Narrative:

While cruising at 33000 ft about 30 mi south of jacksonville, fl, the pressurization valve moved to the full closed position and the cabin pressure started increasing rapidly. The pressurization 'flow' light came on. Since we could not control the pressurization, we immediately requested a lower altitude from ZJX. The center assigned us 24000 ft. We started descending using speed brakes and after changing frequency we received clearance to 15000 ft. After leveling at this altitude, the cabin pressure stabilized and started down. During this time the cabin had climbed to 13000 ft. Although the cabin aural warning sounded briefly, the cabin oxygen masks did not deploy since the cabin did not reach 14000 ft. Dispatch was contacted and after a few mins dispatch advised that if we proceeded to jfk at 15000 ft the fuel required would be 12700 pounds. We continued to jfk cruising at 15000 ft for about 15 mins and then, since everything was normal, we requested 27000 ft. The remainder of the flight was normal and we landed at jfk with 6500 pounds of fuel. I suspect that the left pressurization pack had frozen and failed since we were having trouble earlier controling the left pack temperature. Callback conversation with reporter revealed the following information: the reporting captain believes that the left air conditioning pack froze up because of very high cooling demand, and blocked off air to the cabin. The medium large transport can operate up to 25000 ft with only 1 pack, but this particular aircraft would not maintain pressurization at 33000 ft. Maintenance found and replaced a faulty clamp in the pressurization system that may have contributed to the problem. The report has not flown this aircraft since the incident. No emergency was declared as ATC handling was prompt and superb.

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Original NASA ASRS Text

Title: AN MLG ACR PLT HAD A PARTIAL FAILURE OF THE PRESSURIZATION SYS REQUIRING A RAPID DSCNT.

Narrative: WHILE CRUISING AT 33000 FT ABOUT 30 MI S OF JACKSONVILLE, FL, THE PRESSURIZATION VALVE MOVED TO THE FULL CLOSED POS AND THE CABIN PRESSURE STARTED INCREASING RAPIDLY. THE PRESSURIZATION 'FLOW' LIGHT CAME ON. SINCE WE COULD NOT CTL THE PRESSURIZATION, WE IMMEDIATELY REQUESTED A LOWER ALT FROM ZJX. THE CTR ASSIGNED US 24000 FT. WE STARTED DSNDING USING SPD BRAKES AND AFTER CHANGING FREQ WE RECEIVED CLRNC TO 15000 FT. AFTER LEVELING AT THIS ALT, THE CABIN PRESSURE STABILIZED AND STARTED DOWN. DURING THIS TIME THE CABIN HAD CLBED TO 13000 FT. ALTHOUGH THE CABIN AURAL WARNING SOUNDED BRIEFLY, THE CABIN OXYGEN MASKS DID NOT DEPLOY SINCE THE CABIN DID NOT REACH 14000 FT. DISPATCH WAS CONTACTED AND AFTER A FEW MINS DISPATCH ADVISED THAT IF WE PROCEEDED TO JFK AT 15000 FT THE FUEL REQUIRED WOULD BE 12700 LBS. WE CONTINUED TO JFK CRUISING AT 15000 FT FOR ABOUT 15 MINS AND THEN, SINCE EVERYTHING WAS NORMAL, WE REQUESTED 27000 FT. THE REMAINDER OF THE FLT WAS NORMAL AND WE LANDED AT JFK WITH 6500 LBS OF FUEL. I SUSPECT THAT THE L PRESSURIZATION PACK HAD FROZEN AND FAILED SINCE WE WERE HAVING TROUBLE EARLIER CTLING THE L PACK TEMP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTING CAPT BELIEVES THAT THE L AIR CONDITIONING PACK FROZE UP BECAUSE OF VERY HIGH COOLING DEMAND, AND BLOCKED OFF AIR TO THE CABIN. THE MLG CAN OPERATE UP TO 25000 FT WITH ONLY 1 PACK, BUT THIS PARTICULAR ACFT WOULD NOT MAINTAIN PRESSURIZATION AT 33000 FT. MAINT FOUND AND REPLACED A FAULTY CLAMP IN THE PRESSURIZATION SYS THAT MAY HAVE CONTRIBUTED TO THE PROB. THE RPT HAS NOT FLOWN THIS ACFT SINCE THE INCIDENT. NO EMER WAS DECLARED AS ATC HANDLING WAS PROMPT AND SUPERB.

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.