PMA Questions

But is there such a thing as an STC for a minor modification?

No, by definiton.

An STC is issued as approved data for a MAJOR modification to the type
design.

A minor modification does not require approved data, it only requires
acceptable data.

However, what constitutes a major vs. a minor alteration is open to
interpretation, and to the extent official guidance exists (not often)
it has been known to change over time. For example, once upon a time
any RNAV installation was considered to be major. It required either
an STC or a field approval. If you look at the installation manual for
a KNS-80 VOR-DME RNAV, you will see included an STC for its
installation into a PA-30. The guidance on this has changed. These
days, the installation of a KNS-80 is generally treated as a minor
modification, but when it was new the FAA wanted to maintain control.
IFR GPS installation is going through the same process. Originally, an
STC or field approval was required, but the FAA has issued guidance
that certain simple IFR GPS installations (ones where there is no GPSS,
for example) may be treated as minor.

So there are STC's out there for the installation of a given piece of
equipment into a difference aircraft that are being cited as acceptable
data for a modification that used to be considered major but is now
well enough understood that it is considered minor.

If you're looking for hard and fast answers in this area, realize that
they are not to be found. What is major or minor varies from FSDO to
FSDO, and from year to year in a given FSDO. What will be considered
approved data also varies. I've had a 337 for a Stormscope
installation IAW an FAA APPROVED installation manual (comepte with
revision history) accepted one year, and an almost identical one
rejected a couple of years later. Policy had changed. You will NOT
get clear and unambiguous guidance on this from the FSDO. If you
insist on it, you will be told that everything is major and no field
approvals are available, so either get an STC or hire a DER. Or you
can live with a grey area.

Michael