Narrative:

On mar/mon/98, we were on a scheduled far 121 flight from spokane to seattle, wa. The takeoff was normal and on climb out during a right turn the captain noticed that the yaw damper indication was showing full scale right deflection with my feet flat on the floor. During the climb I adjusted the slip/skid indicator to position the ball in the center of the indicator. We had been at cruise for probably 10 mins when a pop was heard and the aircraft yawed to the right and the autoplt corrected it by putting in 10-15 degrees of bank. The aircraft oscillated a few times, each time the degree of oscillation got smaller. I grabbed hold of the yoke and put my feet on the rudder pedals as soon as the pop was heard and tried to help the autoplt dampen out the oscillations. The captain turned the yaw damper off and then ran the uncommanded yaw checklist. I never called for the checklist as he had the checklist out before I could say let's run the checklist. We did the checklist and I turned the autoplt off per the checklist and started to descend. We had been given discretion to FL240. During our descent the captain said to reengage the autoplt as he thought the yaw damper was the culprit. We continued the flight making sure the passenger and the flight attendants were not hurt. The C flight attendant said he felt the aircraft oscillating back and forth during the flight with the oscillations getting a little more noticeable each time. We landed without incident in seattle and pulled the flight data recorder and the cockpit voice recorder. Callback conversation with reporter revealed the following information: the first officer said that he was flying the B737-300 using the control wheel steering mode of the autoplt when the captain stated that the yaw indicator showed the rudder displaced full (yaw damper limit) to the right. He said, that he had not noticed any abnormal feel or aircraft motion to that time in the flight and since the aircraft continued to fly without any unusual motions they continued the climb. He said, that when he looked at the yaw indicator it was centered. During cruise, when the aircraft yawed to the right the whole airframe popped according to the reporter. Again, he placed his hands and feet on the controls, but he left the aircraft on autoplt since it was correcting properly. He said that he did not feel any great displacement of the controls other than the few autoplt corrective inputs. The captain completed the uncommanded yaw checklist and entered the event in the maintenance log. Upon arrival in sea the crew briefed contract maintenance personnel and then had to depart for the next destination in another aircraft. He has not heard of any finding, but he knows that his regular company maintenance operation sent a team to sea to inspect the aircraft. Supplemental information from acn 396530: while in cruise at FL350 at mach .755 the rudder kicked hard right. The aircraft tail swung violently to the left and the aircraft rolled 12-15 degrees to the right. The autoplt then countered the roll and overcorrected back to about 5-7 degrees left wing down. The yaw damper indicator showed full deflection to the right. The yaw damper was turned off and the aircraft stabilized. QRH items for uncommanded roll were accomplished during the descent.

Related reports:

Google
 

Original NASA ASRS Text

Title: AN ACR B737 FLC EXPERIENCED SEVERAL YAW DAMPER DISPLACEMENTS AND ONE OF THEM WAS VIOLENT ENOUGH TO CAUSE THE AIRFRAME TO 'POP' WHILE IN CRUISE.

