What was found
(CAN) IN CRUISE FLIGHT, AT FL360 WITH THE A/P ON, THE ACFT HAD AN UN-COMMANDED RUDDER TRIM GOING FULL LEFT. THE FLIGHT CREW RETURNED THE RUDDER TRIM TO NEUTRAL AND THERE WERE NO FURTHER ISSUES. MAINTENANCE INVESTIGATION FOUND FROM THE FDR READOUT THAT THE RUDDER TRIM WAS COMMANDED ELECTRICALLY AND THAT THERE WERE CORRESPONDING CHANGES IN HEADING AND THAT THE PILOT INPUT OPPOSITE RUDDER PEDAL TO COUNTERACT THE TRIM MOVEMENT. ALTHOUGH THE DEFECT COULD NOT BE DUPLICATED ON THE GROUND IT IS STRONGLY SUSPECTED THAT THE RUDDER TRIM SWITCH STUCK AFTER A SMALL PILOT INPUT CAUSING THE TRIM TO RUN FULL TRAVEL. THE AILERON/RUDDER TRIM PANEL, PN CC670-51114-3 WAS REPLACED AND THE SYSTEM TESTED SERVICEABLE.