A B777 developed an apparent fuel imbalance in flight but since troubleshooting did not reveal a problem; the crew returned to the departure airport where enroute they found the fuel crossfeed valves unexpectedly open.
Synopsis
A B777 developed an apparent fuel imbalance in flight but since troubleshooting did not reveal a problem; the crew returned to the departure airport where enroute they found the fuel crossfeed valves unexpectedly open.
Narrative
About 2 hours into the flight we noticed a fuel imbalance; unable to figure why. We tried to work on it. We phone-patched Dispatch and Maintenance to help solve the problem. I asked Maintenance if the aircraft had a history of imbalance. I couldn't find anything in the log history. I asked the question; because I had flown an aircraft before; with that problem. We said that it wasn't. After much discussion; we all decided to return to the departure airport as the best option. On returning it was noticed that the crossfeed was open. None of us had touched them. There had been maintenance work going in the cockpit before start for another problem. A number of switches were being activated during the fix. During the before checklist; the fuel panel check was overrode. The aircraft turns off a center pump for load shedding and we all saw that and thought that was the problem. Never seeing the cross feeds open; we closed the crossfeed and the imbalance stopped. It was too late in the flight to turn back and head to our filed destination.
Second reporter narrative
About 3 hours out from departure we noticed a fuel imbalance developing on the center EICAS. Ran the fuel imbalance checklist to try and identify the problem and seek a remedy. The first part of it did not point to a fuel leak and we were hesitant to run the cross feeding part of the checklist due to the Captain's concern about dumping more fuel overboard if indeed a fuel leak existed. It is then we got Dispatch and Maintenance on the SATCOM radio and came up with the plan to return to the departure airport rather than make the Atlantic crossing. Shortly thereafter our Relief Pilot who had recently returned from his rest period noticed the crossfeed valves were both open and had been; knowing what we know now since we walked on the airplane at the gate. On our before start checklist; the checklist override was selected by me on command by the Captain as the only amber light visible was the shed fuel pump light which is normal. We failed to identify that the XFEED were open. I realize now if I had run that second part of the fuel imbalance checklist; and I mentioned it at least twice to the Captain that that was what was left on our fuel imbalance checklist; which involved cross feeding to remedy the imbalance we would have noticed early enough the problem to avoid the diversion.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.