Date: 201812
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Fuel System
Component problem: Malfunctioning
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer
Anomaly: Aircraft Equipment Problem Critical; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed As Precaution; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: On departure; we were cleared direct to position ZZZZZ. We crossed ZZZZZ at FL300. This was 34 minutes into the flight; which was 2 minutes ahead of plan. We had 215;000 lbs of fuel on board; which was 500 lbs under planned burn. 47 minutes later we crossed position ZZZZZ1 and we were 4;200 lbs over planned burn. We made initial contact via SATCOM with [Dispatch] and conferenced [Maintenance] to advise them of a possible fuel leak and to coordinate a diversion to the nearest suitable airport if required. We received a FUEL QTY/USED CHK alert and ran the associated checklist; which directed us to check departure fuel minus total used with present fuel on board. We now had approximately 10;000 lbs of fuel unaccounted for. We [advised ATC about fuel] and requested a diversion back to ZZZ. We continued to run the FUEL QTY/USED CHK checklist and determined there was an abnormal quantity decrease from the aux tank when the aux tank transfer switches were on and tank fill valve switches were selected to ARM/FILL. We now had the fuel leak isolated and contained by deselecting the tank fill valve switches. We then initiated a conference call via SATCOM with [Dispatch]; [Maintenance]; Duty Officer; Fleet Captain; and Technical Pilot. The Technical Pilot advised we might have a fuel gauging problem instead of a fuel leak and recommended pulling and resetting the primary and alternate fuel quantity circuit breakers. We did so and the fuel quantity did not change; indicating we most likely had a fuel leak. We advised our intensions were to land at ZZZ with emergency equipment standing by and we would need to dump fuel to reduce to maximum landing weight. We dumped approximately 47;000 lbs of fuel at top of descent and made an uneventful landing. After landing; emergency services performed an exterior check of the aircraft and advised there were no visible leaks. We continued taxi to the gate and terminated the flight. Maintenance was debriefed and the duty officer was contacted to advise of the safe conclusion of the flight and to pass along our gratitude to all parties involved for their outstanding support. This fuel emergency event went exactly as trained.


Second narrative: On departure; we were cleared direct to position ZZZZZ. We crossed ZZZZZ at FL300. This was 34 minutes into the flight; which was 2 minutes ahead of plan. We had 215;000 lbs of fuel on board; which was 500 lbs under planned burn. 47 minutes later we crossed position ZZZZZ1 and we were 4;200 lbs over planned burn. We made initial contact via SATCOM with [Operations Control] and conferenced [Maintenance Control] to advise them of a possible fuel leak and to coordinate a diversion to the nearest suitable airport if required. We received a FUEL QTY/USED CHK alert and ran the associated checklist; which directed us to check departure fuel minus total used with present fuel on board. We now had approximately 10;000 lbs of fuel unaccounted for. We [notified ATC] and requested a diversion back to ZZZ. We continued to run the FUEL QTY/USED CHK checklist and determined there was an abnormal quantity decrease from the aux tank when the aux tank transfer switches were on and tank fill valve switches were selected to ARM/FILL. We now had the fuel leak isolated and contained by deselecting the tank fill valve switches. We then initiated a conference call via SATCOM with [Operations Control; Maintenance Control]; Duty Officer; Fleet Captain; and Technical Pilot. The Technical Pilot advised we might have a fuel gauging problem instead of a fuel leak and recommended pulling and resetting the primary and alternate fuel quantity circuit breakers. We did so and the fuel quantity did not change; indicating we most likely had a fuel leak. We advised our intentions were to land at ZZZ on runway XXL with emergency equipment standing by and we would need to dump fuel to reduce to maximum landing weight. We dumped approximately 47;000 lbs of fuel at top of descent and made an uneventful landing. After landing; emergency services performed an exterior check of the aircraft and advised there were no visible leaks. We continued taxi to the gate and terminated the flight. Maintenance was debriefed and the duty officer was contacted to advise of the safe conclusion of the flight and to pass along our gratitude to all parties involved for their outstanding support. This fuel emergency event went exactly as trained.


Synopsis: Air carrier flight crew and Dispatcher reported returning to departure airport following indications of a possible fuel leak.