B737-800
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Fuel Issue
General None Reported / Taken
Aircraft
We experienced an unwanted fuel transfer (center to wing tank) on B737. [A maintenance report] entry was completed. We started with 8.65 in the left tank; 5.24 in the center and 8.60 in the right tank. As we taxied for takeoff; the center tank burned normally; the right tank remained at 8.60 and the left tank had increased to 8.70. During the climb; the left tank continued to climb to 8.79 and then stopped; the right tank remained at 8.60 and the center tank was draining at a rate that exceeded our two-engine fuel burn. We determined unwanted fuel transfer was occurring from the center tank to the left tank and the left tank had exceeded capacity; which led to venting of fuel overboard through the fuel vent system. This anomaly of unwanted fuel transfer from the center to the wing tank is insidious and mostly unrecognizable. The first procedural recognition by the flight crew would be the realization of less than flight plan fuel quantity at the top of climb during the cruise check. By that time; the center tank is near completion of its burn and the fuel loss would have been complete. On an individual flight; the loss of 1;000-1;500 pounds of fuel could cause a flight to land below the alternate or reserve fuel; even possibly cause a diversion or return. However; the overlying issue is the long-term loss (or venting) of fuel for the duration of this problem. With this aircraft averaging five flights per day; I calculate the loss to equate to 52;500 pounds per week or 230;000 pounds per month. If this is occurring with merely 20 aircraft in the fleet; we are venting 7.9 million gallons of fuel insidiously overboard annually increasing the fleet operation cost roughly $40 million per year. The recommended corrective action is to add a QRH checklist for 'Main tank fuel quantity increase with center tank fuel.' The steps would include turning off the associated center tank pump to maintain wing tank fuel balance. The most appropriate time to check for an increase in main tank fuel is passing 10;000 feet during the climb phase of flight. The crew should also make an associated [maintenance] write-up for unwanted fuel transfer from the center to the wing tank.
B737-800 Captain reported experiencing unwanted fuel transfer from the center tank to the wing tanks; resulting in fuel being vented overboard.