On April 13, 2005, about 0900 central daylight time, a Cessna 310, N3622D, piloted by an airline transport pilot, sustained substantial damage during landing on runway 18 (3,000 feet by 150 feet, asphalt), at the Leaders/Clear Lake Airport, Clear Lake, Minnesota. The 14 CFR Part 91 personal flight was operating in visual meteorological conditions without a flight plan. The pilot was not injured. The flight originated from the St. Cloud Regional Airport, St. Cloud, Minnesota, at 0845. According to a Federal Aviation Administration Inspector that responded to the accident, the pilot had removed a cotter pin from the nose landing gear door link prior to the flight so that he could gain access to the nose landing gear compartment. The cotter pin was not replaced. During the flight, the nose gear failed to retract and subsequently, the nose landing gear collapsed during the landing and the airplane nosed over. Review of the airplane maintenance records showed that the most recent annual inspection was performed on March 9, 2004, 13 months prior to the accident. No special flight authorization had been granted for the accident flight.