From here
What about the reverse direction, i.e. installation of an EASA STC on an FAA N-reg aircraft?
Specifically, MT Propeller has (for years now) an EASA STC for my Cessna R172K Hawk XP (TCDS 3A17) but there is no equivalent FAA STC. I’ve asked the question to MT and they have no interest in obtaining the FAA equivalent.
chflyer wrote:
What about the reverse direction, i.e. installation of an EASA STC on an FAA N-reg aircraft?
The previous owner of my plane did basically that with an MT propeller, and it was done via a field approval. I assume the European STC was considered to be acceptable data. This was in roughly 2004, so a while ago if that makes any difference.
The FAA-EASA treaty (the BASA) provides for mutual acceptance of data used to obtain an STC.
Not the STC itself of course – that has been the Holy Grail for decades. That does not work in either direction currently.
So the data used to obtain an EASA STC ought to be usable as approved data for an FAA Field Approval, FAA STC, or perhaps even other processes like a DER 8110.
Or perhaps even for an FAA Minor Alteration, in scenarios where the mod does not qualify for a Major Alteration. There are unquestionably STCs out there where no STC was needed; it is a Minor Alteration and the STC was done purely for marketing reasons so the A&P feels “good” about it. Concorde batteries are one example.
A search for BASA digs out loads of leisure reading 
Peter wrote:
So the data used to obtain an EASA STC ought to be usable as approved data for an FAA Field Approval
FWIW (not a lot, it’s only terminology), I think this would be acceptable data versus approved data. Approved data would be an STC or something else allowing a 337 to be filed by an IA without prior interaction with FAA on a given project.
Yes
However, I wonder if there is that old gotcha. The data used to obtain an (EASA or FAA) STC is likely to be confidential.
I know for a fact there are people who know the answers but this is specialised work and they are doing consultancy for money, and won’t likely post anything too useful on a forum. I have some contacts in the US, I could ask, if needed. One is an in-house DAR, who does a lot of EASA to FAA conversion stuff.
In the case of the European STC’d MT propeller on my FAA registered plane, the FAA field approval appears to use the foreign STC itself as acceptable data. The FSDO approval signature on the 337 and short 337 technical write-up are the only difference on that form to installation of an FAA STC’d propeller. The funny thing is that the European STC for my type does not cover electric actuation which my plane/propeller has. Perhaps the previously FAA approved use of almost the exact same model electric CS propeller on e.g. Grumman Tigers was enough to convince the FSDO. However in the case of my plane, the then-owner and his IA were the only people involved.
What is really strange about this particular case is that the MT EASA STC is specifically for the US-manufactured R172K. For this aircraft model, often an EASA STC will be for the French Reims-produced Hawk XP, under licence from Cessna …. i.e. the FR172K…. but AFAIK not in this case. There are, I believe, quite a number of FR172K Hawk XP’s in Europe but very few of the US R172K model. Most of the R172K built are still in the US, so it is rather strange that having gone to the effort of getting the EASA STC there would be no interest by MT USA of adding an FAA STC since the additional effort for them must be very small vs the potential target installed base.
