The cruise mode is activated when the airplane climbs to within 0.25 psi of the
selected FLT ALT. During cruise, the controller maintains the cabin altitude
slightly below the selected LAND ALT, if the differential pressure between the
selected LAND ALT and FLT ALT is less than or equal to 7.8 psid above 28,000
feet or 7.45 psid below 28,000 feet. If the differential pressure between the
selected LAND ALT and FLT ALT is greater than these values, the controller
maintains a pressure differential of 7.8 psid above 28,000 feet and 7.45 psid below
28,000 feet. Deviations from flight altitude can cause the pressure differential to
vary as the controller modulates the outflow valve to maintain a constant cabin
altitude.
The descent mode is activated when the airplane descends to 0.25 psi below the
selected FLT ALT. The cabin begins a proportional descent to slightly below the
selected LAND ALT. The controller programs the cabin to land slightly
pressurized so that rapid changes in altitude during approach result in minimum
cabin pressure changes.
Taxiing in, the controller drives the main outflow valve slowly to full open when
the FLT/GRD switch is positioned to GRD, thereby depressurizing the cabin.
Having the main outflow valve full open also prevents the equipment cooling fan
from depressurizing the airplane to a negative pressure.