You need to check the AP was on the TC for the relevant airframe S/N range.
Socata TB owners got shafted on this, on the “dual GNS box is a Major Mod” totally idiotic crap from EASA. Socata certified a GNS430+430 and a GNS430+530 on their TC, but only for S/N of 2000 and higher.
I surely understand that. In the field, dealing with the A&P IA who in actuality approves the installation, I would maintain that the written STC limitations drive the process. That’s simply because what’s explicitly written in the STC is the only mechanism by which FAA records the existence of “permission”.
The fact that the IA is breaking the law and is not getting caught doesn’t in any way make it legal. If there is an accident and the autopilot is installed without permission of the STC holder, both the IA and aircraft owner will have this as a potential liability. Recording the 337 does not make it legal, it is merely recorded.
The burden of proof would not be on the IA or the owner of the aircraft & autopilot, and there is no record required by FAA for STC holder permission being granted for the owner to install his autopilot in his aircraft. Its hard to prove a negative, and I think it would be a nonsense legal case.
If the owner of both the autopilot and the aircraft is being forced in essence to buy another autopilot by the manufacturer, when he already owns one that was bought originally from the manufacturer, I think anything that can be done to defeat that coercive purpose would be a positive. I am quite happy to hold that opinion and have no interest in changing it 
Again, this is why I don’t in general do business with avionics manufacturers or their representatives.
Here are a few regulations you should be aware of:
Subpart E—Supplemental Type Certificates Sec. 21.120
Responsibility of supplemental type certificate holders to provide written permission for alterations.
A supplemental type certificate holder who allows a person to use the supplemental type certificate to alter an aircraft, aircraft engine, or propeller must provide that person with written permission acceptable to the FAA.
Subpart E—Maintenance, Preventive Maintenance, and Alterations Sec. 91.403
General.
(a) The owner or operator of an aircraft is primarily responsible for maintaining that aircraft in an airworthy condition, including compliance with part 39 of this chapter.
(b) No person may perform maintenance, preventive maintenance, or alterations on an aircraft other than as prescribed in this subpart and other applicable regulations, including part 43 of this chapter.
(c) No person may operate an aircraft for which a manufacturer’s maintenance manual or instructions for continued airworthiness has been issued that contains an airworthiness limitations section unless the mandatory replacement times, inspection intervals, and related procedures specified in that section or alternative inspection intervals and related procedures set forth in an operations specification approved by the Administrator under part 121 or 135 of this chapter or in accordance with an inspection program approved under Sec. 91.409(e) have been complied with.
(d) A person must not alter an aircraft based on a supplemental type certificate unless the owner or operator of the aircraft is the holder of the supplemental type certificate, or has written permission from the holder.
Once again, if an IA fraudulently installs an STC and files it with the FAA, they will be in serious trouble if it is discovered and will likely lose their certificate.
If you remember, NCYankee, I pointed out initially that if the written STC was specific to the aircraft serial number, then the avionics manufacturer had the owner and the IA of the equipment in a position where they couldn’t legally base the installation on that STC, as approved data. There’s no need to endlessly repeat the point. Nor is there any need for me to say again that charging an excessive amount to tweak a few pots or swap an EPROM to suit a different airframe and issue a new approval would be coercion, and in my view morally reprehensible. Whatever, the world operates under more practical constraints. I’d be interested to know if anybody here has ever seen an example of an A&P IA installing an owners used hardware item in the owners aircraft and later facing any implications for doing so without the written permission of the STC holder. Just as factual data.
Moving on, what about a field approval or under the original TC as others mentioned, making no reference to an STC? The owner of the autopilot owns both it and the plane, after all. Tweaking pots (or the electronic equivalent) to tune the unit is precisely what the buyers of experimental autopilots do to get the unit working in their plane. It’s particularly fun in my limited experience when its a two axis autopilot that’s installed in an aircraft that desperately needs the third control axis to turn the plane!
what about a field approval making no reference to the STC?
One can use the contents of an STC as Approved Data to support a 337 in a Field Approval. I did that in this case on the advice of an FAA FSDO inspector in California. The STC was for a Cessna 421 so obviously not applicable to a TB20 but anything relevant in the STC can be used.
