Date: 202007
Local time of day: 0001-0600
Location: ZZZZ.ARTCC, FO
Aircraft: B767-300 and 300 ER
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
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); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Troubleshooting
Anomaly: Aircraft Equipment Problem Critical; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight; Routine Inspection
Result: Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Diverted; Flight Crew Requested ATC Assistance / Clarification; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We had just leveled off at FL320 and I did my First fuel score which showed us up 2.8 on fuel. The next waypoint showed us up 2.4. The next two waypoints continued trending down. We checked the winds but they where within 5 knots of planned. We then went to PROG page 2 to see Totalized and Calculated Fuel which is where we began to see a discrepancy. We initially saw a difference of 1.4 between both. We decided to monitor and the split between them continued to climb. Once we saw the the trend was continuing we called the Mechanics up to see if we could solve this issue. Before departure there was maintenance action on a Center Fuel pump pressure sensor. Also at this time we where burning out of the Center tank so the general feeling was that the suspected leak was in the center. After trying different configurations we found that CTR L pump off; Dual Crossfeeds Open; CTR R pump on slowed the suspected but did not fully stop it. At this point we where approaching 2:45hrs in flight and well over the pacific. We initiated a SATCOM call with Dispatch and Maintenance control. After explaining the situation; various other possibilities such as fuel migration or indication issues discussed and back and forth with them; Captain and I both agreed that it was unsafe to continue as 1. We where operating the fuel system in a Non-Normal way without checklist guidance. 2. Will this Suspected Fuel Leak get worse? 3. Where is the Fuel going? Overboard or somewhere else where it can cause severe issues? 4. If it gets worst later on; we would have to divert into ZZZZ1 with a plane full of American Troops. 5. ZZZ is only 2:30hrs behind us and we easily have enough fuel to return even with the current leak. We then made the decision to return.Dispatch was advised and I requested a change of destination and return to ZZZ through CPDLC with Center. We where shortly cleared direct ZZZ; asked if we wanted to request priority handling and we both agreed it would not be necessary. We continued to monitor the fuel difference on the way back and it continued to climb to 2.7 difference between Totalized and Calculated while operating in the modified Fuel Configuration. Had we not modified the fuel pump configuration we would have easily been more in the 5.0 to 6.0 region of difference. We burned enough gas for a normal and uneventful landing back in ZZZ about 2:30hrs after initiating the return back to ZZZ. After the desicion to Return to ZZZ was made Captain and FO (who where in crew rest) where advised and brought up to speed on what was going on and the course of action we where taking.This was detected through the Fuel Score numbers and the difference between totalized and calculated.Fuel Scores are more important then some people think. It allowed us to catch this early; And not when it would have been potentially a severe problem.




Synopsis: B767 First Officer reported an uneventful air diversion due to a critical fuel system malfunction.