Narrative:

Cabin altitude alert due to cabin altitude at 10000 ft. Usual cabin altitude at cruise is 7000-8000 ft. I did not notice that the cabin had begun a slow increase. Descent with oxygen masks on was initiated to FL330 and cabin returned to normal in approximately 10 mins. Flight to destination was uneventful (atl-den). Maintenance was notified and indicated to us that a cracked heat exchanger could be the cause. However, no such problem was found in den. Maintenance indicated further checks would be undertaken in atl after the return flight (den-atl). Callback conversation with reporter revealed the following information: the reporter stated the crew was surprised when the cabin pressure warning horn sounded and a look at the cabin pressure indicators showed the cabin was climbing very slowly. The reporter said ATC was advised and they were given FL330. Before this altitude was reached the cabin pressure had returned to normal. The reporter stated the pressurization system remained in automatic and operated normally to den where maintenance accomplished a test of the system and no faults were found. The reporter said the aircraft was routed back to atl and the pressurization operated normally in all modes.

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Original NASA ASRS Text

Title: A B757-200 IN CRUISE AT FL390 EXPERIENCED THE CABIN PRESSURE WARNING HORN AND REQUESTED A DSCNT TO FL330 WHERE THE CABIN RETURNED TO NORMAL.

Narrative: CABIN ALT ALERT DUE TO CABIN ALT AT 10000 FT. USUAL CABIN ALT AT CRUISE IS 7000-8000 FT. I DID NOT NOTICE THAT THE CABIN HAD BEGUN A SLOW INCREASE. DSCNT WITH OXYGEN MASKS ON WAS INITIATED TO FL330 AND CABIN RETURNED TO NORMAL IN APPROX 10 MINS. FLT TO DEST WAS UNEVENTFUL (ATL-DEN). MAINT WAS NOTIFIED AND INDICATED TO US THAT A CRACKED HEAT EXCHANGER COULD BE THE CAUSE. HOWEVER, NO SUCH PROB WAS FOUND IN DEN. MAINT INDICATED FURTHER CHKS WOULD BE UNDERTAKEN IN ATL AFTER THE RETURN FLT (DEN-ATL). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE CREW WAS SURPRISED WHEN THE CABIN PRESSURE WARNING HORN SOUNDED AND A LOOK AT THE CABIN PRESSURE INDICATORS SHOWED THE CABIN WAS CLBING VERY SLOWLY. THE RPTR SAID ATC WAS ADVISED AND THEY WERE GIVEN FL330. BEFORE THIS ALT WAS REACHED THE CABIN PRESSURE HAD RETURNED TO NORMAL. THE RPTR STATED THE PRESSURIZATION SYS REMAINED IN AUTO AND OPERATED NORMALLY TO DEN WHERE MAINT ACCOMPLISHED A TEST OF THE SYS AND NO FAULTS WERE FOUND. THE RPTR SAID THE ACFT WAS ROUTED BACK TO ATL AND THE PRESSURIZATION OPERATED NORMALLY IN ALL MODES.

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.