The aircraft's transponder, which identifies the plane and relays details such as altitude and speed to controllers, stopped transmitting.

CNN: Pilot spots large object off Reunion Island

Aircraft equipped with ADS-B are able to automatically send highly precise data, such as aircraft positioning and speed, to air traffic controllers every second.

BBC: The key to preventing lost planes

Burning 3, 785 liters of fuel an hour, they turn left and right and speed up and slow down as air traffic controllers on the ground try to arrange them like a conga line for their final approach to the runway.

FORBES: Magazine Article