Good grief! The reference in post 2 is ‘approved’ by the German government, not the FAA!
92.205 requires you to use an ‘approved’ gps for ifr operations, and your op limits require you to follow 91.205 for ifr. Is the supplemental manual part of that approval? I don’t know, but one came with the 420 and it was easy enough to stuff it into the seat back pocket.
Bob, you are correct, the link I posted was the top google result, but that supplement exists in many forms, approved by whatever government/oversight of aviation in the country your plane is registered in. Here's the same thing, but approved by the FAA.
This is all my understanding, and could definitely be wrong (I had to dive into some of this stuff several years ago looking into an upgrade for my Warrior from a KLN89b):
Basically, to install an IFR GPS into a certified airframe, you need to complete an STC, and part of that STC requires going through the checklist for interference of systems, com's checks, etc.
The normal airplane POH/AFM also needs to be updated with the operations of the GPS. Instead of coming up with all of that, Garmin provides that basic document you can find variations of online in google, or from Garmin directly. You either take it as is, and fill in your airframe SN/GPS combo etc, or you make subtle changes to it (some people take it and remove the requirement for a current database and instead say something like "waypoints are verified for correct lat/long in the database", which gets around having to have a technically current db in the device).
All of that (the confirmation of the install being interference free, the supplement to the AFM etc) is then compiled into the STC application, signed by various responsible, and sent to the FAA for approval. If you used the Garmin example supplement, it's basically rubber stamped, dated and signed, and entered into the airplane record in OKC. You get back a signed, FAA approved copy of the STC, including the AFM supplement, which you then print and throw in the back of the airplane never to be seen again.
I don't think any of that required for our airplanes, being experimental and all, since the POH/AFM is not approved as part of the type certificate.
As to OP, I don't know why your CFII would know or care about the AFM supplement for the Garmin, except as something your DPE might quiz you on, and that you should be familiar with if flying a certified airplane (I believe it should be in the plane based on the same regulation for ARROW, since the Garmin AFM supplement is technically part of the POH after the STC is approved, so if you didn't have it, you aren't technically legal, but I don't know for sure)
I’ve done my share of STC installs as both an IA and certified repair station, the only document that gets sent to FAA is the 337, and they don’t send or approve anything back, that’s just a record keeping exercise . The STC is the approval and it has already been officially blessed by FAA.
To the best of my knowledge there are no changes allowed to any of the STC documents, which includes the AFM supplement.
And none of this is required for EAB aircraft so time to educate the CFI in question.
I poorly summarized the process and reasoning, but if you check out that article, even for entertainment purposes, that is what I trying to describe, and was the only thing I could even think of that your CFII might be referring to.Changing the AFMS
Fortunately, the AFMS can be changed, or a completely new one can be created, but it is a nontrivial job because under FAA (and to some degree similar EASA) rules, changing an AFMS is a Major Alteration. In FAA-land this is done with a Form 337 supported by Approved Data.
In this case, the lack of an aircraft-specific STC (the KLN94 STC is for a Beech Baron) meant that this had to be done as a Field Approval i.e. a Form 337, with some supporting documents, which has to be FAA approved (signed). In the case of a "field approval" one submits any data which the FAA finds acceptable and when the FAA inspector signs block 5 on the 337, that makes the data "approved data" for that installation.
The AFMS is custom written for the particular aircraft and is specific to its registration number.
Generally, a specimen text of the supplement is provided in the back of the installation manual for the particular GPS and this is used as the template for the new AFMS. The KLN94 version is [there]. The installation manuals are not in general circulation and only authorised dealers are supposed to have them, but they are easy enough to find on the internet. I have a huge collection of them myself
Assuming the GPS installation was done correctly, nothing physical on the aircraft changes; all this is purely paperwork!"
As you say, none of this applies to our EAB aircraft, and the only reason I can think of a CFII being concerned about it is just getting into the weeds about anything that might come up on a checkride somehow