On September 8, 2004, at approximately 1415 mountain daylight time, a Beech B-60, N1551L, piloted by a private pilot, was substantially damaged when it impacted terrain while taxiing at Natrona County International Airport (CPR), Casper, Wyoming. Visual meteorological conditions prevailed at the time of the accident. The business flight was being conducted under the provisions of Title 14 CFR Part 91 without a flight plan. The pilot reported no injuries. The cross-country flight originated in Riverton, Wyoming, at approximately 1130. According to the accident report submitted by the pilot, he had been taxiing south on Alpha taxiway and was departing the taxiway en route to the Trajen Air ramp area when the nose gear struck a storm drain. The nose gear collapsed and the airplane skid approximately 6 to 10 feet before coming to rest on its nose. Both propellers struck the ground and the wheel well structure was crushed and torn. According to the ramp agent that was marshalling the airplane to parking, the airplane was taxiing at a slow pace when it impacted the storm drain, bounced, collapsing the nose gear and slid. A postaccident examination of the airplane by the FAA revealed that the nose gear bolt, spacer and nut were missing. No other anomalies were noted. According to the pilot, the bolt had been replaced on September 1, 2004. In an FAA interview with the mechanic, it was discovered that the Beech maintenance procedures had not been followed while servicing the nose gear. The difference in elevation between the storm drain and the abutting pavement was 1 inch. Title 14 CFR Part 139.305, Paved Areas, states that, "The pavement edges must not exceed 3 inches difference in elevation between abutting pavement sections and between pavement and abutting areas."