Narrative:

We remained at 11;000 feet until 25 miles off shore as a [another aircraft] was flying overhead. A climb to 13;000 feet was issued and a frequency change. We were then cleared to FL270. We were logging on to controller pilot data link communication (cpdlc); receiving ETOPS weather and in the process of dismissing the relief first officers. We were cleared to FL340 and ready to leave FL270 when we observed the pressurization system indicator appeared on the EICAS display. It was amber and the cabin altitude was 9;100 feet and climbing at 300 feet per min. The climb was stopped and ATC clearance obtained to 10;000'. All outflow valves were closed and all packs operating normal. The altitude horn sounded momentarily at 10;000'. The cabin then started to descend with the aircraft. ATC was advised of our intention to return to [our departure airport]. A heading was issued and then a direct ZZZ; with [the] arrival. Dispatch was informed of our problem. ATC authorized a fuel dump to begin and it was completed in 20 min; as we descended from 10;000' to 7;000'and 50 miles off shore. The max landing weight of 425;000 lbs and a 30 vref of 152k was accomplished.the aircraft had a deferral of aft ee cooling overboard exhaust. The MEL required the valve to be closed and the circuit breakers (circuit breaker) pulled. However; on landing it was discovered that maintenance opened the valve and pulled the circuit breaker and therefore the aircraft would not pressurize. No emergency was declared and the flight landed normally.

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Original NASA ASRS Text

Title: The Captain of a Boeing 787 reported that they were unable to pressurize the aircraft in flight.

Narrative: We remained at 11;000 feet until 25 miles off shore as a [another aircraft] was flying overhead. A climb to 13;000 feet was issued and a frequency change. We were then cleared to FL270. We were logging on to Controller Pilot Data Link Communication (CPDLC); receiving ETOPS weather and in the process of dismissing the relief first officers. We were cleared to FL340 and ready to leave FL270 when we observed the pressurization system indicator appeared on the EICAS display. It was amber and the cabin altitude was 9;100 feet and climbing at 300 feet per min. The climb was stopped and ATC clearance obtained to 10;000'. All outflow valves were closed and all packs operating normal. The altitude horn sounded momentarily at 10;000'. The cabin then started to descend with the aircraft. ATC was advised of our intention to return to [our departure airport]. A heading was issued and then a direct ZZZ; with [the] arrival. Dispatch was informed of our problem. ATC authorized a fuel dump to begin and it was completed in 20 min; as we descended from 10;000' to 7;000'and 50 miles off shore. The max landing weight of 425;000 lbs and a 30 vref of 152k was accomplished.The aircraft had a deferral of AFT EE cooling overboard exhaust. The MEL required the valve to be closed and the Circuit Breakers (CB) pulled. However; on landing it was discovered that maintenance opened the valve and pulled the CB and therefore the aircraft would not pressurize. No emergency was declared and the flight landed normally.

Data retrieved from NASA's ASRS site 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.