide independent protection if the controller fails.

The failure logic follows directly. If the outflow valve fails open, the escaping flow exceeds the supply and the cabin climbs toward the aircraft altitude. If the supply is lost, there is no inflow to hold against normal leakage, so the cabin climbs even with the valve fully closed. That is the diagnostic point: a cabin climbing with the valve already closed means the fault is upstream, not in the valve.

• Try this: Run an auto climb to 41,000 ft. At what aircraft altitude does the cabin stop following the schedule and start climbing, and what is limiting it?
• Try this: Reduce the maximum differential pressure. What happens to the cabin altitude at cruise, and what does that tell you about hull strength versus cabin comfort?
• Try this: Select Air supply lost at cruise. The outflow valve drives fully closed, yet the cabin altitude still rises. Explain why, and what the crew action would be.