Narrative:

Ground operations and climb out were uneventful with one exception. Prior to pushback; the flight attendant asked us to take a look at the 3R door because; in her opinion; there appeared to be a gap that was larger than normal. The first officer went back to take a look and said that it appeared ok; and the door was armed properly. All the doors were showing closed; so we decided it was safe to proceed. We asked the flight attendant to let us know if it was unusually noisy once we got airborne. We didn't notice anything unusual in the pressurization of the aircraft while climbing out. We didn't have any EICAS messages or notice any status cue displayed either. About an hour into the flight and at FL330; we got a cabin altitude warning. We complied with the QRH procedures while requesting and receiving clearance to descend to 10000 ft. I checked the cabin altitude and rate of climb and it was above 10000 ft and climbing at about 100-200 FPM. My first thought was that the 3R door was leaking; and that was the reason for the slow rate of climb in the cabin altitude. From a physiological standpoint; there were no indications the cabin was at 10000 ft. Kudos for the simulator training that teaches to start an immediate descent. I contacted the flight attendant and informed him that we were descending due to the cabin altitude and he informed me that everything appeared normal in the back. Very soon after we started the descent; the cabin altitude stabilized at 0 FPM rate of climb and by the time we were in the mid twenties it was descending again. The cabin altitude light extinguished at around 14000 ft and the cabin continued to descend to approximately 4500 ft at leveloff. I did not declare an emergency because ATC did not delay granting our request for a lower altitude. We did tell them we were having a cabin pressurization issue after the fact. By the time I reached the step in the QRH that deals with the cabin altitude being uncontrollable; the cabin rate of climb had dropped to 0 FPM; so we considered it controllable. After consultation with the maintenance coordinator; dispatcher and flight attendant; we decided it was safe to continue to ZZZ1. At that point; the only aircraft issue was the overboard exhaust valve that showed open via a status message. The flight attendants said that no one in the back had mentioned or appeared to have noticed that we had descended. There were no comments from any passenger as they deplaned relating to the descent or cabin altitude. When we got to the gate at ZZZ1; we were met by maintenance and described the event. We relayed the sequence of events from ground operations to level off at altitude through descent to 10000 ft. We followed up with them the following day and they said that they suspected a fault in the overboard exhaust valve or controling relays. They replaced the overboard exhaust valve and associated relays. They also found and fixed some issues with the right pack and associated ducting.

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Original NASA ASRS Text

Title: A B757 FLT CREW EXPERIENCED A CABIN PRESSURIZATION PROBLEM IN CRUISE FLT. THEY RAN THE PROCEDURE; REGAINED CONTROL OF THE PRESSURIZATION SYSTEM; AND CONTINUED TO DEST.

Narrative: GND OPS AND CLBOUT WERE UNEVENTFUL WITH ONE EXCEPTION. PRIOR TO PUSHBACK; THE FLT ATTENDANT ASKED US TO TAKE A LOOK AT THE 3R DOOR BECAUSE; IN HER OPINION; THERE APPEARED TO BE A GAP THAT WAS LARGER THAN NORMAL. THE FO WENT BACK TO TAKE A LOOK AND SAID THAT IT APPEARED OK; AND THE DOOR WAS ARMED PROPERLY. ALL THE DOORS WERE SHOWING CLOSED; SO WE DECIDED IT WAS SAFE TO PROCEED. WE ASKED THE FLT ATTENDANT TO LET US KNOW IF IT WAS UNUSUALLY NOISY ONCE WE GOT AIRBORNE. WE DIDN'T NOTICE ANYTHING UNUSUAL IN THE PRESSURIZATION OF THE ACFT WHILE CLBING OUT. WE DIDN'T HAVE ANY EICAS MESSAGES OR NOTICE ANY STATUS CUE DISPLAYED EITHER. ABOUT AN HR INTO THE FLT AND AT FL330; WE GOT A CABIN ALT WARNING. WE COMPLIED WITH THE QRH PROCS WHILE REQUESTING AND RECEIVING CLRNC TO DSND TO 10000 FT. I CHKED THE CABIN ALT AND RATE OF CLB AND IT WAS ABOVE 10000 FT AND CLBING AT ABOUT 100-200 FPM. MY FIRST THOUGHT WAS THAT THE 3R DOOR WAS LEAKING; AND THAT WAS THE REASON FOR THE SLOW RATE OF CLB IN THE CABIN ALT. FROM A PHYSIOLOGICAL STANDPOINT; THERE WERE NO INDICATIONS THE CABIN WAS AT 10000 FT. KUDOS FOR THE SIMULATOR TRAINING THAT TEACHES TO START AN IMMEDIATE DSCNT. I CONTACTED THE FLT ATTENDANT AND INFORMED HIM THAT WE WERE DSNDING DUE TO THE CABIN ALT AND HE INFORMED ME THAT EVERYTHING APPEARED NORMAL IN THE BACK. VERY SOON AFTER WE STARTED THE DSCNT; THE CABIN ALT STABILIZED AT 0 FPM RATE OF CLB AND BY THE TIME WE WERE IN THE MID TWENTIES IT WAS DSNDING AGAIN. THE CABIN ALT LIGHT EXTINGUISHED AT AROUND 14000 FT AND THE CABIN CONTINUED TO DSND TO APPROX 4500 FT AT LEVELOFF. I DID NOT DECLARE AN EMER BECAUSE ATC DID NOT DELAY GRANTING OUR REQUEST FOR A LOWER ALT. WE DID TELL THEM WE WERE HAVING A CABIN PRESSURIZATION ISSUE AFTER THE FACT. BY THE TIME I REACHED THE STEP IN THE QRH THAT DEALS WITH THE CABIN ALT BEING UNCONTROLLABLE; THE CABIN RATE OF CLB HAD DROPPED TO 0 FPM; SO WE CONSIDERED IT CONTROLLABLE. AFTER CONSULTATION WITH THE MAINT COORDINATOR; DISPATCHER AND FLT ATTENDANT; WE DECIDED IT WAS SAFE TO CONTINUE TO ZZZ1. AT THAT POINT; THE ONLY ACFT ISSUE WAS THE OVERBOARD EXHAUST VALVE THAT SHOWED OPEN VIA A STATUS MESSAGE. THE FLT ATTENDANTS SAID THAT NO ONE IN THE BACK HAD MENTIONED OR APPEARED TO HAVE NOTICED THAT WE HAD DSNDED. THERE WERE NO COMMENTS FROM ANY PAX AS THEY DEPLANED RELATING TO THE DSCNT OR CABIN ALT. WHEN WE GOT TO THE GATE AT ZZZ1; WE WERE MET BY MAINT AND DESCRIBED THE EVENT. WE RELAYED THE SEQUENCE OF EVENTS FROM GND OPS TO LEVEL OFF AT ALT THROUGH DSCNT TO 10000 FT. WE FOLLOWED UP WITH THEM THE FOLLOWING DAY AND THEY SAID THAT THEY SUSPECTED A FAULT IN THE OVERBOARD EXHAUST VALVE OR CTLING RELAYS. THEY REPLACED THE OVERBOARD EXHAUST VALVE AND ASSOCIATED RELAYS. THEY ALSO FOUND AND FIXED SOME ISSUES WITH THE R PACK AND ASSOCIATED DUCTING.

Data retrieved from NASA's ASRS site as of May 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.