Narrative:

We noticed a pressurization problem inbound during descent through FL220 to 16000 ft. Cabin suddenly climbed from 6500 ft to 9500 ft. The outflow valve was closed and left and right supply 25 psi. Around 12000 ft, system normalized. I wrote it up and maintenance controller MEL'ed autopressurization. System operated normally on outbound leg to FL330. 50 mins into our flight, we noticed cabin altitude of 8000 ft, but climbing 200 FPM (ice off). Outflow closed pack output 25 psi. We did not locate a communication procedure for this. ATC cleared us from FL330 to FL290. The cabin continued to climb unless the left air conditioning system was almost full hot. Passenger and flight attendant complained so we lowered temperature and cabin climbed. ATC would not give us lower than FL240. As we raised temperature on cabin, we lost left pack and lost cabin. Right pack was not doing anything. We performed emergency procedures, but did not regain cabin. We declared emergency and descended to 10000 ft. Oxygen masks in cabin deployed. Without pressurization, ice protection, and temperature control, we diverted. We changed aircraft and continued. Maintenance discovered the aircraft had a blown duct (2 locations). I feel the duct blew due to a frozen water separator bag. Thus, we operated on 1 pack which also had a frozen water bag. That is why using high temperature (bypasses bag) held cabin. I feel the bags were probably dirty, which contributed to freezing and plugging. WX was 79 degrees F and high humidity. Had I known the exact nature of this problem in msy, I would not have accepted the aircraft.

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Original NASA ASRS Text

Title: A DC9 IN CRUISE AT FL290 DECLARED AN EMER AND DIVERTED DUE TO LOSS OF CABIN PRESSURIZATION. CAUSED BY 2 FAILED PNEUMATIC DUCTS IN THE AIR CONDITIONING PACKS.

Narrative: WE NOTICED A PRESSURIZATION PROB INBOUND DURING DSCNT THROUGH FL220 TO 16000 FT. CABIN SUDDENLY CLBED FROM 6500 FT TO 9500 FT. THE OUTFLOW VALVE WAS CLOSED AND L AND R SUPPLY 25 PSI. AROUND 12000 FT, SYS NORMALIZED. I WROTE IT UP AND MAINT CTLR MEL'ED AUTOPRESSURIZATION. SYS OPERATED NORMALLY ON OUTBOUND LEG TO FL330. 50 MINS INTO OUR FLT, WE NOTICED CABIN ALT OF 8000 FT, BUT CLBING 200 FPM (ICE OFF). OUTFLOW CLOSED PACK OUTPUT 25 PSI. WE DID NOT LOCATE A COM PROC FOR THIS. ATC CLRED US FROM FL330 TO FL290. THE CABIN CONTINUED TO CLB UNLESS THE L AIR CONDITIONING SYS WAS ALMOST FULL HOT. PAX AND FLT ATTENDANT COMPLAINED SO WE LOWERED TEMP AND CABIN CLBED. ATC WOULD NOT GIVE US LOWER THAN FL240. AS WE RAISED TEMP ON CABIN, WE LOST L PACK AND LOST CABIN. R PACK WAS NOT DOING ANYTHING. WE PERFORMED EMER PROCS, BUT DID NOT REGAIN CABIN. WE DECLARED EMER AND DSNDED TO 10000 FT. OXYGEN MASKS IN CABIN DEPLOYED. WITHOUT PRESSURIZATION, ICE PROTECTION, AND TEMP CTL, WE DIVERTED. WE CHANGED ACFT AND CONTINUED. MAINT DISCOVERED THE ACFT HAD A BLOWN DUCT (2 LOCATIONS). I FEEL THE DUCT BLEW DUE TO A FROZEN WATER SEPARATOR BAG. THUS, WE OPERATED ON 1 PACK WHICH ALSO HAD A FROZEN WATER BAG. THAT IS WHY USING HIGH TEMP (BYPASSES BAG) HELD CABIN. I FEEL THE BAGS WERE PROBABLY DIRTY, WHICH CONTRIBUTED TO FREEZING AND PLUGGING. WX WAS 79 DEGS F AND HIGH HUMIDITY. HAD I KNOWN THE EXACT NATURE OF THIS PROB IN MSY, I WOULD NOT HAVE ACCEPTED THE ACFT.

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.