Fuel for the fire. I found this letter from the FAA Milwaukee FSDO in response to an inordinate number of Form 337s being filed for radio installations. It may be old, but I don't know of any change in policy since the date of the letter regarding 337s.

https://groups.google.com/forum/#!to...lt/qodHhczVS1Y



U.S. Department of Transportation
Federal Aviation Administration
Flight Standards District Office
4915 South Howell Avenue
Milwaukee, WI 53207

DATE: June 5, 1995

SUBJECT: Major Alterations

TO: Certified Repair Stations and Inspection
Authorization (IA's)

This office is frequently confronted with questions regarding
what constitutes a major alteration, especially when
installation or removal of avionics and other similar
equipment is involved. The following information is provided
to help mechanics and repair stations understand this
sometimes complicated matter. The regulatory definitions of
major alterations are found in Federal Aviation Regulations
(FAR) 1 and 43, Appendix A (a). These regulations are often
interpreted too conservatively resulting in minor alterations
being treated as major alterations.

Simple alterations that do not affect the structural
integrity of the aircraft, alterations that are made by the
same method as the aircraft manufacturer has used or
installation of equipment listed by the aircraft manufacturer
as "Optional Equipment" may be considered minor alterations.
A simple equipment installation that is made in an aircraft
with units mounted on factory provided racks may be
considered as minor alterations. The certificated person
installing the equipment is responsible to use the
information provided by the equipment manufacturer, and that
information in Advisory Circular (AC) 43.13-1 and 43.13-2
regarding wire size, circuit protection, wire support,
antenna installation, etc. They are also responsible to
record the alteration in the aircraft records in accordance
with FAR 43.9 (logbook entry) and update the weight and
balance and equipment list.

We do not wish to imply that field approvals of equipment
installation is never required. Examples of installations
that would require field approval a

* Alterations that involve the construction of a radio rack
or shelf.

* An installation made in an instrument panel that is a
structural part of the aircraft and requiring an opening
to be cut in areas other than that specified by the
aircraft manufacturer.

* Special FAA policy sometimes requires field approval on
installation of new types of equipment such as the current
requirement for GPS.

If you have any questions regarding this memo, or are in
doubt as to whether a 337 is required, please do no
hesitate to contact this office.


Sincerely,

Thomas L. Lind
Supervisor, Airworthiness
Certificate Management Unit