Auto Mode Operation
The AUTO system consists of two identical controllers, with one controller alternately
sequenced as the primary operational controller for each new flight. The other automatic
controller is immediately available as a backup. In the AUTO or ALTN mode, the
pressurization control panel is used to preset two altitudes into the auto controllers:
- FLT ALT (flight or cruise altitude).
- LAND ALT (landing or destination airport altitude).
Takeoff airport altitude (actually cabin altitude) is fed into the auto controllers at all times
when on the ground. The air/ground safety sensor signals whether the airplane is on the
ground or in the air. On the ground and at lower power settings, the cabin is depressurized
by driving the outflow valve to the full open position.
The cabin begins to pressurize on the ground at higher power settings. The controller
modulates the outflow valve toward close, slightly pressurizing the cabin. This ground
pressurization of the cabin makes the transition to pressurized flight more gradual for the
passengers and crew, and also gives the system better response to ground effect pressure
changes during takeoff. In the air, the auto controller maintains a proportional pressure
differential between airplane and cabin altitude. By increasing the altitude at a rate
proportional to the airplane climb rate, cabin altitude change is held to the minimum rate
required.
An amber OFF SCHED DESCENT light illuminates if the airplane begins to descend without
having reached the preset cruise altitude; for example, a flight aborted in climb and
returning to the takeoff airport. The controller programs the cabin to land at the takeoff field
elevation without further pilot inputs. If the FLT ALT indicator is changed, the automatic
abort capability to the original takeoff field elevation is lost.
