The article says it disappears from radar. When tracking aeroplanes there are two kinds of radar primary and secondary. Primary is actual radar, secondary i

The article says it disappears from radar. When tracking aeroplanes there are two kinds of radar primary and secondary. Primary is actual radar, secondary isn't really radar at all but instead radio transmission of telemetry such as position, altitude, speed etc. This is equivalent to AIS, the system used to track boats that produced your image. So we were tracking the plane but then the plane stopped transmitting tracking info for some reason (no idea if they even had it on primary radar). Someone could have turned it off or the plane could have crashed.