Narrative:

On normal profile descent to sjc, upon leaving FL330, we experienced a bump in the cabin pressurization. We looked at the pressurization indications: cabin altitude = FL500, psi differential 8.0 pounds, cabin vsi -2000 ft descending. No automatic fail light was illuminated. We determined the problem was an over pressurization and elected to attempt to stabilize the cabin with the standby system. The standby mode yielded no noticeable change and we reselected the automatic mode. To my recollection, the cabin immediately showed a cabin vsi of -4000 FPM descent. Our parameters now included a cabin altitude of FL450 and a psi differential of 8.2 pounds. I was PF so my numbers might be rough. We were now at roughly 12000 ft MSL and elected to use the manual AC mode. The cabin immediately equalized to a normal cabin altitude with a noticeable and uncomfortable ear bump and a gush of cold air. We made one last attempt at the automatic mode at roughly 5000 ft MSL. It was now operating normally. We landed in the automatic position. Upon landing and with the advice of maintenance, we ran the outflow valve full open and closed. It acted normally. The captain also performed a walkaround. Maintenance and the captain believed the issue was a frozen outflow valve and that our changing modes and/or the descent to warmer air solved the problem. We continued the trip to ont-las. Las maintenance looked at the problem and determined that the outflow heater gasket had failed. Interesting to note that it is MEL'able under (MEL #21-37). The MEL provides a procedure for operating the valve in-flight in order to prevent freezing. The simple knowledge of this action would have been very helpful in both analyzing the problem and restabilizing the cabin. Supplemental information from acn 602543: normal flight until descent into sjc. Passing FL200, we noticed a pressurization spike. Cabin altitude showed FL500, psi differential [cabin differential pressure] was 7.8 pounds. In our attempt to get control of the pressurization, we went to standby and manual AC, as what concerned us was that the psi differential seemed to be increasing as we were continuing the descent. (The highest psi differential we saw was 8.3 - 8.4.) by the time we descended through 7000 ft MSL, pressurization was back in control (mode selector back in automatic) and cabin altitude was reading normally. While attempting to regain control of the pressurization in the descent, there were 2 separate occasions when everyone in the aircraft (cockpit and cabin) felt a cool rush of air. At no time did any masks deploy in the cabin, nor was there any fogging associated with a rapid depressurization. Landing and taxiing to the gate were normal. Conference call was established between dispatch, maintenance control and myself. Maintenance said what could have happened was that the outflow valve might have froze causing our pressurization problems during the descent. It was working properly in sjc and after discussions with maintenance, we continued our sequence. I told maintenance I would recontact them at our next city. Pressurization was normal for all remaining flts. While in las, maintenance changed the heater gasket to the outflow valve. All remaining flts after sjc were normal. In retrospect, there were 2 mistakes I made: 1) I should have done a better job of informing the passenger on what's going on. This flight was a through flight. As we were leaving sjc, one of the through passenger came up to the front of the aircraft and was upset saying no one had told them what was going on. I apologized, briefly explained the situation and made an announcement. In a descent with pressurization issues and WX, there was a lot going on, tasks needed to be prioritized, but one word like, 'folks, everything is fine, I'll explain at the gate what happened,' would have been helpful. 2) I should have written it up and established an MEL in sjc as opposed to writing it up in las where the actual part was fixed. Even though the pressurization was normal for all remaining flts that evening, the MEL would have more accurately reflected where the problem occurred.

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Original NASA ASRS Text

Title: B737-500 CREW HAD THE AUTO PRESSURIZATION CTL OVER PRESSURIZE THE ACFT TO THE POINT THE EMER RELIEF VALVES OPENED. THE CAPT FLEW 2 MORE LEGS BEFORE LISTING THE DISCREPANCY IN THE MAINT LOGBOOK.

