Fright Recorder Power
3G cutoff of recorder power does seem lower than needed. But this is FAA, where 50G SLF bodies are strapped in place with 9G belts and who knows what strength seat rails.
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PJ2: "Likely so in the scenario I image - the recording devices would simply keep falling back on the "next" source of power until the last source, likely battery-backup, was exhausted, say, after 10 or 15 minutes. Likely longer wouldn't be needed because an airplane at that stage of electrical capability isn't going to be in the air long. (That said, there have been some interesting A320 DC Essential Bus failures, (DFDR is powered by the AC1 bus), which have blacked-out all or partial instruments, as we know). I note for example, that the Bombardier Global Express accident report cited above, indicates that the DFDR is powered by the DC Essential Bus - that type of bus, as indicated by it's name, is "normally" a backup power source for the main DC busses for essential services only so it appears as though Bombardier has considered this scenario - I'm guessing here of course."
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Transport aircraft, as you know, have AC generators on the engines and APU, and use rectifiers to power the DC buses. Essential buses typically have multiple power sources, as does the Emergency DC or Battery Bus. Adding non-emergency items to the battery bus requires installing a larger capacity battery, to maintain the 30 minutes or so required operation after the generators quit. Rolling the dice, you can see the wisdom of not putting the DFDR and CVR on the battery bus, especially the older motorized recorders.
Bizjets like the Global Express, and regional airliners have 28 VDC generators on the engines and APU, and use inverters for AC requirements. You then see why the recorders are on the DC bus in those planes.
GB