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> This is a system that's supposed to have it's own built in redundancy and must not go down.

There are contingencies for radio outages, but yeah, everything going down at once (if that's what's happening here, which I doubt) is bad.

There is satellite-based CPDLC or "datacom", but I think most planes that have CPDLC use radio-based CPDLC.

In the worst case: light guns and wing-rocking or flashing the navigation lights.

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Technically, you do not need radios. There are many many uncontrolled airports in the United States, where you can technically fly around without establishing two-way radio communication. It’s doable, but I don’t know if that’s even a scenario pilots test in the simulator during their semiannual trainings.

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You don’t need radio for IFR (as the whole point of a flight plan is that if you lose contact you continue to fly your plan). But you kind of do need radar.

Right, but isn't a lot of that necessary radio aircraft-to-aircraft, like ADS-B? Obviously not ideal, but I thought IFR traffic could feasibly maintain separation without ATC help.

AM Radios are literally everywhere, and very commonly available.

I would hope that airports keep a battery operated radio + antenna in case of emergency with them, to at least communicate with aircraft within visual range of the airport.

I would bet the problem is not strictly the radios themselves, they will still work, Planes can talk to each other and directly to airports, But that there is a large scale highly integrated control center network that failed, and planes can no longer talk to centralized traffic control.

So it falls back to local control, more in the style of visual flight rules. Pilots in existing flights will have to be vigilant and do their own deconflictation and stop new flights out until the network is back up.

All conjecture at this stage of course, but all the radios won't just fail. It is probably going to be a poster child example about the dangers of large scale integration. The attraction of course is that integration is very efficient, the danger is it tend to fail all at once instead of in smaller pieces.