Narrative:

I was the captain on a commercial air carrier flight. My first officer and I departed ZZZ1 with 1 MEL item outstanding for an inoperative right recirculation fan. During climb out and leveloff we were in and out of clouds and deviating slightly for thunderstorms. Engine anti-ice was on from approximately FL240 to leveloff. Shortly after reaching our assigned cruise flight level of FL360 the aircraft experienced a noticeable depressurization bump accompanied by immediate 'cabin altitude' aural and EICAS warnings. There were no other warnings; cautions; or status messages associated with this event; but cabin altitude was climbing at approximately 1000-1500 FPM; passing through approximately 10000 ft; and was very uncomfortable. At that point we immediately initiated the QRH procedure for a 'cabin altitude warning' and declared an emergency. Maximum cabin altitude attained during the event was approximately 13000 ft. We also requested and were granted an emergency descent to 10000 ft by center. During the descent and leveloff we also requested and were granted course deviations as necessary to avoid thunderstorms in the area. During the entire event there were no indications of any loss of separation; no ATC traffic calls; and a TCAS scan indicated no other aircraft in the vicinity. The remainder of the descent and leveloff were uneventful. During the descent we communicated with cabin crew who did a magnificent job keeping the passenger informed and providing for their welfare throughout the entire event. After we leveled off we were able to regain control of the cabin and all cockpit indications returned to normal. At that point we canceled the emergency and contacted the company. After consultation with both dispatch and maintenance the decision was made to divert to ZZZ2. There were no further abnormal indications during the approach and landing phase of flight. Subsequent discussions with maintenance indicated the aircraft had a prior history of pressurization malfunctions which had resulted in a recent left air cycle machine change. Those discussions also indicated that the most probable cause of the depressurization was a faulty 'aft cargo door seal' failure that only occurred at high differential pressure. That was also the reason for the lack of other EICAS warnings as well as the reestablishment of cabin control at lower altitudes/differential pressure. The aircraft was subsequently ferried back to a maintenance station for further maintenance action.

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Original NASA ASRS Text

Title: A B757 EXPERIENCED LOSS OF CABIN PRESSURE AT CRUISE ALTITUDE. THE FLT CREW DECLARED AN EMERGENCY; EXECUTED AN EMERGENCY DESCENT; AND DIVERTED TO THE NEAREST SUITABLE AIRPORT.

Narrative: I WAS THE CAPT ON A COMMERCIAL ACR FLT. MY FO AND I DEPARTED ZZZ1 WITH 1 MEL ITEM OUTSTANDING FOR AN INOP R RECIRCULATION FAN. DURING CLBOUT AND LEVELOFF WE WERE IN AND OUT OF CLOUDS AND DEVIATING SLIGHTLY FOR TSTMS. ENG ANTI-ICE WAS ON FROM APPROX FL240 TO LEVELOFF. SHORTLY AFTER REACHING OUR ASSIGNED CRUISE FLT LEVEL OF FL360 THE ACFT EXPERIENCED A NOTICEABLE DEPRESSURIZATION BUMP ACCOMPANIED BY IMMEDIATE 'CABIN ALT' AURAL AND EICAS WARNINGS. THERE WERE NO OTHER WARNINGS; CAUTIONS; OR STATUS MESSAGES ASSOCIATED WITH THIS EVENT; BUT CABIN ALT WAS CLBING AT APPROX 1000-1500 FPM; PASSING THROUGH APPROX 10000 FT; AND WAS VERY UNCOMFORTABLE. AT THAT POINT WE IMMEDIATELY INITIATED THE QRH PROC FOR A 'CABIN ALT WARNING' AND DECLARED AN EMER. MAX CABIN ALT ATTAINED DURING THE EVENT WAS APPROX 13000 FT. WE ALSO REQUESTED AND WERE GRANTED AN EMER DSCNT TO 10000 FT BY CTR. DURING THE DSCNT AND LEVELOFF WE ALSO REQUESTED AND WERE GRANTED COURSE DEVS AS NECESSARY TO AVOID TSTMS IN THE AREA. DURING THE ENTIRE EVENT THERE WERE NO INDICATIONS OF ANY LOSS OF SEPARATION; NO ATC TFC CALLS; AND A TCAS SCAN INDICATED NO OTHER ACFT IN THE VICINITY. THE REMAINDER OF THE DSCNT AND LEVELOFF WERE UNEVENTFUL. DURING THE DSCNT WE COMMUNICATED WITH CABIN CREW WHO DID A MAGNIFICENT JOB KEEPING THE PAX INFORMED AND PROVIDING FOR THEIR WELFARE THROUGHOUT THE ENTIRE EVENT. AFTER WE LEVELED OFF WE WERE ABLE TO REGAIN CTL OF THE CABIN AND ALL COCKPIT INDICATIONS RETURNED TO NORMAL. AT THAT POINT WE CANCELED THE EMER AND CONTACTED THE COMPANY. AFTER CONSULTATION WITH BOTH DISPATCH AND MAINT THE DECISION WAS MADE TO DIVERT TO ZZZ2. THERE WERE NO FURTHER ABNORMAL INDICATIONS DURING THE APCH AND LNDG PHASE OF FLT. SUBSEQUENT DISCUSSIONS WITH MAINT INDICATED THE ACFT HAD A PRIOR HISTORY OF PRESSURIZATION MALFUNCTIONS WHICH HAD RESULTED IN A RECENT L AIR CYCLE MACHINE CHANGE. THOSE DISCUSSIONS ALSO INDICATED THAT THE MOST PROBABLE CAUSE OF THE DEPRESSURIZATION WAS A FAULTY 'AFT CARGO DOOR SEAL' FAILURE THAT ONLY OCCURRED AT HIGH DIFFERENTIAL PRESSURE. THAT WAS ALSO THE REASON FOR THE LACK OF OTHER EICAS WARNINGS AS WELL AS THE REESTABLISHMENT OF CABIN CTL AT LOWER ALTS/DIFFERENTIAL PRESSURE. THE ACFT WAS SUBSEQUENTLY FERRIED BACK TO A MAINT STATION FOR FURTHER MAINT ACTION.

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.