What was found
TRIP NR 01787, AIRCRAFT VIBRATING AFTER 290 KTS (SUSPECT RUDDER) WITH AUTO-PILOT ON OR OFF, YAW DAMPER ON OR OFF. VIBRATION INCREASED WITH RUDDER HYDRAULIC POWER IN OFF POSITION. REMOVED AND REPLACED TAB ASSY, RUDDER OPS CHECK ON GROUND GOOD IAW DC8 MM 27-21-2.