SMT CORP ACFT HAS LOSS OF PRESSURIZATION CAPABILITY.

Narrative

WE WERE CLBING OUT AFTER TKOF FROM GNOSS FIELD ON A BEAUTIFUL; CLR SUNNY MORNING FOR SAN DIEGO. WE HAD BEEN INITIALLY CLRED TO FL230 AND CTR JUST CLRED US ON UP TO FL250. I HAD SET THE ALT ON THE PRESSURE CTLR TO ABOUT 26000 FT; WHICH CORRESPONDED TO A CABIN ALT OF ABOUT 6500 FT. MY ATTN WAS FIXED ON THE CABIN ALT INDICATOR SINCE I'D HAD A COUPLE OF INSTANCES OF THE PRESSURE CTLR FAILING TO HOLD THE SET CABIN ELEVATION WHEN IN THE LOW ECS (ENVIRONMENTAL CTL SYS) POS. IN THOSE PREVIOUS CASES; GOING TO THE HIGH POS ALWAYS CORRECTED THE SIT. SINCE OUR MAINT IS PERFORMED OFF FIELD AND THE ACFT WAS GOING TO BE GNDED AT THE END OF THE WK FOR ENG OVERHAULS; I DEFERRED MAINT ON PRESSURIZATION. I DIDN'T SEE ANY URGENCY TO IT. AS THE CABIN ELEVATION PASSED 6500 FT THE RATE OF CLB INDICATOR WAS STILL AT 500 FPM SO I KNEW IT WASN'T STABILIZING. I SWITCHED OVER TO THE HIGH POS AND SAW NO IMPROVEMENT. I HEARD A LOUD WHOOSHING SOUND OF AIR THAT I COULDN'T IDENT. ONE OF MY PAX LATER SAID THAT HE HEARD A THUMP AND ALSO HEARD A WHOOSHING SOUND OF AIR. WE WERE CLBING THROUGH FL220 AND THE CABIN ALT WAS NOW PASSING 15000 FT WITH THE 'CABIN ALT' ALERT SOUNDING AND THE CABIN RATE OF CLB INDICATOR PEGGED. I TRIED THE EMER PRESSURIZATION POS ON THE ECS WITH NO RESPONSE. NOT KNOWING IF THERE WAS A STRUCTURAL PROB WITH THE ACFT; I DECIDED TO EXECUTE AN EMER DSCNT. I ADVISED CTR THAT I WAS EXECUTING AN EMER DSCNT AND THEY ASKED IF I WAS DECLARING AN EMER. AT FIRST I SAID NO. AFTER A COUPLE SECONDS I REALIZED THAT I WAS DSNDING IN VIOLATION OF MY CLRNC TO CLB SO I ADVISED CTR THAT I WAS INDEED DECLARING AN EMER. THEY CLRED ME TO 14000 FT. REACHING 14000 FT CTR HANDED ME OFF TO BAY APCH. I CANCELED MY IFR AND THE EMER WITH BAY AND STAYED WITH THEM FOR ADVISORIES UNTIL I EXITED THEIR AIRSPACE. AFTER DROPPING OFF MY PAX WITH APOLOGIES FOR NOT REACHING OUR DEST; I TOOK THE ACFT TO SANTA ROSA WHERE OUR MAINT IS DONE. THE MECH FOUND A CLAMP HAD BLOWN OFF A LINE GOING FROM THE AIR CYCLE MACHINE TO THE CABIN. THE PREVIOUS INSTANCES THAT HAD BEEN DEALT WITH WERE PROBABLY DUE TO A LOOSE CLAMP. WHEN I DISCUSSED THE FIRST INSTANCE WITH OUR MECH; WE DECIDED IT WAS PROBABLY A STICKY OUTFLOW VALVE SINCE IT HAD ONLY HAPPENED THAT ONE TIME. AFTER IT HAPPENED A COUPLE MORE TIMES; I RATIONALIZED NOT DEALING WITH IT RIGHT AWAY SINCE WE WERE GNDING THE ACFT WITHIN A WK AND I HAD ALWAYS BEEN ABLE TO CORRECT THE PROB IN THE PAST. I TRIED TO AVOID THE COST OF A FLT TO MAINT DURING A BUSY FLT SCHEDULE AND WOUND UP HAVING TO ABORT A FLT WITH PAX. THIS COULD HAVE AND SHOULD HAVE BEEN AVOIDED HAD I HEEDED THE WARNING SIGNS.