Narrative: ON MAR/MON/98, WE WERE ON A SCHEDULED FAR 121 FLT FROM SPOKANE TO SEATTLE, WA. THE TKOF WAS NORMAL AND ON CLBOUT DURING A R TURN THE CAPT NOTICED THAT THE YAW DAMPER INDICATION WAS SHOWING FULL SCALE R DEFLECTION WITH MY FEET FLAT ON THE FLOOR. DURING THE CLB I ADJUSTED THE SLIP/SKID INDICATOR TO POS THE BALL IN THE CTR OF THE INDICATOR. WE HAD BEEN AT CRUISE FOR PROBABLY 10 MINS WHEN A POP WAS HEARD AND THE ACFT YAWED TO THE R AND THE AUTOPLT CORRECTED IT BY PUTTING IN 10-15 DEGS OF BANK. THE ACFT OSCILLATED A FEW TIMES, EACH TIME THE DEG OF OSCILLATION GOT SMALLER. I GRABBED HOLD OF THE YOKE AND PUT MY FEET ON THE RUDDER PEDALS AS SOON AS THE POP WAS HEARD AND TRIED TO HELP THE AUTOPLT DAMPEN OUT THE OSCILLATIONS. THE CAPT TURNED THE YAW DAMPER OFF AND THEN RAN THE UNCOMMANDED YAW CHKLIST. I NEVER CALLED FOR THE CHKLIST AS HE HAD THE CHKLIST OUT BEFORE I COULD SAY LET'S RUN THE CHKLIST. WE DID THE CHKLIST AND I TURNED THE AUTOPLT OFF PER THE CHKLIST AND STARTED TO DSND. WE HAD BEEN GIVEN DISCRETION TO FL240. DURING OUR DSCNT THE CAPT SAID TO REENGAGE THE AUTOPLT AS HE THOUGHT THE YAW DAMPER WAS THE CULPRIT. WE CONTINUED THE FLT MAKING SURE THE PAX AND THE FLT ATTENDANTS WERE NOT HURT. THE C FLT ATTENDANT SAID HE FELT THE ACFT OSCILLATING BACK AND FORTH DURING THE FLT WITH THE OSCILLATIONS GETTING A LITTLE MORE NOTICEABLE EACH TIME. WE LANDED WITHOUT INCIDENT IN SEATTLE AND PULLED THE FLT DATA RECORDER AND THE COCKPIT VOICE RECORDER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FO SAID THAT HE WAS FLYING THE B737-300 USING THE CTL WHEEL STEERING MODE OF THE AUTOPLT WHEN THE CAPT STATED THAT THE YAW INDICATOR SHOWED THE RUDDER DISPLACED FULL (YAW DAMPER LIMIT) TO THE R. HE SAID, THAT HE HAD NOT NOTICED ANY ABNORMAL FEEL OR ACFT MOTION TO THAT TIME IN THE FLT AND SINCE THE ACFT CONTINUED TO FLY WITHOUT ANY UNUSUAL MOTIONS THEY CONTINUED THE CLB. HE SAID, THAT WHEN HE LOOKED AT THE YAW INDICATOR IT WAS CTRED. DURING CRUISE, WHEN THE ACFT YAWED TO THE R THE WHOLE AIRFRAME POPPED ACCORDING TO THE RPTR. AGAIN, HE PLACED HIS HANDS AND FEET ON THE CTLS, BUT HE LEFT THE ACFT ON AUTOPLT SINCE IT WAS CORRECTING PROPERLY. HE SAID THAT HE DID NOT FEEL ANY GREAT DISPLACEMENT OF THE CTLS OTHER THAN THE FEW AUTOPLT CORRECTIVE INPUTS. THE CAPT COMPLETED THE UNCOMMANDED YAW CHKLIST AND ENTERED THE EVENT IN THE MAINT LOG. UPON ARR IN SEA THE CREW BRIEFED CONTRACT MAINT PERSONNEL AND THEN HAD TO DEPART FOR THE NEXT DEST IN ANOTHER ACFT. HE HAS NOT HEARD OF ANY FINDING, BUT HE KNOWS THAT HIS REGULAR COMPANY MAINT OP SENT A TEAM TO SEA TO INSPECT THE ACFT. SUPPLEMENTAL INFO FROM ACN 396530: WHILE IN CRUISE AT FL350 AT MACH .755 THE RUDDER KICKED HARD R. THE ACFT TAIL SWUNG VIOLENTLY TO THE L AND THE ACFT ROLLED 12-15 DEGS TO THE R. THE AUTOPLT THEN COUNTERED THE ROLL AND OVERCORRECTED BACK TO ABOUT 5-7 DEGS L WING DOWN. THE YAW DAMPER INDICATOR SHOWED FULL DEFLECTION TO THE R. THE YAW DAMPER WAS TURNED OFF AND THE ACFT STABILIZED. QRH ITEMS FOR UNCOMMANDED ROLL WERE ACCOMPLISHED DURING THE DSCNT.

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.