On Aug 23, 8:53*pm, "Peter Dohm" wrote:

To add just a little more, and with the understanding that I am a former
avionics technician and have never been a mechanic--much less an IA--my
understanding is that a form 337 is a return to service following a major
repair or a modification that was not sufficient to require an STC.

A 337 is required even if there is an STC. If the change is a major
modification/alteration, effectively that it's not in compliance with
the original certificated design, then you need the 337. Now the
approved data may be a STC or it could be some other "data
acceptable to the administrator."

*In
effect, the modification or repair should not alter the original operating
limitations of the aircraft, which would require that issuance of an STC;
but may alter the weight and balance calculations for the specific aircraft.

A change in the empty weight / balance isn't a major modification
by the way. Only, those things that alter the envelope limits.

You might want to peruse Part 43 appendix A as to what
is considered "major."