How Can an Airliner Disappear?

Secondary radar and the transponder. Air traffic control's primary tool. The ground station interrogates, and the aircraft's transponder replies with identity and altitude. If the transponder is off or fails, the aircraft stops appearing on the controller's screen in the normal way.

Primary radar. Reflects radio energy off the airframe and needs no cooperation. It is the exception — but it has a range of a few hundred kilometres at most, is expensive, and coverage is concentrated near land and around busy airspace. Over an ocean there is none.

ADS-B. The aircraft broadcasts its own GPS position continuously. Excellent, cheap and now near-universal — but again, the aircraft is doing the telling, and ground receivers need to be within a few hundred kilometres. Satellite-based ADS-B reception, which did not exist in 2014, has since closed much of the oceanic gap.

ACARS. Sends operational and maintenance data to the airline by radio or satellite. It can be disabled, and it reports intermittently rather than continuously.

HF radio position reports. Over oceans, crews traditionally reported position by voice at waypoints. That is a report every 30 to 60 minutes, from a crew who choose to make it.

Put these together and the picture over mid-ocean before 2014 was: no primary radar, no ADS-B ground receivers, position known only from what the aircraft chose to transmit and from voice reports every half hour or more.

An aircraft that stops transmitting in that environment is not being watched by anything. Its last known position is wherever it last reported, and its possible position grows by hundreds of kilometres for every subsequent hour.

Over land, a missing aircraft is found: there is terrain to search, people who saw or heard something, and wreckage stays where it lands. Over deep ocean, wreckage sinks to a seabed several kilometres down that is often mapped in less detail than the Moon. Floating debris drifts thousands of kilometres in unpredictable directions. The underwater locator beacons on the recorders historically ran for 30 days.

ICAO now requires aircraft on oceanic routes to report position at least every 15 minutes in normal operations, and autonomous distress tracking to transmit position at least once a minute when an aircraft enters an abnormal state — regardless of what anyone in the cockpit does. Recorder beacon life was extended to 90 days, with an additional low-frequency beacon on the airframe for long over-water operations.

Satellite ADS-B reception now covers the oceans commercially. The combination means a modern aircraft in the same situation would leave a track measured in single-digit kilometres rather than an arc thousands of kilometres long.

It is still possible for an aircraft to be lost at sea. What is now very difficult is for one to be lost without anybody knowing roughly where.