Narrative:

Aircraft had MEL 21-31-1a deactivating left outflow [valve] and locking it in the full closed position. The crew complied with operations procedure portion of the MEL and verified that maintenance portion was complete as well. Engine start; taxi and takeoff were uneventful. During climbout; passing roughly 6;000 ft flight crew monitored ecs page and noticed left outflow valve was displayed in an uncommanded open position. Cabin altitude appeared normal. After passing FL180 crew observed cabin altitude was 9;500 ft and climbing. Crew initially leveled at FL190 and coordinated an immediate descent to safe attitude with ATC. ATC cleared aircraft to descend to 14;000 due to terrain and a right turn to the east to clear terrain and continue the descent. During the descent the cabin altitude climbed above 10;000 ft. Crew performed memory items for cabin altitude; don oxygen masks and established communications; and continued expedited descent to bring cabin altitude below 10;000 ft. The crew then completed QRH checklist for the cabin altitude warning. Crew continued descent to 8;000 ft and asked for holding near the airport. Crew then contacted company to relay information/status and coordinate return due to pressurization capability concerns. Crew then performed fuel jettison QRH checklist and jettisoned fuel from 263;000 pounds to 162;000 pounds to attain maximum landing weight. Approach and landing [to the airport] was uneventful.

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Original NASA ASRS Text

Title: B747 flight crew reported departing with an MEL requiring the left outflow valve to be deactivated and secured in the closed position. During climb the left outflow valve was displayed in an un-commanded open position on the ECS page. Out of FL190 the cabin altitude is at 9;500 FT and climbing. An emergency descent is initiated and after discussion with the company; fuel is dumped down to landing weight and the flight returns to the departure airport.

Narrative: Aircraft had MEL 21-31-1a deactivating left outflow [valve] and locking it in the full closed position. The crew complied with Operations procedure portion of the MEL and verified that maintenance portion was complete as well. Engine start; taxi and takeoff were uneventful. During climbout; passing roughly 6;000 FT flight crew monitored ECS page and noticed left outflow valve was displayed in an uncommanded open position. Cabin altitude appeared normal. After passing FL180 crew observed cabin altitude was 9;500 FT and climbing. Crew initially leveled at FL190 and coordinated an immediate descent to safe attitude with ATC. ATC cleared aircraft to descend to 14;000 due to terrain and a right turn to the east to clear terrain and continue the descent. During the descent the cabin altitude climbed above 10;000 FT. Crew performed memory items for cabin altitude; don oxygen masks and established communications; and continued expedited descent to bring cabin altitude below 10;000 FT. The crew then completed QRH checklist for the cabin altitude warning. Crew continued descent to 8;000 FT and asked for holding near the airport. Crew then contacted company to relay information/status and coordinate return due to pressurization capability concerns. Crew then performed Fuel Jettison QRH Checklist and jettisoned fuel from 263;000 LBS to 162;000 LBS to attain maximum landing weight. Approach and landing [to the airport] was uneventful.

Data retrieved from NASA's ASRS site as of July 2013 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.