B757 crew at FL320 has yaw damper failure which causes a significant left yaw and EICAS message.

Attributes
ACN	724119
Time
Date	200701
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : zzz.airport
State Reference	US
Altitude	msl single value : 32000
Environment
Light	Night
Aircraft 1
Controlling Facilities	artcc : zzz.artcc tower : zzz.tower
Operator	common carrier : air carrier
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	724119
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other flight crewa
Resolutory Action	flight crew : declared emergency flight crew : diverted to another airport
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

In cruise flight at FL320 flying at .81 mach; we were just cleared after the intersection and ZZZ. Aircraft suddenly veered to the left and we received an emergency door message and yaw damper EICAS message. Aircraft returned to stable flight and only emergency doors light went out. Few mins later; emergency door light came back on. At this time; we declared an emergency descending to 15000 ft. We broadcast our descent. We noticed more messages appearing on EICAS; however; upon reaching 15000 ft; aircraft was stable and flying safely. We elected to fly to ZZZ; versus ZZZ1 due to the fact both airports were equal distance; however; flying to ZZZ1 would have been greater headwinds. We landed safely at ZZZ; with no further problems. Callback conversation with reporter revealed the following information: the aircraft was dispatched with one yaw damper on MEL; resulting in the sharp yaw when the second failed. The emergency door warning may have initially been triggered by the yaw and a very sensitive micro switch; however no discrepancy could be found by maintenance after landing. The reporter also states that they had numerous other EICAS messages during the descent that seemed to appear and disappear when the HF transmit switch was pressed. He does not know how this issue was resolved.

Original NASA ASRS Text

Title: B757 CREW AT FL320 HAS YAW DAMPER FAILURE WHICH CAUSES A SIGNIFICANT LEFT YAW AND EICAS MSG.

Narrative: IN CRUISE FLT AT FL320 FLYING AT .81 MACH; WE WERE JUST CLRED AFTER THE INTXN AND ZZZ. ACFT SUDDENLY VEERED TO THE L AND WE RECEIVED AN EMER DOOR MESSAGE AND YAW DAMPER EICAS MESSAGE. ACFT RETURNED TO STABLE FLT AND ONLY EMER DOORS LIGHT WENT OUT. FEW MINS LATER; EMER DOOR LIGHT CAME BACK ON. AT THIS TIME; WE DECLARED AN EMER DSNDING TO 15000 FT. WE BROADCAST OUR DSCNT. WE NOTICED MORE MESSAGES APPEARING ON EICAS; HOWEVER; UPON REACHING 15000 FT; ACFT WAS STABLE AND FLYING SAFELY. WE ELECTED TO FLY TO ZZZ; VERSUS ZZZ1 DUE TO THE FACT BOTH ARPTS WERE EQUAL DISTANCE; HOWEVER; FLYING TO ZZZ1 WOULD HAVE BEEN GREATER HEADWINDS. WE LANDED SAFELY AT ZZZ; WITH NO FURTHER PROBS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE AIRCRAFT WAS DISPATCHED WITH ONE YAW DAMPER ON MEL; RESULTING IN THE SHARP YAW WHEN THE SECOND FAILED. THE EMERGENCY DOOR WARNING MAY HAVE INITIALLY BEEN TRIGGERED BY THE YAW AND A VERY SENSITIVE MICRO SWITCH; HOWEVER NO DISCREPANCY COULD BE FOUND BY MAINTENANCE AFTER LANDING. THE REPORTER ALSO STATES THAT THEY HAD NUMEROUS OTHER EICAS MESSAGES DURING THE DESCENT THAT SEEMED TO APPEAR AND DISAPPEAR WHEN THE HF TRANSMIT SWITCH WAS PRESSED. HE DOES NOT KNOW HOW THIS ISSUE WAS RESOLVED.

Data retrieved from NASA's ASRS site as of January 2009 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.