On August 7, 1995, approximately 0800 hours mountain daylight time, a Cessna TU206F, N777FN, sustained substantial damage when it collided with terrain after encountering strong downdrafts while on approach to the Sportsman's private airport, Ennis, Montana. The commercial pilot and four passengers were uninjured. Variable meteorological conditions existed and no flight plan had been filed. The flight, which was personal, was to have been operated under 14CFR91, and originated from Ennis approximately 0740 hours. The pilot reported in a telephone interview that he was executing a visual approach to a southerly landing after having taken friends for a local flight. He reported that while approaching the private airport he observed thunderstorm clouds nearby and virga coming from the base of the clouds. While approximately one-quarter mile on final, and in the vicinity of power lines, the aircraft suddenly descended vertically impacting the ground. The pilot reported that the indicated airspeed was 100 knots but that the ground speed dropped to zero.