Mike Granby wrote:

Indeed. But is there such a thing as an STC for a minor modification?

Unless I'm misunderstanding something, you're talking about two
different things. A 337 is for a major repair or alteration as defined
in 43 Appendix A. An STC is issued for a *design* modification. So
yes, you can be issued an STC for a minor design modification. 337's
and STC's aren't necessarily tied together, although they often are.

Is there any situation in which a vendor might choose to use an STC to
get a part on to his PMA, but where the existance of the STC does not
per se render the change major and thus in need of a 337?

I don't think it works that way. An STC isn't something you choose to
use, it's either required or not. PMA parts, as far as I can think,
have nothing to do with it.

Just the uneducated opinion of an non-experienced A&P who lets other
people worry about this stuff. Actually, I went to a conference on
aircraft certification last year and can't remember any of it. Should
have taken notes.