AN ACR WDB RAN OFF OF THE RWY AFTER AN UNCOMMANDED RUDDER INPUT.

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I WAS MAKING A CAT III APCH TO FRA ARPT. BROKE OUT BELOW CLOUDS AT 600 FT AND ELECTED TO LAND MANUALLY. KICKED OFF THE AUTOPLT AND AUTO THROTTLE. WIND NO FACTOR; ABOUT 200 DEGS AT 5 KTS. LANDED RWY 25R. SMOOTH TOUCHDOWN. LOWERING NOSE JUST BEFORE NOSE TOUCHED DOWN; GOT UNCALLED FOR FULL R RUDDER AND LOCKED IN THAT POS. COULD NOT PUSH L RUDDER IN. ACFT LEFT THE RWY TO THE R. FO APPLIED L RUDDER WITH ME AND TOGETHER ABLE TO OVERCOME RUDDER LOCK. ACFT TURNED TO THE L AND WENT BACK ONTO RWY 25. I REGAINED CTL OF RUDDER AFTER HITTING AUTOPLT DISCONNECT SWITCH TWICE. STOPPED THE ACFT ON HIGH SPD TXWY JUST OFF THE RWY. AUTOPLT DISCONNECT IN THE AIR SEEMED NORMAL. ACFT FLEW MANUALLY OK. ACFT DAMAGE MINIMAL; NO PAX SERIOUSLY HURT. ALL AGENCIES NOTIFIED. ACFT IS UNDERGOING TESTING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: THE RPTING CAPT STATES THAT THE ACFT TOUCHED DOWN NORMALLY THEN HEADED TO THE R. BOTH PLTS STANDING ON THE RUDDERS GOT THE ACFT BACK ON THE RWY. 1 PAX FAINTED; ANOTHER WAS HIT ON THE HEAD BY FALLING LUGGAGE BUT WAS NOT SERIOUSLY HURT. ONE OF THE ENGS WAS CHANGED BECAUSE IT INGESTED MUD ON THE ROLLOUT. THE RPTR STATES THAT HIS ACR WAS NOT ABLE TO FIND ANYTHING WRONG WITH THE ACFT. IT WAS FERRIED BACK TO ITS AMERICAN MANUFACTURER WHERE THE RPTR FLEW THE ACFT. THE RPTR HAS HEARD THAT THE MANUFACTURER FOUND 'SERIOUS PROBS WITH THE RUDDER' ON THIS PARTICULAR ACFT. THE MANUFACTURER MAY HAVE BOUGHT THE ACFT BACK FROM THE ACR. THE ONLY OTHER DAMAGE OTHER THAN THE ENG WAS 1 BLOWN TIRE.