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Reading into Federal Express Flight 1406 and I noticed when the flight engineer said he was raising the cabin altitude “manually” I assume this has something to do with cutting off the oxygen, but how can it be done manually? This also makes me curious to know if there is another way of doing it, but would be disabled under certain emergency scenarios?

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Tip: accident investigation reports cover the system controls related to an accident. The one in question is here (ntsb.gov), from which:

According to the March 20, 1994, edition of FedEx’s DC-10 Flight Manual, page 8-2-2/8-2-3:

Cabin pressure is controlled and maintained by metered release of conditioned air in either an AUTO, SEMI-AUTO, or MANUAL mode…. The cabin pressure controller maintains the desired cabin pressure in relation to the altitude set…. In the AUTO mode, cabin pressurization is automatically controlled during takeoff, climb, cruise, and descent conditions to the lowest cabin altitude compatible with aircraft and flight requirement.

When MANUAL control is selected, the control wheel engages a locking mechanism to secure the outflow valve in its existing position. After selection, manual control is achieved by pressing the control wheel inward (to disengage the control wheel locking device) and rotating the wheel to position the outflow valve to attain the desired cabin altitude. When the control wheel is released, the outflow valve is again locked in its existing position. Selecting the valve at more than two-thirds open can cause a negative pressure and create a noise in the cabin. At touchdown, the Second Officer [flight engineer] must manually depressurize the aircraft. [emphasis added]

Regarding the outflow valves, see:

And the aforementioned wheel and readout:

enter image description here
Source: wikimedia.org

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