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CPCS Back Up Mode


jetset
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CPCS backup is a mode of the GASC (Global Air Supply Controller) when it loses certain critical sensor inputs (such as cabin pressure differential, or aircraft barometric altitude). Basically the controller then can't provide reasonable cabin altitude control, so it defaults to a "safe-in-all-conditions" cabin altitude of 9800 ft.
Below 9800 ft your automatic outflow valve negative differential protection will make sure the cabin altitude doesn't exceed flight altitude by too much (meaning, the pressure in the cabin would be lower than the outside air). But above 9,800 ft flight altitude, the cabin will essentially become locked to 9,800 cabin altitude and stay there for the entire flight.

For reference, this is in the POH in the ECS section.

Edited by Goran_M
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