On April 29, 2018, about 2230 central daylight time, a Beech 1900C, N172GA, was substantially damaged when the right main landing gear collapsed as the airplane started to taxi for takeoff at Joe Foss Field Airport (FSD), Sioux Falls, South Dakota. The airline transport pilot was not injured. The airplane was registered to and operated by Alpine Aviation, doing business as Alpine Air Express, under the provisions of Title 14 Code of Federal Regulations Part 135 as a non-scheduled domestic cargo flight. Visual meteorological conditions prevailed at the airport at the time of the accident, and an instrument flight rules flight plan had been filed. The flight was originating from FSD, and was destined for Rapid City Regional Airport (RAP), Rapid City, South Dakota. According to Federal Aviation Administration (FAA) inspectors, the airplane had just moved forward to taxi for takeoff when the right main landing gear collapsed. A postaccident examination of the airplane revealed the right main landing gear drag brace bolt failed at the wing forward attach point. The drag brace bolt was not retained for further examination. The strut pivoted aft when the airplane moved forward, and the landing gear collapsed. The aft center wing lower spar cap was dented and deformed up to 0.090". Further examination found pitting corrosion on the spar cap.