BT wrote:
91.215 (b)(4) states that all aircraft operating above Class B and C
airspace up to 10,000MSL require transponders.
The "glider exemption" described in 91.215(b)(3)(ii) states a glider (no
factory electrical generation system) can operate without a transponder
within 30nm of Class B airspace as long at they operate below the ceiling of
Class B or Class C airspace or 10,000MSL WHICH EVER IS LOWER.

Of course, there is a second "glider exemption" in 91.215:

(5) All aircraft except any aircraft which was not originally
certificated with an engine-driven electrical system or which has not
subsequently been certified with such a system installed, balloon, or
glider—
(i) In all airspace of the 48 contiguous states and the District of
Columbia at and above 10,000 feet MSL, excluding the airspace at and
below 2,500 feet above the surface; and

Most ClassB in the US go up to 10,000MSL, so that would be moot, but there
are a few that do not, LAS for example. There is talk of lowering the PHX to
9,000MSL.

The way 91.215(b)(iii) is worded, raises a question. There are many Class C
airspace designations well outside the 30nm ModeC veil associated with Class
B airspace. Can one legally fly above these Class C areas with no
transponder?

Reno Class C is surface to 8400 feet MSL, without transponders we
routinely overfly it above 10,000 feet, but do not fly within the
boundaries and below 10,000 feet (or below 8400 feet with a transponder)
without permission from Reno tower. Of course, we are actively
encouraging people to get transponders, not so much for overflights, but
for the critical traffic areas outside of the boundaries of Class C...

Marc