One cannot make a system that runs on aircraft electrical power that doesn’t have an “off” switch (at least a circuit breaker) because of the risk of electrical fire.
Blueplum’s separate, independent backup system may be more workable, but I’m not sure how to power the system - perhaps the emergency bus? Battery power has a tendency to wane on a system that is never used, and therefore be insufficient when the need arises. Latent failure rises to the probability of no longer being latent.
From an aircraft design perspective, the requirements for such a system are difficult.
Please see my post 43. The Find Me Spot device has it’s own power source, but could easily be provided with an external source of power. If that source is killed, the internal battery can operate for days.
You don’t need to reinvent the black box. You just need archived position data updated every ten minutes by satellite.
In the event of total electrical failure, a small prop that works off ram air will start to spin, providing minimal emergency backup power to the most critical systems, like communications and life support (if it could be called that), maybe some hydraulics.
the 'pinger' on the engines is self-contained, external to the cabin and powered by the engines. Find a way to slap a gps on it, and voila'. Easy peasy.
How about some type of sonar buoy designed to break free from a sinking plane. At least that could give a surface point for reference. Wouldn’t think it would have to be very big and mounted somewhere under the wing.