B737-500 aircraft in cruise had yaw damper malfunction which caused violent rudder deflections. Flight attendants thrown from wall to wall, but no serious injuries. After yaw damper turned off, all operations normal. NTSB obtained flight recorder for further examination.

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Attributes
ACN	365523
Time
Date	199704
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zob
State Reference	OH
Altitude	msl bound lower : 28000 msl bound upper : 28000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zob tower : sju
Operator	common carrier : air carrier
Make Model Name	B737-500
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : commercial pilot : atp pilot : instrument
Experience	flight time last 90 days : 150 flight time total : 10000 flight time type : 2500
ASRS Report	365523
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 : overcame equipment problem flight crew : declared emergency
Consequence	faa : investigated
Supplementary
Primary Problem	Aircraft

Narrative:

In-flight uncontrollable yaw. Yaw damper was turned off. Normal flight for the remaining en route portion of the flight. Uncontrollable yaw report filed. Callback conversation with reporter revealed the following information: callback with captain reporter. Aircraft B737-500 at cruise and 28000 ft. Suddenly there were violent rudder inputs, 2 hard left, 2 hard right and 1 hard left before the flight crew could turn off the yaw damper. Flight attendants hit the walls with each successive hard over, but none were seriously injured. No passenger were injured because all were seated. Captain says that the aircraft was controllable at all times. He did say that the first officer, PF, couldn't get rudder response trying to counteract the yaw inputs to the rudder, but captain believes it may be because he didn't push hard enough. Captain was unable to get rudder throw because his seat was too high. Extensive reports were submitted and discussed with boeing and the NTSB took the flight recorder to investigate the incident further.

Original NASA ASRS Text

Title: B737-500 ACFT IN CRUISE HAD YAW DAMPER MALFUNCTION WHICH CAUSED VIOLENT RUDDER DEFLECTIONS. FLT ATTENDANTS THROWN FROM WALL TO WALL, BUT NO SERIOUS INJURIES. AFTER YAW DAMPER TURNED OFF, ALL OPS NORMAL. NTSB OBTAINED FLT RECORDER FOR FURTHER EXAMINATION.

Narrative: INFLT UNCONTROLLABLE YAW. YAW DAMPER WAS TURNED OFF. NORMAL FLT FOR THE REMAINING ENRTE PORTION OF THE FLT. UNCONTROLLABLE YAW RPT FILED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CALLBACK WITH CAPT RPTR. ACFT B737-500 AT CRUISE AND 28000 FT. SUDDENLY THERE WERE VIOLENT RUDDER INPUTS, 2 HARD L, 2 HARD R AND 1 HARD L BEFORE THE FLC COULD TURN OFF THE YAW DAMPER. FLT ATTENDANTS HIT THE WALLS WITH EACH SUCCESSIVE HARD OVER, BUT NONE WERE SERIOUSLY INJURED. NO PAX WERE INJURED BECAUSE ALL WERE SEATED. CAPT SAYS THAT THE ACFT WAS CONTROLLABLE AT ALL TIMES. HE DID SAY THAT THE FO, PF, COULDN'T GET RUDDER RESPONSE TRYING TO COUNTERACT THE YAW INPUTS TO THE RUDDER, BUT CAPT BELIEVES IT MAY BE BECAUSE HE DIDN'T PUSH HARD ENOUGH. CAPT WAS UNABLE TO GET RUDDER THROW BECAUSE HIS SEAT WAS TOO HIGH. EXTENSIVE RPTS WERE SUBMITTED AND DISCUSSED WITH BOEING AND THE NTSB TOOK THE FLT RECORDER TO INVESTIGATE THE INCIDENT FURTHER.

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.