Narrative:

While en route to phl at 6000 ft and 250 KIAS, 30 NM southwest of jfk on the jfk R-210, the aircraft experienced an uncommanded rhythmic yawing from left to right. The yawing ceased after slowing below 220 KIAS. We elected to return to jfk, declared an emergency, and landed without incident. Callback conversation with reporter revealed the following information: the reporter stated the aircraft was a jetstream 4100 and the yaw damper system was deferred as inoperative per the MEL. The reporter said the rudder inputs were at a 2-3 second frequency and the yaw damper system had been deferred and rendered inoperative per the MEL procedures. The reporter said maintenance investigated this incident and discovered the MEL requires one circuit breaker to be pulled and locked out but should have required an additional circuit breaker to be pulled to prevent yaw inputs to the rudder actuator. The reporter said the company has started procedures to correct and upgrade the MEL procedures requiring two circuit breakers to be pulled and locked out to prevent unwanted rudder inputs.

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Original NASA ASRS Text

Title: A JETSTREAM 4100 IN CRUISE AT 6000 FT DECLARED AN EMER AND DIVERTED DUE TO UNCOMMANDED RUDDER INPUTS FROM THE YAW DAMPER SYS.

Narrative: WHILE ENRTE TO PHL AT 6000 FT AND 250 KIAS, 30 NM SW OF JFK ON THE JFK R-210, THE ACFT EXPERIENCED AN UNCOMMANDED RHYTHMIC YAWING FROM L TO R. THE YAWING CEASED AFTER SLOWING BELOW 220 KIAS. WE ELECTED TO RETURN TO JFK, DECLARED AN EMER, AND LANDED WITHOUT INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A JETSTREAM 4100 AND THE YAW DAMPER SYS WAS DEFERRED AS INOP PER THE MEL. THE RPTR SAID THE RUDDER INPUTS WERE AT A 2-3 SECOND FREQ AND THE YAW DAMPER SYS HAD BEEN DEFERRED AND RENDERED INOP PER THE MEL PROCS. THE RPTR SAID MAINT INVESTIGATED THIS INCIDENT AND DISCOVERED THE MEL REQUIRES ONE CIRCUIT BREAKER TO BE PULLED AND LOCKED OUT BUT SHOULD HAVE REQUIRED AN ADDITIONAL CIRCUIT BREAKER TO BE PULLED TO PREVENT YAW INPUTS TO THE RUDDER ACTUATOR. THE RPTR SAID THE COMPANY HAS STARTED PROCS TO CORRECT AND UPGRADE THE MEL PROCS REQUIRING TWO CIRCUIT BREAKERS TO BE PULLED AND LOCKED OUT TO PREVENT UNWANTED RUDDER INPUTS.

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.