There are many Avidyne / GFC500 installations up and running, I doubt they are all in violation of the STC.
Also note how the STC requires the previous or concurrent installation of a G5, G3X or a GI275 but no navigator restriction is mentioned in the text.
The navigator box restriction is indirect, via what can drive a G5 or a GI275 which is in turn required for the GFC500. I saw this on a US site:
Then someone contacted Avidyne and got this
The GI-275 is not listed in our IM. As for getting a field approval for an autopilot installation I have doubts that most FSDO inspectors are capable or confident enough to do so. If in fact the GI-275 STC requires a Garmin GPS source I don’t see a way clear that an installer could provide documentation to support a field approval for the GI275, Avidyne IFD, GFC500 combo.
One thing I’m confused on is how I was able to install the GI275 in the first place, if the STC requires a Garmin GPS. I do remember having to install a Garmin glareshield antenna to satisfy the STC but I don’t have a Garmin GPS.
and the reply was
So, yes, as the guy who contacted Avidyne found out, an IFD will drive a KI275 just fine, because IFDs pretty well emulate GTNs/GNSs, but this opinion, FWIW, is that it is not legal. But we all know you can line up five people and get five different answers, especially if there is money to be made 
The GI275 STC at no point mentions the requirement of a Garmin navigator.
Where is the STC?
I wonder what @wigglyamp thinks of this?
Avidyne did reply that since the GI275 is not in their IM, that combination can’t be legally installed. I don’t buy that (of course ) but it is a common legal position that only stuff in the IM can be connected. The guy in the US who wrote that text is well known for taking the most absolutely strict legal position on everything.
I can’t comment on the GI-275 situation, but it seems that the Avidyne/G5/GFC500 combination is (becoming?) quite popular in the US. The GI-275 STC may have specific requirements/restrictions. Avidyne’s position, though, is that anything approved with a GNSx30 is approved with a slide-in IFD if no installation-specific rewiring is required and this seems to be commonplace in the US.
A report on the Avidyne forum of discussions with their tech support is that if installation is done according to the existing approved paperwork, then a GFC500 with dual G5’s and Avidyne IFD is legal. The G5 is approved to work with the Avidyne IFD series, and the GFC500 doesn’t need a direct connection to the IFD, only the G5. I have a G5 AI and IFD 540 (but not, yet, GFC500) along with a Sandel 3500 and it all works very well together.
chflyer wrote:
it seems that the Avidyne/G5/GFC500 combination is (becoming?) quite popular in the US.
It that for uncertified aircraft? The G5 STC IM doesn’t show any possibility of connecting other than Garmin navigators (or Garmin NAV receivers) to G5’s.
Seemed to work fine for me a few weeks ago…
Peter wrote:
Where is the STC?
Here’s the GI275 STC certificate.
FAA_STC_SA02658SE_01_pdf
Like most STCs it has the phrase:
The specific GPS requirement is stated as:
The STC installation manual does provide a list of compatible GPS sources:
My own view would be that if Avidyne’s IFD STC allows for replacement of any GNS then using the Avidyne unit with a GI275 would be acceptable based on the statement in the GI275 STC certificate that’s it down to me as the installer to assess compatibility.
Avidyne have this statement in their STC installation manual which does leave the door open to interpretation and doesn’t seem to provide undue limitation:
2.3 IFD5XX/4XX Interfaces
The IFD5XX/4XX can interface with a host of other avionics equipment. The following list
represents the proven interfaces. There may be other devices that can be configured the same
as one on the below list but Avidyne has not tested it and can therefore not make any
compatibility claims.
Airborne_Again wrote:
It that for uncertified aircraft? The G5 STC IM doesn’t show any possibility of connecting other than Garmin navigators (or Garmin NAV receivers) to G5’s.
No, it’s a C172. The Avidyne IFD series is being installed under FAA rules as a plug-compatible replacement for the GNS series using the IM instructions for connections to them. This is a major sales argument for Avidyne and very widely done in the US along the lines described by @wigglyamp. My avionics shop here (I fly N-reg) had no issue signing off the 337 form referencing the G5 IM and STC.
I think that argument has to be right, otherwise Avidyne would never be able to sell a plug-compatible box into the Garmin market.
The vast majority of other stuff already connected to the GNS will not have anything “Avidyne” in their IM, so it must be the Avidyne STC which facilitates the IFD installation.
This however means that some Garmin product whose IM includes a GTN source but does not include a GNS source (perhaps there is no such Garmin product currently) would not be applicable to an IFD installation. So if e.g. an airframe maker had the GTN on a TC but did not have a GNS there, an IFD could not be installed (on the basis of the IFD STC alone).
