As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ENGAGING THE AUTOPILOT CAUSED AN UN-COMMANDED AND ERRATIC NOSE UP PITCH MOVEMENT. THE AUTOPILOT WAS DISCONNECTED AND AIRCRAFT RESPONDED TO HAND TRIMMING TO STABILIZE. THE INITIAL TAKEOFF HAD A STARK NOSE UP PITCH, WHICH WE TRIMMED OUT BY HAND. THEN APPROACHING OUR INITIAL LEVEL OFF ALTITUDE, WE ENGAGED THE AUTOPILOT. THE AIRCRAFT IMMEDIATELY AND RAPIDLY BEGAN TO PITCH UP AGAIN, LEADING TO US DECIDING TO TAKE IT BACK TO BUR DUE TO CONCERN OF THE TRIM OF THE PLANE.