What is a PMA, Really?

[YankeeClipper]

Stemming from the woes of trying to establish whether or not I may use a weldon pump to replace the Duke pump that recently failed in my 69 337D, I decided to take a deep dive into what it is a PMA really is. Like all things FAA, I still know very little. What I do know is that it's about an approval to sell a replacement part, versus the STC which is about an original design.

What virtually none of the literature out there covers is eligibility or applicability. One must search far and wide to figure out--not whether a PMA allows me to sell a part--, but to whom, or more specifically for what. As I stated in my other thread, it has been established that the weldon pump may surely be installed in a 337E or later assuming the part that it replaces was originally used in that aircraft.

But what about aircraft where the part it replaces was indeed originally used, but aren't specified in the PMA? I'd be curious to know if any of you can find or know of any authoritative language on this. My IA feels that he is not allowed to install a part unless the application is specifically mentioned in the PMA. Further, from an FAA document:

"Product Eligibility • Identify the eligible aircraft, engines or propellers for proposed installation of your article – List each product by make, model or series, and if appropriate serial numbers, as specified on the applicable type certificate data sheet (TCDS)
• Also identify the corresponding article from the type design by a descriptive name and part number – Show this article’s location in its respective products"

The fact that you are asked to specify the application would imply that that is what the approval will be for. But once again, that is not explicitly stated and since the PMA is often based on so-called identicality, it wouldn't be unreasonable to ask the question of whether or not a PMA allows me to install a part anywhere the OEM that it replaces was originally installed.