Narrative:

I was the flying pilot on flight X. We released brakes with 21;300 lbs of fuel and taxied out uneventfully. Taxiing on to the runway fuel quantity was still normal. After departure and climbing out of 10;000 I turned the autopilot on and looked down noticing the left main tank was now indicating approximately 7;700 lbs and we had a fuel imbalance message. We were still operating on center fuel; and the right main tank still indicated 8.7. I started watching the quantity closely and did not notice any drop in left main tank quantity while we burned the remainder of the center fuel. I was uncertain if we had a leak or just an indication problem. We subtracted fuel burned off of the ff (fuel flow) meter from what we started with and there was a difference of about 1;000 lbs. I contacted dispatch on arinc and asked to conference [maintenance control] for any history of fuel quantity errors. I was closely monitoring the fuel quantity to see if it was dropping faster than the right main tank; and it had seemed to stabilize; and actually the difference between tanks decreased on the quantity gauge to about 900 lbs. That was short lived though and while I was on the radio with dispatch it seemed that the fuel quantity in left main started to decrease more quickly again. The difference got to 1;400 lbs. The first officer (first officer) and I had already discussed diversion options; and we both agreed based on our present position that ZZZ seemed the best option. I quickly decided based on the fact that our route of flight was going to take us out over the water after ZZZ1; that it would not be wise to continue on. At that time we requested a divert to ZZZ and began a turn. We alerted ATC and got cleared directly to ZZZ. Upon landing in ZZZ; crash fire rescue equipment (crash fire rescue) met the airplane and advised us that they detected a leak; and to shut down. We started the APU; and shut the engines down. They still detected a leak and I turned off all fuel and hydraulic pumps. They no longer detected any leaking fluids; and inspected the fluids that had already leaked out. Crash fire rescue equipment advised us that the fluid was not a 'hazmat' fuel or hydraulic fluid; and it should be safe to start up and taxi in. After start up they detected another leak; and we shut down and got towed in. I debriefed the mechanics after the passengers deplaned and gave them the fuel upload slip from [the departure airport]. We did the math and could not account for about 1;200 lbs of fuel.I feel like for the most part we handled the situation well; but I did identify some errors in the way I managed the diversion. I initially handed the aircraft to the first officer to have him fly while I notified the flight attendants and passengers what was going on. After that I resumed the flying pilot duties; and that was a mistake. My mind was moving rapidly; and I still had coordinating and communicating left to do. I always brief prior to takeoff that if we have a situation; once things are stabilized that I will hand the aircraft to the first officer so I can manage the situation. I should have stuck to that. I noticed that I was trying to do too much; fly and coordinate. As soon as I realized that; I handed the aircraft back to the first officer and let him fly the remainder of the flight so I could make decisions and communicate them with everyone that I needed to. Once we did that; things went much more smoothly.

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Original NASA ASRS Text

Title: B737 Captain reported encountering a fuel anomaly in-flight.

Narrative: I was the flying pilot on flight X. We released brakes with 21;300 lbs of fuel and taxied out uneventfully. Taxiing on to the runway fuel quantity was still normal. After departure and climbing out of 10;000 I turned the autopilot on and looked down noticing the left main tank was now indicating approximately 7;700 lbs and we had a fuel imbalance message. We were still operating on center fuel; and the right main tank still indicated 8.7. I started watching the quantity closely and did not notice any drop in left main tank quantity while we burned the remainder of the center fuel. I was uncertain if we had a leak or just an indication problem. We subtracted fuel burned off of the FF (Fuel Flow) meter from what we started with and there was a difference of about 1;000 lbs. I contacted dispatch on ARINC and asked to conference [Maintenance Control] for any history of fuel quantity errors. I was closely monitoring the fuel quantity to see if it was dropping faster than the right main tank; and it had seemed to stabilize; and actually the difference between tanks decreased on the quantity gauge to about 900 lbs. That was short lived though and while I was on the radio with Dispatch it seemed that the fuel quantity in left main started to decrease more quickly again. The difference got to 1;400 lbs. The FO (First Officer) and I had already discussed diversion options; and we both agreed based on our present position that ZZZ seemed the best option. I quickly decided based on the fact that our route of flight was going to take us out over the water after ZZZ1; that it would not be wise to continue on. At that time we requested a divert to ZZZ and began a turn. We alerted ATC and got cleared directly to ZZZ. Upon landing in ZZZ; CFR (Crash Fire Rescue) met the airplane and advised us that they detected a leak; and to shut down. We started the APU; and shut the engines down. They still detected a leak and I turned off all fuel and hydraulic pumps. They no longer detected any leaking fluids; and inspected the fluids that had already leaked out. CFR advised us that the fluid was not a 'hazmat' fuel or hydraulic fluid; and it should be safe to start up and taxi in. After start up they detected another leak; and we shut down and got towed in. I debriefed the Mechanics after the passengers deplaned and gave them the fuel upload slip from [the departure airport]. We did the math and could not account for about 1;200 lbs of fuel.I feel like for the most part we handled the situation well; but I did identify some errors in the way I managed the diversion. I initially handed the aircraft to the FO to have him fly while I notified the flight attendants and passengers what was going on. After that I resumed the flying pilot duties; and that was a mistake. My mind was moving rapidly; and I still had coordinating and communicating left to do. I always brief prior to takeoff that if we have a situation; once things are stabilized that I will hand the aircraft to the FO so I can manage the situation. I should have stuck to that. I noticed that I was trying to do too much; fly and coordinate. As soon as I realized that; I handed the aircraft back to the FO and let him fly the remainder of the flight so I could make decisions and communicate them with everyone that I needed to. Once we did that; things went much more smoothly.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.