Incident Description
During a descent through 15,500 feet, the flight crew reported encountering heavy mixed icing conditions on the windscreen with some ice accumulation aft of the de-ice boots. Subsequently, the aircraft experienced two uncommanded lateral control movements. The pilots noted that the controls felt stiff and they perceived a 'buzzing' sensation in the aileron and rudder controls. At the time of the event, the autopilot was not engaged and the aircraft was in a clean configuration. The crew did not report any turbulence until reaching lower altitudes.
Passenger Reports
Passenger accounts of the event varied. Some passengers reported no significant turbulence or nothing excessive, while others described the turbulence as 'scary' or 'too much'.
Investigation Findings
A thorough examination of the flight controls, associated systems, and equipment revealed no malfunctions. Analysis of the flight data recorder (FDR) data showed some lateral control movement that is typically associated with turbulence. The investigation did not identify any mechanical or system anomalies.