That's Airborne Warning and Control System. This is technology used by aircraft to "detect aircraft, ships and vehicles at long ranges and perform control and
command of the battle space." The technology can also take over and electronically "hijack" airplanes, including planes for passenger flights.
Stone presents an interesting case for this theory. He gets the flight number incorrect and makes a questionable comparison to the allegations of planes hitting the World Trade Center towers on 9/11, but explains the AWACS theory well. He states, among other things, that AWACS can cause a plane to fall off civilian radars but not military radars (which apparently happened here). The facts of cell phones ringing also support an AWACS hijack and landing, as does the fact of "missing" black boxes. Give it a read!