What was found
DURING DESCENT, AUTOPILOT BEGAN TO ASCILLATE (ROLL). DISENGAGED THE AUTOPILOT. AFTER LEVELING AR 15,000, WE ATTEMPTED TO RE-ENGAGE THE AUTOPILOT, BUT WERE UNABLE. CONCURRENTLY, THE "YD FAIL LOWER B" CAS MESSAGE WAS OBSERVED. AUTOPILOT WOULD NOT ENGAGE. ON THE GROUND, ATTEMPTED TO SWITCH FGCS, BUT WERE UNABLE. "YD FAIL LOWER B" MESSAGE REMAINED ILLUMINATED. REMOVED AND REPLACED AILERSON SERVO DRIVE ASSY 10. REMOVED AND INSTALLED "INSPECTED" SERVO DRIVE ASSY IAW MM SEC 22-10-01 REV 38. P/T DRIVE. F/TED OK. INSTALLED PANEL REMOVED FOR ACCESS.