I think that is the point RR_NDB wants to bring forward :
If the recorders have their own independent power supply, they will keep recording.
My point of logic was: if you have no power for instruments and sensors there is absolutely nothing to record.
Sure, you can verify that any specific data was missing, but if the data cannot be generated or transferred, it cannot be recorded. If you loose all power on a modern aircraft there is nothing left to record if you do not create a completely independent array of sensors powered equally independently
Some additional viewpoints to this issue.
I don't really understand this fuss about independent power supply.
First: From a flight safety viewpoint, surely the most important data the FDR/CVR can provide is why the main electrical power supply/supplies failed? Any subsequent loss of control or a/c gyrations are likely to be a consequence, not a primary accident cause. And that all-important pre-power-failure data is in all likelihood already recorded by current recording systems.
Second: Correct me if I'm wrong but I believe flight recorders are already now powered by a redundant power supply system ("essential bus" or equivalent).
I realize there is a case for an independent recorder power supply. It would help the recording e.g. in situations where the aircraft's main electrical power supply systems go into cyclic restart situations or other nasty states. However, an independent power supply brings its own troubles such as increased maintenance and the risk of failure of the independent power supply (for example, I've had my PC harddisks fried when the PC power supply failed, allowing overvoltage to reach the harddisks). Will the benefits outweigh the costs?
Would there be any value in scattering a few extra pingers around the airframe? There'd be no guarantee, of course, that any that worked would end up in the vicinity of the recorders. But even ones far from the recorders would likely be near something worth recovering.
This might be a feasible avenue forward, and indeed was one of the improvement suggestions which got a high score in the IMO excellent BEA
study on the topic which was
referred to by AlphaZuluRomeo a few pages back (in post #641).