HISTORY OF FLIGHTOn January 19, 2017 about 1148 mountain standard time, a Beech 99A airplane, N326CA, experienced a landing gear collapse while landing at the Billings Logan International Airport, Billings, Montana. The commercial pilot was not injured; the airplane sustained substantial damage. The airplane was registered to and operated by Alpine Aviation Inc., d.b.a. Alpine Air, as a Title 14 Code of Federal Regulations Part 135 cargo transport flight. Visual meteorological conditions prevailed, and an instrument flight rules flight plan had been filed but was not activated. The flight departed from Dawson Community Airport, Glendive, Montana about 0945, with a planned destination of Sidney-Richland Municipal Airport, Sidney, Montana. The pilot reported that after departure, he retracted the landing gear and heard a "thud" sound. The landing gear "unsafe" light was illuminated, and the left landing gear light remained illuminated green, an indication that it was still extended despite the landing gear handle being in the retracted position. When reaching his initial destination of Sidney, the pilot performed a low pass over the runway to enable maintenance personnel on the ground to do a visual assessment. A mechanic reported to the pilot that the nose gear was in trail. The pilot diverted to Billings and the air traffic control personnel confirmed observing the nose gear extended at a 450 angle. The pilot further stated that he aligned with runway 28R and just prior to touch down, he feathered the propellers and decreased the airspeed. During touchdown, the right main landing gear collapsed, and the airplane came to rest on the centerline. As a result of the impact, the right aileron and right-wing spar was substantially damaged. TESTS AND RESEARCHPost-accident examination of the left landing gear actuator, part number (p/n) 99-388-008-1, revealed that it appeared to be fractured allowing the hydraulic fluid to pass freely from the downside to the upside, prohibiting pressure to control the landing gear's hydraulic system. Based on the pilots' statements about the landing, the fractured left landing gear was shipped to Textron Aviation, the airframe manufacturer, for examination. The piston rod, p/n 4A125C4, was separated into two pieces through several threads. According to the report, the damaged threads displayed crack propagation by fatigue. The final fracture was by overload and resulted in deformation of the damaged threads. The National Transportation Safety Board Materials Laboratory reviewed the Textron Aviation Materials report and pictures. The lab specialist reported that the fractography indicated that the piston rod failed in the threaded region by fatigue cracking over about 60% of the cross section due primarily to axial tension force. Contact wear was observed on the faying surface of the retaining nut, the shoulder of the piston rod, and on the faying surfaces of the piston head. These signatures were consistent with the nut not being tight enough to mitigate relative sliding of the piston head relative to the piston rod shoulder and the retaining nut. It could not be determined if the manufacturer required a specific torque to be applied to the nut during assembly. The airplane operator reported that the airplane had amassed 46,416.3 hours of total time in service. According to the maintenance records, the left actuator was last overhauled in November 2001, at which time the piston rod was replaced. The operator further reported that the part had acquired 7,445.2 hours equating to 8,267 cycles. The Beechcraft Model 99 Series Maintenance Manual stated that the landing gear hydraulic actuators should be overhauled or replaced every 10,000 hours.