Narrative:

While en route from slc to sfo, upon beginning level off at FL310, aircraft on autoplt and first officer at controls approximately 500 ft prior to level began to experience a slight left to right 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 first officer noted displacement of the yaw damper indicator. At this time I assumed control of the aircraft, accomplished the immediate action items for emergency procedure involving uncommanded yaw. Disconnected yaw damper and autoplt, as I did this, the aircraft yawed approximately 15-20 degrees left and began a roll moment left of 15 degrees. I countered with control inputs, directed the first officer to obtain clearance to FL280 which was done immediately. After brief analysis the decision was made to return to slc. The aircraft was now fully and positively controllable. I returned control of the aircraft to the first officer and proceeded with the reference items for the uncommanded yaw emergency procedure. No further special assistance was required. The flight attendants were called and informed of the situation. No one on board was injured, however the yaw was detected in the cabin by the attendants. 1 attendant was jarred off balance but totally uninjured. The flight back to slc was uneventful. The crew, flight attendants, ATC all performed in a very timely and professional manner. Supplemental information from acn 350435: my experience with yaw damper failures, 10 yrs and 4500 hours in the C141B, were extremely similar to what I experienced with this B737- 200. Disconnected the yaw damper and descending alleviated the problem in all the uncommanded yaw sits I have encountered.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.