Date: 201702
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
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: Fuel Quantity-Pressure Indication
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting; Workload; Confusion; Time Pressure
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Fuel Issue
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; Flight Crew Diverted; Flight Crew FLC complied w / Automation / Advisory; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: After level off at FL330; turned off seat belt sign; and made PA. Shortly after; FO noticed a fuel imbalance of about 2;000 lbs and noted left main tank fuel indicator decreasing approximately 100 lbs every 30-60 seconds. Center tanks empty and had secured center tank pumps earlier. Ran QRH fuel leak checklist and discovered cross feed valve open. Closed cross feed valve and noted fuel continued to decrease at the same rate. Discussed diversion airports with FO. Handed over ATC comms to FO and instructed her to start towards divert airport. Weather was clear. Notified FAs diverting for possible fuel leak; 15 mins to landing; no special instruction or evacuation preparations; turned on seatbelt sign; made PA; sent ACARS message to dispatch; and asked commuting pilot in the cabin to visually inspect left engine/wing for a fuel leak. They reported back no leaks noted. Continued QRH Fuel Leak checklist down to Engine Shutdown. I paused and handed the QRH to FO and took over Pilot Flying duties. Reviewed current ATC clearance and distance from the divert airport. FO re-ran the QRH Fuel Leak checklist. Slowed aircraft to 250 KIAS; then 220 KIAS while following vectors and attitude assignments from ATC and continued to diagnose fuel leak. Left main tank fuel indications continued to decrease at the same rate. ATC relayed current airport weather. Paused FO with QRH at Engine shutdown to brief visual approach backed up by ILS; FMC programming; and setup flight instruments and frequencies. We also briefed Engine Out approach which required an ATC delay vector to accomplish this then continued vectors to intercept ILS. Configured and intercepted LOC/GS and proceeded to runway visually. Continued QRH with securing the left engine and the rest of the QRH. FO was most of the way thru the One Engine In-Operative Landing checklist (stopped at 'Wing Anti-Ice Not Required') but due to proximity to the airport (1500 AGL) I elected to continue approach and landing and forego the rest of that checklist. Wanted to concentrate on safely landing the aircraft. Landed without incident; taxied clear and stopped for Fire/Rescue to inspect aircraft. Received all clear and taxied to gate. Fire/ Rescue recommended we deplane the aircraft at the gate as Maintenance inspected the aircraft. Complied and debriefed with maintenance and dispatch. Aircraft inspected by Maintenance; cleared; and returned to service. Deadheaded on the same aircraft for the next flight. After landing; follow-on pilots noted similar fuel decrease on left main tank indicator on climb out. The fuel issue written up and aircraft taken out of service.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 flight crew reported a fuel leak was discovered in flight and the crew elected to divert. Maintenance determined an aircraft fuel quantity indication anomaly was the cause.