737 MAX Flight Crew Operations Manual
Air Systems -
Pressurization System Description
2.40.4
MN-FLT-OH-201
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.
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 lowest possible
cabin altitude based on the differential pressure limits indicated in the table below.
In certain circumstances the selected LAND ALT may exceed the target cabin
altitude determined by the differential pressure limits. In these cases, the
controller will maintain a cabin altitude slightly below the selected LAND ALT.
Deviations from flight altitude can cause the pressure differential to vary as the
controller modulates the outflow valve to maintain a constant cabin altitude.
March 1, 2021
