Event Summary
A pilot reported that during a soft field landing on a grass airstrip, the aircraft touched down on the main landing gear. Immediately after the nose wheel contacted the ground, grass and dirt obscured the windshield and the airplane decelerated rapidly, subsequently nosing over.
Post-Accident Findings
Following the accident, it was observed that the nose wheel had separated from the nose wheel fork. The axle bolt, both buckets, and a locknut were recovered from the grass airstrip along the aircraft's ground track.
Maintenance and Parts Inspection
Inspection of the separated components and a review of maintenance records revealed evidence that the nose gear and tire installation were for a Piper PA32. The accident aircraft was a Cessna U206B. Although the two installations are similar, the Piper installation uses an axle bolt secured with a lock nut (which was installed on the accident aircraft), whereas the Cessna installation requires an axle bolt drilled for a castle nut and cotter key.
An examination of the nut and bolt threads was conducted at the NTSB Materials Laboratory. The specialist reported that the threads on both the bolt and nut showed no evidence of excessive loading or stripping damage. Due to the lack of damage to the axle bolt and nut, it appeared that the bolt was not locked down at the last maintenance inspection and subsequently backed off.