A B727 FLC LOSES CABIN PRESSURE AT FL270 ON THEIR CLBOUT AND RETURN LAND AT MSP; MN.

Narrative

AT FL270; CABIN ALT WARNING HORN SOUNDED. LEVELED THE ACFT AT FL280 AND TRIED TO GAIN CTL OF CABIN PRESSURIZATION. WENT TO STANDBY MODE ON ELECTRONIC PRESSURE SYS WITH NO EFFECT. USED MANUAL AC AND DC MODES TO CLOSE OUTFLOW VALVES WITH NO EFFECT. COULDN'T CTL CABIN SO WE STARTED A RAPID DSCNT TO 10000 FT. AN EMER WAS NOT DECLARED BECAUSE THERE WAS NO OTHER TFC IN THE AREA AND ATC WAS ABLE TO ACCOMMODATE OUR DSCNT REQUEST IMMEDIATELY. WE RETURNED TO THE MINNEAPOLIS ARPT WITHOUT FURTHER INCIDENT. AS TRAINED; THE FO TOOK OVER THE FLYING AND RADIO COMS OF THE ACFT AND THE CAPT WORKED WITH THE FE ON THE PROB. THE FE WAS ALSO PERFORMING IOE ON A NEW STUDENT AT THE TIME. THE CABIN REACHED AN ALT OF 13500 FT AS THE ACFT DSNDED THROUGH 14000 FT. THE CREW HAD MASKS ON UNTIL 10000 FT AND THE PAX MASKS WERE NEVER DEPLOYED. SUPPLEMENTAL INFO FROM ACN 512398: CLBING THROUGH FL280; OUR SO ADVISED THE CABIN WAS CLBING RAPIDLY. TROUBLESHOOTING EFFORTS STABILIZED THE RATE OF CLB TO 300 FPM WHILE THE CABIN WAS AT 13000 FT. ATC CLRED US FOR DSCNT TO 10000 FT WHERE WE FLEW DEPRESSURIZED AND RETURNED TO MSP UNEVENTFULLY. IT APPEARED THE R AIR CONDITIONING PACK MIX VALVE WAS FROZEN IN THE FULL COLD POS ALLOWING VERY LITTLE AIR INTO THE CABIN. WE COORDINATED WITH DISPATCH; MAINT AND OUR FLT ATTENDANTS THROUGHOUT THE EVENT. COMS WERE EFFECTIVE WHILE ON OXYGEN. THE CREW EACH CONTRIBUTED IDEAS ON HOW TO TROUBLESHOOT THE PROB.