Skywagon, I will disagree about the degree of difficulty between wiring a 430 vs a KX170, lol.
But you are spot on with the question of log book, 337, or STC. Years ago we argued with the FSDO up here about doing avionics installs as log book entries. I have a copy of the letter they gave out stating that installing equipment such as radios and intercoms do not constitute a major installation. So, ever since that, I've been installing most equipment as log entry only. Then some equipment started coming from the manufacturers with STCs. When I've asked about this, the most common answer I get is that as the equipment gets more sophisticated the path of least resistance to the feds acceptance, is to get an STC. Personally, I feel that it amounts to an unnecessary STC, but from a business stand point I guess I can understand. Less dollars spent fighting the feds equals more profit.
And then, as discussed above, some companies hold control over their products by keeping the STC/paperwork as dealer only items. Contact a Garmin dealer and they will tell you if your aircraft is on their AML. Contact a Bendix/King dealer and they ask you what you want to buy. I love most of the Garmin equipment I've installed, but getting that equipment for my customers and playing their dealer only game has become a problem. So if you need an item from one of these companies, there are two ways to do it. First, find a dealer that will work with you or your installer. Just make sure that all warranty issues are routed back through that dealer. Second, buy factory repaired units. The Garmin 430 and 530 items that are factory repaired come with a Garmin warranty and have been running about half the price of new. The best part of this is that there are no limits to who can purchase and install them.
Just from observation, the units that have IFR level navigation or auto pilot functions seem to, now, come with STCs. Most radios do not, and intercoms never do. And most audio control panels, originally, never even had a TSO on them.
But, back to your original point; no, installation of avionics equipment, in of itself, does NOT meet the requirements of a major alteration, as listed in Part 43. Even auto pilots.
As Arsenio Hall used to say "things that make you say hmmm"?
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