What was found
FUEL CROSSFEED VALVE TRIPPED IN FLIGHT. CROSSFEED VALVE HAD BEEN CLOSED AND HAD NOT BEEN OPENED BY PILOTS. WHEN CENTER TANK EMPTIED OF FUEL, IMBALANCE BEGAN TO BE EVIDENT. SURMISED THAT VALVE HAD OPENED AND WAS NOW STUCK OPEN. BALANCE MAINTAINED BY SELECTIVE USE OF MAIN TANK BOOST PUMPS. RESET CIRCUIT BREAKER REF AMM 20-60-01-860-801, PERFORMED OPS CK OF CROSS FEED VALVE, REF AMM 28-22-21-701-801. CROSSFEED OPS NORMAL INSP CONNECTOR AND WIRING NO FAULTS NOTED. GROUND VERIFICATION SUCCESSFUL. TSR ISSUED. REPLACED FUEL CROSSFEED VALVE ACTUATOR REF AMM 28-22-21-000-804 & 28-22-21-400-804. OPS CHECK GOOD REF AMM 28-22-21-710-801.