To amplify on what Prof Chris Reed has posted, I have probably the only glider that my club that could physically accommodate a Trig 21 transponder, but contrary to what others have posted, I cannot do it, because there is no EASA approved modification, in sufficient detail to satisfy the CAA, for my glider. I have a hole in the instrument panel ready to accommodate the control unit; there is enough spare weight capacity to accommodate the extra batteries required for long flights; but there is no paperwork to cover it.
It is no good people saying that they have them on the continent. I have been in correspondence with people on the continent who have fitted transponders in gliders, and at least some of them did it before there was an approved EASA modification. Those who did it on my model of glider have subsequently seen an approved modification submitted by the glider manufacturer. Unfortunately, it does not cover installation of extra batteries, nor the installation of a separate power unit located in the fuselage and not the panel which is what the Trig 21 has - and that is the only one that would fit in my glider, I believe. The BGA, and the inspector I have most recently had doing work on my glider, both tell me that without that sufficiently detailed approved modification, I cannot legally fit a transponder. It is no good saying they did it on the continent. It is no good saying that I should go ahead and have it installed illegally. The inspector won't do it. The next ARC would not be renewed if it is there.
Unless the posters on here, who say it can be done, are themselves the right person with the right authority in the CAA, they just don't know what they're talking about.
If you are the right person in the CAA, and do have the right authority, why have you and/or your colleagues told the BGA and my inspector differently?
So Flarm and PCAS is all I can do at the moment to have technology assist me in collision avoidance, unless I invest in a completely new instrument panel and layout, with extra batteries, and the cost and inconvenience of drawing up a very detailed modification proposal covering all that which would be required, and paying for it to be officially approved - a burden which is prohibitively expensive and time-consuming, requiring skills I do not have.
Chris N