Air traffic controllers track aircraft using two types of radar. "Primary" radar determines a plane's position by analyzing signals that bounce back off the aircraft; the "secondary" or "enhanced" type requests information from each plane, which is then sent by a piece of equipment aboard an airplane known as a transponder.
Radar facilities are based on land, and each one has a range of about 200 miles (320 kilometers). So you might have a commercial airline go off the radar for a bit (though they have other means of getting their info to ATC) on a transoceanic flight but where this plane went missing there would be continuous radar coverage
Going so low as to not be seen on radar is only going to happen if large mountains prevent line of sight contact, also not an issue in this case.
Had they just glided down to a low altitude the air traffic controller would have seen that, and likely questioned why they were descending. The fact that they disappeared from the scope only means one thing. They crashed into the water, and it was probably an extremely rapid descent. I’m still going with loss of control. They were cloudy, stormy weather and the faa public records (
www.faa.gov) for the pilot (who was issued the same type of license he had in the United Kingdome -what they call private pilot/foreign based) shows he was not licensed to fly commercially nor in the clouds (no instrument rating).