Narrative:

Cruising at 37000 ft, where it had been assigned for some 2 hours, just north of dca, flight crew noticed an acute change of cabin pressure manifested by ear discomfort. Crew immediately noticed cabin rate of climb 'pegged' at 4000 FPM and cabin altitude approaching 10000 ft. Crew donned oxygen masks and established communication with each other. No aural or light warnings were observed. First officer manually selected standby mode of cabin pressurization with no noticeable effect on the cabin's ascent. First officer then selected manual mode and using the dc system toggled the outflow valve toward the closed position. Cabin pressure was stabilized just under 10000 ft and then repressurized to approximately 8000 ft. Appropriate quick reaction checklist was then initiated and proceeded without incident until the air conditioning packs were switched to 'high.' at this point the cabin again started ascending at 4000 FPM and toggling the outflow valve toward closed seemed to have no effect on the cabin rate of climb. The crew surmised the cabin pressure was uncontrollable and elected to make an emergency descent. Declared an emergency to ZDC and selected transponder to 7700. Center responded immediately with clearance to 28000 ft. Other lower altitudes were issued in a timely manner by center thus negating descending to an unclred altitude. Flight requested a radar vector to dulles, the closest company airport not inundated with traffic. Diversion was selected due to aircraft structure and fuel considerations. Landing at dulles was routine. Upon inspecting the aircraft nothing abnormal was discovered except a 3 ft square piece of fiberglass duct insulation lodged in the main (aft) outflow valve. Insulation was removed and normal valve operation observed. Flight continued on to original destination after securing necessary release from flight control. The aircraft was given a complete pressurization check at newark and finding no discrepancies was released for service.

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Original NASA ASRS Text

Title: CTL OF CABIN PRESSURIZATION IS LOST WHEN LOOSE MATERIAL BECOMES LODGED IN THE OUTFLOW VALVE.

Narrative: CRUISING AT 37000 FT, WHERE IT HAD BEEN ASSIGNED FOR SOME 2 HRS, JUST N OF DCA, FLC NOTICED AN ACUTE CHANGE OF CABIN PRESSURE MANIFESTED BY EAR DISCOMFORT. CREW IMMEDIATELY NOTICED CABIN RATE OF CLB 'PEGGED' AT 4000 FPM AND CABIN ALT APCHING 10000 FT. CREW DONNED OXYGEN MASKS AND ESTABLISHED COM WITH EACH OTHER. NO AURAL OR LIGHT WARNINGS WERE OBSERVED. FO MANUALLY SELECTED STANDBY MODE OF CABIN PRESSURIZATION WITH NO NOTICEABLE EFFECT ON THE CABIN'S ASCENT. FO THEN SELECTED MANUAL MODE AND USING THE DC SYSTEM TOGGLED THE OUTFLOW VALVE TOWARD THE CLOSED POS. CABIN PRESSURE WAS STABILIZED JUST UNDER 10000 FT AND THEN REPRESSURIZED TO APPROX 8000 FT. APPROPRIATE QUICK REACTION CHKLIST WAS THEN INITIATED AND PROCEEDED WITHOUT INCIDENT UNTIL THE AIR CONDITIONING PACKS WERE SWITCHED TO 'HIGH.' AT THIS POINT THE CABIN AGAIN STARTED ASCENDING AT 4000 FPM AND TOGGLING THE OUTFLOW VALVE TOWARD CLOSED SEEMED TO HAVE NO EFFECT ON THE CABIN RATE OF CLB. THE CREW SURMISED THE CABIN PRESSURE WAS UNCTLABLE AND ELECTED TO MAKE AN EMER DSCNT. DECLARED AN EMER TO ZDC AND SELECTED XPONDER TO 7700. CTR RESPONDED IMMEDIATELY WITH CLRNC TO 28000 FT. OTHER LOWER ALTS WERE ISSUED IN A TIMELY MANNER BY CTR THUS NEGATING DSNDING TO AN UNCLRED ALT. FLT REQUESTED A RADAR VECTOR TO DULLES, THE CLOSEST COMPANY ARPT NOT INUNDATED WITH TFC. DIVERSION WAS SELECTED DUE TO ACFT STRUCTURE AND FUEL CONSIDERATIONS. LNDG AT DULLES WAS ROUTINE. UPON INSPECTING THE ACFT NOTHING ABNORMAL WAS DISCOVERED EXCEPT A 3 FT SQUARE PIECE OF FIBERGLASS DUCT INSULATION LODGED IN THE MAIN (AFT) OUTFLOW VALVE. INSULATION WAS REMOVED AND NORMAL VALVE OP OBSERVED. FLT CONTINUED ON TO ORIGINAL DEST AFTER SECURING NECESSARY RELEASE FROM FLT CTL. THE ACFT WAS GIVEN A COMPLETE PRESSURIZATION CHK AT NEWARK AND FINDING NO DISCREPANCIES WAS RELEASED FOR SVC.

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.