B747-400
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Maintenance; Inflight Event / Encounter Fuel Issue
Flight Crew Diverted; Flight Crew Landed As Precaution; General Maintenance Action
Aircraft; Procedure
Prior to departing; maintenance was being performed to address a status message regarding the number four fuel transfer valve. The mechanic and maintenance control had elected to defer the item. During this period; the local mechanic ask the senior FO and myself; which valves he should safety wire closed. We both replied to him that we were not mechanics and that he should refer his question to [Company] maintenance control. It occurred to me then that perhaps this mechanic did not fully understand the [maintenance document]; since English was not his primary language and the manual was written in English. Losing confidence in his ability to successfully complete the procedure; I called [Company] maintenance control to speak with the administrative mechanic on duty. The mechanic on duty told me that he was working with the local mechanic. He told me that the local mechanic had understood which electrical cannon plugs to remove; which fuel transfer valves to close and assured me that it was safe for us to depart. After an hour and fifty minutes past our scheduled departure time; with the maintenance log book signed off; we pushed back; started engines and began our flight.After reaching cruise altitude while resting in the crew bunk; I received an unscheduled wake-up call from the cockpit asking me to come forward to address a problem. [The other crewmembers] explained to me that an uncontrollable fuel imbalance existed between main tanks one and four as well as between main tanks two and three as shown on the fuel synoptic page as well as EICAS messages. They further explained that the number four main tank quantity was dropping significantly more than the number one tank. It appeared the number one tank fuel quantity was being burned at a normal rate. Furthermore; the fuel quantity in main tank number three was increasing over time and the fuel quantity in main tank number two was dropping at a normal rate. Please note that the center fuel tank was now at 1.0 and the center fuel tank boost pump switches were off in accordance with procedures. Additionally; the calculated fuel versus the fuel quantity on Progress page 2 showed nearly the same amount; indicating that a fuel leak was unlikely. [They] also explained that even after manipulating the fuel cross feed valves and boost pumps on the fuel panel; they were unable to stop the increasing fuel imbalance between all four main fuel tanks. I also attempted to correct this situation by opening fuel cross-feed valves one and four; closing cross-feed valves two and three and switching off both boost pumps in the number four main tank; thus trying to feed engines one and four from main tank number one. Even under these conditions; the fuel quantity in the number four main tank continued to drop. We tried other combinations of opening and closing fuel cross feed valves and switching on and off fuel boost pumps to correct the increasing imbalance to no avail.At this time; after our crew exhausted QRH and [flight manual] procedures available to us; I called Dispatch via satellite phone to speak with maintenance control. I explained the situation to them and they advised me to follow the procedures outlined in the QRH and [flight manual]. I told them we had; but that the procedure was not working. Dispatch then added [the] Chief Pilot to our sat phone conversation. We had a detailed conversation to troubleshoot the problem. He came to the same conclusion that our crew had already come to; that the number four transfer valve had been mistakenly safety wired to the open position rather than the closed position as had been directed by the [manual procedure] by the mechanic in [departure station]. This was causing the fuel in the number four main tank to gravity feed into the number three main tank; as space became available in the number three main tank. Naturally; all of us realized that we could not continue with this situation. I asked Dispatch to select a diversion airport. ATC was notified and we were rerouted to [the diversion airport]. Upon arriving into Approach Control airspace; we asked for and were given holding instructions to jettison fuel down to a landing gross weight below the max landing weight. This action of jettisoning fuel also resolved most of the fuel imbalance issue. An additional 10 knots was carried on final to remain just above the maneuvering caution zone on the airspeed indicator ('yellow pencil') which was needed due to the remaining fuel imbalance between main tanks one and four. The approach and landing was uneventful.After securing the aircraft in the parking bay; I spoke with the mechanic on duty to explain the problem. He investigated and discovered that the number four transfer valve had indeed been mistakenly safety wired in the open position rather than the closed position. Our crew went into crew rest. The next day; the local mechanic explained to us that he had replaced the number four transfer valve and the deferral was now cleared. It is clear to our crew that this event could have been easily avoided by following the proper maintenance procedures by the local mechanic and by proper oversight by [Company] Maintenance Control.It is important to note that upon landing in [the diversion airport]; we still had about 1.1 of fuel in the number four main fuel tank. Had this situation occurred further away from a suitable airfield; it would have required more fuel burn to reach such an airport. The fuel quantity in main tank number four would inevitably drop to zero; due to the uncommanded fuel transfer and fuel burn prior to making an approach. In such a scenario the number four engine would have continued to run due to fuel flowing from main tank number three through the number three and four cross-feed valves. However; once the flaps begin to extend in preparation for landing; the number three cross-feed valve closes automatically by the fuel management computer. This would cut off the fuel supply to engine number four; causing an engine flameout. Obviously; this would have added to the complexity of the situation. Luckily; we were fairly close to a suitable landing airport.
B747 Captain reported diverting to an alternate airport after it became clear the fuel system was compromised by improper maintenance at a foreign station.