"Jay Honeck" wrote
I just purchased a Garmin GTX-327 off of Ebay (installed for one day, before
the owner decided he REALLY wanted the Mode S GTX-330!), and am lining up
installation. I was surprised to hear that a 337 form had to be filed for
this installation.

It's not a big deal for the avionics shop to do, but wouldn't you think
Garmin would have STC'd this unit for the Cherokee line?

It's not likely that the unit is STC's for the Cherokee line, but you
can check easily enough. www.airweb.faa.gov/stc

By definition, if an STC is required, then the alteration is major.
That means a Form 337 is required. The STC also constitutes approved
data.

If no approved data are available, you also need a field approval
(which is almost impossible to get these days).

In general, avionics are installed on a Form 337. This should not be
necessary. There is FAA guidance that explicitly states that the
installation of a single radio in a non-pressurized airplane is a
minor alteration, but many (most) FSDO's feel otherwise. So without
an STC, what kind of data do we use? Well, mostly we wing it.

The avionics are TSO'd, and that means the installation manual is
TSO'd as well. A TSO'd manual is approved data. So we cite the
installation manual and the relevant (sometimes barely relevant) parts
of AC 43.13 as the data, and call it good. Most FSDO's seem to be
happy to accept this. Is it really legal? Probably not, but it works
so we don't make too many noises about it.

What your shop is doing is normal practice. Leave it be.

Michael