[sliverstorm]

You can't just "fail over to a lower freq". Propagation beyond the radio horizon at all is at the whim of the ionosphere, which is the only reason long-distance radio works in the first place (it reflects back down, when the conditions are right)

[fnordfnordfnord]

With full duplex communication you can.

[sliverstorm]

I... what? What does that have to do with radio wave propagation & the ionosphere?

[wglb]

How would that work?

[fnordfnordfnord]

When the aircraft stops receiving signal from the ground on one channel, it searches for a beacon on another channel.

[wglb]

And what I am saying is that in bad HF conditions, it won't hear anything, and thus won't say anything. Putting the likelihood of a missed communication higher.

Duplex doesn't fix no propagation.

[fnordfnordfnord]

Under what conditions in an airplane's flight routine is there no radio propagation on any frequency?

[sliverstorm]

http://en.wikipedia.org/wiki/HF_radio#Propagation_characteri...

I'm no expert in HF, but my general understanding is that while auto band-hopping could be helpful, you are never guaranteed there is a usable band.

[wglb]

From my comment ancestor to this thread:

Keep in mind that over-the-horizon radio communication is subject to the vagaries of the ionosphere and is not totally reliable.

Non-line-of-sight radio communications are short wave (well, also medium wave and long wave, but those modes require significant power). If ionospheric conditions are bad, as they are at many times of day (worse at night for some bands), at many times during the sunspot cycle, and during a solar storm, and sometimes during atmospheric events, then you won't get a radio signal through. One famous example of this was just prior to the Pearl Harbor attack, communications via radio between US and Hawaii were very poor or nonexistent for key parts of the time.

So to put a number to your question "likely better than half the time".