Earlier, Rex wrote:

Most mechanics that work closely with a FAA inspector will tell you
that even the smallest addition of equipment is considered a
modification that requires approval data. Non tso'd varios and gps nav
systems are consider by many to be non-essential equipment and belived
to be leagal to install in type certificated gliders. The only people
that can approve the installation of this equipment in these sailplanes
is the glider manufacturer. Many inspectors and DARs look the
otherway and do not take issue with these instruments.
But the FARs are clear...

Ah, if the FARs are clear, please cite them by number.

On the topic of TSOs, 337s, and "Major Alterations," I still think that
this 1996 Rod Farlee post sums up the situation best:

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

Quoted:

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 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!

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com