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?

Even if there is an STC for the cherokees you would still need to file a
337. If there isn't an STC, if installing a transponder is considered a
major alteration, then you would need a field approval. So you send the
337 to the FSDO for approval before installation, when the approval
comes back you install the transponder. After installation and IA
signoff you send the 337 back to the FSDO. They then forward it down to
OKC for it to go into the permanent records of your aircraft.