Flight Crew reported a fuel imbalance that increased in flight. The Flight Crew ran the checklists and QRH. An in flight shut down of the left engine was accomplished and a request for priority handling was made. The Flight Crew elected to divert and make a precautionary landing.
Synopsis
Flight Crew reported a fuel imbalance that increased in flight. The Flight Crew ran the checklists and QRH. An in flight shut down of the left engine was accomplished and a request for priority handling was made. The Flight Crew elected to divert and make a precautionary landing.
Narrative
When we boarded the plane in ZZZ1 we noticed that the night before the plane had been written up for a large fuel imbalance during the flight that night. The write up had been checked and signed off so I noted the issue but felt since it had been signed off that it was safe to fly. We then departed for ZZZ2 with no issues during departure. During cruise approximately 1:30 into the flight; I noticed a 500 pound imbalance in the left wing from the right wing while still using center tank fuel. We discussed the possibility of a fuel leak since a similar write up had been made on the plane the night before. We had received a 'using reserve fuel' on the FMC and the left side wing tank had 8.12 versus the right side of 8.52. The center tank had approximately 1.1-1.4 and since we were still using center tank fuel we knew that the left side should not have been reduced at that point. We talked with the Flight Attendants and had them inspect the wing for any fuel that could be coming out of the wing or engine spar. They reported that nothing appeared to be coming out of the wing. We then contacted Dispatch and Maintenance Control and felt it was best to divert to ZZZ. After beginning to divert we used the remaining center tank fuel at which point the left wing fuel began to drastically drop (approximately 400 pounds in about 5 minutes). At this point we decided to run the fuel leak checklist which recommended that we shut down the engine. We did that and requested priority handling. We then ran the appropriate engine out checklists and proceeded to land in ZZZ with no further incident.
Second reporter narrative
The aircraft had overnighted in ZZZ1 and had been written up and serviced for a fuel imbalance on the inbound flight. The logbook had been closed out properly and signed off. We departed out of ZZZ1 without any issues and climbed to FL350. While at cruise (approximately 1.5 hours into flight) the Captain noticed we had a fuel Imbalance while we were still operating out of the center wing tank. We had received a 'using reserve fuel' on the FMC. The left side wing tank had 8.12 versus the right side of 8.52. center tank had approximately 1.1-1.4. We started discussing the possibility of us losing fuel on the left side. I began logging our fuel imbalance. XA55 - LT 8.12/ RT 8.52. We ensured that the appropriate fuel pumps were on and that the cross feed was closed. We coordinated with the Flight Attendants and had them inspect the wing for any fuel that could be coming out of the wing or engine spar. They reported that nothing appeared to be coming out of the wing. At this point we were monitoring the fuel imbalance and began looking at the flight manual for a potential fuel leak checklist as well as considering drift down altitudes and diversion options and contingencies. By XB11 our fuel imbalance was LT 8.01/RT 8.51. At this point we had used all center wing tank fuel and had started operating from the wing tanks. The left tank fuel was leaving the tank distinctly faster than the right tank and both engine fuel flow rates were relatively similar and normal. XB15 - LT 7.85/ RT 8.28. By this point we had already made the decision to divert to ZZZ. While heading south we separated duties. I flew while the Captain coordinated with Dispatch and Maintenance Control. In their discussion they had recommended we follow the appropriate QRH. XB24 - LT 7.34/ RT 7.74. We remained at FL350 while preceding south to ZZZ until we got to the point in the checklist where we needed to shutdown and secure the left side engine for a suspected fuel leak. The Captain continued to run the checklist and set us up for the single engine approach and landing. We coordinated with ZZZ ATC and requested priority handling and proceeded in through the arrival (originally cleared ZZZZZ 3 arrival). As we got closer to ZZZ we got assigned direct to the field and then vectors for a left downwind for XXL. We transferred fight controls so the Captain could make the landing. The First Officer (FO) completed the remainder of the checklist; securing the pneumatics and the deferred landing checklist. The approach was visual and the landing was safe and without incident. We stopped off the runway at XX and coordinated with CFR and had our left engine inspected for any fuel leakage before proceeding to the gate without any further incident.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.