The automatic pressure control system is designed to maintain cabin altitude
and rate of change at preselected values. The proper rate of change must be
calculated considering climb/descent time and maximum differential pressure.
The system is normally operated and controlled by the cabin pressure controller, incorporating the controls required for selecting desired cabin altitude, rate of change, and barometric setting.
The manual pressurization control system is used to override the automatic system if a malfunction should occur. When using the manual control system, the automatic system will be deactivated and the actuator declutched.
In MANUAL position (down) the clutch is opened and the valves are controlled by the cabin pressure manual control wheel.
The AUTO/MANUAL selector on this picture is in position AUTO and the OUTFLOW VALVE POSITION INDICATOR (2) is in position fully opened.
In manual mode the position of the outflow valve is controlled by the CABIN
PRESSURE MANUAL CONTROL WHEEL (1). The wheel is cable connected to the drive shaft
of the outflow valve actuator. The wheel is locked into position and must be
pressed down before it can be rotated to desired position.
Seen here is a position that is normal during cruise. The AUTO/MANUAL selector is auto.