On April 25, 2008, at approximately 1123 central daylight time, a Beech C90, N21SP, was substantially damaged when the right main landing gear collapsed during the landing roll. The commercial pilot and five passengers were not injured. The airplane was owned by Southwest Aviation Specialties LLC and operated by the United States Marine Corps (USMC). The local flight originated at Baton Rouge Metropolitan Airport (BTR), Baton Rouge, Louisiana at approximately 1030. Visual meteorological conditions prevailed and a visual flight rules (VFR) flight plan was filed for the Title 14 Code of Federal Regulations Part 91 public-use flight. In a telephone interview, the pilot reported the accident occurred after landing on runway 13 at BTR. During the landing roll, the pilot felt the airplane shake "as if we hit a pothole." Furthermore, the pilot reported that the airplane began to shimmy, the right wing dropped, and the airplane began to slide. Once the airplane came to a halt, the pilot and his passengers were able to egress without assistance. The responding Federal Aviation Administration (FAA) inspector discovered that the upper torque knee failed which resulted in a collapse of the right main gear. Examination of the upper torque knee revealed a preexisting fracture with associated coloring consistent with corrosion. Another fracture located on the same component was found to be shiny and without corrosion. On January 22, 2002, airworthiness directive (AD) 2002-01-10 required an inspection of the main landing gear upper and lower torque knees for fatigue cracks. An inspection was to be performed at the next 100 hour inspection and at a subsequent interval not to exceed 1,000 hours time in service. A review of the logbooks revealed that the last recorded inspection of the torque knees occurred on January 3, 2003 at which time the airframe had 9,327.7 hours. On July 27, 2007 a logbook entry recorded the airframe time as 10,306.8 hours. The only hours that could be verified after this logbook entry was from the USMC pilot. The pilot's logbook recorded his flights in N21SP starting on October 1, 2007. From October 1 to October 23, the pilot logged over 23 hours in the airframe requiring an inspection to be in compliance with AD 2002-01-10. There is no record of the compliance with the AD despite continued operation. The airplane owner leased the airplane to a company who provided an airplane and maintenance by contract to the USMC.