CABIN PRESSURE LOST AS ACFT CLBS TO 31000 FT. DSND LAND.

Narrative

NOTICED CABIN ALT AT 9000 FT AFTER LEVELING AT FL310. TRIED MANUAL PRESSURIZATION CTL BUT COULD NOT GET CABIN TO PRESSURIZE PROPERLY. THE CABIN CONTINUED A SLOW CLB. WE TOLD CTR OF THE PROB AND MADE RAPID DSCNT TO 10000 FT. WHEN WE REDUCED PWR FOR DSCNT; THE CABIN ALT SEEMED TO CLB FASTER. THE OXYGEN MASKS DROPPED AT 14000 FT CABIN ALT. AT 10000 FT THE CABIN SLOWLY PRESSURIZED AND GOT DOWN TO ABOUT 3000 FT. THE COMPANY ADVISED US TO LAND AT IND TO BETTER ACCOMMODATE THE PAX. I BRIEFED THE PAX AS TO WHAT OCCURRED AND THERE WAS NO PROB WITH THE SAFETY OF THE FLT. I TALKED TO MAINT LATER AND WAS UNOFFICIALLY INFORMED THAT AN AVIONICS COOLING OUTFLOW VALVE STUCK OPEN PREVENTING PROPER PRESSURIZATION. WE RECEIVED NO ALERTS AS TO ANY SYS MALFUNCTION. SUPPLEMENTAL INFORMATION FROM ACN 249955: OUR CTL OVER THE PRESSURIZATION VALVES GOT WORSE AND WE WERE FORCED TO GO TO MANUAL PRESSURIZATION FOR THE FINAL SEGMENT INTO IND. AT THIS POINT WE GOT SOME COMPLAINTS FROM PAX CONCERNING THEIR EARS. LANDED WITHOUT INCIDENT WITH CABIN ALT FINALLY DOWN TO FIELD ELEVATION. PAX'S EARS SEEMED TO RECOVER AND WE DID NOT GET ANY MORE COMPLAINTS AS THEY EXITED THE ACFT.