But as you say that still leaves you with a Field Approval which means an FAA FSDO has to approve it before it can be installed. Then the IA signs off the finished job as IAW the FAA-approved 337 and sends off the whole package to AFS-750.
The “European problem” is that there isn’t an FSDO which can officially help you. The Frankfurt one is shut, and the NY IFU (which is supposed to be doing it) is not functioning. The only ways that are open to European based N-regs are
There are UK avionics shops which use the second route. They have a tie-up with a DER, presumably US-based, whereby they do the design, the DER does a bit, and the customer is charged a minimum of GBP 2000 + VAT for the design package. I recall derassociates.com can do this for $300 per sheet, if they don’t need to do any actual work (but that firm does not communicate with end users).
The first route is an obvious business opportunity for US based avionics installers 
As regards whether the FAA verifies the STC permission, what I have been told is that nobody at AFS-750 checks anything they receive. But if you were to export the aircraft, and/or somebody has a reason to get the FAA aircraft record (available electronically for some small fee) and they do it diligently, the alleged lack of STC permission would be discovered.
Reading between the lines, IMHO, loads of STC-based 337 jobs are done without the STC holder’s permission in any form, because you only have to read the US flying forums for 5 minutes to realise that both owners and shops out there have difficulties reading the simplest of regs
My view on the whole business of STCs is mixed. On the one hand, the STC holder is entitled to make a return on his work in getting the STC pushed through the system. I could see an STC for a new autopilot servo would involve a lot of work (been there) though pretty obviously the way to deal with the ROI is to sell the hardware itself, not the permission to use the STC! On the other hand, probably 99.9% of avionics STCs involve zero engineering effort beyond opening up the applicable installation manuals and generating a composite diagram based on what is found there IOW, somebody collects a load of public-domain FAA-approved data, collates it into a package, and charges for the “intellectual property” (what “intellectual property”?).
I suppose somebody might take the view that they don’t make money on a secondhand piece of kit so they charge a lot for the permission to use the STC which is required even for the install of the secondhand item. It would look much less of a corporate greed exercise if they charged for the overhaul of the kit and did the STC permission for nothing
Peter, when a 337 is filed referencing an STC, in my (recent, direct, I mailed the envelope) experience the STC itself is not it part of the package. The electronic record is therefore unaffected by the STC holder approval issue. FAA is mainly concerned with safety and airworthiness.
It’s not just the owners and shops that tend to be illiterate, it’s also the FSDO people. The world carries on regardless – literacy is probably overrated
I think a lot of people could benefit from reading your post, that minor point notwithstanding.
when a 337 is filed referencing an STC, in my (recent, direct, I mailed the envelope) experience the STC itself is not it part of the package
Yes – the final 337 is at the end of the article I wrote
But….. it is still a Field Approval, with all the Major Hassle you get in Europe with that.
That is why a 337+STC is so much better in Europe, because you just do the install and mail the stuff to the FAA.
And for an illiterate installer an STC is a must even for changing the door handle
Peter, what I meant to say is that when you install an item on an N-registered under an STC, the 337 is submitted to FAA, and it references the STC. A copy of the STC is not submitted with the 337. Therefore there is no FAA record of the STC applicability, and reviewing the CD will provide no visibility to whether STC holder permission was in place, other than the A&P IA indicating that he has inspected the installation for STC conformity.
Hmmm, I get your drift but I have no idea. I thought the STC had to accompany the 337, together with the STC owner’s permission.
When I was doing that EHSI job, one of the FAA docs (the one that lists all the categories of what is allowed as approved data) said that the FAA will require the STC owner’s permission. Obviously, whether they check is another matter. The 337+STC package doesn’t go to an FSDO; it goes to the AFS-750 “file-only” address and I don’t think they have anybody there who looks at the stuff. Checking that sort of thing for consistency requires a certain IQ; above about 20.
I know for a fact a lot of field approvals get sent to AFS-750 also The FSDO inspector told me that. There isn’t anything they can do about it…
