SEA83FA038

Before takeoff, the plt received a wx briefing & filed an IFR flt plan to Idaho falls, id at 1300 ft. he took off at 1804 PST. At 1814 PST, just prior to calling level at 13,000 ft, he reported he was encountering light turbulence & light rime ice. Aprx 4 min later, the plt reported experiencing heavy vibrations. He requested & received a clearance toreturn to portland & descend to 7000 ft. shortly after that, the acft entered a tight descending turn which the plt reported he was having difficulty arresting. He reported regaining directional control at 6000 ft & said the acft had a heavy load of ice. Shortly thereafter, the acft crashed in mountains at the 3130 ft level. Due to damage from impact & wreckage retrieval, the preimpact condition of all deicing components was not verified. The acft was not certified for flt in known icing conditions. Moderate mixed icing condition & moderate turbulence were forecast. The plt was briefed there had been reports of moderate icing thru-out the area.