What happened
During a descent through 15,500 feet, the crew of the aircraft encountered heavy mixed icing on the windscreen and ice accumulation aft of the de-ice boots. Following this encounter, the aircraft experienced two uncommanded lateral control movements.
The pilots reported that the flight controls felt stiff and noted a "buzzing" sensation within both the aileron and rudder controls. At the time of these events, the aircraft was in a clean configuration and the autopilot was not engaged. While the crew did not report any turbulence until reaching lower altitudes, passenger accounts varied; some passengers reported no significant turbulence, while others described the turbulence as "scary" or "too much."
The investigation
An examination of the flight controls, associated systems, and equipment was conducted, which revealed no malfunctions. Additionally, an analysis of the flight data recorder showed lateral control movements that are typically associated with turbulence.