The pilot's failure to lower the flaps during the approach and maintain sufficient airspeed while flying in instrument meteorological and icing conditions and the accumulation of ice on the wings’ leading edges, which resulted in the exceedance of the airplane’s critical angle of attack and subsequent aerodynamic stall. Contributing to the accident was the pilot’s lack of proper qualification to operate the airplane under a single-pilot exemption due to his lack of total turbine time, which led to task saturation and his failure to properly configure the flaps for landing.