PMA Questions

He's out of his tree. There are now a couple of different companies
that make replacement blades for the more popular Bell helicopters -
completely PMA'd parts. But you are still going to need a heck of an
STC in order to mount one on your Cessna 172. G

If I just had to guess, I would say that where he got confused is that
if you are replacing a part on your aircraft with a part that is a
PMA'd *replacement*, then you don't need an STC. But that's because
it's already covered by the TC (or by a previous STC). The PMA says,
in effect, you can treat these parts as APPROVED parts (including
approved replacement parts). Doesn't necessarily mean you can stick it
on your plane anywhere you feel like.

To be fair to him, what he meant was that if you had a product that was
PMA/STCed for a given airframe (for example the B&C standby alternator
referened at [1] below) then the installation of this product did not
require a 337, even thought it was being installed using an STC. He
says that the PMA approval removes the need for additional paperwork.
My view is that you still need the 337 if it's a major alteration,
which this would be, and that the PMA-ness of the parts is merely
something that allows the manufacturer to put them on the market,

[1] http://www.bandcspecialty.com/PA32desc.html