B777 flight crew reported a fuel quantity indication malfunction which was treated as a fuel leak during cruise flight in oceanic airspace. The flight diverted to an alternate airport and landed; maintenance engine run ups did not finding indications of a fuel leak.
Synopsis
B777 flight crew reported a fuel quantity indication malfunction which was treated as a fuel leak during cruise flight in oceanic airspace. The flight diverted to an alternate airport and landed; maintenance engine run ups did not finding indications of a fuel leak.
Narrative
I was one of two relief pilots who had assumed our flight duties for about 1/2 an hour prior and the primary crew were in the crew bunks. We received an EICAS msg notifying us of a fuel imbalance (roughly 4000-4500lbs) and ran the abnormal checklist. We did not initially suspect a fuel leak so balanced the tanks. The right tank was the low tank and the center tanks were empty. We noticed after a few minutes into the balancing that the right tank continued to go down. Both right tank pumps were off and one crossfeed open per the checklist. It took about 12-14 minutes to balance the tanks and the right tank lost another several hundred pounds during this time. We hacked a clock and discovered the right tank was losing 100 lbs. every 4 minutes. Once the tanks were balanced we started a new clock and within 25 minutes the right tank was over 1000 pounds less than the left. The totalizer was also about the same amount less than the calculated. At this point I decided to wake the Captain and both primary pilots came back to the flight deck. Maintenance Control and Dispatch were called and it was determined we had a probable fuel leak. At this point the decision was made to divert to ZZZZ. We shutdown the right engine in accordance with the checklist; off set and descended to our eng out alt using the drift down checklist. We picked up a clearance to ZZZZ; finished the fuel leak and all other checklists; calculated eng out landing distance; notified the flight attendants; passengers; etc… and landed flaps 20 at ZZZZ without further incident. I learned later that just prior to the initial crew change; the operating pilots had shut off the center tank pumps once they were depleted. The First Officer (FO) noticed both wing tanks were balanced at the time. Additionally we later read in supplementary section of the Flight Manual (FM) that there is a known issue with the 'C' model regarding flow valves I believe.
Second reporter narrative
The Captain and I had been on our rest break (second break/double augmented) for approximately 1 hour when the relief pilots called us to report a suspected fuel leak. We joined the relief pilots on the flight deck and they briefed us on the situation. They had received a FUEL IMBALANCE EICAS indication and noted left main tank quantity approximately 53.2 and right main tank approximately 48.8. They had attempted to balance fuel with right tank pumps off; and noticed that right tank fuel indicated a continuous reduction in quantity at a rate of approximately 100lbs every 4 minutes. Furthermore; a change in fuel imbalance of greater than 1000 lbs in 30 minutes had occurred; which met the criteria for engine tank leak confirmation (Step 7 on Fuel Leak checklist). The relief pilots also observed totalizer fuel quantity lower than calculated fuel. While the relief pilots initially remained in the flying seats; I went back to the cabin to attempt to look for any fuel spray coming from the right engine or wing; and observed none; although my view from the aisle through an overwing window was limited. Prior to my exchanging places with the relief pilot in the right seat; and while the Captain took his seat from the relief pilot and set up a SAT call with Dispatch and Maintenance Control; I reviewed the Divert Guide in the FOM Quick Reference Guide (QRG) and began previewing charts for ZZZZ which was our critical point (CP) Alternate and the nearest suitable airport for a diversion; approximately 45-60 minutes from our position.I then exchanged seats with the relief pilot in the right seat and resumed pilot flying (PF) duties. The four of us continued to thoroughly discuss the situation; studied and continued running through the fuel leak checklist; and were confident that we had performed the checklist properly; and that the situation met the criteria for engine shutdown and diversion per the fuel leak checklist. Maintenance Control had little guidance to add to what we had already observed; and we elected to continue the checklist and shutdown the right (#2) engine and to divert to ZZZZ in cooperation and coordination with Dispatch. We shut down the engine; requested priority handling; performed the Driftdown checklist; and began drifting down at Engine Out speed of approximately 260kts while flying toward a Left 5nm offset. Upon reaching the 5nm offset; I increased speed to 280 kts and descended in FLCH toward Engine Out altitude FL210 in order to get below FL290 asap. Meanwhile ZZZZ Radio handed us off to ZZZZ Control who gave us clearance to fly direct to ZZZZ upon reaching below FL290. Once below FL290; I flew direct ZZZZ; increased speed to 300 kts; determined Engine Out cruise altitude at that speed which if I recall correctly was FL190; and continued descent to that altitude. ZZZZ was approximately 45 min flight away when we started flying direct.The Captain delegated some communications to the relief pilot in the center jumpseat; including giving information to Flight Attendants (FAs) and talking to Operations at ZZZZ. I don't recall who made a PA to the passengers but that was done. 10 minutes prior to descent I exchanged flight control with the Captain; set up the ZZZZ ILS Runway XX; and briefed the approach. The second relief pilot got the Non- Normal One Engine Inoperative Flaps 20 landing data. Weather at ZZZZ was OVCST 31 with good visibility and light winds. ZZZZ Approach gave us unrestricted speed below 10;000 ft.; a couple of descent clearances to intermediate altitudes; Direct to ZZZZZ (IAF); and then cleared us for the approach. I remained above 250kts below 10;000 ft; slowing incrementally in accordance with ability to comply with my stabilized approach plan. Once below the overcast layer at approximately 3000ft; I disconnected the Autopilot and hand flew the remainder of the approach and landing. Landing was uneventful. We taxied to a remote location per Tower instructions and shut down the number 1 engine.Subsequent events on the ground included a 5min engine run up and a couple of maintenance logbook entries RE oil pressure at time of engine shutdown and time of engine windmilling (45min) at Maintenance Control's request; and coordination with chief pilot; ZZZZ operations; and engineering technicians at ZZZZ. The engine run up did not reveal any fuel leaks at the engine. I spent some time in the cabin talking to passengers. One of the relief pilots spent some time talking to FAs. The Captain and other relief pilot spent some time on the flight deck communicating with chief pilot; engineering technicians; and Operations. Post flight debriefing discussions during the layover; which included looking at Flight Manual (FM) Supplementary Procedures FUEL Anomalies; concluded that we couldn't think of anything we should have done differently and that our decisions had been appropriate and effective; except that there might have been one or two small items in the Anomalies section that were not performed; e.g. I don't know if the relief pilots took an EICAS snapshot of the FUEL IMBALANCE.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.