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Oftentimes gliders are not equipped with transponders (at least in Europe). Even if they are, they do not necessarily operate it at all times. This is allowed by SERA.13001:

(a) When an aircraft carries a serviceable SSR transponder, the pilot shall operate the transponder at all times during flight [...]

(c) Except for flight in airspace designated by the competent authority for mandatory operation of transponder, aircraft without sufficient electrical power supply are exempted from the requirement to operate the transponder at all times.

Modern gliders give a bad target for primary surveillance radars, because they are small and contain little reflective material. Thus gliders maybe unknown to air traffic controllers and flight information services. A transponder would give ATC secondary surveillance radar data which could help to avoid mid air collisions between gliders and other planes, especially airliners which may be too fast for practical "see and avoid" (as e.g. demonstrated in the event of an airprox between a glider and an A321 near Hamburg, Germany).

I could not find definite sources about the power consumption of transponders, however this question implies an average consumption somewhere between 2W and 6W. Considering that USB power banks with a capacity of 100Wh are small, light, cheap and could easily provide the energy needed for >16 hours (a time I expect should cover >99.99% of all glider flights) it seems that technically the power supply should not be the reason for not operating a transponder.

Another reason may simply be money. A quick google search implies about 2000 € for equipping a glider with a transponder which seems quite a lot considering that this is quite a simple device (e.g. compared to a smartphone). Nevertheless it is quite an easy measure to increase flight safety.

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    $\begingroup$ So this received 2 downvotes. I have no idea why. It would be nice if the next one leaves a comment about what is wrong. $\endgroup$
    – yankee
    Sep 15 at 7:05
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    $\begingroup$ I see nothing wrong with this question. While not required, it would be very helpful to leave constructive criticism in the comments when downvoting. $\endgroup$
    – Bianfable
    Sep 15 at 8:10
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    $\begingroup$ "without sufficient electrical power supply" seems to be your answer... No engines = no generators (I'm sure glider pilots don't want ram air turbines either). What if the pilot forgets to charge the battery at home? $\endgroup$
    – user253751
    Sep 15 at 8:41
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    $\begingroup$ A useful question, clear and well researched, offering apparently valid solutions and asking why these solutions are not used to prevent a risk of collision... one upvote, 3 downvotes. $\endgroup$
    – mins
    Sep 15 at 10:03
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    $\begingroup$ Perhaps because collisions of airliners and gliders are vanishingly rare. This SE question found exactly one instance, in which both planes landed safely: aviation.stackexchange.com/questions/80936/… $\endgroup$
    – jamesqf
    Sep 15 at 17:20
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Gliders in Europe are not usually equipped with transponders because most of them have FLARM.

As of November 2017, over 35,000 manned aircraft already have a FLARM system installed, including both the legacy Classic FLARM models and the PowerFLARM models. Most FLARM installations are in Europe. Over 50% of all GA aircraft in Europe already have FLARM and thousands of installations are currently being made each year.

In Switzerland, almost all powered airplanes and helicopters have FLARM. In the rest of Europe, most powered airplanes and helicopters have FLARM.

In Switzerland, Germany and France, all gliders have FLARM. In rest of Europe, almost all gliders have FLARM.

In the UK, over 7,000 aircraft already have FLARM, of which over 50% is powered airplanes and helicopter.

Additionally, FLARM is in use in over 20,000 drones/UAS

FLARM is lower cost and lower power consumption than most ADS-B or Mode-S, and better fits the way that gliders and light GA use the airspace. Gliders tend to fly in Class-G airspace and/or close to their base airfield. The traffic they usually encounter is mostly other gliders or powered GA flying VFR. As jamesqf pointed out, encounters between gliders and airliners are extremely rare. TCAS is designed for IFR traffic and its purpose is to maintain IFR minimums, and while gliders are allowed to fly IFR, I think it's usually well out of the way of airliners.

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  • $\begingroup$ Exactly. This should be the accepted answer. $\endgroup$ Sep 17 at 3:03
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    $\begingroup$ The problem with FLARM is that it does not help to avoid collisions between light aircraft and airliners (in airspace class-E), because airliners are not FLARM equipped (something that might change in the future). While encounters between airliners and gliders are rare, this is still a huge topic in commercial aviation, as a single accident will have a huge effect on the airline's safety record. So basically I interpret your answer as "Within the glider community the risk of conflict between gliders and airliners plays a hardly any role and within the community FLARM is used" $\endgroup$
    – yankee
    Sep 18 at 5:33
  • $\begingroup$ TCAS will generate a resolution advisory with VFR traffic, so no, it is not just for IFR traffic. TCAS is not programmed to maintain separation minima, but to prevent collisions. $\endgroup$
    – RetiredATC
    Sep 21 at 5:51
  • $\begingroup$ Since low power ADS-B out is already available for medium to large drones, I can envision that gliders might start to adopt it. Likely we'll see "dual" systems for both ADS-B and FLARM in gliders in the near future, and I would predict FLARM being slowly phased out. $\endgroup$ Sep 21 at 14:35
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This is anecdotal, but as a glider owner I can tell you why my glider doesn't have a transponder:

  1. Monetary cost: A transponder is a substantial investment which requires recurring inspection. Avionics shops tend not to set up shop out in the boondocks where gliders fly.
  2. Energy: a transponder consumes a fair amount of energy and the battery in my plane is not easy to access to remove. And anyway there's no convenient way to charge it when it's sitting in a trailer several hundred meters away from the nearest electric socket. FWIW, 2-6W is a huge power load for a glider, the electric variometer I use can run a year off a 9V battery.
  3. Weight: we go to great lengths to make the glider light. Adding back in a radio and suitable battery would work counter to that.
  4. Regulatory burden: If I had an ADSB transponder, then the plane would not be legal to fly with the transponder off. This means that if the battery is dead, the plane is grounded.
  5. Value: Ignoring the specific problem of gliders flying in a gaggle-- a situation where a transponder doesn't help-- mid-air collisions are extremely rare. And when we're flying in the mountains, ATC couldn't receive the signal anyway. A FLARM, as Dave Gremlin describes in his answer, system is far more practical.

Of course, change the equation variables and my response changes. I don't speak for other owners, but I suspect they have similar priorities.

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