Auto Mode Operation (Contd.)
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 8.35 psid above 37, 000 feet, 7.80 psid with the FLT ALT between 28,000
and 37,000 feet, and 7.45 psid with FLT ALT less than 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 8.35 psid above 37,000
feet, 7.80 psid with the FLT ALT between 28,000 and 37,000 feet, and 7.45 psid with FLT
ALT less than 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 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. While taxiing in, the
controller drives the outflow valve slowly to the full open position depressurizing the cabin.
