Hi @all,
I was wondering what the restrictions are to adding particular avionics when there have been e.g. engine or slow flight mods (with STCs) applied to the aircraft. For example, the G500TXi AML only lists models (as the name suggests), this suggests that this allows to add a G500TXi to any aircraft on the AML no matter its modifications?
I assume this looks different when installing an autopilot. I assume it would need to be certified on a particular engine, e.g. if someone has the IO-520 to 550 upgrade on a Bonanza A36 or a turbo normalizer?
Thanks,
Jonas
My understanding is that compatibility between several STCs is the responsibility of the installer, his/her expertise and his/her common sense.
For example, in my opinion (but I am not an aircraft mechanic), installation of a G500TXi in any plane requires that the correct stalling speed, best glide speed, best rate/angle of climb speed, etc are configured into the G500TXi. I assume that can be done at installation time by the mechanic, and then no problem, he/she just has to take the new values from the slow flight mod. But if Garmin insists on hardcoding them only at the factory, and only based on the standard POH of the standard plane (not the other STC installed), then indeed we have a problem.
This is a grey area. I have never seen any clarifications with respect to avionics or autopilots, relative to engine or slow flight mods.
Some installers take the view that the STC model list must be read exactly.
Others take a more flexible view.
And others make what you might call mistakes, installing an item for which the “others” would regard the STC as not applying; post-discovery these jobs are kept quiet and the aircraft gets sold ASAP 
In the meantime the company offers to sign off the plane at each Annual 
I could give several examples but it might point at the persons, or the buyer might recognise his newly bought aircraft… You can imagine such an aircraft is more or less worthless, if avionics into 5 digits was installed. And if I know of some, there are bound to be many more because most people will keep very quiet about it.
I think that if a well known company did the job, then it is “ok” because they (not you) are the “experts” and you are entitled to rely on their expert advice, so you would have a legal recourse if it got found out later.
Practically speaking an autopilot might not actually quite fully work with say a canard mod, if you work on the assumption that it is supposed to be stable all around the envelope.
There is the example of the Jetprop issue but I think that is more a practical/economic one in that you could solve it by ripping out the G1000 and replacing it with a G500/etc.
I imagine @pilot_dar may have come across this situation.
Peter, lionel,
Thanks for your answers and thanks for the linked threads Peter, interesting reads. Does the owner of an STC have any say to which it deems it to be compatible to?
I think it may depend on the STC wording. Some require the installer to obtain permission to use it. Others are just for anybody to use. Also many STC holders have vanished years ago and are not contactable.
I would be surprised if the STC developer had a say in the interpretation, but there are people here who have written many STCs and who might know e.g. @wigglyamp @mh.
EDIT: above two have probably moved on (clearly, as far as assisting people here is concerned ); @antonio and @pilot_dar may know more. But as I say, I am not aware of any regulation which clarifies this.
