Narrative:

Aircraft was OTS all night due to a repeat write-up regarding a vibration in the main cabin. The problem was thought to be associated with a cargo door seal that may have been affecting the pressurization system. Maintenance was still working on the aircraft at departure time which included a pressurization test on the ground. The aircraft was signed off by maintenance as airworthy. Due to the recurring problem; the captain conversed with both maintenance and operations before accepting the aircraft and they all agreed that the maintenance problem was idented and rectified. I was the monitoring pilot for the flight and due to the pressurization issue; I was watching the pressurization system more diligently. The initial climb out was normal and the pressurization was working on its normal schedule. Climbing through approximately FL180; a non-revenue pilot seated in the main cabin called the flight deck on the interphone to advise us that a vibration had started in the forward portion of the main cabin. We observed that the pressurization system was still working on schedule so we continued our climb. Climbing through FL280; the cabin rate of climb started to increase from 500 FPM to 1000 FPM; but the cabin altitude remained on schedule. Climbing through FL300; the differential pressure started to slowly decrease; and the cabin altitude started to increase faster than normal. The cabin rate of climb also increased to match the aircraft's actual rate of climb. We stopped our climb before reaching cruise altitude and asked ATC for a lower altitude. We decided to descend down to an altitude where the pressurization was previously working normally and we would be able to run the abnormal pressurization checklist in the QRH. As we began our descent; the cabin altitude started to increase at a very rapid rate and continued past 10000 ft at which time the cabin altitude warning horn sounded. We then completed all memory items and emergency checklists; and asked ATC to continue our descent down to 10000 ft MSL. We also declared an emergency with ATC and requested a return to our departure airport. We landed safely without any further incident. This maintenance problem was much bigger than originally thought. A more thorough pressurization system check should probably have been completed or even a functional check flight. This plane probably shouldn't have been returned to revenue service. Callback conversation with reporter revealed the following information: 718924: the reporter states that the aircraft was finally repaired when the cargo door seal was replaced. Most seals perform better as pressure differential increases. This seal was damaged in such a way that seal effectiveness decreased with increasing pressure.

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Original NASA ASRS Text

Title: B737 CREW RPTS PRESSURIZATION PROBS SHORTLY AFTER THE START OF VIBRATIONS IN THE MAIN CABIN. CABIN ALT WARNING HORN SOUNDED. EMER DECLARED FOLLOWED BY AN EMER DSCNT.

Narrative: ACFT WAS OTS ALL NIGHT DUE TO A REPEAT WRITE-UP REGARDING A VIBRATION IN THE MAIN CABIN. THE PROB WAS THOUGHT TO BE ASSOCIATED WITH A CARGO DOOR SEAL THAT MAY HAVE BEEN AFFECTING THE PRESSURIZATION SYS. MAINT WAS STILL WORKING ON THE ACFT AT DEP TIME WHICH INCLUDED A PRESSURIZATION TEST ON THE GND. THE ACFT WAS SIGNED OFF BY MAINT AS AIRWORTHY. DUE TO THE RECURRING PROB; THE CAPT CONVERSED WITH BOTH MAINT AND OPS BEFORE ACCEPTING THE ACFT AND THEY ALL AGREED THAT THE MAINT PROB WAS IDENTED AND RECTIFIED. I WAS THE MONITORING PLT FOR THE FLT AND DUE TO THE PRESSURIZATION ISSUE; I WAS WATCHING THE PRESSURIZATION SYS MORE DILIGENTLY. THE INITIAL CLBOUT WAS NORMAL AND THE PRESSURIZATION WAS WORKING ON ITS NORMAL SCHEDULE. CLBING THROUGH APPROX FL180; A NON-REVENUE PLT SEATED IN THE MAIN CABIN CALLED THE FLT DECK ON THE INTERPHONE TO ADVISE US THAT A VIBRATION HAD STARTED IN THE FORWARD PORTION OF THE MAIN CABIN. WE OBSERVED THAT THE PRESSURIZATION SYS WAS STILL WORKING ON SCHEDULE SO WE CONTINUED OUR CLB. CLBING THROUGH FL280; THE CABIN RATE OF CLB STARTED TO INCREASE FROM 500 FPM TO 1000 FPM; BUT THE CABIN ALT REMAINED ON SCHEDULE. CLBING THROUGH FL300; THE DIFFERENTIAL PRESSURE STARTED TO SLOWLY DECREASE; AND THE CABIN ALT STARTED TO INCREASE FASTER THAN NORMAL. THE CABIN RATE OF CLB ALSO INCREASED TO MATCH THE ACFT'S ACTUAL RATE OF CLB. WE STOPPED OUR CLB BEFORE REACHING CRUISE ALT AND ASKED ATC FOR A LOWER ALT. WE DECIDED TO DSND DOWN TO AN ALT WHERE THE PRESSURIZATION WAS PREVIOUSLY WORKING NORMALLY AND WE WOULD BE ABLE TO RUN THE ABNORMAL PRESSURIZATION CHKLIST IN THE QRH. AS WE BEGAN OUR DSCNT; THE CABIN ALT STARTED TO INCREASE AT A VERY RAPID RATE AND CONTINUED PAST 10000 FT AT WHICH TIME THE CABIN ALT WARNING HORN SOUNDED. WE THEN COMPLETED ALL MEMORY ITEMS AND EMER CHKLISTS; AND ASKED ATC TO CONTINUE OUR DSCNT DOWN TO 10000 FT MSL. WE ALSO DECLARED AN EMER WITH ATC AND REQUESTED A RETURN TO OUR DEP ARPT. WE LANDED SAFELY WITHOUT ANY FURTHER INCIDENT. THIS MAINT PROB WAS MUCH BIGGER THAN ORIGINALLY THOUGHT. A MORE THOROUGH PRESSURIZATION SYS CHK SHOULD PROBABLY HAVE BEEN COMPLETED OR EVEN A FUNCTIONAL CHK FLT. THIS PLANE PROBABLY SHOULDN'T HAVE BEEN RETURNED TO REVENUE SVC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: 718924: THE RPTR STATES THAT THE ACFT WAS FINALLY REPAIRED WHEN THE CARGO DOOR SEAL WAS REPLACED. MOST SEALS PERFORM BETTER AS PRESSURE DIFFERENTIAL INCREASES. THIS SEAL WAS DAMAGED IN SUCH A WAY THAT SEAL EFFECTIVENESS DECREASED WITH INCREASING PRESSURE.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.