Narrative:

During a climb to FL330, while passing through FL310, a cabin altitude warning horn was heard. At that point, the cabin altitude was climbing through 10000 ft. Engine bleed valves were confirmed on and the main outflow valve confirmed full closed. The pressurization controller was switched from automatic to standby with no change in the cabin rate of climb. We immediately contacted ZAB and requested a lower altitude in an attempt to stop the slow climb and at least stabilize the cabin altitude and/or lower it with the descent. We made an incremental ATC coordinated descent to FL290, FL270, and FL240 in an attempt to slow the cabin pressure altitude from climbing. While descending to FL240 it became apparent that we could not stop the slow depressurization and continued a maximum descent to 10000 ft. During the descent the highest cabin altitude noted was 13500 ft, the oxygen masks dropped shortly after descent was initiated. After leveling at 10000 ft, an uneventful landing was made in las. An emergency was not declared during the descent since we had clearance to all altitudes we requested while attempting to solve the problem. In reviewing our actions we may have been able to avoid the masks deploying if we had declared an emergency immediately and gotten a descent to 10000 ft, instead of attempting to work out the problem, leveling twice along the way due to ATC constraints. Callback conversation with reporter revealed the following information: aircraft was a B737-200. Reporter's flight had left phx and diverted to las. The maintenance person, supervisor, that the flight crew spoke with mentioned that there had been a similar problem at las 2 yrs ago and it dealt with an 'orange cone' on the APU duct. This 'incident' aircraft had been operating with the APU MEL'ed for an air source problem or malfunction. Reporter believes that the ground crew changed the aircraft's pressurization control panel, the outflow valve (or controller) and the positive (pressure?) relief valve. The aircraft was back in service the following day. The air carrier has a full report of the incident.

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Original NASA ASRS Text

Title: B737 EXPERIENCES A LOSS OF CABIN PRESSURE IN BOTH AUTO AND MANUAL CTL. OXYGEN MASKS DEPLOYED AS CABIN ALT REACHED 13500 FT. FO RPTR FELT THAT CREW ERRED IN NOT DSNDING DIRECTLY TO 10000 FT FROM FL310 VIA DECLARING AN EMER INSTEAD OF A SLOW DSCNT AND TROUBLESHOOTING THE PROB.

Narrative: DURING A CLB TO FL330, WHILE PASSING THROUGH FL310, A CABIN ALT WARNING HORN WAS HEARD. AT THAT POINT, THE CABIN ALT WAS CLBING THROUGH 10000 FT. ENG BLEED VALVES WERE CONFIRMED ON AND THE MAIN OUTFLOW VALVE CONFIRMED FULL CLOSED. THE PRESSURIZATION CTLR WAS SWITCHED FROM AUTO TO STANDBY WITH NO CHANGE IN THE CABIN RATE OF CLB. WE IMMEDIATELY CONTACTED ZAB AND REQUESTED A LOWER ALT IN AN ATTEMPT TO STOP THE SLOW CLB AND AT LEAST STABILIZE THE CABIN ALT AND/OR LOWER IT WITH THE DSCNT. WE MADE AN INCREMENTAL ATC COORDINATED DSCNT TO FL290, FL270, AND FL240 IN AN ATTEMPT TO SLOW THE CABIN PRESSURE ALT FROM CLBING. WHILE DSNDING TO FL240 IT BECAME APPARENT THAT WE COULD NOT STOP THE SLOW DEPRESSURIZATION AND CONTINUED A MAX DSCNT TO 10000 FT. DURING THE DSCNT THE HIGHEST CABIN ALT NOTED WAS 13500 FT, THE OXYGEN MASKS DROPPED SHORTLY AFTER DSCNT WAS INITIATED. AFTER LEVELING AT 10000 FT, AN UNEVENTFUL LNDG WAS MADE IN LAS. AN EMER WAS NOT DECLARED DURING THE DSCNT SINCE WE HAD CLRNC TO ALL ALTS WE REQUESTED WHILE ATTEMPTING TO SOLVE THE PROB. IN REVIEWING OUR ACTIONS WE MAY HAVE BEEN ABLE TO AVOID THE MASKS DEPLOYING IF WE HAD DECLARED AN EMER IMMEDIATELY AND GOTTEN A DSCNT TO 10000 FT, INSTEAD OF ATTEMPTING TO WORK OUT THE PROB, LEVELING TWICE ALONG THE WAY DUE TO ATC CONSTRAINTS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ACFT WAS A B737-200. RPTR'S FLT HAD LEFT PHX AND DIVERTED TO LAS. THE MAINT PERSON, SUPVR, THAT THE FLC SPOKE WITH MENTIONED THAT THERE HAD BEEN A SIMILAR PROB AT LAS 2 YRS AGO AND IT DEALT WITH AN 'ORANGE CONE' ON THE APU DUCT. THIS 'INCIDENT' ACFT HAD BEEN OPERATING WITH THE APU MEL'ED FOR AN AIR SOURCE PROB OR MALFUNCTION. RPTR BELIEVES THAT THE GND CREW CHANGED THE ACFT'S PRESSURIZATION CTL PANEL, THE OUTFLOW VALVE (OR CTLR) AND THE POSITIVE (PRESSURE?) RELIEF VALVE. THE ACFT WAS BACK IN SVC THE FOLLOWING DAY. THE ACR HAS A FULL RPT OF THE INCIDENT.

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.