A DORNIER 328 ON APCH AT 2500 FT HAD WHAT APPEARED TO BE JAMMED RUDDER PEDALS CAUSED BY YAW DAMPER STILL ENGAGED ON APCH.

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RUDDER PEDALS JAMMED ON APCH DURING REPOSITIONING FLT. ON PREFLT; FLT CTL SUB-PAGE ON MFD SHOWED AMBER 'X' IN PLACE OF NORMAL RUDDER INDICATION. WHEN I CYCLED THE PEDALS; MFD SHOWED NORMAL RUDDER SYMBOL WITH FULL TRAVEL L; BUT ONLY 3/4 TRAVEL R. I WROTE UP ACFT. MECH CLAIMED RUDDER POS INDICATOR WAS DEFECTIVE; DEFERRED IT; AND SIGNED OFF LOGBOOK. I COMPLETED WITH MEL 'OPERATIONAL' PROC; VISUALLY CONFIRMING FULL RUDDER TRAVEL. RUDDER PEDALS MOVED NORMALLY DURING TAXI/TKOF. ON CLBOUT; AFTER YAW DAMPER ACTIVATED; I NOTICED INCLINOMETER 'BALL' ALMOST FULLY DEFLECTED TO THE R; SO I TRIMMED R AND CTRED BALL. HAND FLEW THE 20 MIN REPOS FLT; WITH YAW DAMPER ON AND AUTOPLT OFF. (ABOVE 160 KTS; RUDDER PEDALS ARE NORMALLY 'LIMITED' (RUDDER SPRING TAB NO LONGER PROVIDES ASSIST TO RUDDER -- ESSENTIALLY IMMOBILIZING RUDDER PEDALS). BELOW 155 KTS; RUDDER LIMITER DISENGAGE; AND RUDDER PEDALS MOVE FREELY.) ON APCH; FULLY CONFIGURED; BELOW 155 KTS; AND SLOWING; I ENCOUNTERED WAKE TURB FROM A B757 5 MI AHEAD. I SLOWED TO 120 KTS TO INCREASE SEPARATION FROM THE B757; BUT ENCOUNTERED MORE WAKE TURB. AS I PUSHED THE RUDDER PEDALS TO MANEUVER; THEY FELT JAMMED; JUST LIKE IN THE SIMULATOR AFTER THE RUDDER LIMITER ACTIVATES. HAVING RECENTLY SEEN THIS IN THE SIMULATOR; I TRIED TO FREE THE RUDDER USING THE RUDDER UNLIMIT PROC. HOWEVER; PRESSING THE 'RUDDER UNLIMIT/ BUTTON HAD NO EFFECT. (WE DID NOT SEE A 'RUDDER LIMITED' CAS MESSAGE; WHICH SHOULD HAVE APPEARED IF THE RUDDER LIMITER REMAINED ON BELOW 155 KTS; BUT I NEVERTHELESS ASSUMED THE RUDDER LIMITER HAD JAMMED.) I FELT THE RUDDER PEDALS MOVE. THERE WAS NO SUDDEN JOLT OR BREAK -- RATHER; THE PEDALS SIMPLY BEGAN TO MOVE NORMALLY AGAIN; UNDER PRESSURE THAT I HAD ATTEMPTED TO APPLY TO THE R RUDDER PEDAL THROUGHOUT THE FLARE AND TOUCHDOWN. AT THE SAME TIME; WE GOT AN AMBER CAUTION LIGHT/CHIME AND AN AMBER 'YAW DAMPER OFF' CAS MESSAGE. PER SOP; I CANCELED THE MASTER CAUTION LIGHT AS WE CLRED THE RWY; AND READ THE CAS MESSAGE ALOUD. AT THAT POINT; IT DAWNED ON ME THAT WE HAD PROBABLY EXPERIENCED A YAW DAMPER MALFUNCTION; RATHER THAN A JAMMED OR LIMITED RUDDER. THE YAW DAMPER WAS TURNED ON SHORTLY BEFORE I NOTICED THE OUT-OF-TRIM RUDDER; AND WAS LEFT ON BECAUSE I FORGOT TO ASK THE FO TO TURN IT OFF PRIOR TO LNDG. (DISENGAGING THE YAW DAMPER IS NOT ON THE BEFORE LNDG CHKLIST.) COMPANY PROCS ASSUME THE AUTOPLT WILL BE ENGAGED EVERY FLT; AND THE YAW DAMPER AUTOMATICALLY DISCONNECTS WHEN THE AUTOPLT IS TURNED OFF. AS WE TAXIED TO THE GATE; THE FO TOOK IT UPON HIMSELF TO NOTIFY GND CTL THAT WE HAD SUFFERED A 'FLT CTL FAILURE.' IN HINDSIGHT; HOWEVER; I THINK THE YAW DAMPER; RATHER THAN THE RUDDER; MALFUNCTIONED. UPON REVIEWING THE VOLUMINOUS EMER CHKLISTS MANUAL AFTER WE PARKED; I NOTED A 'RUDDER CTL JAMMED' PROC THAT I PROBABLY SHOULD HAVE COMPLETED; INSTEAD OF THE 'RUDDER LIMITED' PROC. THE FORMER CALLS FOR TURNING OFF THE YAW DAMPER. I LATER CONSULTED A COMPANY MECH WHO SAID THE YAW DAMPER AUTOMATICALLY DISCONNECTS WHEN THE NOSEWHEEL SQUAT SWITCH DETECTS WT-ON-WHEELS. THAT WOULD EXPLAIN WHY THE RUDDER PEDALS MOVED AGAIN AFTER THE NOSEWHEEL TOUCHED DOWN; AND WHY WE RECEIVED THE 'YAW DAMPER OFF' CAS MESSAGE UPON LNDG. COMPANY MECHS INSPECTED THE RUDDER LATER THAT EVENING AND FOUND NO DEFECTS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE APPEARANCE OF JAMMED RUDDER PEDALS WAS CAUSED BY ALLOWING THE YAW DAMPER TO BE ENGAGED ON APCH WHILE FLYING THE ACFT MANUALLY. THE RPTR SAID THE LNDG CHKLIST CALLS FOR AUTOPLT DISCONNECT WHICH NORMALLY DISENGAGES THE YAW DAMPER. THE RPTR STATED IN THIS INCIDENT THE ACFT WAS BEING FLOWN MANUALLY WITH THE YAW DAMPER ENGAGED AND REQUIRED THE PLT TO WORK AGAINST THE YAW DAMPER FORCES. THE RPTR SAID THIS SIT WAS EXPERIENCED IN THE SIMULATOR; BUT NOT RECALLED AND WAS OPERATOR ERROR. THE RPTR STATED THE LNDG CHKLIST SHOULD HAVE AUTOPLT OFF AND ADDITIONAL YAW DAMPER OFF INCLUDED WHEN MANUALLY OPERATING THE ACFT.