The diaphragm that separates cabin pressure from vacuum pressure utilises two springs, which are biased by the pilot using the altitude selector knob. Changing this spring pressure through the altitude selector knob, directly affects the reference pressure controlling the movement of the outflow valve.
Cabin Altitude selector
Cabin Alt selector and biased springs inside press. controller.
A cabin rate controller is also employed in the pressurisation controller to vary the rate at which cabin pressure is changed during climbs and descents.
This rate can be increased or decreased by the pilot manually dependant on aircraft performance and passenger comfort.
Cabin Rate selector
|Cabin Rate control inside the pressurisation controller|
The cabin rate controller works by allowing small volumes of cabin air pressure into the vacuum source pressure which once again varies the amount of movement of the outflow valve.
A combined cabin altitude and cabin differential pressure guage is located behind the power and condition levers on the instrument panel to indicate the current cabin altitude and cabin differential pressure.
Combined Cabin pressure and Cabin differential guage (left) ROC/RoD guage (right)
The cabin rate of climb indicator is located directly beside the cabin altitude and cabin pressure guage and indicates to the pilot the rate of cabin climb or descent.
The rate controller can be adjusted as desired by reference to this guage to either increase or decrease the cabin rate as desired.
The pressurisation system also incorporateds a manual dump toggle valve which is operated in the event that the cabin needs to be depressurised immediately.
This toggle valve is operated by a guarded DUMP selector located next to the cabin altitude selector at the bottom of the centre pedestal.
Manual cabin DUMP toggle valve selector
This is a two position valve that must be selected up to deprssurise the cabin.
Selecting DEPRESS on the toggle valve pneumatically actuates the safety valve to full open by allowing vacuum to be sent to the safety valve therefore depressurising the cabin completely.
Pressurisation In flight Scenario De-pressurised