How does the pressurization system（B737）work if airplane climbs again after descending from the original cruising level？

Question

Does the pressurization system（B737ng） still remain in descent mode？

Answer 1

Assuming AUTO mode operation, descent mode would activate when the aircraft descends 0.25 psi below the selected FLT ALT (≈2000 ft at 37,000ft). The cabin begins a proportional decent to slightly below the selected LAND ALT.

If a climb was then initiated, ambient static pressure would start to decrease, altitude would increase and thrust levers would advance. Ambient static pressure and altitude changes are detected by the air data inertial reference units (ADIRUs) and thrust lever position are detected by both stall management computers (SMYDs). The cabin pressure controller receives inputs from the ADIRUs and SMYDs and would act to maintain a proportional pressure differential between airplane altitude and cabin altitude. It would therefore no longer be in descent mode.

If the FLT ALT window wasn’t altered, and the aircraft climbed again to the previous cruising level, the cruise mode would activate again when the airplane climbs within 0.25 psi of the FLT ALT (≈2000 ft at 37,000ft). In cruise mode the cabin pressure controller seeks to maintain the lowest possible cabin altitude based on differential pressure limits.

If the FLT ALT window wasn’t altered, and the aircraft climbed again above the previous cruising level, the cabin pressure controller would act to maintain a proportional pressure differential between airplane altitude and cabin altitude. This may result in a higher cabin altitude than if the FLT ALT window was adjusted to the actual new cruising altitude. The cabin pressure controller is not in cruise mode. Furthermore, as the system is not anticipating a new FLT ALT it may take a short amount of time to “settle” at a stable cabin altitude when the new cruising altitude has been reached. This delay happens because the cabin pressure controller needs to adjust the outflow valve position to balance the pressure differential and achieve a new stable cabin environment.