CHI07LA168 · Cessna R172K

The pilot reported that the right nose wheel separated from the float approximately 210 feet from where the airplane started the takeoff roll, and about 35 feet later, the left nose wheel separated from the float. After the left nose wheel separated, the left nose wheel strut dug into the turf causing the aircraft to nose over. The inspection of the airplane revealed that the nose wheel struts had separated from the castoring assemblies. The left and right nose wheel struts and castoring assemblies were sent to the National Transportation Safety Board's (NTSB) Materials Laboratory for a metallurgical examination. The visual examination of the parts showed that the castoring nose wheels from both floats had fractured at the bearing boss welds to the strut arms. Both strut arms were bent upward. The right arm was bent about 30 degrees and the left was bent about 5 degrees. The strut fractures were located through the fusion zones of the strut to bearing boss welds with some tearing of the boss material at the upper edges of the welds. Microscopic inspection of the fractures on both struts revealed matte gray textured fracture surfaces typical of overstress fractures. Deformation and rearward displacement of the boss material adjacent to the upper edge tears and fracture regions, along with the smearing contact of the lower fracture areas, were consistent with bending overstress forces on the weld joint. On August 8, 2007, Baumann Floats LLC issued Service Bulletin No. 001. The reason for the issuance of the service bulletin was "to improve the service and reliability of the nose gear assembly." The service bulletin modified the G-129 nose gear tube assembly. Compliance with the service bulletin was considered mandatory by the manufacturer upon receipt of the service bulletin, and specifically prior to operating off grass or soft field areas.

Verbatim NTSB analysis and probable cause from the NTSB dataset