Narrative:

As we approached our descent point for tus in cruise flight at FL330, the cabin began to climb at 1000 FPM. Thrust was at cruise power, duct pressure was normal and outflow valve showed closed. Standby and manual AC/dc (with toggle switch held closed) were all tried to no avail. The cabin continued to climb so an emergency descent was initiated per the checklist. Flight attendants were notified via interphone that we were having pressurization problems, that the masks may drop and to standby for further instructions. ATC was told that we were having some pressurization problems, but an emergency was not declared as we had clearance all the way to 10000 ft due to our proximity to tus. The cabin continued to climb all through the descent with both packs/bleeds on and the outflow valve showing closed until we equalized at 10000 ft. Cabin exceeded 10000 ft momentarily and the warning horn sounded. Tus operations was called about the problem and asked to call dispatch and contract maintenance. An uneventful landing was made in tus. Per air carrier X maintenance, contract maintenance pressurized the aircraft to check for leaks and operation of the outflow valve. Contract stated that the outflow valve closed normally, but after a few minutes it popped open approximately an inch or two while the cockpit control was in manual air conditioning and the valve position indicator showed full closed. Air carrier X maintenance control and dispatch decided to MEL the entire pressurization system and fly the aircraft with passengers, unpressurized, to phx. Near our descent point for phx the duct pressure split with #1 near zero with no caution lights or other indications. Bleed switch was cycled and bleed valve circuit breaker was cycled with no change. Duct pressure had been normal during taxi, takeoff and cruise to that point. Landed in phx uneventfully and briefed maintenance. Problem could not be duplicated as described by contract maintenance (outflow valve popping open after a time), but outflow valve was changed. We were then given a different aircraft while maintenance continued to work on aircraft. Ironically I had a similar problem about 3 yrs ago with the same set of circumstances, cabin suddenly began to climb in cruise flight, etc., but the masks did deploy by the time we could get down from FL350. However, that incident did not have the coincidence of the bleed valve failure while en route, unpressurized, during the flight back to phx.

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Original NASA ASRS Text

Title: LOSS OF PRESSURIZATION AT CRUISE ALT.

Narrative: AS WE APCHED OUR DSCNT POINT FOR TUS IN CRUISE FLT AT FL330, THE CABIN BEGAN TO CLB AT 1000 FPM. THRUST WAS AT CRUISE PWR, DUCT PRESSURE WAS NORMAL AND OUTFLOW VALVE SHOWED CLOSED. STANDBY AND MANUAL AC/DC (WITH TOGGLE SWITCH HELD CLOSED) WERE ALL TRIED TO NO AVAIL. THE CABIN CONTINUED TO CLB SO AN EMER DSCNT WAS INITIATED PER THE CHKLIST. FLT ATTENDANTS WERE NOTIFIED VIA INTERPHONE THAT WE WERE HAVING PRESSURIZATION PROBS, THAT THE MASKS MAY DROP AND TO STANDBY FOR FURTHER INSTRUCTIONS. ATC WAS TOLD THAT WE WERE HAVING SOME PRESSURIZATION PROBS, BUT AN EMER WAS NOT DECLARED AS WE HAD CLRNC ALL THE WAY TO 10000 FT DUE TO OUR PROX TO TUS. THE CABIN CONTINUED TO CLB ALL THROUGH THE DSCNT WITH BOTH PACKS/BLEEDS ON AND THE OUTFLOW VALVE SHOWING CLOSED UNTIL WE EQUALIZED AT 10000 FT. CABIN EXCEEDED 10000 FT MOMENTARILY AND THE WARNING HORN SOUNDED. TUS OPS WAS CALLED ABOUT THE PROB AND ASKED TO CALL DISPATCH AND CONTRACT MAINT. AN UNEVENTFUL LNDG WAS MADE IN TUS. PER ACR X MAINT, CONTRACT MAINT PRESSURIZED THE ACFT TO CHK FOR LEAKS AND OP OF THE OUTFLOW VALVE. CONTRACT STATED THAT THE OUTFLOW VALVE CLOSED NORMALLY, BUT AFTER A FEW MINUTES IT POPPED OPEN APPROX AN INCH OR TWO WHILE THE COCKPIT CTL WAS IN MANUAL AIR CONDITIONING AND THE VALVE POS INDICATOR SHOWED FULL CLOSED. ACR X MAINT CTL AND DISPATCH DECIDED TO MEL THE ENTIRE PRESSURIZATION SYS AND FLY THE ACFT WITH PAXS, UNPRESSURIZED, TO PHX. NEAR OUR DSCNT POINT FOR PHX THE DUCT PRESSURE SPLIT WITH #1 NEAR ZERO WITH NO CAUTION LIGHTS OR OTHER INDICATIONS. BLEED SWITCH WAS CYCLED AND BLEED VALVE CIRCUIT BREAKER WAS CYCLED WITH NO CHANGE. DUCT PRESSURE HAD BEEN NORMAL DURING TAXI, TKOF AND CRUISE TO THAT POINT. LANDED IN PHX UNEVENTFULLY AND BRIEFED MAINT. PROB COULD NOT BE DUPLICATED AS DESCRIBED BY CONTRACT MAINT (OUTFLOW VALVE POPPING OPEN AFTER A TIME), BUT OUTFLOW VALVE WAS CHANGED. WE WERE THEN GIVEN A DIFFERENT ACFT WHILE MAINT CONTINUED TO WORK ON ACFT. IRONICALLY I HAD A SIMILAR PROB ABOUT 3 YRS AGO WITH THE SAME SET OF CIRCUMSTANCES, CABIN SUDDENLY BEGAN TO CLB IN CRUISE FLT, ETC., BUT THE MASKS DID DEPLOY BY THE TIME WE COULD GET DOWN FROM FL350. HOWEVER, THAT INCIDENT DID NOT HAVE THE COINCIDENCE OF THE BLEED VALVE FAILURE WHILE ENRTE, UNPRESSURIZED, DURING THE FLT BACK TO PHX.

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.