Andrew Gideon wrote:

Ah, so a PMA is aircraft-specific?

Not exactly. If the item is (for example) a Cessna wing flap, then the answer
is yes. If it's a replacement for something more generic, it can be used to
replace that item on any aircraft that uses it. A good example would be one of
these lightweight starter motors. Since the part is legally a direct replacement
part, no STC is required.

The 337 is required for any change that the local FSDO believes is a "major"
modification.

An STC is legally a change to the original type certificate that allows you do
do something the original manufacturer didn't do.

George Patterson
A man who carries a cat by the tail learns something that can
be learned no other way.