Narrative: ON NORMAL PROFILE DSCNT TO SJC, UPON LEAVING FL330, WE EXPERIENCED A BUMP IN THE CABIN PRESSURIZATION. WE LOOKED AT THE PRESSURIZATION INDICATIONS: CABIN ALT = FL500, PSI DIFFERENTIAL 8.0 LBS, CABIN VSI -2000 FT DSNDING. NO AUTO FAIL LIGHT WAS ILLUMINATED. WE DETERMINED THE PROB WAS AN OVER PRESSURIZATION AND ELECTED TO ATTEMPT TO STABILIZE THE CABIN WITH THE STANDBY SYS. THE STANDBY MODE YIELDED NO NOTICEABLE CHANGE AND WE RESELECTED THE AUTO MODE. TO MY RECOLLECTION, THE CABIN IMMEDIATELY SHOWED A CABIN VSI OF -4000 FPM DSCNT. OUR PARAMETERS NOW INCLUDED A CABIN ALT OF FL450 AND A PSI DIFFERENTIAL OF 8.2 LBS. I WAS PF SO MY NUMBERS MIGHT BE ROUGH. WE WERE NOW AT ROUGHLY 12000 FT MSL AND ELECTED TO USE THE MANUAL AC MODE. THE CABIN IMMEDIATELY EQUALIZED TO A NORMAL CABIN ALT WITH A NOTICEABLE AND UNCOMFORTABLE EAR BUMP AND A GUSH OF COLD AIR. WE MADE ONE LAST ATTEMPT AT THE AUTO MODE AT ROUGHLY 5000 FT MSL. IT WAS NOW OPERATING NORMALLY. WE LANDED IN THE AUTO POS. UPON LNDG AND WITH THE ADVICE OF MAINT, WE RAN THE OUTFLOW VALVE FULL OPEN AND CLOSED. IT ACTED NORMALLY. THE CAPT ALSO PERFORMED A WALKAROUND. MAINT AND THE CAPT BELIEVED THE ISSUE WAS A FROZEN OUTFLOW VALVE AND THAT OUR CHANGING MODES AND/OR THE DSCNT TO WARMER AIR SOLVED THE PROB. WE CONTINUED THE TRIP TO ONT-LAS. LAS MAINT LOOKED AT THE PROB AND DETERMINED THAT THE OUTFLOW HEATER GASKET HAD FAILED. INTERESTING TO NOTE THAT IT IS MEL'ABLE UNDER (MEL #21-37). THE MEL PROVIDES A PROC FOR OPERATING THE VALVE INFLT IN ORDER TO PREVENT FREEZING. THE SIMPLE KNOWLEDGE OF THIS ACTION WOULD HAVE BEEN VERY HELPFUL IN BOTH ANALYZING THE PROB AND RESTABILIZING THE CABIN. SUPPLEMENTAL INFO FROM ACN 602543: NORMAL FLT UNTIL DSCNT INTO SJC. PASSING FL200, WE NOTICED A PRESSURIZATION SPIKE. CABIN ALT SHOWED FL500, PSI DIFFERENTIAL [CABIN DIFFERENTIAL PRESSURE] WAS 7.8 LBS. IN OUR ATTEMPT TO GET CTL OF THE PRESSURIZATION, WE WENT TO STANDBY AND MANUAL AC, AS WHAT CONCERNED US WAS THAT THE PSI DIFFERENTIAL SEEMED TO BE INCREASING AS WE WERE CONTINUING THE DSCNT. (THE HIGHEST PSI DIFFERENTIAL WE SAW WAS 8.3 - 8.4.) BY THE TIME WE DSNDED THROUGH 7000 FT MSL, PRESSURIZATION WAS BACK IN CTL (MODE SELECTOR BACK IN AUTO) AND CABIN ALT WAS READING NORMALLY. WHILE ATTEMPTING TO REGAIN CTL OF THE PRESSURIZATION IN THE DSCNT, THERE WERE 2 SEPARATE OCCASIONS WHEN EVERYONE IN THE ACFT (COCKPIT AND CABIN) FELT A COOL RUSH OF AIR. AT NO TIME DID ANY MASKS DEPLOY IN THE CABIN, NOR WAS THERE ANY FOGGING ASSOCIATED WITH A RAPID DEPRESSURIZATION. LNDG AND TAXIING TO THE GATE WERE NORMAL. CONFERENCE CALL WAS ESTABLISHED BTWN DISPATCH, MAINT CTL AND MYSELF. MAINT SAID WHAT COULD HAVE HAPPENED WAS THAT THE OUTFLOW VALVE MIGHT HAVE FROZE CAUSING OUR PRESSURIZATION PROBS DURING THE DSCNT. IT WAS WORKING PROPERLY IN SJC AND AFTER DISCUSSIONS WITH MAINT, WE CONTINUED OUR SEQUENCE. I TOLD MAINT I WOULD RECONTACT THEM AT OUR NEXT CITY. PRESSURIZATION WAS NORMAL FOR ALL REMAINING FLTS. WHILE IN LAS, MAINT CHANGED THE HEATER GASKET TO THE OUTFLOW VALVE. ALL REMAINING FLTS AFTER SJC WERE NORMAL. IN RETROSPECT, THERE WERE 2 MISTAKES I MADE: 1) I SHOULD HAVE DONE A BETTER JOB OF INFORMING THE PAX ON WHAT'S GOING ON. THIS FLT WAS A THROUGH FLT. AS WE WERE LEAVING SJC, ONE OF THE THROUGH PAX CAME UP TO THE FRONT OF THE ACFT AND WAS UPSET SAYING NO ONE HAD TOLD THEM WHAT WAS GOING ON. I APOLOGIZED, BRIEFLY EXPLAINED THE SIT AND MADE AN ANNOUNCEMENT. IN A DSCNT WITH PRESSURIZATION ISSUES AND WX, THERE WAS A LOT GOING ON, TASKS NEEDED TO BE PRIORITIZED, BUT ONE WORD LIKE, 'FOLKS, EVERYTHING IS FINE, I'LL EXPLAIN AT THE GATE WHAT HAPPENED,' WOULD HAVE BEEN HELPFUL. 2) I SHOULD HAVE WRITTEN IT UP AND ESTABLISHED AN MEL IN SJC AS OPPOSED TO WRITING IT UP IN LAS WHERE THE ACTUAL PART WAS FIXED. EVEN THOUGH THE PRESSURIZATION WAS NORMAL FOR ALL REMAINING FLTS THAT EVENING, THE MEL WOULD HAVE MORE ACCURATELY REFLECTED WHERE THE PROB OCCURRED.

Data retrieved from NASA's ASRS site as of July 2007 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.