B747-200 CREW HAD THE LOWER YAW DAMPER MALFUNCTION.

Narrative

DURING CLBOUT TO FL350 WITH THE FO FLYING AND AUTOPLT B ENGAGED; I NOTICED THE AUTOPLT REQUIRING ABOUT A 10 DEG R WING DOWN DEFLECTION TO MAINTAIN COURSE. THE LOWER RUDDER INDICATOR WAS SLIGHTLY OUT TO THE L. I ASKED THE FO TO CHK THE TRIM OF THE ACFT WHICH SHE DID BY DISCONNECTING THE AUTOPLT AND TRIMMING RUDDER. THE ACFT REQUIRED APPROX 1 1/2 UNITS OF L RUDDER TRIM TO MAINTAIN WINGS LEVEL. SHORTLY AFTER THE FO REENGAGED THE B AUTOPLT; WE EXPERIENCED A SIGNIFICANT AND UNCOMMANDED YAW. I TOOK THE ACFT; DISCONNECTED THE AUTOPLT AGAIN AND STOPPED THE CLB. WE EXPERIENCED ANOTHER YAW. AS THE SO WAS GETTING OUT THE COM PROCS FOR THE APPROPRIATE MALFUNCTION CHKLIST; I ASKED HIM TO DISENGAGE THE YAW DAMPERS (AFTER THE FIRST YAW; I HAD MY FEET ON THE RUDDERS AND NOTED NO RUDDER MOVEMENT WITH THE SECOND YAW AND I KNOW FROM PREVIOUS EXPERIENCE THAT WE CAN OPERATE WITHOUT EITHER YAW DAMPER). WE ACCOMPLISHED THE COM PROC 'UNCOMMANDED RUDDER MOVEMENT' AND FOUND THAT THE LOWER YAW DAMPER WAS THE PROB. WE OPERATED THE REMAINDER OF THE FLT TO TPE WITH THE LOWER YAW DAMPER OFF. ALL OTHER SYS INCLUDING HYDS WERE NORMAL. ALSO; NORMAL FLT CTL FUNCTION OF BOTH YAW DAMPERS HAD BEEN OBSERVED ON THE TAXI-OUT CHK.