The pressurisation control system uses a variable pressure controller to drive the outflow valves which modulate the flow of pressurised air out of the cabin back to atmosphere. Both the outflow valve and the safety valve are referenced to ambient static air pressure and will automatically open should cabin pressure exceed 6.3 PSI.
Variable pressure controller
Outflow and Safety Valve
Both the outflow and safety valve is connected to a cabin altitude limit controller solenoid which automatically overrides any pressurisation control system failure that would allow the cabin altitude to exceed 13,500 ft, i.e. it tries to close the outflow valve to maintain 13,500 ft cabin altitude.
The cabin dump solenoid is used to direct vacuum to either the outflow and safety valves during ground operations or to the pressurisation controller in flight.
During ground operations a squat switch mounted on the right hand main landing gear leg energises the cabin dump solenoid allowing vacuum to the outflow and safety valves via the cabin vent valve which will open the valves to prevent the cabin being pressurised on the ground.
The outflow and safety valves use spring pressure to close the valves (pressurising the cabin) and are vacuum operated to open the valves (depressurising the cabin).
In flight, the cabin dump solenoid and cabin vent valve are de-energised. This allows vacuum pressure to the pressurisation controller which regulates the cabin pressure by varying the amount of vacuum pressure against spring pressure therefore regulating cabin altitude.
Pressurisation In flight Scenario -- Pressurised
Pressurisation In flight Scenario -- De-pressurised
|Vacuum pressure opens the outflow and safety valves Spring pressure closes them. Static pressure varies the spring pressure.|
The outflow valve and the safety valve are identical valves which with the safety valve taking over all functions of the outflow valve should it fail for any reason.
The Cabin vent solenoid valve and the Cabin dump solenoid are the only two valves in the pressurisation system that require electrical power for operation and will allow the aircraft to operate in a pressurised sense should electrical power be lost.
The system utilises a red CABIN ALTITUDE annunciator light on the right hand annunciator panel which will illuminate should the cabin altitude reach 11 500 ft.
This light is controlled by a separate barometric switch and is used to indicate to the pilot that the use of oxygen is required.
Cabin Altitude annunciator
The Pressurisation controller is responsible for metering the control air (vacuum) to the outflow valve located at the rear pressure bulkhead. The pressurisation controller is connected to both regulated vacuum pressure and cabin pressure. These two pressures adjust a diaphragm which controls the reference pressure which regulates the outflow valve.