Originally Posted by
pattern_is_full
Antiquated idea of how "radar" works these days. What controllers see on their screens is a transmission from the aircraft (transponder, or as Airbubba says, now ADS-B). A flip of an on/off switch in the cockpit can make a plane "disappear" instantaneously. As can anything that cuts off the transmission: loss of electrical power, loss of airframe continuity. The aircraft does not have to "descend below radar coverage."
The old-school, bouncing radio waves off aluminum, primary radar is available if desired, but if the aircraft is now a descending shower of confetti, won't help much after the fact. It will take some time to check the primary returns on tape to see what shows up there.
All we really know right now is that the aircraft abruptly ceased transmitting while still at 37000 feet.
This explanation is also incorrect. Think of surveillance in two ways, cooperative (the aircraft tells you something) and non-cooperative (you are not dependent on the aircraft telling you). ATC radar is (for safety reasons) non cooperative using either primary radar (a skin paint) or secondary radar where more than one radar detects the signal from the transponder. ADS B is a cooperative technology, the aircraft's transponder squits out the position of the aircraft and that gets picked up by a ground receiver, which is why you can do it on your home PC for a few bucks spent on an aerial. Very few ATC units utilise ADS B only.
ADS B has benefits in the amount of data which gets sent, like Mode S aircraft ID, the tail number and callsign. No need for a squawk code but not all aircraft are equipped with ADS B and you, on the ground, cannot be certain that what the aircraft is telling you (position reporting) is absolutely correct. Whilst flight radar 24 etc is an interesting toy to check on the status of where auntie Doris aeroplane is when she is coming to visit it is not what ATC (in general) are using to track aircraft.