Date: 201912
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Aircraft: B737-900
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: Powerplant Fuel Distribution
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot 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 Inflight Shutdown; Flight Crew Landed in Emergency Condition; Flight Crew Diverted; General Flight Cancelled / Delayed
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Flight departed ZZZ1 under normal winter conditions after deicing. Our fuel amount was above planned minimum fuel at takeoff. The FO (First Officer) was the PF (Pilot Flying). The CA (Captain) was the PM (Pilot Monitoring). The flight was proceeding without incident and all fuel indications were normal until the Captain was preparing to shut off the center fuel pumps. He noticed a fuel imbalance of 300-400 lbs. The CA corrected the fuel imbalance. It then became imbalanced again in a relatively short amount of time. It was at this time that we both observed a drastic drop in fuel on the left main tank. It showed a loss of approximately 500 lbs. in about 3 minutes. I know that fuel fluctuations can be normal when the center fuel pumps are shut off but this seemed excessive. The rate of fuel loss from the left tank seemed to slow after that initial drop but we were closely monitoring the amounts and observed an imbalance of approx. 900 lbs. Once the fuel amount on the left tank stabilized we started discussing the possible cause of the fluctuations. And it was at that time that we started watching the total fuel burns more intently. Prior to the all of this happening; the recorded fuel burn at ZZZ2 on the flight plan was 400 lbs. over scheduled burn which seemed slightly unusual for the conditions but as we watched the FMC planned burn at ZZZ3 we realized that we were going to be over the scheduled burn by almost 2;000 lbs.; which was excessive. According to the scheduled flight plan fuel burn; the fuel amount at ZZZ3 should have been 15.4. We crossed ZZZ3 with 13.5 indicated. We believed that we could have a fuel leak due to the amount of fuel loss in such a short amount of time and the fact that the total amount of fuel onboard was much lower than planned.The CA went into the QRH and went to the Fuel Leak - Engine checklist. We agreed that we met the criteria for the checklist and continued. The CA proceeded with the checklist; talked to the FAs (Flight Attendants); and contacted Dispatch and Maintenance. We asked the FAs to check for evidence of a fuel leak. The FA said they saw moisture on the left wing; but not the right wing; and the FA did not see mist coming from the wing. The CA's wife was also onboard the plane. She is a professional pilot so we asked her to check for evidence of a fuel leak. She reiterated what the FA said about moisture on the wing; but no mist spotted. While the CA was doing that; I was pulling the weather for ZZZ4 and ZZZ. Both airports looked good but the nearest airport was ZZZ. I selected the frequency and course for the runway listed on ATIS in case we decided to go there. We proceeded with the checklist. The CA requested a descent to our drift down altitude of 22;000 ft.; briefed the FAs; advised the passengers; requested priority handling; pulled landing data for ZZZ; and shut down the left engine per the checklist. I was flying the aircraft and talking to ATC when needed.While all this was going on we were trying to communicate with Dispatch. They were asking a lot of questions via ACARS; we were very busy; and it seemed quicker to ask them to contact us via the ZZZ Operations frequency since the aircraft didn't have SATCOM. This proved difficult because we couldn't get a hold of ZZZ Operations and Dispatch couldn't hear us reply on that frequency. We finally talked to Operations once we were much closer but we never had a chance to talk to Dispatch in the air.I landed the aircraft single engine and slightly overweight at approximately 146;000 lbs. for a Flaps 15 landing; normally max of 144;200. The landing was uneventful and at a normal rate of descent. We taxied to the gate under our own power.


Second narrative: Flight XXX departed ZZZ under normal winter conditions after deicing. Our fuel amount was above planned minimum fuel at takeoff. The FO was the PF. The CA was the PM. The flight was proceeding without incident and all fuel indications were normal until the captain was preparing to shut off the center fuel pumps. He noticed a fuel imbalance of 300-400 lbs. The CA corrected the fuel imbalance. It then became imbalanced again in a relatively short amount of time. It was at this time that we both observed a drastic drop in fuel on the left main tank. It showed a loss of approximately 500lbs in about 3 minutes. I know that fuel fluctuations can be normal when the center fuel pumps are shut off but this seemed excessive. The rate of fuel loss from the left tank seemed to slow after that initial drop but we were closely monitoring the amounts and observed an imbalance of approx. 900lbs. Once the fuel amount on the left tank stabilized we started discussing the possible cause of the fluctuations. And it was at that time that we started watching the total fuel burns more intently. Prior to the all of this happening; the recorded fuel burn at ZZZ2 on the flight plan was 400 over scheduled burn which seemed slightly unusual for the conditions but as we watched the FMC planned burn at ZZZ3 we realized that we were going to be over the scheduled burn by almost 2;000 lbs; which was excessive. According to the scheduled flight plan fuel burn; the fuel amount at ZZZ3 should have been 15.4. We crossed ZZZ3 with 13.5 indicated. We believed that we could have a fuel leak due to the amount of fuel loss in such a short amount of time and the fact that the total amount of fuel onboard was much lower than planned.The CA went into the QRH and went to the Fuel Leak - Engine checklist. We agreed that we met the criteria for the checklist and continued. The CA  proceeded with the checklist; talked to the FAs; and contacted Dispatch and Maintenance. We asked the FAs to check for evidence of a fuel leak. The FA said they saw moisture on the left wing; but not the right wing; and the FA did not see mist coming from the wing. While the CA was doing that; I was pulling the weather for ZZZ2 and ZZZ1. Both airports looked good but the nearest airport was ZZZ1. I selected the frequency and course for the runway listed on ATIS in case we decided to go there. We proceeded with the checklist. The CA requested a descent to our drift down altitude of 22;000; briefed the FAs; advised the passengers; requested priority handling; pulled landing data; and shutdown the left engine per the checklist. I was flying the aircraft and talking to ATC when needed.While all this was going on we were trying to communicate with Dispatch. They were asking a lot of questions via ACARS; we were very busy; and it seemed quicker to ask them to contact us via the Ops frequency since the aircraft didn't have SATCOM. This proved difficult because we couldn't get a hold of Ops and Dispatch couldn't hear us reply on that frequency. We finally talked to Ops once we were much closer but we never had a chance to talk to Dispatch in the air.I landed the aircraft single engine and slightly overweight at approx 146;000 lbs for a Flaps 15 landing; normally max of 144;200. The landing was uneventful and at a normal rate of descent. We taxied to the gate under our own power.


Synopsis: Flight Crew reported increasing fuel loss in flight; which resulted in an engine shutdown and diversion.