On March 11, 1998, a Boeing 737-205 (registration N733AR) operated by Arco Alaska, Inc. was involved in an aviation accident near Anchorage, AK. Investigators recorded the probable cause as: Loss of grease to the yaw rate gyro spin bearings, and corresponding damage to the spin and gimbal bearings. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 2 related events involving the same aircraft type or operator are linked below.
While parked at the gate, an airplane experienced rapid rudder oscillations that shook the entire aircraft. Post-incident testing found the oscillation was limited to yaw damper authority and caused by loss of grease and damage to the yaw rate gyro bearings.
Incident
During parking at the gate, the airplane's rudder began a rapid oscillation that caused the entire airframe to shake. No corresponding movement of the cockpit rudder pedals was observed.
Investigation
Postincident testing determined that the rudder was oscillating approximately 1.5 degrees on either side of the centerline. The oscillation was confined solely within the authority of the yaw damper system. Further examination revealed a loss of grease on the yaw rate gyro spin bearings, along with corresponding damage to the spin and gimbal bearings. The yaw damper coupler and gyro assembly had accumulated 24,850 hours in service prior to the event. The testing concluded that the rudder movement was produced by the yaw damper itself, as no other cause was identified.
Probable cause
Loss of grease to the yaw rate gyro spin bearings, and corresponding damage to the spin and gimbal bearings.
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