BOEING COMMERCIAL AIRPLANE GROUP
FLIGHT OPERATIONS TECHNICAL BULLETIN
NUMBER: 737-09-3
DATE: July 1, 2009
... Investigation of pressurization incidents in the fleet has indicated that some operators are setting the FLT ALT before takeoff to an intermediate altitude, and then resetting the FLT ALT one or more times during climb until the final cruise altitude or flight level is reached. Although this method of operation will also provide a safe and comfortable cabin altitude for the duration of the flight, there are some considerations which may make this method of operation less desirable:
a) Multiple adjustments of the FLT ALT unnecessarily increase crew workload
during climb.
b) Intermediate FLT ALT settings are likely to result in higher cabin altitudes than if the final planned cruise flight altitude is selected. This is because the pressure schedule for lower altitudes uses lower differential pressure limits.
c) If the flight crew does not reset the FLT ALT when appropriate, flying above the selected FLT ALT can result in an overpressure situation and has resulted in activation of the pressure relief valves. Boeing has investigated several events where the airplane altitude exceeded the selected FLT ALT. When this happens, the system will overpressurize the cabin. When the differential pressure limit is reached, the system will first try to relieve pressure by opening the outflow valve. Under these conditions the pressurization system will allow the normal cabin rate limitations to be exceeded which will result in high cabin rates. Depending on the magnitude of the difference between the selected and actual FLT ALT, the pressure relief valves may be activated.
So, that's Boeing's position.
