ELT's were discussed way back in the thread. The C5 I think had a jettisonable ELT on the tail fin. The ones in current commercial aircraft tend to be fixed in the fuselage, or on life rafts or even as separate manually initiated buoys that the flight attendants can access. Given the thoughts on what happened with MH370 only a detachable external ELT would have given any location of crash as the cabin crew are thought to have been disabled by depressurization. ELTs do not run all the time as they are on an HF emergency frequency so they would only allow easy finding of the crash site.