Air carrier flight crew reported a fuel imbalance in flight was misdiagnosed to be a fuel leak and performed an inflight shut down and diversion to a precautionary landing.
Synopsis
Air carrier flight crew reported a fuel imbalance in flight was misdiagnosed to be a fuel leak and performed an inflight shut down and diversion to a precautionary landing.
Narrative
I operated Aircraft X ZZZ-ZZZ1 on DATE with a suspected fuel leak that resulted in an engine shutdown and diversion to ZZZ2.I was the Pilot Monitoring and the First Officer was the Pilot Flying on this flight. Around the ZZZ3 area the left center tank low pressure lights flickered and I turned the left center pump off with about 400 pounds remaining in the center tank. I didn't see a fuel imbalance between the left and right tanks at this time. Several minutes later the center fuel tank was empty and I turned the right center pump switch off. In addition at this time I didn't notice anything abnormal about the cross feed valve selector or light.At approximately 70 miles SE of ZZZ2; I heard the First Officer say we had a fuel imbalance and looked over to see the fuel IMBAL light illuminated on the right fuel tank. At this point I noted a 1;000 pound fuel imbalance and that in my experience the fuel in the right tank was decreasing at an abnormally fast rate. It had only been around 10 minutes since I turned off the center fuel tanks. At this point the First Officer and myself thought a fuel leak was plausible due to a 1;000 pound fuel imbalance occurring in around 10 minutes and observing an abnormally high rate of fuel decreasing from the right tank. I proceeded to run the fuel leak-engine QRH and contacted the flight attendants to request one of them check for a fuel leak/mist coming from the back of the right (#2) engine. While running the QRH I felt time pressures to stop the fuel imbalance before it led to adverse control issues. At step 5 I recorded the total fuel and time (I don't remember what I recorded) and proceeded to the condition statement in step 6. After reading the condition statements and based on the abnormally high rate of fuel decrease in the right tank I proceeded to step 7. At this point I thought I confirmed an engine fuel leak because we were now at 1;200 pounds imbalance in 10-15 minutes. Far greater than the 500 pounds in less than 30 minutes that the QRH states. By this time the FA reported not seeing any fuel leaking from behind the right (#2) engine. Knowing that we were going to shut down the #2 engine I requested priority handling and requested a lower altitude. In addition we requested vectors to ZZZ2. By the time we shut down the #2 engine we had a 1;400 pound fuel imbalance. I'd like to add that after shutting down the #2 engine the QRH calls for the cross feed selector to be opened. The cross feed valve opened normally with no abnormal indications and closed normally with no abnormal indications several minutes later when we decided to even out the imbalance by burning fuel from the left tank.We completed the QRH; received landing data from dispatch and briefed the FAs and PAX. Finally we were vectored to an ILS XXR in visual conditions. On the ground in ZZZ2 we waited 30-45 minutes for Customs to arrive and spoke to the PAX about the delayed deplaning. I don't have any additional information to add to the ASAP report than what I reported above in the event narrative.While rereading the Fuel leak-engine QRH step 6 after the event; I realize that my decision to proceed to step 7 was based on what I read the step 6 condition statement to say of 'the fuel quantity is decreasing at an abnormal rate out of the right tank'. Rather than basing it on what the condition statement actually said 'or the total fuel quantity is decreasing at an abnormal rate'. This was a mistake.I can say that the fuel imbalance QRH checklist would probably have been more appropriate to call first. However this wasn't an imbalance that took time to develop. We experienced the fuel IMBAL light and an abnormally high rate of fuel decrease from the right tank around 10 minutes after turning the center pumps off. I didn't notice any fuel imbalance prior to turning the center tanks off. I don't think that it was unreasonable to run the fuel leak-engine QRH in these circumstances.
Second reporter narrative
I operated Aircraft X from ZZZ-ZZZ1 on DATE and was Pilot Flying. Northbound; about 70 miles SE of ZZZ2 and approximately 10 minutes after the Captain had turned off the center tank fuel pumps; I noticed a fuel imbalance; and the right fuel quantity indication was amber. I informed the Captain that we had a fuel imbalance. While looking at the fuel quantity; I noticed that quantity in the right tank was decreasing at a higher-than-normal rate; and the fuel imbalance was approximately 1;000 pounds. The Captain pulled out the QRH. I also may have verbalized 'do we have a fuel leak;' or 'is this a fuel leak.' This was probably confirmation bias on my part. I recently completed my annual simulator training; about 2.5 months ago. The scenario that I had on day 3 was depart ZZZ4 for ZZZ5. During that sim session we had a fuel imbalance shortly after takeoff; which was actually a fuel leak. So; the scenario we were experiencing in the airplane seemed similar to a recent training event. Since the Captain and I both thought a fuel leak was possible; he started to run the Fuel Leak checklist in the QRH. The Captain also contacted the flight attendants and asked them to check the right wing and engine for any visible fuel spray. The flight attendants reported back to us that they didn't see anything. However; based on the fuel imbalance rapidly getting worse; exceeding the 500 pounds within 30 minutes the Captain and I confirmed a fuel leak. We requested priority handling; lower altitude requested; as well as vectors for ZZZ2. Once on a localizer intercept vector; the Captain took controls and landed the airplane. We taxied clear of the runway; and emergency personnel (crash fire and rescue) visual inspected the airplane to make sure fuel was not leaking from the aircraft. We then taxied towards the gate. Since I was the Pilot Flying; and the first person to notice the imbalance I should have asked for imbalance checklist first.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.