Narrative:

We were level at FL330 approximately 80 NM west of eed when the cabin altitude warning horn, unaccompanied by an autofail or master caution light, sounded. Having made a bleeds off takeoff from bur, I immediately checked to ensure that both packs and engine bleeds were on, which they were. Looking at the cabin altimeter I noticed that the cabin altitude was about 10000' and increasing at a noticeable rate. Realizing that the psu's would deploy their O2 masks in a very short length of time it was decided that if we were to best the cabin to 14000', a descent had to be initiated immediately. This was done. The first officer and I both donned our O2 masks and the outflow valve was closed in man. During the descent I noticed pressure on my ears. Upon checking the cabin rate indicator, I observed a 3000 FPM rate of descent, whereupon I returned the pressurization controller to automatic to see what effect this would have. The cabin rate decreased immediately to -500 FPM. Remembering a past pressurization problem, caused by a defective door seal, that displayed very similar symptoms, I elected to continue the descent and analyze the problem at a low altitude and in level flight west/O the added distraction of intermediate level offs. After leveling off at 12000' we observed a cabin altitude of 5000' and apparently normal operation of the pressurization controller. Given that our CDU's were partially inoperative (no ZFW or fuel information) we had not quick way of confirming a fuel burn to phx. The aircraft was also not equipped with atscall. This would have made a communication patch through las to dispatch very laborious at best, especially given the low altitude and the fact we would be flying away from las. Since we were only 75 mi from las, and any delay in diverting for any reason would only serve to increase that distance we elected to proceed to las. This was accomplished west/O further incident. Callback conversation with reporter revealed the following: captain did not declare emergency for his descent since they had time to obtain a clearance from center for the descent. Precautionary landing at las. Maintenance found a sticky outflow valve. Flight had enough fuel to continue to destination but manually controling the pressurization is a time consuming operation requiring almost constant attention by first officer.

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Original NASA ASRS Text

Title: PRESSURIZATION CTL ERRATIC CAUSED FLT CREW TO DSND TO 1000' WHERE PRESSURE CTL WAS REGAINED MANUALLY. PRECAUTIONARY DIVERSION TO LAS FOR LNDG AND MAINTENANCE.

Narrative: WE WERE LEVEL AT FL330 APPROX 80 NM W OF EED WHEN THE CABIN ALT WARNING HORN, UNACCOMPANIED BY AN AUTOFAIL OR MASTER CAUTION LIGHT, SOUNDED. HAVING MADE A BLEEDS OFF TKOF FROM BUR, I IMMEDIATELY CHKED TO ENSURE THAT BOTH PACKS AND ENG BLEEDS WERE ON, WHICH THEY WERE. LOOKING AT THE CABIN ALTIMETER I NOTICED THAT THE CABIN ALT WAS ABOUT 10000' AND INCREASING AT A NOTICEABLE RATE. REALIZING THAT THE PSU'S WOULD DEPLOY THEIR O2 MASKS IN A VERY SHORT LENGTH OF TIME IT WAS DECIDED THAT IF WE WERE TO BEST THE CABIN TO 14000', A DSNT HAD TO BE INITIATED IMMEDIATELY. THIS WAS DONE. THE F/O AND I BOTH DONNED OUR O2 MASKS AND THE OUTFLOW VALVE WAS CLOSED IN MAN. DURING THE DSNT I NOTICED PRESSURE ON MY EARS. UPON CHKING THE CABIN RATE INDICATOR, I OBSERVED A 3000 FPM RATE OF DSNT, WHEREUPON I RETURNED THE PRESSURIZATION CTLR TO AUTO TO SEE WHAT EFFECT THIS WOULD HAVE. THE CABIN RATE DECREASED IMMEDIATELY TO -500 FPM. REMEMBERING A PAST PRESSURIZATION PROB, CAUSED BY A DEFECTIVE DOOR SEAL, THAT DISPLAYED VERY SIMILAR SYMPTOMS, I ELECTED TO CONTINUE THE DSNT AND ANALYZE THE PROB AT A LOW ALT AND IN LEVEL FLT W/O THE ADDED DISTR OF INTERMEDIATE LEVEL OFFS. AFTER LEVELING OFF AT 12000' WE OBSERVED A CABIN ALT OF 5000' AND APPARENTLY NORMAL OPERATION OF THE PRESSURIZATION CTLR. GIVEN THAT OUR CDU'S WERE PARTIALLY INOP (NO ZFW OR FUEL INFO) WE HAD NOT QUICK WAY OF CONFIRMING A FUEL BURN TO PHX. THE ACFT WAS ALSO NOT EQUIPPED WITH ATSCALL. THIS WOULD HAVE MADE A COM PATCH THROUGH LAS TO DISPATCH VERY LABORIOUS AT BEST, ESPECIALLY GIVEN THE LOW ALT AND THE FACT WE WOULD BE FLYING AWAY FROM LAS. SINCE WE WERE ONLY 75 MI FROM LAS, AND ANY DELAY IN DIVERTING FOR ANY REASON WOULD ONLY SERVE TO INCREASE THAT DISTANCE WE ELECTED TO PROCEED TO LAS. THIS WAS ACCOMPLISHED W/O FURTHER INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: CAPT DID NOT DECLARE EMER FOR HIS DSNT SINCE THEY HAD TIME TO OBTAIN A CLRNC FROM CENTER FOR THE DSNT. PRECAUTIONARY LNDG AT LAS. MAINT FOUND A STICKY OUTFLOW VALVE. FLT HAD ENOUGH FUEL TO CONTINUE TO DEST BUT MANUALLY CTLING THE PRESSURIZATION IS A TIME CONSUMING OPERATION REQUIRING ALMOST CONSTANT ATTN BY F/O.

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.