This is a good discussion. I'm an A&P, hard in the books studying for the IA test. I see some definite errors in thinking in some of the above comments.
First, everything the FAA does is covered by CFRs and not by woms (word of mouth, like we sometimes had in the military). And in searching, I find nothing in writing that says an STC requires a 337. Some have said - "an STC is by definition a major alteration". I find this not to be true. If that "definition" is in the CFRs, I can't find it so please let me know if you do.
A major alteration is defined in 14 CFR part 1 and further spelled out in part 43, Appendix A. Nowhere do I see it written that an STC has anything to do with the concept of major/minor.
The Bowers letter, like many legal documents has to be read with extreme attention to detail. This whole argument keys on one word in the letter (second main paragraph) - the word "certain". Counsel Peter uses the word "certain" i.e. "certain STCs require the completion of an FAA 337" and I think that resolves the question right there. Obviously, as we all know, applying an STC that involves a major alteration will absolutely require a 337 - not because of the STC but because of the major alteration. And therefore, contrarily and logically, applying an STC that involves a minor alteration does not require further documentation than the aircraft log book.
If Counsel Peter had not used the word "certain" the story would be completely different and all STC applications would require a 337 instead of "certain" ones.
The second to last paragraph of the letter seems to contradict this and require the 337 but actually does not - "The person performing an alteration in accordance with an STC must comply with all the requirements of section 43.9". And that makes perfect sense. 43.9 directs that "each person who (blah blah) alters an aircraft" make an entry in the maintenance record. It further requires in paragraph 43.9 (d) that major repairs and alterations be documented per Appendix B where we find the 337. Again, there's no mention of the term "STC".
So what do we learn? To me, the Bowers letter confirms what we already knew about documentation requirements for major and minor alterations and does not direct us to fill out a 337 just because we use an STC as data to make the alteration. That makes perfect sense to me and that's what I plan to do going forward. If it's a minor alteration, I'll reference the STC as approved data in the log book and not file a 337. Of course, the title of the form is in fact - "Major Repair or Alteration" and therefore by definition is not relevant for minor alterations as defined in 43 Appendix A.
Some have said, "just file the 337". Ok, you could do that but IMO, you're wasting time and money, using the wrong form, and not properly doing the job as an IA if the alteration is clearly minor. But if the alteration could be interpreted to be major, of course, file a 337.
Obviously though, many alterations we make to our airplanes are actually minor so they should be done by an A&P with a log book entry referencing "acceptable data".
Please let me know if you find I've got my facts wrong. I think this is an important concept for all A&Ps and IAs to understand. And I appreciate what everyone has written about this since it forced me to dig in the books.
Thanks - John McClanahan, Atlanta
