Narrative:

In cruise approximately 3 hours and 15 minutes into the flight; the crew noticed a fuel imbalance situation developing between the left and right main tanks with approximately 2.7K remaining in the center tank. The left main fuel tank had approximately 40k and the right had approximately 38K with the 'fuel config' light illuminated. The crew balanced the fuel between tanks. We noticed that not only had the center tank not burned all its fuel; but the fuel quantity in the center tank was increasing slightly. The QRH was consulted. Nothing there seemed to apply to this situation. We contacted dispatch and asked for a link to maintenance for guidance. We relayed all the info up to that point to the maintenance representative and informed him of the steps we had taken to keep the fuel balanced. We determined that the rate of transfer from the right main tank to the center was approximately 3.1K per hour. At that point we were informed by the maintenance representative that once the main tanks reached the halfway point in their burn (or about 20k per tank) the fuel transfer from the right tank to the center would cease. With that information I elected to continue the flight expecting to land in ZZZ1 with approximately 18k in the center and approximately 8k in each main tank. We put together a plan to divert to several locations as the situation developed. We then spent time figuring out various scenarios to determine the options for safety; weather; maintenance; passenger servicing etc. Climbed to FL380 as soon as ATC allowed it for slightly better range and enroute weather avoidance. The rate of [fuel] transfer to the center tank began to accelerate gradually as we approached ZZZZ it became clear that this rate would not allow us to reach ZZZ1 safely. Upon reaching ZZZZ airport we declared an emergency and elected to divert to ZZZZ. At that point the fuel tanks had about 16k in each main and approximately 18 to 19k in the center. I had asked the relief pilot to take over the monitoring of the fuel panel as the first officer and I dealt with getting the aircraft on the ground in ZZZZ. Upon leaving FL380 the rate of transfer from the right tank into the center began to accelerate again. By the time we reached ZZZZ; the main tanks were down to approximately 5.5K and the center 35k and climbing. We asked to direct to the field and were given clearance to runway xx. After seeing that was too far away from the field; we were given direct XXXX for the ILS. Again; not feeling comfortable with the distance from the end of the runway (XXXX was 12 miles); we called 'field in sight' and headed directly toward the end of the runway. At about 20 miles from the field; I felt the possibility of losing one or both engines was a real possibility. I was determined to get to a 3-mile final with at least 2;000 ft to 2;500 ft of altitude in case of a dual engine failure. Once we were close enough to the field we flew through final to gain spacing and brought the aircraft onto final on runway xx and were in the slot by 500 ft and landed without incident. The relief pilot reported seeing approximately 2.5k in the left and 2.0k in the right tank as we crossed the threshold.

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Original NASA ASRS Text

Title: B767-300 Captain describes the efforts of his flight crew to reach an airport to avoid losing one or both engines. Uncommanded fuel was being transferred from the right main wing tank to the center tank; while creating a fuel imbalance on the left wing. Their aircraft crossed the landing threshold with 2.5K of fuel in the left main tank and 2.0 in the right; center tank had over 35K.

Narrative: In cruise approximately 3 hours and 15 minutes into the flight; the crew noticed a fuel imbalance situation developing between the left and right main tanks with approximately 2.7K remaining in the center tank. The left main fuel tank had approximately 40k and the right had approximately 38K with the 'Fuel Config' light illuminated. The crew balanced the fuel between tanks. We noticed that not only had the center tank not burned all its fuel; but the fuel quantity in the center tank was increasing slightly. The QRH was consulted. Nothing there seemed to apply to this situation. We contacted Dispatch and asked for a link to Maintenance for guidance. We relayed all the info up to that point to the Maintenance Representative and informed him of the steps we had taken to keep the fuel balanced. We determined that the rate of transfer from the right main tank to the center was approximately 3.1K per hour. At that point we were informed by the Maintenance Representative that once the Main tanks reached the halfway point in their burn (or about 20k per tank) the fuel transfer from the right tank to the center would cease. With that information I elected to continue the flight expecting to land in ZZZ1 with approximately 18k in the center and approximately 8k in each Main tank. We put together a plan to divert to several locations as the situation developed. We then spent time figuring out various scenarios to determine the options for safety; weather; maintenance; passenger servicing etc. Climbed to FL380 as soon as ATC allowed it for slightly better range and enroute weather avoidance. The rate of [fuel] transfer to the center tank began to accelerate gradually as we approached ZZZZ it became clear that this rate would not allow us to reach ZZZ1 safely. Upon reaching ZZZZ Airport we declared an emergency and elected to divert to ZZZZ. At that point the fuel tanks had about 16k in each Main and approximately 18 to 19k in the center. I had asked the Relief Pilot to take over the monitoring of the Fuel panel as the First Officer and I dealt with getting the aircraft on the ground in ZZZZ. Upon leaving FL380 the rate of transfer from the right tank into the Center began to accelerate again. By the time we reached ZZZZ; the main tanks were down to approximately 5.5K and the Center 35k and climbing. We asked to direct to the field and were given clearance to RWY XX. After seeing that was too far away from the field; we were given direct XXXX for the ILS. Again; not feeling comfortable with the distance from the end of the runway (XXXX was 12 miles); we called 'Field in sight' and headed directly toward the end of the runway. At about 20 miles from the field; I felt the possibility of losing one or both engines was a real possibility. I was determined to get to a 3-mile Final with at least 2;000 FT to 2;500 FT of altitude in case of a dual engine failure. Once we were close enough to the field we flew through final to gain spacing and brought the aircraft onto final on Runway XX and were in the slot by 500 FT and landed without incident. The Relief Pilot reported seeing approximately 2.5k in the left and 2.0k in the right tank as we crossed the threshold.

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.