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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 left yaw damper message appeared on EICAS and the left yaw damper is turned off; stopping the oscillations.
| Attributes | |
| ACN | 1140978 |
| Time | |
| Date | 201401 |
| Local Time Of Day | 0001-0600 |
| Place | |
| Locale Reference | ZZZ.ARTCC |
| State Reference | US |
| Environment | |
| Flight Conditions | VMC |
| Light | Night |
| Aircraft 1 | |
| Make Model Name | B757-200 |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Cruise |
| Flight Plan | IFR |
| Component | |
| Aircraft Component | Autoflight Yaw Damper |
| Person 1 | |
| Function | Captain Pilot Flying |
| Experience | Flight Crew Last 90 Days 150 Flight Crew Total 20000 Flight Crew Type 15000 |
| Person 2 | |
| Function | First Officer Pilot Not Flying |
| Experience | Flight Crew Last 90 Days 120 Flight Crew Total 25000 Flight Crew Type 8000 |
| Events | |
| Anomaly | Aircraft Equipment Problem Critical |
Narrative:
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 left yaw damper message appeared on EICAS. The first officer accomplished the checklist for that condition; and we landed without further incident.
Original NASA ASRS Text
Title: 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.
Narrative: 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.
Data retrieved from NASA's ASRS site 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.