Narrative:

We departed enroute to vhhh. When reaching cruise we got an EICAS message for a fuel imbalance between the #1 main fuel tank and the #4 main fuel tank. They were imbalanced by approximately 5000 lbs. The configuration didn't make sense because all the fuel was being drawn from the stab tanks and then the center tanks. We checked the fuel load sheet and verified that the fuel was in fact boarded properly; it was. We balanced the fuel by burning gas tank to engine on the #1 main tank and after achieving a balance; reconfigured the fuel system to normal operations. While taking our fuel scores at each waypoint; we realized that we were low on fuel at every check point. It started out as 1500 lbs low but was progressively getting worse. We scored 5000 lbs low after about 2 hours of flying and we were forecasted to be 6600 lbs low at the next waypoint. All other items on the flight plan were normal. We ran all applicable check list; the first being the imbalance checklist then the fuel leak checklist. We had signs of a leak but the totalized fuel was consistently greater than the calculated fuel. This caused some confusion as to why this was happening if we had a leak. We woke up the 2 relief pilots and got them involved with the situation. After we all determined that we had some sort of leak and that we no longer had enough fuel to reach vhhh; we elected to return. We needed to jettison fuel before arriving in order to be at max landing weight on arrival. It was determined that we could begin this process 50 minutes prior to landing so as to be completed before we descended to land. During the jettison process the fuel became imbalanced again and we stopped the jettison. This left us with about 8000 pounds of excess gas so we flew lower and configured the airplane to burn the fuel more rapidly. We landed at 631.8 lbs glw (gross landing weight) without incident and the emergency crews met us off the runway to inspect the airplane. Nothing was noted by them so we returned to the gate. On arrival into the gate; the calculated fuel was 2000 lbs greater than the totalizer fuel. We briefed the mechanics and then left the airplane.

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Original NASA ASRS Text

Title: Two B747 First Officers describe a fuel anomaly that may be the result of a fuel leak. Several hours into the flight the crew elects to return to the departure airport due to insufficient fuel to reach their destination. The flight is rescheduled to depart 14 hours after returning but one of the First Officers is unable to get meaningful sleep in the allotted time frame and calls in fatigued.

Narrative: We departed enroute to VHHH. When reaching cruise we got an EICAS message for a fuel imbalance between the #1 main fuel tank and the #4 main fuel tank. They were imbalanced by approximately 5000 lbs. The configuration didn't make sense because all the fuel was being drawn from the stab tanks and then the center tanks. We checked the fuel load sheet and verified that the fuel was in fact boarded properly; it was. We balanced the fuel by burning gas tank to engine on the #1 main tank and after achieving a balance; reconfigured the fuel system to normal operations. While taking our fuel scores at each waypoint; we realized that we were low on fuel at every check point. It started out as 1500 lbs low but was progressively getting worse. We scored 5000 lbs low after about 2 hours of flying and we were forecasted to be 6600 lbs low at the next waypoint. All other items on the flight plan were normal. We ran all applicable check list; the first being the imbalance checklist then the fuel leak checklist. We had signs of a leak but the totalized fuel was consistently greater than the calculated fuel. This caused some confusion as to why this was happening if we had a leak. We woke up the 2 relief pilots and got them involved with the situation. After we all determined that we had some sort of leak and that we no longer had enough fuel to reach VHHH; we elected to return. We needed to jettison fuel before arriving in order to be at max landing weight on arrival. It was determined that we could begin this process 50 minutes prior to landing so as to be completed before we descended to land. During the Jettison process the fuel became imbalanced again and we stopped the jettison. This left us with about 8000 pounds of excess gas so we flew lower and configured the airplane to burn the fuel more rapidly. We landed at 631.8 lbs GLW (Gross Landing Weight) without incident and the emergency crews met us off the runway to inspect the airplane. Nothing was noted by them so we returned to the gate. On arrival into the gate; the calculated fuel was 2000 lbs greater than the totalizer fuel. We briefed the mechanics and then left the airplane.

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.