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Given the histories of flight data recorders and cockpit voice recorders, I somewhat understand their origins as two separate physical devices. Given their separate tracks of development, regulatory requirements, and storage mechanisms, it stands to reason that they historically existed as two separate "things".

However, I'm under the impression that the vast majority of the commercial airliners operating today are utilizing solid state memory to store both flight data and cockpit voice recordings. Both devices share many similar requirements in regards to being capable of withstanding extreme heat, g-forces, impact resistance, submersion in water, etc. Both devices are equipped with underwater locator beacons. Even the bright orange color of both recorders are the same!

The Wikipedia article on flight recorders briefly mentions that "the FDR and CVR can be manufactured in one fireproof, shock proof, and waterproof container as a combined digital Cockpit Voice and Data Recorder (CVDR)". However, whenever I read about modern aviation accident investigations--1990s, 2000s, 2010s--investigators are always searching for and analyzing the two separate black boxes. I don't think I've ever heard of an accident that involved a single combined black box, so their usage still doesn't appear to be very widespread.

Why not? Since the only thing I can see that distinguishes a modern CVR from a modern FDR is the type of data being stored, why didn't the integration of these separate devices become the norm decades ago? The best explanation I can come up with is some sort of notion of redundancy, but this doesn't make sense if they're storing two separate sets of data.

Even if a single combined recorder was deemed to be too risky, how hard could it be to just record both sets of data on two identical recorders? At least that way, there's true redundancy being maintained. The fact that "flight data" and "cockpit voice" data are still being stored on two separate physical devices seems rather absurd in this day and age.

It seems about as logical as buying and maintaining two separate external hard drives--one for my music collection, one for my movie collection. It'd be closer to life in the early 90s, when I had a collection of VHS tapes and a collection of cassettes. But now that everything is digital, it makes sense (logically and financially) to just use one device to store both collections. Why isn't it the same with modern-day FDRs and CVRs?

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Welcome to Aviation.SE! Excellently done first question, well thought out and written. Shows your research. –  CGCampbell Aug 24 at 15:18
    
Well presented facts and analogies. This question was briefly asked and answered in past, but your version is detailed. –  Farhan Aug 25 at 16:37

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You probably wouldn't believe the effort needed to certify anything that flies on an aircraft. This is a multi-year process, especially if it is so much security-related as the CVR and FDR. And it needs to be done with all national authorities of your intended customers.

This is a case of good-enough. The advantage from combining both is real, but so small, that nobody so far thought it worthwhile to go through the certification process. And regarding redundancy, transmitting the recorded data to some ground station would be even better than recording twice on the same aircraft. If that would have been installed in MH370, most of the search effort could have been saved.

I expect something to happen with the transmitting of data, and then both devices could conceivably merged into one.

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It's odd to call CVR and FDR 'security' related, since they don't really protect passengers from accidents. –  florisla Aug 25 at 6:43
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@florisla They do protect passengers. Not of the affected flight, but of all future flights if a serious flaw is found. –  yo' Aug 25 at 6:51
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Also, if one is damaged (or can't be found) and can't be read, chances are good the other may still be able to yield data. Combine both into a single device and you lose that. So it adds a bit of redundancy. –  jwenting Aug 25 at 10:25

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