Narrative:

Aircraft was at 26000 ft per abnormal procedure for a cracked R-4 window which happened earlier in the flight. Cabin altitude was 10000 ft and differential pressure was 5 psi (per procedure, controller in standby). Cabin altitude horn sounded as cabin slowly climbed above 10000 ft. The captain and I donned oxygen masks and captain inquired to flight attendants about passenger oxygen masks, while I began descent to 12000 ft per ATC clearance. An emergency was declared. Passenger masks dropped at 14000 ft cabin altitude and aircraft altitude met cabin altitude at approximately 15000 ft. Cabin altitude warning or rapid depressurization checklist was accomplished and aircraft was leveled at 12000 ft. Control of cabin was regained in the manual mode and a cabin altitude of 8000 ft was achieved. Diverting options were discussed and slc was chosen since distance to den was approximately the same and 12000 ft provided terrain separation all the way. Captain made several attempts to call company, but none worked so ATC was asked to relay our situation to company. 10 mins later, the outflow valve began to open and cabin altitude rose to 12000 ft (aircraft altitude). Flight deck oxygen masks were redonned and captain was able to regain control of the cabin in manual mode to 5000 ft. Flight was continued at 12000 ft (cabin at 5000 ft) until top of descent into slc. Descent, approach, and arrival were uneventful. Callback conversation with reporter revealed the following information: the first officer stated in this callback that one of the air conditioning packs failed, and that failure allowed cabin pressurization to that the pack in addition to preventing the failed pack from supplying pressure. So it appears that there were multiple failures. Analyst asked the reporter about identing the cabin altitude warning horn sound with the rising cabin, and he stated that because of the cracked window the crew had reviewed the rapid depressurization procedure, so they knew the cabin warning horn sounds. They were able to cancel the horn rapidly.

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Original NASA ASRS Text

Title: A B737-300 AFTER EXPERIENCING A SIDE WINDOW MIDDLE LAYER CRACK, DSNDED TO 26000 FT AND SUBSEQUENTLY EXPERIENCED A PARTIAL LOSS OF CABIN PRESSURE BECAUSE OF A PACK FAILURE.

Narrative: ACFT WAS AT 26000 FT PER ABNORMAL PROC FOR A CRACKED R-4 WINDOW WHICH HAPPENED EARLIER IN THE FLT. CABIN ALT WAS 10000 FT AND DIFFERENTIAL PRESSURE WAS 5 PSI (PER PROC, CTLR IN STANDBY). CABIN ALT HORN SOUNDED AS CABIN SLOWLY CLBED ABOVE 10000 FT. THE CAPT AND I DONNED OXYGEN MASKS AND CAPT INQUIRED TO FLT ATTENDANTS ABOUT PAX OXYGEN MASKS, WHILE I BEGAN DSCNT TO 12000 FT PER ATC CLRNC. AN EMER WAS DECLARED. PAX MASKS DROPPED AT 14000 FT CABIN ALT AND ACFT ALT MET CABIN ALT AT APPROX 15000 FT. CABIN ALT WARNING OR RAPID DEPRESSURIZATION CHKLIST WAS ACCOMPLISHED AND ACFT WAS LEVELED AT 12000 FT. CTL OF CABIN WAS REGAINED IN THE MANUAL MODE AND A CABIN ALT OF 8000 FT WAS ACHIEVED. DIVERTING OPTIONS WERE DISCUSSED AND SLC WAS CHOSEN SINCE DISTANCE TO DEN WAS APPROX THE SAME AND 12000 FT PROVIDED TERRAIN SEPARATION ALL THE WAY. CAPT MADE SEVERAL ATTEMPTS TO CALL COMPANY, BUT NONE WORKED SO ATC WAS ASKED TO RELAY OUR SIT TO COMPANY. 10 MINS LATER, THE OUTFLOW VALVE BEGAN TO OPEN AND CABIN ALT ROSE TO 12000 FT (ACFT ALT). FLT DECK OXYGEN MASKS WERE REDONNED AND CAPT WAS ABLE TO REGAIN CTL OF THE CABIN IN MANUAL MODE TO 5000 FT. FLT WAS CONTINUED AT 12000 FT (CABIN AT 5000 FT) UNTIL TOP OF DSCNT INTO SLC. DSCNT, APCH, AND ARR WERE UNEVENTFUL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FO STATED IN THIS CALLBACK THAT ONE OF THE AIR CONDITIONING PACKS FAILED, AND THAT FAILURE ALLOWED CABIN PRESSURIZATION TO THAT THE PACK IN ADDITION TO PREVENTING THE FAILED PACK FROM SUPPLYING PRESSURE. SO IT APPEARS THAT THERE WERE MULTIPLE FAILURES. ANALYST ASKED THE RPTR ABOUT IDENTING THE CABIN ALT WARNING HORN SOUND WITH THE RISING CABIN, AND HE STATED THAT BECAUSE OF THE CRACKED WINDOW THE CREW HAD REVIEWED THE RAPID DEPRESSURIZATION PROC, SO THEY KNEW THE CABIN WARNING HORN SOUNDS. THEY WERE ABLE TO CANCEL THE HORN RAPIDLY.

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.