Date: 201503
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B747 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Fuel System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Other / Unknown; Distraction; Confusion; Situational Awareness; Troubleshooting
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew FLC complied w / Automation / Advisory; Flight Crew Diverted; Flight Crew Landed As Precaution; Flight Crew Took Evasive Action; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I and the other relief pilot were asleep on our rest break when we were called up into the cockpit at approximately 3 hours into our scheduled flight.  The captain briefed us that we had a fuel problem that was either a gauge error; a transfer malfunction; or a leak.  The problem had begun when they had reached cruise and had started the routine transfer of fuel from the horizontal stabilizer into the center wing tank.  When the stabilizer tank pump switches were set to ON; fuel in the #4 main tank began to rapidly decrease.  When they turned the pumps off; the decrease in quantity indication stopped.  The captain reported that they had run the checklists and spoke with maintenance and dispatch and that we were going to stop enroute to have maintenance check out the issue.  We would dump fuel to obtain landing weight.  Maintenance had felt there was just a gauge error. However; the flying crew felt that the decrease in fuel quantity in then #4 tank was directly correlated to the transfer of fuel from the stab tanks.  So; all of us on the crew concurred that landing enroute to check out the problem was the best course of action as there was both maintenance and a crew base to keep the passengers on their way to [destination] if prolonged maintenance action was necessary.  Dispatch and Maintenance were in on this decision; as well.During their initial evaluation of the malfunction; the flying crew did not believe there to be an engine leak.  Totalizer fuel and calculated fuel were always within a few hundred pounds of agreement. At the first point of decrease of quantity indication in the #4 tank and then immediately afterward; total fuel was very close to the flight plan. Due to the unknown nature of the malfunction and the concern that the fuel was somehow transferring into the jettison manifold or leaking at a point other than the #4 engine; it did not seem prudent to attempt to balance the #1 and #4 main tanks as we had plenty of fuel to make it to [divert airport] without causing either lateral control issues or risking fuel starvation to the # 4 engine.  During tank to engine configuration after dumping; it appeared there may have been a continued slight decrease in the #4 tank relative to the #1 tank; but nowhere near what had been observed during the fuel transfer from the stab.  Engine indications were normal throughout.The plan had been to land at 630K to prevent an overweight landing.  On downwind at the [divert airport]; it became apparent that we would touchdown about 2-3 thousand pounds overweight.  The captain elected to accept that rather than take an airborne delay to as he felt there was still not certainty where the fuel was going.  The rest of the crew concurred.After an uneventful visual approach and landing; maintenance drip sticked the tanks.  Actual fuel quantity in the tanks agreed with the gauge indications. There was no initial explanation as to what may have occurred with the fuel in the # 4 tank.




Synopsis: A B747 crew diverted after horizontal stabilizer fuel transfer initiation resulted in a rapid fuel tank 4 quantity decrease. Tank to engine fuel feed was near normal.