What happened
While the aircraft was parked at the gate, the rudder began to oscillate rapidly. This movement was vigorous enough to shake the entire airframe. The oscillation occurred without any corresponding movement of the rudder pedals in the cockpit.
The investigation
Post-incident testing determined that the rudder was oscillating approximately 1.5 degrees either side of centerline. The motion was controlled solely by the yaw damper system. Further examination revealed a loss of grease to the yaw rate gyro spin bearings. This lack of lubrication resulted in corresponding damage to both the spin and gimbal bearings.
Findings
The yaw damp coupler and gyro had accumulated 24,850 hours in service before the failure occurred. The mechanical degradation of the gyro bearings led to the uncommanded rudder movement.