In my former occupation I have seen and had experiences with 'fixed ELT's' which were not always 'nice'.
The established failure to operate when needed for a fixed ELT is greater than 90%, with 100% being the rate if the aircraft is underwater.
This was confirmed by the CASA post implementation review of the current rules. Fixed ELT (including in airline size aircraft) are a complete waste of money.
If any other piece of aeronautical equipment had that failure rate in service --- there would be an outcry --- but such is the politics within CASA, that mandatory fixed ELT have, once again, made a reappearance in draft legislation --- Part 135.
Tootle pip!!