B737-200 CLBING TO FL310 BEGAN TO EXPERIENCE UNCOMMANDED YAW OSCILLATIONS. CAPT TOOK ACFT FROM FO; PF; AND DISCONNECTED YAW DAMPER AND AUTOPLT. REQUESTED DSCNT AND REGAINED FULL CTL OF ACFT. RETURNED TO SLC; NO FURTHER PROB.

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WHILE ENRTE FROM SLC TO SFO; UPON BEGINNING LEVEL OFF AT FL310; ACFT ON AUTOPLT AND FO AT CTLS APPROX 500 FT PRIOR TO LEVEL BEGAN TO EXPERIENCE A SLIGHT L TO R YAW. BOTH CREW MEMBERS QUESTIONED IF IT COULD BE ATMOSPHERIC IN NATURE; HOWEVER CONDITIONS WERE SMOOTH. THE YAW BEGAN TO INCREASE AND OSCILLATE AGAIN; NEITHER CREW MEMBER HAD THEIR FEET ON THE RUDDER AT THE TIME; THE YAW INCREASED; AND FO NOTED DISPLACEMENT OF THE YAW DAMPER INDICATOR. AT THIS TIME I ASSUMED CTL OF THE ACFT; ACCOMPLISHED THE IMMEDIATE ACTION ITEMS FOR EMER PROC INVOLVING UNCOMMANDED YAW. DISCONNECTED YAW DAMPER AND AUTOPLT; AS I DID THIS; THE ACFT YAWED APPROX 15-20 DEGS L AND BEGAN A ROLL MOMENT L OF 15 DEGS. I COUNTERED WITH CTL INPUTS; DIRECTED THE FO TO OBTAIN CLRNC TO FL280 WHICH WAS DONE IMMEDIATELY. AFTER BRIEF ANALYSIS THE DECISION WAS MADE TO RETURN TO SLC. THE ACFT WAS NOW FULLY AND POSITIVELY CONTROLLABLE. I RETURNED CTL OF THE ACFT TO THE FO AND PROCEEDED WITH THE REF ITEMS FOR THE UNCOMMANDED YAW EMER PROC. NO FURTHER SPECIAL ASSISTANCE WAS REQUIRED. THE FLT ATTENDANTS WERE CALLED AND INFORMED OF THE SIT. NO ONE ON BOARD WAS INJURED; HOWEVER THE YAW WAS DETECTED IN THE CABIN BY THE ATTENDANTS. 1 ATTENDANT WAS JARRED OFF BAL BUT TOTALLY UNINJURED. THE FLT BACK TO SLC WAS UNEVENTFUL. THE CREW; FLT ATTENDANTS; ATC ALL PERFORMED IN A VERY TIMELY AND PROFESSIONAL MANNER. SUPPLEMENTAL INFO FROM ACN 350435: MY EXPERIENCE WITH YAW DAMPER FAILURES; 10 YRS AND 4500 HRS IN THE C141B; WERE EXTREMELY SIMILAR TO WHAT I EXPERIENCED WITH THIS B737- 200. DISCONNECTED THE YAW DAMPER AND DSNDING ALLEVIATED THE PROB IN ALL THE UNCOMMANDED YAW SITS I HAVE ENCOUNTERED.