A combination pressure and vacuum valve meant to prevent cabin pressurization from exceeding safe limits. The valve normally combines the functions of overpressure relief valve, vacuum relief valve, and dump valve. The overpressure relief valve prevents cabin pressure from exceeding specific values; the vacuum relief valve ensures that ambient pressure is less than cabin pressure by allowing ambient air to enter the cabin whenever ambient air exceeds cabin pressure; and the dump valve permits depressurization of the cabin by a cockpit-operated switch.
Normal flight.
Pressure relief.
Cabin altitude selector knob (selects isobaric setting)
