B747 Undifferentiated or Other Model
Flight Crew Last 90 Days 200; Flight Crew Total 13000; Flight Crew Type 200
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Flight Crew Diverted; Flight Crew Landed As Precaution; General Maintenance Action
Aircraft
During cruise we turned on Stab fuel pumps and several minutes later we had imbalance of 3000 between 1 and 4 main fuel tanks. We ran imbalance checklist which led us to fuel leak checklist. But checklists did not seem to address issue. The fuel imbalance stopped at 9000 lbs. or more and after Stab fuel tank empty. We turned off #4 main pumps trying to stop what appeared to be a leak. It seemed to work. Maintenance Control seemed to think it was a gauge error but things did not add up with fuel Totalizer; calibrated and between individual tanks. The plane seemed in trim. The FP fuel was not far off yet we were not comfortable flying over the ocean not knowing for sure if there was a gauge error or leak. So we diverted after jettisoning fuel. The jettison system had some difficulty keeping the fuel in balance on the right side. After landing the mechanics did a fuel stick and discovered that the gauges were correct. Which led us to believe we had lost 7 or 8 thousand pounds of fuel somehow. We may have landed over weight by about 2000 pounds but the landing was very smooth.
As a crew we accomplished the fuel dump procedure due a diversion. We planned on landing at max gross landing weight. However; upon touchdown we noticed that we landed 2-3 thousand lbs. overweight. Normal touchdown in the touchdown zone and rollout accomplished.
B747 flight crew experiences a fuel imbalance several minutes after turning on the stab fuel pumps in cruise. Whether a leak or a gauge anomaly is involved cannot be determined and the crew elects to return to the departure airport.