What was found
RECEIVED UNCOMMANDED YAW INPUT WHICH RESULTED IN AUTOPILOT DISENGAGEMENT. *S/D* YAW DAMPER ACTUATOR HAS BEEN ISOLATED AS THE PROBLEM. THIS UNIT HAS NOT BEEN REPLACED ON THE AIRCRAFT AS OF 2/21/99. BUT THERE IS NO UNCOMMANDED YAW INPUT WITH THE YAW DAMPER NR 1 DISCONNECTED AND THE HYD LINES CAPPED AND STOWED. THIS UNIT IS A KNOWN PROBLEM UNIT, ESPECIALLY ON THE NR 1 SIDE (LEFT). THE LEFT SYSTEM SEES A HIGHER CYCLE RATE FROM THE HYD SYSTEM AS CONFIRMED ON AIRBUS MESSAGE 953.0811/99.