On September 30, 1995, approximately 1515 mountain daylight time, a Piper PA-18A, N1953P, received substantial damage when it nosed over following a main landing gear wheel separation on landing roll at the pilot's private airstrip near Roberts, Idaho. The private pilot-owner of the two-seat airplane, an adult passenger, and one child passenger (5 years old) received minor injuries. One passenger, a two-year-old child, was not injured. The flight was a local 14 CFR 91 flight originating from the pilot/owner's airstrip, where the accident occurred. Visual meteorological conditions prevailed and no flight plan had been filed. The pilot reported: "On landing, plane touched down and a gust of wind lifted plane off ground about three feet. Left front wheel fell off at this time. On second touchdown, left landing gear dug into dirt runway causing plane to flip upside down." Statements from the pilot, adult passenger, witnesses, and responding law enforcement officials, contained in the Jefferson County sheriff's report of the accident, indicated that after the accident, the wheel retaining nut and cotter pin were not observed to be in place on the left main gear axle. Additionally, the statements in the report indicated that the individuals at the scene observed that there was no cotter pin securing the retaining nut on the right main landing gear wheel although the right main wheel retaining nut was still in place. The left main wheel was located about 50 feet from the airplane after the accident. The sheriff's report statements indicated that a search was made on the airstrip for the left wheel retaining nut but that it was not located. The pilot's NTSB accident report indicated that the airplane's last annual inspection was on April 12, 1995, 12.16 flight hours before the accident. The pilot stated in a telephone query by the investigator on March 11, 1996, that Hart Enterprises of Rexburg, Idaho, had recently rebuilt the aircraft, and that the annual inspection was performed in conjunction with this rebuild. He also stated during this telephone query that he performed a preflight inspection on the day of the accident but had not checked the main wheel cotter pins, and that there are no wheel covers installed on the main wheels. In a telephone inquiry to Hart Enterprises on March 8, 1996, a representative of the company stated to the investigator: "Three people missed the checking of the cotter key, if you can figure that out." The adult passenger reported in his sheriff's report statement that shortly before the accident landing, he unfastened his seat belt since the children, who were riding in his lap, were interfering with the travel of the control stick.