This (local copy) has come out recently.
I think it was posted here recently in connection with something else but it’s worth another mention.
For example when I was doing the SN3500 installation, the FAA office in NY gave me a load of complete bull about an EHSI being an EFIS which thus needs an STC! This kind of document, which was originally just internal FAA guidance, would have been very useful.
The other thing is to notice how far the goalposts have moved since the general guidance in FAR 43 Appendix A.
Peter, without having reference to FAR 43 appendix A, are you saying that Field Approvals are now harder to obtain? If so, that’s certainly consistent with experience.
The document is interesting but other than providing a ‘we won’t field approve this type of mod’ justification for the FSDO, the more interesting thing to me is the definition of Approved Engineering Data. In the past a lot of Field Approvals were done on the basis of a simple (non DER) package. From what I hear, the FSDOs are now being populated by ex-military people with no GA or meaningful engineering experience and those people obviously will want to pass the buck to a DER… assuming they can’t use this document to say no to anything but an STC! The FSDOs know a lot less than they used to which has its problems… and perhaps its ‘practical’ benefits too (they hide in the office)
All the modifications and approval routes covered by this document (FAA STCs and Field Approvals) are major alterations filed under a Form 337 as the top level document.
I wrote down all I know on this topic in the above-linked SN3500 writeup, under Certification Issues.
NCYankee is the specialist here, since I have never done this in the USA, but what I see is that US based owners use a particular FSDO inspector for exactly the same reason they use the same AME – until he drops dead 
A Major Mod done as a Field Approval should not need a DER package unless
The last one is thus popular with certain European avionics shops, who prepare the job, get some DER to sign it off, give him a few hundred $, and bill the package to the client at about €2000. I got that once… on a box which had an AML STC! Then they can use the DER package to get an EASA STC which they can sell to other shops
FAA equipment substitution concession for TSOd items
This appeared on a US site
https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_20-41A.pdf local copy
It is pretty old but – if current – could be very useful. There are some exclusions but the potential usefulness of this concession is huge.
Interesting, so the clock, possibly a (non-nav) radio, some switches, circuit breakers, panel lights can be swapped to panel LED’s etc
Colleagues, please correct me if I am wrong, but it seems me this is one the basic intentions of the entire TSO system – to provide for interchangeability between items of different design but same function.
I think using TSO’d parts interchangeably isn’t an undiscovered concession, it’s instead the basic system by which stuff can be installed in planes… notwithstanding the inexplicable way in which people tend to think STCs are required for everything including some installations, avionics etc, that may actually only require a mechanic’s log book entry.
I agree with the last two but very few people know about this; most people think that changing almost anything is a Major mod and needs an STC blah blah etc…
There’s a very helpful flowchart in fig 3.2 of the following AC which makes the decision-making process for Major/Minor alterations much more straightforward:
https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_43-210A_CHG_1.pdf local copy
One Q is whether AC43-210 overrides the other document…
43-210 is great because loads of things which most people would think are Major are actually Minor. The problem is that few A&Ps are willing to read this stuff, let alone stick their neck out. I’ve been reading US forums for many years and they really struggle with this.
And there are exceptions anyway e.g. an “EFIS” product needs an STC… these are spread around the FAA regulatory “library”.
I think the TSO substitution concession is implicit in the context of a Minor alteration. Very few people know this but for example you can completely re-work the RHS panel in an N-reg GA plane, all as a Minor mod, provided you comply with various requirements e.g. not removing any items specified on the TC, no autopilot connection, etc. The Q would be whether the concession is usable in the context of a Major alteration and I don’t think it is.
I have merged this thread into an older one on more or less the same topic.
