2010-12 · NASA ASRS report 925583
B737 flight crew experiences a cabin altitude warning horn passing through FL210 during climb. Crew is able to control the outflow valve manually and continue to destination. The aircraft had a history of pressurization problems.
Climbing through FL210 the intermittent warning horn sounded. I donned my oxygen mask and configured the radios for communication. I reported to ATC that we had a pressurization issue and needed to stop climbing. The Captain tried to regain cabin altitude control by selecting the (analog) mode first to standby; and when that failed; to manual. The outflow valve quickly moved towards the close position and control was quickly regained and the cabin altitude descended rapidly. I read the appropriate QRH checklists and we left the mode controller in manual. ATC initially re-cleared us to 10;000 FT; but once the pressurization control was regained we decided to level off at FL230 and remained there for the rest of the flight. Adjustments were made to maintain the optimal cabin altitude for our flight level and also while descending and landing. Note: Passing through 10;000 FT; the Climb Checklist was accomplished and the aircraft seemed to be pressurizing normally.We were running approximately five hours late and we were given an aircraft from the hangar; which had previously been written up for the same type of malfunction at FL260. The corrective action showed that the outflow valve had just been replaced so we were cognizant of the plane's recent history and it allowed for a more timely reaction to the event. However; it's clear the original problem was not resolved.
The pressurization system appeared to be working normally while climbing through 10;000 FT. While climbing through about 21;000 FT; we heard the cabin altitude warning horn. As we started the memory items; I immediately selected manual and the cabin started descending. The pressurization was controllable in manual. We didn't hear the horn for more than thirty seconds. We did notify ATC and asked to level at 23;000 FT. We then coordinated with Dispatch and together elected to continue to destination. The flight proceeded uneventfully from that point on.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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