Date: 201705
Local time of day: 0001-0600
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Aircraft: B757 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Fuel System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Aircraft Equipment Problem Critical; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Landed As Precaution; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: [While enroute]; the R Center Fuel Pump Press illuminated with the corresponding EICAS message. We turned off both L and R Center Fuel Pumps; as per SOP.  Everything appeared normal at that point. Approximately 10 minutes later we got a Fuel Config Advisory EICAS message and I looked up and saw the Fuel Config light illuminated on the Fuel Panel. At first; I assumed it was just a 2000 lb fuel imbalance and told my FO to pull out the QRH to balance the fuel per SOP. She did pull out the QRH; but said; 'We have a fuel leak!' I stared up at fuel panel and also saw the quantity in the right fuel tank dropping a few hundred pounds just as I was watching it. At that point we both confirmed that we had a fuel leak. I quickly typed a message to Dispatch; saying; 'Call me ASAP' and started the Fuel Leak Checklist in the QRH. The checklist specifically said; 'Do NOT try to balance the fuel (i.e.; Using the Fuel Config checklist); but once you determine an actual fuel leak; land immediately at the nearest suitable airport.' I called back to the A Flight Attendant and asked her to look out the right window for any possible fuel leaking from the engine or the wing--she called back promptly and reported no fluid/fuel leaking at all.Dispatch sent us a VHF frequency and I immediately called her; telling her that we had a definite fuel leak and that we needed to divert immediately. I asked Dispatch where would be a good divert field and she recommended [an alternate airport]; about :30 minutes away. My FO and I agreed. Somewhere during this time I told my FO that I would take over the flying and that she work the radios and the checklist. I told her to declare an Emergency with ATC and we immediately get a vector toward [our alternate]. Simultaneously I put [the alternate] in the ALTERNATE page of the FMC and executed the 'Divert Now' prompt. Once headed towards [the alternate]; I called back to the A Flight Attendant and went over the [cabin preparation] components; telling her that we had a fuel leak; to prep for evacuation (at that point; I wasn't sure how bad the fuel imbalance would be when we got to [the alternate] (it was at 3000 lbs and getting worse); or if the aircraft could even be safely landed; so I wanted her to prep for evac; no special instructions; and time to landing was :30 minutes). I then got on the PA and made an announcement to the passengers; saying we have a fuel leak and we needed to divert and that we would be landing in 30 minutes. Dispatch contacted again about 5 minutes later and asked us to divert to [a different alternate]. I told my FO to tell Dispatch; 'Negative--we've [advised ATC] and are preceding directly to [the first alternate]'. I understand that [the second alternate] is our maintenance hub and it would be easier to get our passengers to their final destinations if we went [there]; however; [it] is NOT the 'Nearest suitable airport'; and most importantly; our fuel imbalance would be significantly worse if we had to fly [to the second alternate].My FO and I completed the Fuel Leak Checklist. There was a part that mentioned shutting down an engine so that you can balance the fuel. I thought about it and told my FO that when I got to [the alternate] and clicked off the auto pilot; if the aircraft rolled excessively or was unable to land safely; then we would shut the engine down and balance the fuel (we had plenty of fuel; so fuel wasn't a problem). Until then; I was not going to shut down an engine. We landed safely. There was a 4000 lb fuel imbalance when we landed. The mechanic also told me when I went back to get my bags a few hours later; that [the Company] called him and ask that he [defer] it so that we could continue on. He told them 'No; that there was a fuel imbalance of 4000 lbs and that there was a definite fuel leak.'




Synopsis: B757 Captain reported they diverted to an alternate after experiencing a fuel leak.