Narrative:

At FL290 in level cruise flight experienced a rapid and abrupt, uncommanded rudder input. Autoplt was immediately disengaged followed shortly by the yaw damper. When the yaw damper switch was turned off the rudder input ceased and the rudder centered. Flight continued to destination uneventfully with yaw damper off. Supplemental information from acn 386919: flying from roc to bos, at the gate in roc, the aircraft was free of any type of frost, snow or ice. We did not get deiced because the aircraft was clean. We departed on runway 22. During the climb out, the captain and myself both felt the plane performed as though the yaw damper was not working. The yaw damper showed it was working (switch on/light out). Once the plane climbed into smooth air, the yaw damper seemed to perform normal. About 20 mins later, level FL290, smooth flight, autoplt on, captain flying airplane with the autoplt, all of a sudden the airplane put in a rapid left rudder input (uncommanded) to where the autoplt compensated by adding 15-20 degree yoke deflection to the right. The autoplt maintained control with no heading change (autoplt was on heading select) and no altitude change. The captain disconnected the autoplt, maintained control and the yaw damper indicator went from full left deflection to centered when the yaw damper switch was turned off. The checklist was completed and we spoke to the flight attendants. We landed safely in bos with the yaw damper off. No emergency was declared. The aircraft had a history of yaw damper write-ups. (Problems.) callback conversation with reporter of acn 386597 revealed the following information: this captain was flying a B737- 200 when he experienced the uncommanded rudder input. He recalled, from his review of the logbook, that the rudder power control unit had been replaced on this aircraft. He reacted as he had been trained by disconnecting the autoplt and turning off the yaw damper. After hand flying the aircraft for awhile and feeling no further problem, he said, that he turned the aircraft over to the first officer and called his company. He informed maintenance control and dispatch about the problem and as he approached alb he told operations at that station. After landing the company set up a conference call with a number of people, including boeing and other manufacturers and the maintenance personnel, but he was unsure if the FAA was involved. The reporter said that he and his first officer have flown the B737-300 and -400 also, but this was their first uncommanded input. He has not learned what maintenance did to clear his logbook write- up.

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Original NASA ASRS Text

Title: AN ACR B737 FLC EXPERIENCED AN UNCOMMANDED RUDDER INPUT WHILE CRUISING AT FL290. THE FLC DISCONNECTED THE YAW DAMPER AND COMPLETED THE FLT WITH NO FURTHER PROB. WHILE STILL INFLT THEY INFORMED THEIR OPS AND MAINT PERSONNEL OF THEIR ACTIONS AND THE ACFT'S CONFIGN.

Narrative: AT FL290 IN LEVEL CRUISE FLT EXPERIENCED A RAPID AND ABRUPT, UNCOMMANDED RUDDER INPUT. AUTOPLT WAS IMMEDIATELY DISENGAGED FOLLOWED SHORTLY BY THE YAW DAMPER. WHEN THE YAW DAMPER SWITCH WAS TURNED OFF THE RUDDER INPUT CEASED AND THE RUDDER CTRED. FLT CONTINUED TO DEST UNEVENTFULLY WITH YAW DAMPER OFF. SUPPLEMENTAL INFO FROM ACN 386919: FLYING FROM ROC TO BOS, AT THE GATE IN ROC, THE ACFT WAS FREE OF ANY TYPE OF FROST, SNOW OR ICE. WE DID NOT GET DEICED BECAUSE THE ACFT WAS CLEAN. WE DEPARTED ON RWY 22. DURING THE CLBOUT, THE CAPT AND MYSELF BOTH FELT THE PLANE PERFORMED AS THOUGH THE YAW DAMPER WAS NOT WORKING. THE YAW DAMPER SHOWED IT WAS WORKING (SWITCH ON/LIGHT OUT). ONCE THE PLANE CLBED INTO SMOOTH AIR, THE YAW DAMPER SEEMED TO PERFORM NORMAL. ABOUT 20 MINS LATER, LEVEL FL290, SMOOTH FLT, AUTOPLT ON, CAPT FLYING AIRPLANE WITH THE AUTOPLT, ALL OF A SUDDEN THE AIRPLANE PUT IN A RAPID L RUDDER INPUT (UNCOMMANDED) TO WHERE THE AUTOPLT COMPENSATED BY ADDING 15-20 DEG YOKE DEFLECTION TO THE R. THE AUTOPLT MAINTAINED CTL WITH NO HEADING CHANGE (AUTOPLT WAS ON HEADING SELECT) AND NO ALT CHANGE. THE CAPT DISCONNECTED THE AUTOPLT, MAINTAINED CTL AND THE YAW DAMPER INDICATOR WENT FROM FULL L DEFLECTION TO CTRED WHEN THE YAW DAMPER SWITCH WAS TURNED OFF. THE CHKLIST WAS COMPLETED AND WE SPOKE TO THE FLT ATTENDANTS. WE LANDED SAFELY IN BOS WITH THE YAW DAMPER OFF. NO EMER WAS DECLARED. THE ACFT HAD A HISTORY OF YAW DAMPER WRITE-UPS. (PROBS.) CALLBACK CONVERSATION WITH RPTR OF ACN 386597 REVEALED THE FOLLOWING INFO: THIS CAPT WAS FLYING A B737- 200 WHEN HE EXPERIENCED THE UNCOMMANDED RUDDER INPUT. HE RECALLED, FROM HIS REVIEW OF THE LOGBOOK, THAT THE RUDDER PWR CTL UNIT HAD BEEN REPLACED ON THIS ACFT. HE REACTED AS HE HAD BEEN TRAINED BY DISCONNECTING THE AUTOPLT AND TURNING OFF THE YAW DAMPER. AFTER HAND FLYING THE ACFT FOR AWHILE AND FEELING NO FURTHER PROB, HE SAID, THAT HE TURNED THE ACFT OVER TO THE FO AND CALLED HIS COMPANY. HE INFORMED MAINT CTL AND DISPATCH ABOUT THE PROB AND AS HE APCHED ALB HE TOLD OPS AT THAT STATION. AFTER LNDG THE COMPANY SET UP A CONFERENCE CALL WITH A NUMBER OF PEOPLE, INCLUDING BOEING AND OTHER MANUFACTURERS AND THE MAINT PERSONNEL, BUT HE WAS UNSURE IF THE FAA WAS INVOLVED. THE RPTR SAID THAT HE AND HIS FO HAVE FLOWN THE B737-300 AND -400 ALSO, BUT THIS WAS THEIR FIRST UNCOMMANDED INPUT. HE HAS NOT LEARNED WHAT MAINT DID TO CLR HIS LOGBOOK WRITE- UP.

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.