This 16 September 1996 rec.aviation.owning post by Rod Farlee is the
best summary of 14 CFR 91 and TSO issues I know of:

http://groups.google.com/group/rec.a...cf7fe9b9e52da3

...The FCC specifies operational requirements for most
avionics. FAA TSO specs are environmental (temperature,
humidity, vibration, shock), and in some cases, such as
IFR GPS, are operational. A TSO has nothing to do with
the manufacturing process (this is the concern of the
FAA PMA, parts manufacturing approval, process).

Try to buy a TSO'd DME, VFR GPS, GPS/COM or intercom and
you'll find there are no TSOs for these items. Also
there is no requirement for TSOs for anything used for
FAR Part 91 operations except the transponder and ELT.

To install non-TSO'd equipment, there is no requirement
for an STC. A 377 "Major Alteration" form is needed
only if the installation requires structural
modifications to the airplane or fabrication of a
mounting tray. Otherwise, it requires only a logbook
entry by a radio shop or A&P with avionics inspection
authorization that the physical installation conforms
to AC 43 standard practices, and noting any change in
aircraft weight and balance.

There are some requirements for TSO'd equipment on
airliners in FAR 121, but there are many exceptions
(DME, entertainment, etc).

There is enough confusion among FAA FSDO inspectors
over the new PMA requirements that some of them seem
to be making up there own rules in this area, but
let's not make up our own!

- Rod Farlee

Thanks, and best regards to all

Bob K.