Narrative:

I was the international relief officer on [flight] to ZZZZ. The event and most of the trouble shooting happened while I was on my break in the crew rest facility. The flight crew called me back to the flight deck about 30 minutes prior to coast out. They briefed me that there was a fuel anomaly occurring and they were working on the problem. At the time I returned we were showing a large fuel imbalance [of] approximately 4;700 lbs. Lower in left tank than right tank. The crew indicated that the imbalance and the split in the difference between the totalizer and calculated started when the center tank pumps were transferring last amount of fuel from center tank. Once the scavenge pumps had finished pushing the remaining fuel the totalizer and calculated started to equalize. The fuel imbalance remained but at that point stopped increasing. Soon after I returned we made a satcom call to maintenance control. They recommended balancing the fuel. We pushed the balance button. Soon after we started to balance the fuel the calculated and totalizer started to split again. We deselected the balance button and left the imbalance for the remainder of the flight. We also noticed that the planned fuel burn between two fixes at this time also went from slightly ahead to several hundred pounds behind in a little more than an hour. The large imbalance; the split in the calculated and totalizer and then loss of planned fuel in a short period of time all were factors in returning to ZZZ. We felt that returning to the base was safe since there were multiple airport options on the way back in case the situation got worse. We landed without incident but still had a 4;000 lb. Plus fuel imbalance when arriving at the gate.

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Original NASA ASRS Text

Title: B787 flight crew reported a recurring fuel imbalance that resulted in a return to departure airport.

Narrative: I was the IRO on [flight] to ZZZZ. The event and most of the trouble shooting happened while I was on my break in the crew rest facility. The flight crew called me back to the flight deck about 30 minutes prior to coast out. They briefed me that there was a fuel anomaly occurring and they were working on the problem. At the time I returned we were showing a large fuel imbalance [of] approximately 4;700 lbs. lower in left tank than right tank. The crew indicated that the imbalance and the split in the difference between the totalizer and calculated started when the center tank pumps were transferring last amount of fuel from center tank. Once the scavenge pumps had finished pushing the remaining fuel the totalizer and calculated started to equalize. The fuel imbalance remained but at that point stopped increasing. Soon after I returned we made a Satcom call to Maintenance Control. They recommended balancing the fuel. We pushed the balance button. Soon after we started to balance the fuel the calculated and totalizer started to split again. We deselected the balance button and left the imbalance for the remainder of the flight. We also noticed that the planned fuel burn between two fixes at this time also went from slightly ahead to several hundred pounds behind in a little more than an hour. The large imbalance; the split in the calculated and totalizer and then loss of planned fuel in a short period of time all were factors in returning to ZZZ. We felt that returning to the base was safe since there were multiple airport options on the way back in case the situation got worse. We landed without incident but still had a 4;000 lb. plus fuel imbalance when arriving at the gate.

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.