121.5 mHz ELT: location accuracy 12000 meters

I'm not sure how to interpret this. An ELT (of any flavor) transmits a continuous signal, for as long as the batteries hold out. It can be received at a range much greater than 12 km, and can lead a searcher to within a few meters of the ELT. When an ELT signal correlates with a report of a missing pilot, a search commences promptly and continues until the crash site is located (or, possibly, until some time after the ELT signal quits).

I've been involved in 2 glider S&R missions for which a 121.5 MHz ELT signal was extremely valuable. In the first, the pilot crashed in a hard-to-access area and was killed on impact. He might still be there if not for his ELT, which continued to transmit until the crash site was located the next morning. (It's worth noting that a search - with its considerable attendant risks - continues whether or not the pilot has survived.)

The second was a successful search for a crashed (and seriously injured) glider pilot in another remote and difficult area. There's little question that his chance of survival would have been much lower without the ELT, which continued to operate for the 23 hours that he lay in the wreckage, in near-blizzard conditions.

The often-cited 50% rate of successful operation of 121.5 MHz ELTs deserves some explanation. According to an Air Force study, the four most common reasons for ELT failure a
1. Destroyed in crash
2. Destroyed in post-crash fire
3. Bad batteries
4. Improper installation
#2 should be very unlikely in a pure glider - and probably uncommon even in a motorglider. #1 and #4 can be addressed by proper installation in the area behind the cockpit, which tends to stay reasonably intact even in serious crashes. #3 is under the control of the aircraft owner. So a moderately careful glider pilot can expect his ELT to perform properly at a rate much higher than average.