What was found
(CAN) DURING A TRAINING FLIGHT AND FOLLOWING TAKEOFF THE RUDDER TRIM MOVED TO THE FULL LT TRIM POSITITION UNCOMANDED. THE AIRCRAFT RETURNED AND CONDUCTED AN OUT OF TRIM LANDING. TROUBLESHOOTING LATER THE FAULT WAS DUPLICATED ON THE GROUND. FOLLOWING THE AUTOPILOT PREFLIGHT TEST THE RUDDER TRIM WOULD RUNAWAY TO FULL LT POSITION. INVESTIGATION SHOWED THE FAULT TO BE ISOLATED TO THE AUTOPILOT YAW SERVER WHICH WAS INCORECTLY SIGNALING THE AUTO TRIM TO MOVE. THE SERVO WAS REPLACED.