B747-400
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Flight Crew Diverted
Aircraft
While enroute; about 1.5 hours or so into the flight; we got a Fuel Imbal 2-3 EICAS (Engine Indicating and Crew Alerting System) message. We looked at the fuel page; and tank 3 level was 3 tons more than tank two. At this time; the fuel totalizer level was maybe .3 higher than calculated amount. Given that the tank levels on the ground were not disparate; it was apparent that as the center tank was burning off; it was also pumping fuel into tank three. We ran the checklist; looked for signs of a fuel leak; and given that the totalizer was higher than the calculated level; engine burns were equal; and engine calculated levels were equal; we decided that we did not have a fuel leak situation. At this time; we started balancing tank two and three; to get tank three within 2700 kg of tank two. Since we still had fuel in the center tank; I decided to contact Dispatch/Maintenance Control on the sat phone; to affirm that it was probably better to deplete the center tank before completely balancing tank 2 and 3. We agreed on that approach. After the center got down to approximately 1.4 kg; and the center override pump lights came on; we turned off the center tank override pumps; and proceeded to balance tank two and three. At this time; totalizer level was still approximately .3 higher than calculated. After balancing 2/3; shortly thereafter the totalizer level started dropping below calculated level fairly rapidly. Also; the level in the center tank did not start emptying below 1.3 kg; and the level in tank 3 started fluctuating between .6 below; to .7 above tank 2 level. Given that the Fuel Imbal 2-3 checklist states that a totalizer level lower than the calculated level could be a sign of a fuel leak; and the rapidity with which the totalizer level dropped; and that it didn't start until we turned off the center pumps and balanced tanks 2/3; we thought we might have a problem on our hands. At this time we were in ZZZZ's airspace; heading towards ZZZ. Once past ZZZ; we had 8 hours of remote ZZZZ1/ZZZZ2 airspace ahead of us. So I contacted Dispatch/Maintenance Control on the sat phone again. Neither I; nor the Dispatcher; thought it was overly prudent to continue over that remote airspace if there was a potential fuel problem; which it looked like we had. I wasn't eager to see what the totalizer difference might be 8 hours down the road; and considering that the forecast winds were 290 20/35kts at the time of our arrival; with low tank levels in that turbulence; I didn't think continuing into that situation was prudent. After consulting with Maintenance Control; and the Duty Pilot; and [a fellow pilot] briefly; and hearing nothing persuading me that this was an issue to be taken lightly; or hearing a benign system explanation for what was happening; in conjunction with the Dispatcher; we decided to divert into ZZZ to let our mechanics troubleshoot the problem. In order to get below maximum landing weight on arrival into ZZZ; we held on the way in; rather than dumping fuel. We did not know what dumping fuel would do to a possibly compromised fuel system; and we didn't have much fuel to burn off. We held; and then proceeded with an uneventful arrival/landing into ZZZ. It should be noted that during this situation; I looked through two old logbooks; and saw no write ups reference the fuel system. Maintenance Control made no mention of any prior fuel system issues. But the mechanics in ZZZ said that there were multiple write ups concerning this issue in the recent past. It would have been nice if that information would have been relayed to me while I was talking to Maintenance Control/Dispatch on the sat phone. One suggestion would be to fix these problems when they happen. How many times can you say 'can not duplicate;' or 'system checks OK;' in the face of repeated write ups that demonstrate that there is some kind of problem. This aircraft had repeated instances of the totalizer level dropping below the calculated level. The difference this time was that we had a fair amount of center tank fuel; and the first indication of a problem was when fuel was moving from the center tank; into tank 3. To me; that indicates that irrespective of what is going on with the totalizer; there are other issues with this fuel system. Just a non-mechanics view. Also; the totalizer level again dropped below calculated level on the continued flight. I wrote that up upon landing at [our destination].[The First Officer] was at one time an Aircraft Mechanic. Having that perspective was a pleasant addition to the CRM (Crew-Resource Management) mix. The other two crew members were at rest during this time. We woke them when we decided to divert into ZZZ. All contributed to a successful conclusion of this issue; and a safe flight diversion into ZZZ.
B747-400 Captain reported a fuel imbalance and fuel totalizer indication issue resulting in a diversion.