B757-200
Flight Crew Last 90 Days 150; Flight Crew Total 20000; Flight Crew Type 15000
Situational Awareness
Aircraft Equipment Problem Critical
Flight Crew Landed in Emergency Condition; Flight Crew Diverted; General Declared Emergency; General Maintenance Action
Aircraft
We were in cruise flight at FL400; and suddenly our aircraft started to yaw violently.  The nose was yawing both left and right; and oscillating back and forth in a rapid and rhythmic manner. The ball in the slip indicator was going from stop to stop.  There were no EICAS messages displayed; no warning lights illuminated; and all engine parameters were normal.  The autopilot was not controlling the aircraft very well; so I disengaged it.  This enabled us to control the aircraft better.  We then initiated a descent to FL300; and slowed the aircraft.  After we had descended a couple thousand feet; the yawing stopped as suddenly as it had started. With the aircraft stabilized; we assessed our situation.  We attempted to get a connection with Dispatch; but were unsuccessful.  We were able to get a relay to Dispatch and Maintenance through another company flight crew; who had SATCOM.  While we were talking with them; the yawing problem returned.  We immediately declared an emergency; and proceeded direct to ZZZ to land.  The other crew relayed to us that the company advised us to accomplish the appropriate procedure.  We continued to ZZZ; where ATC gave us vectors to the localizer; for a visual approach.  On approach; a L YAW DAMPER message appeared on EICAS.  The First Officer accomplished the checklist for that condition; and we landed without further incident.
The reporter states that as a crew; they were reluctant to turn off the yaw damper without an EICAS message thinking that the yawing might be the result of some unknown malfunction and the yaw dampers were preventing the situation from being worse. When the left yaw damper was turned off during approach the yawing stopped immediately. Maintenance changed the left yaw damper and released the aircraft but problems related to yaw damping persisted. The aircraft is approximately 30 years old.
Flying at FL400 our aircraft began yawing back and forth in a violent motion in smooth air . The oscillations seemed to be accruing approximately 2/sec and increasing ; the autopilot was having increasing trouble maintaining control . The Captain turned off the autopilot and began flying the aircraft; I preceded to communicate to ATC that we would like a lower altitude. While in a descent to FL300 and slowing from .81 to .75 Mach the oscillations stopped at approximately FL360. After leveling at FL300 we began to look at our options and technical assistance from the company. Unable to communicate with the company directly; several other company aircraft  with SATCOMs relayed the situation to them. With the company unable to provide any support other than complete the appropriate actions. This was very difficult with the absence of any flight deck warnings or cautions. During this time the yawing returned. We immediately declared an emergency and preceded to ZZZ airport. With the assistance of ATC we chose to land on the longest runway with a minimum amount of maneuvering. I proceeded to set the aircraft up for landing. After setting up for the visual with ILS backing us up we began the approach. At approximately 800 AGL our first EICAS message appeared; L YAW DAMPER. At this point the QRH procedure was accomplished and the aircraft continued for an uneventful landing.
B757 flight crew experiences violent yawing at FL400 with no EICAS messages present. The autopilot is turned off and a descent to FL300 requested. During the descent the yawing stops. While communicating with the company the yawing returns and the crew declares an emergency and diverts. At 800 feet AGL during the visual approach; an L YAW DAMPER message appeared on EICAS and the left yaw damper is turned off; stopping the oscillations.