Narrative:

At about 7 1/2 hours into the flight lax-nrt, we experienced a fault in the manual portion of our automatic pressure controller, which initially caused a slow to medium loss of pressurization at initial cabin altitude of approximately 6500 ft to over 10000 ft cabin altitude. So attempted to remedy the runaway cabin altitude, but inadvertently opened the outflow valves more and caused cabin pressure loss to accelerate. Cabin exceeded 14000 ft and passenger masks dropped. Crew donned oxygen masks, established communications. Captain assisted so in evaluating problem and determined solution to manually close outflow valves to re-establish cabin altitude below 10000 ft cabin altitude. Captain reached for red bordered checklist, as so opened cockpit operating manual to 'rapid depressurization' checklist and followed through with the procedure. Cabin altitude control was reestablished manually. Cabin altitude returned to approximately 6500 ft. Passenger oxygen system was attempted to reset, but after a short while captain noticed that passenger oxygen was leaking. Captain contacted dispatch control in msp via commercial radio phone patch and discussed descending to lower altitude of FL240 for remainder of flight at long range cruise speed. Fuel calculations were verified through dispatch, and flight descended to FL240 for last 2 hours of the flight prior to descent for normal approach and landing at tokyo narita, japan. Callback conversation with reporter revealed the following information: the reporter stated the aircraft after 7.5 hours into the flight was experiencing a slow to medium cabin pressurization loss and experiencing pressure 'bumps' caused by cycling of the cargo heat system. The reporter said the flight engineer in attempting to control the cabin pressure manually inadvertently opened the outflow valves and the cabin pressure was lost. The reporter stated the pressure was regained but not before the oxygen masks were deployed. The reporter said this aircraft was dispatched with the cabin pressure automatic control system deferred as inoperative per the MEL. The reporter stated this system should not be deferred on long overwater flts. The reporter said maintenance replaced the cabin pressure controller.

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Original NASA ASRS Text

Title: A B747-200 IN CRUISE AT FL330 WITH A DEFERRED AUTO PRESSURE PRESSURIZATION INOP LOST THE CABIN PRESSURE ON THE MANUAL SYS.

Narrative: AT ABOUT 7 1/2 HRS INTO THE FLT LAX-NRT, WE EXPERIENCED A FAULT IN THE MANUAL PORTION OF OUR AUTO PRESSURE CONTROLLER, WHICH INITIALLY CAUSED A SLOW TO MEDIUM LOSS OF PRESSURIZATION AT INITIAL CABIN ALT OF APPROX 6500 FT TO OVER 10000 FT CABIN ALT. SO ATTEMPTED TO REMEDY THE RUNAWAY CABIN ALT, BUT INADVERTENTLY OPENED THE OUTFLOW VALVES MORE AND CAUSED CABIN PRESSURE LOSS TO ACCELERATE. CABIN EXCEEDED 14000 FT AND PAX MASKS DROPPED. CREW DONNED OXYGEN MASKS, ESTABLISHED COMS. CAPT ASSISTED SO IN EVALUATING PROB AND DETERMINED SOLUTION TO MANUALLY CLOSE OUTFLOW VALVES TO RE-ESTABLISH CABIN ALT BELOW 10000 FT CABIN ALT. CAPT REACHED FOR RED BORDERED CHKLIST, AS SO OPENED COCKPIT OPERATING MANUAL TO 'RAPID DEPRESSURIZATION' CHKLIST AND FOLLOWED THROUGH WITH THE PROC. CABIN ALT CONTROL WAS REESTABLISHED MANUALLY. CABIN ALT RETURNED TO APPROX 6500 FT. PAX OXYGEN SYS WAS ATTEMPTED TO RESET, BUT AFTER A SHORT WHILE CAPT NOTICED THAT PAX OXYGEN WAS LEAKING. CAPT CONTACTED DISPATCH CTL IN MSP VIA COMMERCIAL RADIO PHONE PATCH AND DISCUSSED DSNDING TO LOWER ALT OF FL240 FOR REMAINDER OF FLT AT LONG RANGE CRUISE SPD. FUEL CALCULATIONS WERE VERIFIED THROUGH DISPATCH, AND FLT DSNDED TO FL240 FOR LAST 2 HRS OF THE FLT PRIOR TO DSCNT FOR NORMAL APCH AND LNDG AT TOKYO NARITA, JAPAN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT AFTER 7.5 HRS INTO THE FLT WAS EXPERIENCING A SLOW TO MEDIUM CABIN PRESSURIZATION LOSS AND EXPERIENCING PRESSURE 'BUMPS' CAUSED BY CYCLING OF THE CARGO HEAT SYS. THE RPTR SAID THE FE IN ATTEMPTING TO CTL THE CABIN PRESSURE MANUALLY INADVERTENTLY OPENED THE OUTFLOW VALVES AND THE CABIN PRESSURE WAS LOST. THE RPTR STATED THE PRESSURE WAS REGAINED BUT NOT BEFORE THE OXYGEN MASKS WERE DEPLOYED. THE RPTR SAID THIS ACFT WAS DISPATCHED WITH THE CABIN PRESSURE AUTO CTL SYS DEFERRED AS INOP PER THE MEL. THE RPTR STATED THIS SYS SHOULD NOT BE DEFERRED ON LONG OVERWATER FLTS. THE RPTR SAID MAINT REPLACED THE CABIN PRESSURE CONTROLLER.

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.