http://www.ntsb.gov/NTSB/brief.asp?e...06X01297&key=2

For you amateur FAR lawyers to chew on:

"The glider was equipped with a panel mounted communication radio,
global positioning system (GPS) unit, and a transponder; however, the
pilot did not turn on the GPS and transponder. The transponder's
activation is not required for glider operations (for more details see
14 CFR Part 91.215). According to the glider pilot, he did not turn on
the transponder because he was only intending on remaining in the local
glider area, and because he wanted to reserve his batteries for radio
use. The glider was equipped with two batteries (one main and one
spare), however, due to the previous glider flights, the pilot was
unsure of the remaining charge in the battery."