Date: 201912
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Center ZZZ
Aircraft component: Powerplant Fuel Distribution
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
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 Diverted; Flight Crew Landed in Emergency Condition; Flight Crew Inflight Shutdown; General Flight Cancelled / Delayed
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I first noticed something awry as the center pumps were nearing empty. The mains should have been perfectly balanced; yet they weren't. When the center tank was empty; I did a quick crossfeed to balance the 3 to 400 pound imbalance.Soon after; we crossed the first fix on our fuel/position estimates printout. We were 2 minutes early; 400 pound over burn. Usually; our flight plans are more accurate than that. We had leveled off at our first cruise altitude; but already we were ready to climb to our final cruise; so we requested and received clearance to climb. The mains indicated another imbalance occurring; approximately 500 pounds. We leveled off again; but my concern over the situation was growing slowly - I balanced the mains; and then watched the results carefully. After going back to four pumps on; crossfeed closed; the left main dropped 500 pounds in approximately 3 minutes. At this; I was on high alert. I pulled out the QRH; and began going through the checklist. I didn't want to take action too quickly; so I stopped short of taking any irreversible action for a few minutes.I called the FAs; and requested they look for signs of fuel leak. They didn't observe any misting behind the engine; but they did see liquid around the engine pylon; and at the trailing edge of the flaps. This was inconclusive; since we'd deiced fully (type 1 and 4) before takeoff.The initial drop to 500 pound imbalance tapered off; but continued; so over the next ten minutes or so; the imbalance reached 900 pounds. We were approaching our next fix on the plan; and we were now 1900 pounds over expected fuel burn.At this point; I decided it was time to commit to the checklist; which required shutting down the leaking engine; and divert. Company outstation was convenient at that point in time; so we made that the plan. I shut the engine down per the QRH; and walked the panel -- Dispatch; FAs; and passengers.Communication was difficult as we descended to our drift down altitude. We were in mountainous terrain; and had trouble reaching the Local Approach Controller; as well as raising Operations on company frequency. I didn't have the chance to try DTMF microphone before were able to get back in contact with ATC; and this point; I was too busy to get back to Dispatch over the radio; when the essentials of re-dispatch had already been accomplished.We landed slightly overweight; approximately 46;000 pounds; with a flaps 15 max of 44;200. The landing was smooth; with a minimal sink rate. I think the whole situation proceeded very smoothly. It's a very busy checklist; so I did my best to take my time; and get everything done properly. By the time we landed; I felt we were ready and well-prepared; rather than rushed or stressed.




Synopsis: Captain reported unusual fuel loss during flight; requiring an inflight engine shutdown and diversion.