PJ2:
Notwithstanding Feynman's example of "amateur sleuthing", the best example of a non-specialist doing superb investigative aviation work is the Moshansky Commission and it's Report in the accident at Dryden, the recommendations of which were largely though not wholly adopted by Transport Canada, (which was heavily criticized in the Report, and to which strong criticisms have since been directed regarding SMS (which I have called "the de-regulation of flight safety"), by the same man). Here was a man who knew nothing about aviation yet had tremendous access to wide-ranging information, all with good results.
Moshansky was not a man who knew nothing about aviation... he was in fact a licensed pilot and aicraft owner. Furthermore, I would argue that this was in fact an example of an
expert doing a superb investigation... maybe not an expert in aircraft accident investigation, but certainly an expert at reasoning and sifting through tremendous amounts of information to get to the truth, which is what was needed.