STC's for GA Aircraft
 

If you have a Day VFR light aircraft that is fitted with traditional vacuum instruments (AI and DG) can you legally fit modern electronic instruments to replace them without an STC? All of the replacement candidates have STC's for most common GA types but not the aircraft that I would seek to install in.

Because these instruments are over and above what is required for Day VFR it seems ridiculous to even need an STC but that is what my avionics guy tells me is required plus an electrical load analysis (these instruments use absolutely minimal power).

Have a look at the Dynon gear, they've got STC's for billions of GA planes, if they haven't for yours, they might just do one for you. And it's bloody good gear.

Quick answer - any change to a type rated aircraft will need an STC - so yes.
(STC not required for maintenance, repairs, painting, but just about anything else...)

Yes 717 - or an EO, though depending on what is changing, that could be v. expensive.

What about an Engineering Order?

An EO for the Dynon stuff isn't overly expensive. I think I have a quote somewhere for one being around $1,500 to install the Dynon AP in a Comanche, so don't rule that route out 'just because' something isn't STC'd for your particular application.