A B737-200 in climb at FL250 experienced an uncommanded instantaneous yaw and 20 degree bank to the right caused by a failed yaw damper computer.

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Attributes
ACN	408181
Time
Date	199807
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : lch
State Reference	LA
Altitude	msl bound lower : 25000 msl bound upper : 25000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B737-200
Operating Under FAR Part	Part 121
Flight Phase	climbout : intermediate altitude
Route In Use	departure sid : sid enroute airway : zhu
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 150 flight time total : 17000 flight time type : 180
ASRS Report	408181
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : critical other anomaly other
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Climbing through FL250 on a flight from hou to atl, first officer flying with #2 autoplt engaged, aircraft experienced uncommanded yaw to the right and immediately entered a right bank. First officer disconnected autoplt going through 20 degree bank, and leveled wings, but yaw to the right remained. I turned off yaw damper and yaw disappeared. After consulting dispatcher and maintenance, flight continued on to atl at FL270. No further adverse yaw occurred. I feel it was the yaw damper computer/rudder pack at fault. Callback conversation with reporter revealed the following information: the reporter stated the autoplt was engaged when the yaw damper system failed. The reporter said the aircraft yaw and bank was instantaneous and was corrected by the first officer after disconnecting the autoplt. The reporter stated turning off the yaw damper system corrected the yaw. The reporter said maintenance found the yaw damper computer failed and replaced the unit. The reporter stated the system has operated for several flts with no yaw reports.

Original NASA ASRS Text

Title: A B737-200 IN CLB AT FL250 EXPERIENCED AN UNCOMMANDED INSTANTANEOUS YAW AND 20 DEG BANK TO THE R CAUSED BY A FAILED YAW DAMPER COMPUTER.

Narrative: CLBING THROUGH FL250 ON A FLT FROM HOU TO ATL, FO FLYING WITH #2 AUTOPLT ENGAGED, ACFT EXPERIENCED UNCOMMANDED YAW TO THE R AND IMMEDIATELY ENTERED A R BANK. FO DISCONNECTED AUTOPLT GOING THROUGH 20 DEG BANK, AND LEVELED WINGS, BUT YAW TO THE R REMAINED. I TURNED OFF YAW DAMPER AND YAW DISAPPEARED. AFTER CONSULTING DISPATCHER AND MAINT, FLT CONTINUED ON TO ATL AT FL270. NO FURTHER ADVERSE YAW OCCURRED. I FEEL IT WAS THE YAW DAMPER COMPUTER/RUDDER PACK AT FAULT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AUTOPLT WAS ENGAGED WHEN THE YAW DAMPER SYS FAILED. THE RPTR SAID THE ACFT YAW AND BANK WAS INSTANTANEOUS AND WAS CORRECTED BY THE FO AFTER DISCONNECTING THE AUTOPLT. THE RPTR STATED TURNING OFF THE YAW DAMPER SYS CORRECTED THE YAW. THE RPTR SAID MAINT FOUND THE YAW DAMPER COMPUTER FAILED AND REPLACED THE UNIT. THE RPTR STATED THE SYS HAS OPERATED FOR SEVERAL FLTS WITH NO YAW RPTS.

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.