B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 220; Flight Crew Total 17000; Flight Crew Type 10000
Aircraft Equipment Problem Critical; Inflight Event / Encounter Fuel Issue
Flight Crew Returned To Departure Airport
Aircraft
Fuel imbalance at 390; center tank feeding engines; rapid fuel loss in left tank.  Rapid fuel loss only slowed down when the left thrust lever brought to idle.  We returned to our departure airport.
Upon leveling off at FL390 we received a fuel imbalance alert. There was 3;500 LBS in center tank; 8;500 in right main and about 7;400 in left main. The aircraft was configured to be using fuel from the center tank but the left main was decreasing. We then set up for crossfeed by leaving the crossfeed open and turning the left main pumps off. The fuel amount continued to decrease in the left main at a good rate. Referencing the QRH; we found engine shut down procedures and troubleshooting to discover where leek is. We were confident the problem was in the left main [tank] and did not want to exacerbate the situation in any way. Deciding to return with a constant descent we brought left [engine] to idle and used thrust from only the right [engine] as needed. This was the only
B737 crew experiences a fuel imbalance upon leveling off at FL390 while feeding both engines from the center tank; with the left tank being low. With the cross feed open and the left tank pumps off the imbalance continues to increase. Flight returns to departure airport.