Air Carrier flight crew reported insufficient fuel messages indicated a possible fuel leak resulting in a return to the departure airport.
Synopsis
Air Carrier flight crew reported insufficient fuel messages indicated a possible fuel leak resulting in a return to the departure airport.
Narrative
On climb out of ZZZ we received an insufficient fuel message; and the total fuel number turned amber on the EICAS (Engine Indicating and Crew Alerting System). We 1st checked to see if the reserve fuel amount on the perf init (Performance Initialization) page was set correctly. It had been set 2000lbs below the planned Remf (Remaining Fuel) of 23.1k; it was set to 21.1k we then checked the progress page and it showed us landing with 18.1 in ZZZ1. This accounted for the insufficient fuel message; but when we checked the progress page against the Howgozit; the Howgozit showed landing with 21.8. We decided to pull new wind data to see if there was some inaccuracy in what was loaded. After the new winds were loaded the progress page returned to 21.1k Remf. The winds were light and variable for the entire flight. Thinking that was the solution we continued but the total fuel was still showing in amber. We checked the next waypoint on the Rte (Route) data page vs the Howgozit and discovered there was over a 6000lb difference with the FMC being the lower of the two. Shortly after that the progress page showed us landing in ZZZ1 with 15.1k some 8000lbs below the Maintenance Computer System flight plan. Checking both calculated and totalizer which were within 800lbs of each other; comparing engine burn which were matched; a fuel leak didn't seem probable but we ran the checklist anyway. We could not confirm a fuel leak; no spray trailing engines or wings; no imbalances; no unequal engine fuel burn; the fuel totalizer continued to show we were using fuel at an abnormal rate. Given that the fuel checklist says that a fuel leak should be suspected for 'Total fuel quantity is decreasing at an abnormal rate' I attempted to call Dispatch on Satcom; but it didn't work. I then sent an ACARs message saying we were planning a return to ZZZ for a suspected fuel leak. I with the concurrence of my FO elected to return to ZZZ. We were approximately 240 miles NE. The fuel totalizer at that point showed 115k fuel remaining. I elected to advise ATC and since we were still in communication with ZZZ ATC got direct to ZZZ VOR (direct to the airport). We completed the non-normal checklists; briefed the approach for the ILS to XXL. Looking at landing data we determined that if we didn't reduce our fuel load we would have to land overweight. I briefed that we would get closer to ZZZ then we would dump fuel to our max landing weight. When 70 miles from the field in the descent we followed the procedures and successfully reduced our fuel to max landing weight. Now showing some 110k fuel remaining. Vectors to the ILS and the approach were uneventful. We did request for crash fire and rescue be standing by in case we would be spilling fuel all over the place after landing. After landing we cleared the runway and stopped to allow the fire team time to assess if we had a visible fuel leak. They reported that we did not appear to be leaking fuel. We then taxied to the gate and shutdown. Maintenance reported that they received word that we had a fuel probe showing fault during the flight. I called Dispatch to give him all the details; he did mention they were having problems with Satcom. He was in agreement that the safest course of action was to return to ZZZ given the FMC prediction of landing in ZZZ1 with a large negative value of fuel compared to the Maintenance Computer System flight plan.
Second reporter narrative
On initial climbout of ZZZ we received an insufficient fuel EICAS and the total fuel number turned amber as well on the EICAS (Engine Indicating and Crew Alerting System). We checked to see if the RES (reserve) fuel quantity on the perf init page was set correctly. I initially loaded 2000lbs below the planned REMF (Remaining Fuel) of 23.1k; which was 21.1k Next; we checked the progress page and it showed us landing with 18.1k in ZZZ. This accounted for the insufficient fuel message; but when we checked the progress page against the Howgozit; the Howgozit showed landing with 21.8k. We decided to pull new wind data to see if there was some inaccuracy in what was loaded. After the new winds were loaded; the progress page returned to 21.1k Remf. (The winds were light and variable for the entire flight). Thinking that was the solution; we continued but the total fuel was still showing in amber. We compared the next waypoint on the Rte (Route) Data page vs the Howgozit and discovered there was over a 6000lb difference with the FMC being the lower of the two. Shortly after that the progress page showed us landing in ZZZ1 with 15.1k (8000lbs below the saber flight plan). Checking both calculated and totalizer which were within 800lbs of each other; engine burn rates were equal; a fuel leak didn't seem probable but we ran the checklist anyway. We could not confirm a fuel leak; no spray trailing engines or wings; no imbalances; no unequal engine fuel burn; but the fuel totalizer continued to show we were using fuel at an abnormal rate. Given that the fuel checklist says that a fuel leak should be suspected for 'Total fuel quantity is decreasing at an abnormal rate'; we discussed a diversion. We determined diverting back to ZZZ was the safest course of action; due to limited resources for our route. We were approximately 240 miles NE of ZZZ. The fuel totalizer at that point showed 115k fuel remaining. We advised ATC with ZZZ ATC and got direct to ZZZ VOR (direct to the airport). We completed the non-normal checklists; briefed the approach for the ILS to XXL. Looking at landing data we determined that if we didn't reduce our fuel load we would have to land overweight. Capt. XX and I briefed that we'd dump fuel to our max landing weight when we got near ZZZ. When we were 70 miles from the field in the descent we followed procedures and successfully reduced our fuel to max landing weight (approximately 105.8k) fuel remaining. All checklists were completed and we received Vectors to the ILS XXL. The approach and landing were uneventful. We requested ARFF (Airport Rescue and Firefighting) and they were standing by in case we were spilling fuel after landing. After landing; we cleared the runway and stopped to allow the fire team time to assess if we had a visible fuel leak. They reported that we did not appear to be leaking fuel. We taxied to the gate and shutdown. Maintenance reported that they received word that we had a fuel probe showing fault during the flight. We contacted Dispatch to give the flight details; who mentioned they were having problems with Satcom. He agreed the safest course of action was to return to ZZZ given the FMC prediction of landing in ZZZ1 with a large negative value of fuel compared to the Maintenance Computer System Flight plan.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.