Date: 201709
Local time of day: 0601-1200
Location: ZZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B767 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
ATC / advisory: Center ZZZZ
Aircraft component: Fuel Tank Cap
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Relief Pilot
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting; Communication Breakdown
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport
Contributing factors: Aircraft; Procedure
Primary problem: Aircraft

Narrative: We arrived at the aircraft unaware of the fuel system problem that had occurred on the inbound flight. The inbound defect report was empty and there were no fuel system deferrals. During my walk-around a mechanic approached me and said that the right wing gravity refueling cap was leaking and that they were in the process of replacing the seal. I immediately brought this information to the captain and we both went out on the ramp to have a better look and to speak with maintenance. Nothing abnormal was visible from the ground. The maintenance technician said that the existing seal appeared to be degraded and that this could explain the leak. From inside the cabin I could see that the top of the wing near the fuel cap appeared damp with fuel and that maintenance had been swabbing a bit with a towel as they worked. Maintenance completed the seal replacement and then checked the fuel cap for leaks after the wing tanks were filled for flight. We delayed boarding until the tanks were full and the aircraft was signed off.While the Captain; the flying FO; and I were waiting for repairs; we discussed how a fuel leak might affect our flight. This was a 180 minute ETOPS flight with one critical point and two ETOPS alternates; both of which were more than one thousand miles from the flight path. Our route also took us over [another alternate]; but gusty winds and IFR were forecast shortly after our scheduled time of passing. We studied the fuel leak QRH and that lead us to question precisely when the fuel leak appeared on the inbound flight and how the previous crew's experience might help us should the problem reappear. The captain made several phone calls to get more information while the flying FO and I had a productive argument about whether it would be acceptable to continue the flight with fuel misting from the fuel cap. In my experience with other aircraft it is not uncommon that fuel can temporarily mist from fuel vents shortly after takeoff with full fuel. But continuing the flight; even over land with suitable alternates; was a difficult position to defend. A leaking fuel cap would be an abnormal case. We wouldn't know where all of the leaked fuel is going and if it might cause other systems problems. The Fuel Leak QRH has a step instructing us monitor the fuel imbalance to confirm a leak but we determined; I believe correctly; and that this guidance does not override a following step that asks us to visually check for a leak. In the case where a fuel leak is confirmed the QRH leads to a 'land at the nearest suitable airport' statement.The captain briefed us and the company of the plan: that should a fuel leak be discovered after takeoff we would be returning to [the departure airport]. He asked one of the flight attendants to observe the wing after takeoff and I went to the cabin with her to point out where she should look. A passenger in the exit row overheard our conversation.After takeoff the flight attendant reported that there appeared to be a stain on the wing; or that it was wet with fuel. I knew the wing had been wet before so I was unsure if she was referring to a residual stain that perhaps appeared differently under different light or if there was actually a leak. The captain asked me to go check things out near 10;000 ft in the climb.From the right front overwing exit window I saw fuel running aft from the fuel cap and turning to mist near the trailing edge. The passenger who had earlier overheard me briefing the flight attendant said that he didn't notice very much flow immediately after takeoff but now the leak was very apparent. As I understand it; the flight attendant checked right after liftoff when there was little to no flow and the passenger asked her to come and have another look once he noticed the wing appeared wet. At that point she called us. I asked this passenger to let us know if anything changed; snapped a photo of the leak with my phone; and returned to the flight deck.Since wehad already decided to return in case of a leak things went pretty smoothly. I'm glad the captain briefed a good plan for the contingency. Immediately after takeoff I had observed 500 lbs more fuel on the right wing totalizer than the left. After I returned to the flight deck less than 5 minutes later the indicated imbalance was 300 lbs. I don't know if this was gauge fluctuations or an actual reading of leaked fuel because the imbalance stayed constant after that.As IRO I ran the fuel leak QRH with the captain while the FO flew the aircraft. The Fuel Leak QRH has some drastic measures once a fuel leak is confirmed and we felt that following these would increase our workload unreasonably and decrease our safety margins given what we knew about the leak. We did not turn off autothrust nor shutdown an engine. Once we had taken as much guidance from this checklist as was reasonable the captain began coordinating with ATC; company; and flight attendants. I calculated our maximum landing fuel to avoid an overweight landing. We dumped center tank fuel and then put the gear down to burn the wing tanks down to maximum landing weight. In retrospect I feel that the captain made the best choice in choosing to land under the maximum landing weight. There was no point in adding brake energy and other considerations to our palate given what we knew about the leak. We didn't feel it was an emergency; but we didn't want to continue across the Atlantic either. In total we dumped approximately 17;000 lbs from the center tank and once that was complete we had a little more than 10;000 lbs to burn from the wing tanks before landing. Once we had burned some fuel out of the wing tanks the captain asked a flight attendant to check the leak. It had stopped. The captain told the flight attendants to expect a normal landing. We did not declare an emergency. We covered our bases with diversion planning guide in the QRH and landed. The authorities directed us to park at a remote stand and passengers were bussed to the terminal.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B767 flight crew reported dealing with a fuel leak from an overwing fuel cap requiring a return to the departure airport.