Narrative:

While in cruise autoplt engaged we were experiencing light, with occasional moderate turbulence and chop. There were sigmets out for this and possibly severe. The aircraft began a slow dutch roll tendency but was hard to figure, at the time, due to the ride we were getting. Plus the yaw damper indicator was giving a normal indication (it did this the whole time). The dutch roll began to get much worse so I kicked off the autoplt to see if it would make a difference. At this time it began to get excessive. I instructed the first officer to request a lower altitude immediately, which he did. The controller instructed us to standby. By this time I had tried disengaging the yaw damper, but I couldn't tell if it made a difference due to the turbulence. I began a descent to a lower altitude for safety while slowing the aircraft, and told my first officer to tell center we were descending to a lower altitude probably around FL200. This I figured would still give them plenty of space to move traffic away from us and if the aircraft still could not be controled we'd continue to a lower altitude. Controller finally had given us FL200 and asked the nature of our problem. I responded by telling him of the autoplt or yaw damper malfunction and about the turbulence. (Aircraft limitation for this aircraft is it will not be dispatched into areas of moderate or greater turbulence with yaw damper inoperative). At the lower smoother altitude, I tried to reengage the yaw damper it immediately began yawing the aircraft with increasing intensity until I turned it off at which time it stopped. All this time it gave no abnormal indication on the respective indicator on the instrument panel (no movement). I received no other comments from ATC on conflicts or a problem with my descent. Flight continued uneventfully to destination. Wrote it up in maintenance log. Callback conversation with reporter revealed the following information. This aircraft may be flown to FL240 west/O the yaw damper but in turbulence it is imperative to have the yaw damper operative. Yaw damper was definitely inducing roll. Maintenance fixed the yaw damper and wrote it off but it again malfunctioned on a later flight. Several days later I was assigned to pick up the aircraft in utica, ny, for a ferry to our maintenance base and saw the yaw damper was again inoperative. Flew it home west/O the yaw damper. Hopefully maintenance now has a fix on the problem.

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Original NASA ASRS Text

Title: ACFT HAD INDUCED UNCONTROLLABLE ROLL. CALLED FOR DESCENT CLRNC AND DESCENDED BEFORE RECEIVING THE CLRNC.

Narrative: WHILE IN CRUISE AUTOPLT ENGAGED WE WERE EXPERIENCING LIGHT, WITH OCCASIONAL MODERATE TURB AND CHOP. THERE WERE SIGMETS OUT FOR THIS AND POSSIBLY SEVERE. THE ACFT BEGAN A SLOW DUTCH ROLL TENDENCY BUT WAS HARD TO FIGURE, AT THE TIME, DUE TO THE RIDE WE WERE GETTING. PLUS THE YAW DAMPER INDICATOR WAS GIVING A NORMAL INDICATION (IT DID THIS THE WHOLE TIME). THE DUTCH ROLL BEGAN TO GET MUCH WORSE SO I KICKED OFF THE AUTOPLT TO SEE IF IT WOULD MAKE A DIFFERENCE. AT THIS TIME IT BEGAN TO GET EXCESSIVE. I INSTRUCTED THE F/O TO REQUEST A LOWER ALT IMMEDIATELY, WHICH HE DID. THE CTLR INSTRUCTED US TO STANDBY. BY THIS TIME I HAD TRIED DISENGAGING THE YAW DAMPER, BUT I COULDN'T TELL IF IT MADE A DIFFERENCE DUE TO THE TURB. I BEGAN A DSNT TO A LOWER ALT FOR SAFETY WHILE SLOWING THE ACFT, AND TOLD MY F/O TO TELL CTR WE WERE DSNDING TO A LOWER ALT PROBABLY AROUND FL200. THIS I FIGURED WOULD STILL GIVE THEM PLENTY OF SPACE TO MOVE TFC AWAY FROM US AND IF THE ACFT STILL COULD NOT BE CTLED WE'D CONTINUE TO A LOWER ALT. CTLR FINALLY HAD GIVEN US FL200 AND ASKED THE NATURE OF OUR PROB. I RESPONDED BY TELLING HIM OF THE AUTOPLT OR YAW DAMPER MALFUNCTION AND ABOUT THE TURB. (ACFT LIMITATION FOR THIS ACFT IS IT WILL NOT BE DISPATCHED INTO AREAS OF MODERATE OR GREATER TURB WITH YAW DAMPER INOPERATIVE). AT THE LOWER SMOOTHER ALT, I TRIED TO REENGAGE THE YAW DAMPER IT IMMEDIATELY BEGAN YAWING THE ACFT WITH INCREASING INTENSITY UNTIL I TURNED IT OFF AT WHICH TIME IT STOPPED. ALL THIS TIME IT GAVE NO ABNORMAL INDICATION ON THE RESPECTIVE INDICATOR ON THE INSTRUMENT PANEL (NO MOVEMENT). I RECEIVED NO OTHER COMMENTS FROM ATC ON CONFLICTS OR A PROB WITH MY DSNT. FLT CONTINUED UNEVENTFULLY TO DEST. WROTE IT UP IN MAINT LOG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. THIS ACFT MAY BE FLOWN TO FL240 W/O THE YAW DAMPER BUT IN TURB IT IS IMPERATIVE TO HAVE THE YAW DAMPER OPERATIVE. YAW DAMPER WAS DEFINITELY INDUCING ROLL. MAINT FIXED THE YAW DAMPER AND WROTE IT OFF BUT IT AGAIN MALFUNCTIONED ON A LATER FLT. SEVERAL DAYS LATER I WAS ASSIGNED TO PICK UP THE ACFT IN UTICA, NY, FOR A FERRY TO OUR MAINT BASE AND SAW THE YAW DAMPER WAS AGAIN INOPERATIVE. FLEW IT HOME W/O THE YAW DAMPER. HOPEFULLY MAINT NOW HAS A FIX ON THE PROB.

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.