Several times in the past I've come across statements that particular aircraft designs stopped production, or only continued under grandfather rights, due to airworthiness regulations introduced in 1937. The Douglas DC-3 and Aeronca C-3 being two aircraft in particular.
In the case of the Aeronca C-3, it has been written that its multi-strand wire bracing was outlawed, hence the introduction of the strut equipped Aeronca K. Given that a lot of modern ultralights/microlights use multi-strand wire bracing, why was it banned, and what makes it acceptable today?
In the case of the DC-3, I believe it was the control cable sizes which were the issue. Why was it deemed necessary to go to larger sizes, as clearly the ones used must have been sufficient?
I presume these are the regulations in question:
DOT Online Database