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In one of the MAYDAY Air Crash Investigation episodes (about the Payne Stewart crash), the flight's emergency manual had the following information under the cabin pressurisation section:

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NOTE: At 10,000 (± 500) Feet Cabin Altitude, control pressure to the outflow valve is trapped. This deactivates the Automatic Mode and stops cabin altitude from rising higher if the failure is in the automatic control system.

What does it actually mean? As the NTSB investigators mentioned, it sure does sound confusing.

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If the pilots know the system, it isn't confusing at all. It simply states that a solenoid valve closes causing the cabin pressurization system to go into manual mode and the pressurization controller cannot control the cabin pressure.

It is a feature of Learjets to prevent cabin pressurization issues in case the controller is faulty.

Since you reference the Payne Stewart accident you should also know there were other issues at play that did not cause the pressurization controller to fail. It was one of the early Learjet 35's without an emergency pressurization system.

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I haven't watched that episode, and normally wouldn't take a studio prop and the overdramatization seriously. But what it means can be broken down easily:

Cabin altitude is a measure of the cabin's pressurization. 10,000 feet means the pressure inside the cabin is that of 10,000 feet outside the plane. The higher this value, the lower the pressure.

Trapping the outflow valve means the outflow valve closes. The outflow valve opens and closes to regulate the cabin pressure.

When it is fully closed, the cabin pressure won't decrease unless there's structural damage causing the pressure to escape. The valve closes if the cabin altitude reaches 10,000 feet.

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  • $\begingroup$ The outflow doesn't close when the outflow is trapped. Stuck at a certain position is more appropriate... definitely not closed. $\endgroup$ – wbeard52 Aug 11 '16 at 22:54

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