AN ACR DC9 FLC EXPERIENCED UNCOMMANDED RUDDER MOVEMENT THAT; APPARENTLY; ORIGINATED FROM THE YAW DAMPER.

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RUDDER OSCILLATIONS THAT REQUIRED US TO TURN OFF YAW DAMPER AND POWERED RUDDER. REFERRED TO COM (COCKPIT OPERATING MANUAL) AND PERFORMED APPROPRIATE PROC; DISCUSSED SIT WITH DISPATCHER AND COMPANY MAINT. CONTINUED FLT AS SCHEDULED WITHOUT FURTHER PROBS. DISCUSSED EVENT WITH HEAD OF SAFETY DEPT AND BASE CHIEF PLT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT WAS FLYING A DC9-30 WHEN THE RUDDER MOVED UNEXPECTEDLY. THE AFT CABIN ATTENDANT RPTED THE MOVEMENT AS THE CREW STARTED THROUGH THEIR UNCOMMANDED CTL MOVEMENT CHKLIST. LATER SHE CAME TO THE FLT STATION AND TOLD THEM OF OTHER MOVEMENT WHEN THE CREW WAS FOLLOWING SOME TROUBLESHOOTING SUGGESTIONS FROM THEIR MAINT CTL PERSONNEL AND HAD TURNED ON THE YAW DAMPER AND RUDDER HYD PWR. THE UNUSUAL MOVEMENT STOPPED WHEN THE CREW TURNED OFF THE YAW DAMPER AND THE HYD PWR TO THE RUDDER. THEY INFORMED THE DISPATCHER; MAINT CTL AND ARTCC AND CONTINUED TO DEST USING MANUAL RUDDER CTL. A LARGE GROUP OF COMPANY AND EMER PERSONNEL MET THE ACFT AFTER THEY ARRIVED AND AT THAT TIME THEY DISCOVERED THAT THE CTLR HAD DECLARED AN EMER FOR THEM. THE MAINT PERSONNEL COULD NOT SAY WHAT SPECIFIC FAILURE CAUSED THE MOVEMENT; THEY WERE SURE THAT IT WAS A YAW DAMPER COMPONENT.