Narrative:

Upon liftoff and during initial phase of climb out aircraft experienced back and forth yawing motion that increased significantly with airspeed. Crew initially accelerated to clean speed; however; at this speed the back and forth yawing motion had evolved into an abrupt jerking motion. Crew elected to level off at 3000 ft MSL and slow to approximately 200 knots to avoid any further increase in intensity of aircraft yawing motion. Flight attendants immediately informed flight crew of aircraft shaking and violent back and forth motion. Due to the nature of the problem; crew elected not to engage autopilot. Captain hand flew aircraft maintaining control while the back and forth motion associated with the malfunction continued. Crew referenced QRH for applicable checklist; however; there was none. Crew discussed flight characteristics and the potentiality of over stressing the rudder due to the abrupt left and right yawing of the aircraft. At this point the crew decided to declare an emergency. ATC then instructed crew to climb to 5000 ft. Because of systems knowledge; crew focused attention on yaw damper left and right checklist. Checklist directed turning off the yaw dampers if yaw damper light illuminated. However; this was not the case as we did not have an illuminated yaw damper inoperative light. Because of system knowledge of the yaw dampers; crew elected to turn off both yaw dampers. Yawing motion stopped. Crew left both yaw dampers in off position for remainder of the flight. Crew communicated with flight attendants; passenger; maintenance and flight control. Crew considered factors associated with an overweight landing and elected to return to ZZZ. Crew performed uneventful; overweight landing at ZZZ. Crew made log book entry for aircraft malfunction and overweight landing. Crew coordinated with maintenance control and determined an aircraft trim report was not required. Flight attendants and passengers remained seated throughout flight. No injuries were reported.callback conversation with reporter revealed the following information: the reporter stated that the air carrier maintenance has not told the crew what the failure was that caused the yaw. The reporter does not believe the yaw dampeners themselves failed although it is difficult to say for sure because both of the systems were turned off simultaneously. The aircraft's motion was severe enough as the aircraft accelerated that the crew did not consider examining the EICAS flight control page for rudder movement. The rudder pedals did not move during the time the yawing movement was occurring. The autopilot was not engaged at any time during the flight.

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Original NASA ASRS Text

Title: A B757 DEVELOPED A BACK AND FORTH YAW THAT BECAME A JERKING MOTION. WHEN YAW DAMPENERS WERE TURNED OFF; MOTION CEASED. AN EMER WAS DECLARED WITH A RETURN TO LAND.

Narrative: UPON LIFTOFF AND DURING INITIAL PHASE OF CLIMB OUT AIRCRAFT EXPERIENCED BACK AND FORTH YAWING MOTION THAT INCREASED SIGNIFICANTLY WITH AIRSPEED. CREW INITIALLY ACCELERATED TO CLEAN SPEED; HOWEVER; AT THIS SPEED THE BACK AND FORTH YAWING MOTION HAD EVOLVED INTO AN ABRUPT JERKING MOTION. CREW ELECTED TO LEVEL OFF AT 3000 FT MSL AND SLOW TO APPROXIMATELY 200 KNOTS TO AVOID ANY FURTHER INCREASE IN INTENSITY OF AIRCRAFT YAWING MOTION. FLIGHT ATTENDANTS IMMEDIATELY INFORMED FLIGHT CREW OF AIRCRAFT SHAKING AND VIOLENT BACK AND FORTH MOTION. DUE TO THE NATURE OF THE PROBLEM; CREW ELECTED NOT TO ENGAGE AUTOPILOT. CAPTAIN HAND FLEW AIRCRAFT MAINTAINING CONTROL WHILE THE BACK AND FORTH MOTION ASSOCIATED WITH THE MALFUNCTION CONTINUED. CREW REFERENCED QRH FOR APPLICABLE CHECKLIST; HOWEVER; THERE WAS NONE. CREW DISCUSSED FLIGHT CHARACTERISTICS AND THE POTENTIALITY OF OVER STRESSING THE RUDDER DUE TO THE ABRUPT LEFT AND RIGHT YAWING OF THE AIRCRAFT. AT THIS POINT THE CREW DECIDED TO DECLARE AN EMERGENCY. ATC THEN INSTRUCTED CREW TO CLIMB TO 5000 FT. BECAUSE OF SYSTEMS KNOWLEDGE; CREW FOCUSED ATTENTION ON YAW DAMPER L AND R CHECKLIST. CHECKLIST DIRECTED TURNING OFF THE YAW DAMPERS IF YAW DAMPER LIGHT ILLUMINATED. HOWEVER; THIS WAS NOT THE CASE AS WE DID NOT HAVE AN ILLUMINATED YAW DAMPER INOP LIGHT. BECAUSE OF SYSTEM KNOWLEDGE OF THE YAW DAMPERS; CREW ELECTED TO TURN OFF BOTH YAW DAMPERS. YAWING MOTION STOPPED. CREW LEFT BOTH YAW DAMPERS IN OFF POSITION FOR REMAINDER OF THE FLIGHT. CREW COMMUNICATED WITH FLT ATTENDANTS; PAX; MAINTENANCE AND FLIGHT CONTROL. CREW CONSIDERED FACTORS ASSOCIATED WITH AN OVERWEIGHT LANDING AND ELECTED TO RETURN TO ZZZ. CREW PERFORMED UNEVENTFUL; OVERWEIGHT LANDING AT ZZZ. CREW MADE LOG BOOK ENTRY FOR AIRCRAFT MALFUNCTION AND OVERWEIGHT LANDING. CREW COORDINATED WITH MAINTENANCE CONTROL AND DETERMINED AN AIRCRAFT TRIM REPORT WAS NOT REQUIRED. FLIGHT ATTENDANTS AND PASSENGERS REMAINED SEATED THROUGHOUT FLIGHT. NO INJURIES WERE REPORTED.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT THE ACR MAINTENANCE HAS NOT TOLD THE CREW WHAT THE FAILURE WAS THAT CAUSED THE YAW. THE REPORTER DOES NOT BELIEVE THE YAW DAMPENERS THEMSELVES FAILED ALTHOUGH IT IS DIFFICULT TO SAY FOR SURE BECAUSE BOTH OF THE SYSTEMS WERE TURNED OFF SIMULTANEOUSLY. THE ACFT'S MOTION WAS SEVERE ENOUGH AS THE ACFT ACCELERATED THAT THE CREW DID NOT CONSIDER EXAMINING THE EICAS FLT CONTROL PAGE FOR RUDDER MOVEMENT. THE RUDDER PEDALS DID NOT MOVE DURING THE TIME THE YAWING MOVEMENT WAS OCCURRING. THE AUTOPILOT WAS NOT ENGAGED AT ANY TIME DURING THE FLT.

Data retrieved from NASA's ASRS site as of May 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.