<< >> NASA ASRS Report 321064, November 1995

B737's out-of-limits yaw damper coupler caused uncommanded yaw during descent. Aircraft equipment problem yaw damper system.

Attributes
ACN	321064
Time
Date	199511
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : lga
State Reference	NY
Altitude	msl bound lower : 7000 msl bound upper : 7000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : n90
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent : approach descent other landing other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : cfi pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 180 flight time total : 12000 flight time type : 1200
ASRS Report	321064
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : cfi pilot : flight engineer
Experience	flight time last 90 days : 180 flight time total : 14000 flight time type : 4000
ASRS Report	320668
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other flight crewa
Resolutory Action	none taken : detected after the fact other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

On approach to lga, 7000 ft, and 210 KTS clean, experienced uncommanded yaw. Maximum turn 20 degrees. Captain turned off autoplt and yaw damper, and landed at lga runway 31. Callback conversation with reporter revealed the following information: reporter recently received a letter from the company accident investigator stating that the uncommanded yaw was the result of an out-of-limits yaw damper coupler. The first officer (reporter) states that after the B737-300 yawed to the left, the autoplt attempted to compensate with a roll (control wheel was 45 degree right) back to wings level. He immediately recognized that this was a yaw damper problem because the yaw damper indicator was displaced completely to the right. When the yaw damper was turned off, the indication came back to center. The uncommanded yaw and the subsequent efforts to correct the situation were abrupt enough to throw 2 flight attendants onto the cabin floor.

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Original NASA ASRS Text

Title: B737'S OUT-OF-LIMITS YAW DAMPER COUPLER CAUSED UNCOMMANDED YAW DURING DSCNT. ACFT EQUIP PROB YAW DAMPER SYS.

Narrative: ON APCH TO LGA, 7000 FT, AND 210 KTS CLEAN, EXPERIENCED UNCOMMANDED YAW. MAX TURN 20 DEGS. CAPT TURNED OFF AUTOPLT AND YAW DAMPER, AND LANDED AT LGA RWY 31. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR RECENTLY RECEIVED A LETTER FROM THE COMPANY ACCIDENT INVESTIGATOR STATING THAT THE UNCOMMANDED YAW WAS THE RESULT OF AN OUT-OF-LIMITS YAW DAMPER COUPLER. THE FO (RPTR) STATES THAT AFTER THE B737-300 YAWED TO THE L, THE AUTOPLT ATTEMPTED TO COMPENSATE WITH A ROLL (CTL WHEEL WAS 45 DEG R) BACK TO WINGS LEVEL. HE IMMEDIATELY RECOGNIZED THAT THIS WAS A YAW DAMPER PROB BECAUSE THE YAW DAMPER INDICATOR WAS DISPLACED COMPLETELY TO THE R. WHEN THE YAW DAMPER WAS TURNED OFF, THE INDICATION CAME BACK TO CTR. THE UNCOMMANDED YAW AND THE SUBSEQUENT EFFORTS TO CORRECT THE SIT WERE ABRUPT ENOUGH TO THROW 2 FLT ATTENDANTS ONTO THE CABIN FLOOR.

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.