Date: 201910
Flight conditions: IMC
Aircraft: B787 Dreamliner Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
Aircraft component: Fuel System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Time Pressure; Situational Awareness; Distraction; Troubleshooting
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Fuel Issue
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed As Precaution; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport; Flight Crew Requested ATC Assistance / Clarification; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

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.


Second narrative: In cruise flight at FL290; we received a fuel imbalance EICAS message. All fuel pumps were on; including the center tank pumps; but the left main tank quantity was less than the right main tank quantity and although fuel was being used from the center tank it appeared that the left main tank was also being depleted. We began the fuel imbalance checklist but stopped at the point of attempting to determine if we might have a fuel leak. Total fuel on board was noted to be several hundred pounds more than the FPF at that positioning the totalizer and calculated fuel was very close. At this point we did not suspect a fuel leak but noted that the fuel system was not functioning as it should with fuel quantity dropping in the left main tank with fuel in the center tank and the center pumps on. The fuel imbalance increased to more than 4;000 lbs. About 20 minutes prior to (coast out) having communicated with Dispatch and Maintenance Control; they suggested we select the fuel balance system on which we did not want to begin at that moment as the scavenge pumps were operating at that time to use the remainder of the center tank fuel.When the center tank pumps shut off we re-ran the fuel balance checklist from the beginning and attempted to balance the main tanks. In a very short time the difference between Totalizer and Calculated fuel was diverging and the imbalance increased to 4;700 lbs. The difference between totalizer and calculated fuel was 1;000 lbs. and [later] the difference was 1;600 lbs. We de-selected the fuel imbalance switch which seemed to stabilize the fuel situation; but at this point just a few minutes from [coast out] we noted that the fuel on board would be 600 lbs. below the FPF. Captain and both First Officers concurred that the safest course of action was to turn around where diversion options were plentiful. Clearance back to ZZZ was issued and a discussion ensued about whether or not [to request priority handling]. Again all three pilots concurred that a [priority handling request] was not needed at that time as long as the situation did not deteriorate resulting in a divert to an airport short of ZZZ. Upon turning back to ZZZ; we initially encountered strong headwinds which when we extrapolated the fuel usage indicated an arrival fuel at ZZZ of approximately 5;000 lbs. in the left main and approximately 10;000 lbs. in the right main though after we descended out of the high winds the expected fuel on board on arrival improved to where we actually arrived with approximately 9;000 lbs. in the left main and 14;000 lbs. in the right main. Maintenance met the aircraft on arrival where we followed up on our report with a thorough debrief to the mechanics.


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