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.