Mike Schumann wrote:
Can ATC see you without a transponder?

Mike Schumann

Sometimes. In a non-transponder equipped glider, ATC can usually see
your primary radar return if your groundspeed is above 60 knots (this
depends on several factors). When my groundspeed decreases
significantly (common when climbing in mountain wave), ATC usually
informs me that they have lost radar contact, and ask me for a position
update.

I almost received a waiver from ATC to climb VFR above FL180 once, but
a supervisor stepped in at the last minute and denied my request.
Gliders are not exempt from having transponders in Class A airspace; I
don't think he had a problem with the VFR waiver.

Interesting stuff.

Chris Fleming, F2
El Paso, Texas