In this AVweb video, Paul Bertorelli takes a hard look at 406 MHz emergency locator transmitters for aircraft. While an ELT of some kind is required for most aircraft, historically, these devices haven't performed very well. They still aren't an impressive value for the money.

Additional information and links:

Registration site for 406 beacons: https://beaconregistration.noaa.gov/R...

NASA ELT study: https://ntrs.nasa.gov/citations/20170...

Summary of five year ELT accident sweep between 2013 and 2017 in which NTSB reports keyworded ELT:

81 total accidents
29 percent aided in crash location
65 percent did not aid or did not activate
6 percent were unknown