The original fatigue crack was not due to a manufacturing defect.
Then what caused it? Are they saying it was the operator's fault? What did the operator do to cause it so everyone can avoid doing the same procedure which initiated this crack? Cracks just don't "happen"!
If the crack was not due to a manufacturing fault, then t he alternatives are:
1. The blade design and certification testing may have been deficient.
2. There may have been an issue with the heat treatment for the alloy sheets used to make the blade skin.
3. There may have been an issue with orientation of the rolling direction of the metal. (Sheet metal is strongest in the rolling direction.)
4. The investigators may not have examined all alternatives which may have initiated the crack. For example did they consider the possibility that a bond defect may have led to a redistribution of stress which caused the crack to initiate?
It has been my experience that the level of understanding of adhesive bond failure forensics within the aviation safety investigation community and even manufacturers is open to question.
Regards
Blakmax