A B727 IN CLB AT FL230 MADE AN EMER DSCNT AND DIVERTED DUE TO INABILITY TO CTL THE CABIN PRESSURIZATION CAUSED BY A FAILED L PACK MIXING VALVE.

Narrative

OUT OF FLL WHILE ON CLBOUT; WHEN ATTEMPTING TO ADJUST CABIN TEMP; I NOTICED THAT THE L MIX VALVE WAS STUCK IN THE FULL COLD POS; THE R MIX VALVE WAS WORKING NORMALLY AND I POSITIONED IT TO THE MID POS. I ADVISED THE CAPT OF THE SIT AND PROCEEDED TO TROUBLESHOOT THE PROB; IE; DETERMINING IF IT'S A FAILURE OF THE MIX VALVE POS INDICTOR OR A FAILURE OF THE MIX VALVE ITSELF. I DETERMINED IT WAS A FAILURE OF THE MIX VALVE BY OBSERVING NO TEMP INCREASE WHILE THE SELECTOR WAS HELD IN THE WARM POS. SELECTED AUTO ON THE SELECTOR AND THE MIX VALVE WAS STILL STUCK ON FULL COLD -- TO NO AVAIL. AT THIS TIME; I'M LOOKING FOR THE EMER/ABNORMAL PROC IN MY AOM WHEN THE CABIN ALT WARNING HORN WENT OFF. THE ONLY INDICATION WE HAD THAT THERE WAS A PROB WAS THE CABIN ALT WARNING HORN; FOR THE CABIN HAD CLBED AT A COMFORTABLE RATE. AT THE TIME OF THE HORN; THE CABIN HAD CLBED TO 10000 FT AT FL220. THE OUTFLOW VALVE WAS FULLY CLOSED. CAPT REQUESTED LEVELOFF TO ATC. WHEN THE CABIN REACHED 10500 FT; I ADVISED THE CAPT THAT I CANNOT CTL THE CABIN. WE ACCOMPLISHED THE RAPID DEPRESSURIZATION AND THE EMER DSCNT CHKLISTS. CAPT GOT CLRNC AND WE DSNDED TO 14000 FT IN LESS THAN 2 MINS. AT ITS HIGHEST; THE CABIN ASCENDED TO 12000 FT WHILE THE ACFT WAS ON DSCNT. PURSER WAS NOTIFIED BY CAPT OF THE SIT AND WE DIVERTED TO MIA.