Bob Noel 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...".


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"? Is one
required for an STCed alteration?

- Andrew