Date: 201904
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 10
Light: Night
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Fuel System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We were scheduled to operate a revenue flight from ZZZ to ZZZ1. We arrived at the aircraft approximately 40 minutes prior to block and began our preflight. While accomplishing the cockpit preflight the fueler came into the cockpit to see if we were good with the current fuel load. At this time we noted that the aircraft had more than the planned departure fuel since the aircraft was originally scheduled as a hot spare. My planned total fuel was 26.7 and our current total was 29.7. The fueler asked if we were okay with the current fuel imbalance which was only 400 pounds; left side being 400 pounds less that the right. We noted that our center tank contained 1;500 pounds and that we would have the 'Fuel Config' EICAS message during our initial phases of operations. Stating we had more than our planned fuel and the current imbalance wasn't significant enough to be a concern; he departed. Approximately 30 minutes before scheduled departure we turned on the APU and continued our preflight duties. The sort was running late and we didn't block out until 29 minutes late. Our APU had been on for approximately 1 hour; possibly leading to an extra 200 pounds imbalance on the left side.I was the flying pilot for this leg. Start up; taxi out; takeoff and initial climb out were completely normal with no irregularities noted. Passing through 10; 000 feet I called for the First Officer to turn on the Left Center fuel pump to begin fixing our fuel imbalance. At this time we noticed that there was now a 2300 pound split between the Left and Right fuel quantity indicators where there had only been a 400 pound imbalance earlier. Concerned that we now had exceeded the 1800 pound imbalance limit for the B757 I called for him to open the fuel crossed valve and secure the left engine fuel pumps to help expedite the fuel transfer. At this time the First Officer noted that the fuel quantity in the left tank indicator was falling and that we looked like we were losing fuel. I called for the autopilot and began watching the quantity indicator. Our recollection was that we saw the left fuel quantity go from 12.0 to 11.8 in less than 1 minute. Fearing a significant fuel leak; I called for the First Officer to run the 'Fuel Leak Engine' checklist. In accordance with the checklist; we began diagnosing the problem. We noted that of the 7 possible indicators of a potential fuel leak; we had 2 (EICAS Fuel Config and observed left tank quantity falling at an excessive rate). Continuing with the checklist we secured the center pump switches and fuel crossfeed switches. At this point I do not recall how much increase in the difference there continued to be between the Left and Right fuel quantity indicators; but we had at least a 2;300 pounds imbalance between the two tanks in less than 30 minutes. Based on this observation; we felt we had a condition the checklist states should be considered an engine fuel leak.Leveling off at FL180 we requested a turn back toward ZZZ and a descent. The First Officer continued with the checklist while I sent an ACARS message to Dispatch that we were diverting back to ZZZ for a mechanical. In accordance with the checklist we secured the left engine; which was the side we suspected of having a fuel leak; and the First Officer [advised ATC]. We continued our descent and received vectors for the ILS to XXR. We reviewed the 'Fuel Leak Engine' checklist and set up for a single engine approach. We accomplished the 'Approach Checklist' and 'Landing Checklist' and landed without further incident. We had CFR (Crash Fire Rescue) inspect the engine while we were stopped on the runway and told that we didn't appear to have any leakage at that time. We then received clearance to taxi clear of the runway and into [the] ramp. We blocked in and executed the 'Shutdown and Secure Checklist' without problems. To the best of my recollection we still had a 600-800 pounds imbalance on shutdown even after operating single engine for at least 20minutes.


Second narrative: Approximately 40 minutes prior to block out; we began our pre-flight. During pre-flight the aircraft fueler checked to ensure our current fuel load was adequate and the 400 pound split was okay. We had about 3000 pounds more fuel than planned and the left side was 400 pounds lower than right. We also noted the center tank contained 1500 pounds of fuel. The Captain told the fueler our fuel load was adequate and the split was within limits. We also discussed the fact the fuel configuration light would be illuminated during our initial climb out. During our preflight; the APU was operating and with our loading delay the approximate run time was one hour. The Captain was the flying pilot and I was the pilot monitoring. During initial takeoff and climb out no irregularities were noted and while passing through 10000 feet the Captain called for the left center tank pump to be activated to begin balancing fuel. It was during this time we noticed a 2300 pound fuel imbalance with the left side being low.The Captain then requested I open the cross-feed valve and secure the left engine fuel pumps. Upon setting up cross-feed operations I noticed the left side fuel quantity went from 12.0 to 11.9 then 11.8 in approximately one minute with both left fuel pumps off and the cross-feed valve open. The Captain called for the autopilot and began monitoring the fuel panel. During the decrease in the left side fuel quantity the crew discussed potential of engine fuel leak and the Captain called for the 'Fuel Leak Engine' checklist after noting a decrease in the left side fuel quantity and the fuel configuration EICAS message. The Captain also requested me to coordinate a level off at 18000 feet. I ran the checklist and following crew discussion we decided the most appropriate response; considering the 2300 pound imbalance in less than 30 minutes; should be considering this an engine fuel leak.Per the Captain's request I coordinated a turn back to ZZZ while the Captain sent a message to Dispatch notifying them of our [return to departure airport]. The left engine was secured in accordance with the checklist and [ATC was advised]. We continued the checklist; began our descent and coordinated an arrival via the ILS XXR to ZZZ. We reviewed the checklist and set up for the ILS approach. The approach and landing checklist was completed and we landed without further incident. Once coming to a stop Tower requested we remain on the runway while crash fire rescue inspected the left engine/wing. No leak was noted and we received clearance to taxi to [the] ramp. We blocked in and completed the 'Shutdown and Secure' checklist. I do not recall what the final fuel imbalance was upon shutdown; the Captain recalls approximately 6 to 800 pounds which makes sense considering the 20 minutes of single engine operations.


Synopsis: B757-200 flight crew reported fuel leak shortly after takeoff.