(…) The bolt that sheared off, according to Bell records obtained by the safety board, was one of two made in the same batch in 1999. The second was discarded, but no reason was recorded in the detailed manufacturing records that aircraft manufacturers are required to keep.
The safety board document shows that agency investigators and engineers, working with Bell, examined the remaining portion of the sheared bolt and found abnormal, brittle characteristics in the heat-treated, high-strength steel, including pre-existing microscopic cracks.
Newly manufactured bolts like those used in the aircraft were then tested by Bell at the safety board’s direction. One of those pins was purposely heat-treated at less than the required 900 degrees.(…)
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