Narrative:

We were climbing out after takeoff from gnoss field on a beautiful, clear sunny morning for san diego. We had been initially cleared to FL230 and center just cleared us on up to FL250. I had set the altitude on the pressure controller to about 26000 ft, which corresponded to a cabin altitude of about 6500 ft. My attention was fixed on the cabin altitude indicator since I'd had a couple of instances of the pressure controller failing to hold the set cabin elevation when in the low ecs (environmental control system) position. In those previous cases, going to the high position always corrected the situation. Since our maintenance is performed off field and the aircraft was going to be grounded at the end of the week for engine overhauls, I deferred maintenance on pressurization. I didn't see any urgency to it. As the cabin elevation passed 6500 ft the rate of climb indicator was still at 500 FPM so I knew it wasn't stabilizing. I switched over to the high position and saw no improvement. I heard a loud whooshing sound of air that I couldn't identify. One of my passenger later said that he heard a thump and also heard a whooshing sound of air. We were climbing through FL220 and the cabin altitude was now passing 15000 ft with the 'cabin altitude' alert sounding and the cabin rate of climb indicator pegged. I tried the emergency pressurization position on the ecs with no response. Not knowing if there was a structural problem with the aircraft, I decided to execute an emergency descent. I advised center that I was executing an emergency descent and they asked if I was declaring an emergency. At first I said no. After a couple seconds I realized that I was descending in violation of my clearance to climb so I advised center that I was indeed declaring an emergency. They cleared me to 14000 ft. Reaching 14000 ft center handed me off to bay approach. I canceled my IFR and the emergency with bay and stayed with them for advisories until I exited their airspace. After dropping off my passenger with apologies for not reaching our destination, I took the aircraft to santa rosa where our maintenance is done. The mechanic 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 mechanic, 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 grounding the aircraft within a week and I had always been able to correct the problem in the past. I tried to avoid the cost of a flight to maintenance during a busy flight schedule and wound up having to abort a flight with passenger. This could have and should have been avoided had I heeded the warning signs.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.