Regional Jet 900 (CRJ900)
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe
Flight Crew Landed in Emergency Condition
Aircraft
At FL250 received a Yaw Damper 2 (YD2) INOP status message. Shortly after; experienced sudden yawing motion. Suspecting the yaw damper; disconnected it; which also disconnected the autopilot. Yawing motion continued; both left and right and short; sudden motion; as well as long; sustained ones. Re-engaged YD2 and the autopilot; with the uncommanded motion diminished. Were then cleared to descend via the arrival when the yaw damper disconnected; resulting in a caution message and an autopilot disconnect. Ran the QRH. Decided to leave the yaw damper off. At the same time the yaw motions started again; the cause being uncommanded rudder movements observed on the flight controls synoptic page.Advised ATC unable to continue the arrival due to no autopilot; and requested step-down instructions. Was unable to determine the cause of these rudder movements; and found no reference in the QRH. As the yawing was not diminishing (it was quite noticeable; requiring constant corrections by the flying First Officer); and the cause was unknown; we decided to land as soon as possible. [Requesting priority handling] with ATC. Advised the cabin of sterile procedures and completed all checklists; but decided to contact Dispatch once on the ground in order to have both pilots monitor the aircraft's behavior. Kept speed and configuration changes slow and gentle; and as the aircraft remained controllable; decided not to brace the cabin. Landing was normal. Once on the ground; while being externally inspected by the emergency trucks; briefed the flight attendants and the passengers. At the gate; contacted Dispatch and Maintenance.
CRJ-900 Captain reported a yaw damper INOP status message received in cruise; followed by uncommanded rudder movements. Captain requested priority handling to a normal landing.