On October 4, 2014, about 1028 central daylight time, a Raytheon 58, N345PG, entered an uncontrolled descent from cruise flight near Dixon, Illinois. The pilot recovered the airplane and it was landed without further incident. Both pilots were not injured and the airplane sustained substantial damage. The airplane was owned and operated by the pilot as a 14 Code of Federal Regulation Part 91 as a personal flight. Instrument meteorological conditions prevailed for the cross-country flight that was operating on an instrument rules flight plan. The flight originated from Chicago Executive Airport (PWK), Wheeling, Illinois, and was originally destined to University of Illinois-Willard Airport (CMI), Savoy, Illinois, but diverted to Dixon Municipal Airport-Charles R. Walgreen Field (C73), Dixon, Illinois. The airplane entered an uncontrolled descent after attaining a cruise altitude of 10,000 feet mean sea level (MSL), about 10 nautical miles southwest of C73. The airplane was piloted by the aircraft owner (first pilot) and a Windy City Flyers flight instructor (second pilot). The pilot reported that during a climb to 8,000 feet MSL, light ice accumulated on the deicing boots. Activation of the boots eliminated about 30-40 percent of the ice climbing through instrument meteorological to attain an initial altitude of 8,000 feet MSL, prior to a second climb to 10,000 feet MSL. Upon level off, the there was a "flutter" in engine speed followed by the airplane rolling "quickly to the left." The pilot stated that the flight instructor applied control inputs, and the airplane's spin was arrested about 2,500 MSL. The airplane landed without further incident, following the recovery, at C73. The airplane sustained substantial damage to the right wing and elevator control surfaces, which was consistent with forces from excessive accelerations (over-g). Radar data showed that the airplane descended from its cruise to recovery altitude in about 30 seconds. A weather briefing for the flight showed an AIRMET for moderate icing conditions was in effect and light showers, rain, and, snow were forecast. The pilot stated that he did not fly in such weather conditions unless accompanied by an experienced pilot. The pilot stated that he assumed that ice accumulation, which was not visually evident, resulted in the airplane departing controlled flight. Postaccident examination/testing of the airplane revealed no mechanical anomalies that would have precluded normal operation. The flight instructor was provided with, but did not return a National Transportation Safety Board Pilot/Operator Accident/Incident Report.