ELT's - more info
Too add to the ELT info. I don't know the specifics of the 777 ELT's (some others on here have more info on this). However, the following is general information that will apply to detection and localization of them.
Modern ELT's transmit a periodic 5 watt data burst on 460.25, which is received by low earth orbiting SAR satellites. They also transmit a lower power continuous "whoop-whoop" on the old ELT frequencies of 121.5 and 243, for direction finding during the final phase of localization.
The 5 watt ELT signal, even inside an aircraft like a 777, should be detectable at quite a range, even with the shielding effect of the aircraft hull. I suspect the SARSAT's would pick it up.
ELT's are frequently disabled by crashes (as mentioned by another poster earlier). The antenna can be severed, reducing the range to tens of meters, or the ELT can be destroyed. They are really designed for survivable crashes. They often have G-switches. The batteries are supposed to last a couple of days. I once found an older (121.5) ELT in an aircraft that had crashed and burned on landing - the heat from the fire had moved the ELT well off frequency, but it was still whooping.
Until 2009, COSPAS/SARSAT monitored 121.5 and 243.0. Now they only monitor 406.025. The latter frequency is sent with much higher power and better carrier stability, so the ELT location, derived from doppler as perceived by the satellite, is more accurate than with the older ELT's. ELT's on 463.0 are now monitored by both geosynchronous satellites and low earth orbit satellites (which do doppler processing, decode the digital beacon, and transmit the results to ground stations). Note that the geosynchronous satellites can provide near instantaneous notification of ELT activation, but may not give position.
Newer ELT's may also have GPS's and thus transmit GPS coordinate. They are also registered, so reception of a signal identifies the ELT. I once found one which was also shouting MAYDAY on 121.5 (recorded voice), making us think there was a pilot in distress, when in reality it was an accidentally triggered ELT in an parked aircraft. Grrr.
I suspect an ELT in the 777, built into the vertical stabilizer, might have much higher crash resistance than those in smaller aircraft, and the tail may stay intact.