Heres the thing.
If it was out of vhf range at time of reporting severe turbulence and if over water and oceanic im assuming then only HF radio wouldve been available to use. For those who dont know High Frequency is the form of radio used for oceanic crossings and we have Brisbane Flight Service Station (FSS) here, theres Nadi FSS I believe and Oakland for West Coast USA.
So when a flight is in 'Oceanic' it cannot be seen on a radar as primary radar targets cant be seen and the FSS controller relies on the pilots broadcasting their level, mach speed, estimates for passing positions to know a.) where they are and b.) to ensure planes dont pass over a point at same flight level for example.
So the fact that reports have said that it was lost off radar could potentially be quite misleading as in fact if it was on HF frequency and oceanic, it would have been miles before it was transferred from Brazilian controller with radar.
As I said though potentially because facts arent totally known yet.
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