121.5 no longer is processed by the satellites and therefore no longer can provide a Doppler position. As such, it also isn't a very good alerting device either. It is only useful, to a very limited extent, if heard by an aircraft in the vicinity.

It does serve as a homing beacon, with limited range, but only for as long as it transmits. Lots of things can cause it to transmit for less than the minimum 24 hours, as was clearly the case in this instance.

406 provides satellite alerting, more precise Doppler location, even better GPS location if so equipped, almost instant alerting within the U.S. and also has a 121.5 homer. Both CAP and USCG now have equipped their aircraft to home on 406, which has a homing range of upwards of 100 miles.

The same things that can "kill" a 121.5 ELT will kill a 406 ELT, except there's a far better chance that the 406 will get an accurate location very early in the process, which shoudl improve the odds somewhat. How useful an ELT is in a crash is very debatable and depending upon what statistics you use and how you work them, from 30% - 70% of the time they don't work effectively.

That's why I always say the ELT is back-up to a PLB and also one reason that so far 406 has not been mandated for GA aircraft. The FCC tried last year, in an absurd example of the govt. screwing up as only it can do, but got jumped on and backed off. I suspect at some point we will see a mandate with a very long phase in. Other countries have mandated them, or tried, with variable results.