No, they're not. It's only "after the fact". You really don't want the FAA probing after your part fails, but up until it does there's pretty much no oversight. We have to trust that you haven't fraudulently filled out your paperwork.
There is a significant amount of oversight. This includes inspections of the manufacturing facility and involvement from FAA personnel and FAA designees in both the design, testing, and approvals of parts for PMA and STC work for certificated aircraft applications.
https://www.faa.gov/aircraft/air_ce...e_prop/engine_approvals/engines_pma_approvals
There's a reason that an engine rebuild on a small aircraft can run you $50k, and it isn't the material cost. The certification work required and the level of quality control and inspections for the parts is not insubstantial.
I'm currently working on a PMA for a parts kit on one of my projects. It literally consists of two pieces of bent sheet aluminum, an electrical diode, three mil-spec circuit breakers, and a mil-spec relay. The part design and inspection work has probably consumed about 15 hours of engineering time. The compliance reports for the electrical certification aspects is probably another 5 hours. The compliance report for the flammability aspects is another 5 hours(plus both reports have to be reviewed and signed off by an FAA designee, in my case that is my boss since we have an organizational delegation from the FAA) plus the compliance report and sign off by the structural engineer and FAA structural unit member(probably another ~5 hours).
All of that is just to get the part approved to be added to the STC. Once it's added to the STC and the STC update is signed off, we then have to send a big bunch of documents off to our designated PMA guy who then spends at least a day processing and checking all of our work before he can issue a PMA letter. Once we get that letter we can then add a label to the part that designates it as PMA under that STC. Part of that is checking that the drawing we have for the parts and suppliers has enough detail in it to allow our incoming inspectors to make sure the part is not counterfeit and meets the specifications that the FAA delegates have determined is important.
The entire organization is audited by the FAA at least annually. We also have FAA personnel coming to the facility for meetings and tours randomly throughout the year for specific projects or for them to provide oversight as they decide they want to. Usually it is only if there is something particularly new or concerning to them being developed.
There was just a huge AD issued for continental engines for them shedding counterweights in flight. They had pre-emptively identified that there was a problem, what the problem was, and identified an inspection procedure and fix that was issued in a mandatory service bulletin before the FAA officially got involved. There is actually a process for this kind of thing and it starts with the manufacturer notifying the FAA that there is a problem and then keeping them apprised of the situation as its dealt with. Mistakes can be made by anyone, but the FAA can, and has in the past, revoked a manufacturers right to produce parts and get STC's. They can revoke an ODA at any point too if the manufacturer isn't doing the right thing. The end of the process usually has the FAA issuing an AD for whatever the issue ends up being.
https://www.aopa.org/news-and-media...continental-engine-issue-prompts-immediate-ad
