B737 crew had uncommanded rudder inputs while in cruise.

Attributes
ACN	448786
Time
Date	199909
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : mem.airport
State Reference	TN
Altitude	msl single value : 33000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zme.artcc
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : cfi pilot : atp
Experience	flight time last 90 days : 100 flight time total : 10600 flight time type : 1500
ASRS Report	448786
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : multi engine pilot : instrument
Events
Anomaly	aircraft equipment problem : critical
Independent Detector	other flight crewa other flight crewb
Resolutory Action	aircraft : automation overrode flight crew flight crew : landed as precaution flight crew : overrode automation flight crew : overcame equipment problem
Consequence	other
Supplementary
Problem Areas	FAA Aircraft
Primary Problem	Aircraft

Narrative:

Level at FL330 just after exiting an area of light chop, we experienced what seemed to be a quick rudder kick. The movement of the aircraft was mild, and neither one of us made mention of it at that time. Within the next 10-15 mins we experienced the same kick 2 additional times. No pedal movement was ever felt, however, we did notice 1/2 - 3/4 needle deflection of the yaw damper indicator. The movements occurred while we were in and out of light chop. We believed there was a possibility that the kicks were turbulence induced and we discussed this. I then told my first officer that if it happened again we would divert to the nearest suitable airport. At this point, I referenced the flight handbook. I then phone patched dispatch and maintenance control and advised them of the situation. Maintenance control was unsure of the cause and we discussed the possibility of it being a yaw damper problem, since there was no pedal movement just yaw damper indicator movement. I then inquired about aircraft limitations with the yaw damper off. They were unaware of any airspeed or altitude limitations. I told them if it happened again I would turn off the yaw damper, slow, descend, and divert to the nearest suitable airport. No objections received, end of patch. Approximately 25 NM from top of descent it occurred again. Notified dispatch, slowed, descended, and turned off yaw damper. This time we noticed 1 full needle width deflection. Decided to divert/continue to atl. Asked flight attendants if they felt anything. They said no. Landed uneventfully in atl. Callback conversation with reporter revealed the following information: reporter said all problems stopped when the yaw damper was turned off. The captain was debriefed by air carrier maintenance, engineering, flight safety, and pilot union safety representatives. This aircraft had the old analog yaw damper system installed. Aircraft are being modified to a new digital yaw damper system. The air carrier sent maintenance to atl to modify this aircraft with the new system. No further problems have been reported.

Original NASA ASRS Text

Title: B737 CREW HAD UNCOMMANDED RUDDER INPUTS WHILE IN CRUISE.

Narrative: LEVEL AT FL330 JUST AFTER EXITING AN AREA OF LIGHT CHOP, WE EXPERIENCED WHAT SEEMED TO BE A QUICK RUDDER KICK. THE MOVEMENT OF THE ACFT WAS MILD, AND NEITHER ONE OF US MADE MENTION OF IT AT THAT TIME. WITHIN THE NEXT 10-15 MINS WE EXPERIENCED THE SAME KICK 2 ADDITIONAL TIMES. NO PEDAL MOVEMENT WAS EVER FELT, HOWEVER, WE DID NOTICE 1/2 - 3/4 NEEDLE DEFLECTION OF THE YAW DAMPER INDICATOR. THE MOVEMENTS OCCURRED WHILE WE WERE IN AND OUT OF LIGHT CHOP. WE BELIEVED THERE WAS A POSSIBILITY THAT THE KICKS WERE TURB INDUCED AND WE DISCUSSED THIS. I THEN TOLD MY FO THAT IF IT HAPPENED AGAIN WE WOULD DIVERT TO THE NEAREST SUITABLE ARPT. AT THIS POINT, I REFED THE FLT HANDBOOK. I THEN PHONE PATCHED DISPATCH AND MAINT CTL AND ADVISED THEM OF THE SIT. MAINT CTL WAS UNSURE OF THE CAUSE AND WE DISCUSSED THE POSSIBILITY OF IT BEING A YAW DAMPER PROB, SINCE THERE WAS NO PEDAL MOVEMENT JUST YAW DAMPER INDICATOR MOVEMENT. I THEN INQUIRED ABOUT ACFT LIMITATIONS WITH THE YAW DAMPER OFF. THEY WERE UNAWARE OF ANY AIRSPD OR ALT LIMITATIONS. I TOLD THEM IF IT HAPPENED AGAIN I WOULD TURN OFF THE YAW DAMPER, SLOW, DSND, AND DIVERT TO THE NEAREST SUITABLE ARPT. NO OBJECTIONS RECEIVED, END OF PATCH. APPROX 25 NM FROM TOP OF DSCNT IT OCCURRED AGAIN. NOTIFIED DISPATCH, SLOWED, DSNDED, AND TURNED OFF YAW DAMPER. THIS TIME WE NOTICED 1 FULL NEEDLE WIDTH DEFLECTION. DECIDED TO DIVERT/CONTINUE TO ATL. ASKED FLT ATTENDANTS IF THEY FELT ANYTHING. THEY SAID NO. LANDED UNEVENTFULLY IN ATL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SAID ALL PROBS STOPPED WHEN THE YAW DAMPER WAS TURNED OFF. THE CAPT WAS DEBRIEFED BY ACR MAINT, ENGINEERING, FLT SAFETY, AND PLT UNION SAFETY REPRESENTATIVES. THIS ACFT HAD THE OLD ANALOG YAW DAMPER SYS INSTALLED. ACFT ARE BEING MODIFIED TO A NEW DIGITAL YAW DAMPER SYS. THE ACR SENT MAINT TO ATL TO MODIFY THIS ACFT WITH THE NEW SYS. NO FURTHER PROBS HAVE BEEN RPTED.

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.