In article ne.com,
Andrew Gideon wrote:

If the part is not PMA'd for your aircraft, you'll need a different
"basis" for the approval of the installation in your aircraft. This
does not need to be via an STC.

Ah, so a PMA is aircraft-specific? That's a little suprising - although
reasonable in retrospect - as a number of aftermarket instrument lighting
vendors advertise that their product is "PMA certified", or some such.
No
mention is made of "...for the following aircraft...".

I'm sorry. I was sloppy with my language. I shouldn't have
said "not PMA'd for your aircraft", but something more like
"not a PMA replacement for a part for your aircraft."


As an example, a digital OAT probe I installed in my cherokee
was PMA'd for certain beechcraft aircraft but not my cherokee.
The installation this digital OAT probe was approved via a 337.

I'm confused about the role of a 337. When is one required?

One example is what you've cited: a change for which neither PMA nor STC
exists. But is one required for a replacement with a PMAed "part"?

It is my understanding that replacing an existing part with
a PMA replacement part wouldn't require a 337.

Is one required for an STCed alteration?

Yes.

are we confused yet?

--
Bob Noel