Aircraft normally communicate with the ground using a number of systems, all of which, in this instance, appear to have failed.
How are aircraft normally tracked?
Air traffic control combines the location-measuring properties of basic radar with the signals provided by aircraft transponders to give a detailed picture of traffic in the sky.
All commercial aircraft are equipped with cockpit transponders (an abbreviation of "transmitter responder"), which automatically transmit electronic signals back to the ground when they receive a radio signal.
The most basic types send only the aircraft's altitude and 4-digit flight code, but radar stations are able to establish the plane's speed and direction by monitoring successive transmissions.
Radar coverage usually ends around 150 miles from shore, and while flying over the sea air crew keep in touch with the ground and other aircraft using high-frequency radio.
Transponders can be switched off manually in mid-air, but in the case of Flight MH370 it is not known whether the loss of signal was caused by deliberate human action or from a catastrophic event.
The last radio message received by air traffic control - "Alright, roger that" - suggests everything was normal on board minutes before it went missing over the South China Sea.
What if a transponder fails, or is turned off?
If a transponder stops sending a signal, aircraft can still be tracked using what is essentially still the same form of radar developed in the 1930s.
Primary radar tracks anything in the sky which reflects transmitted radio signals. As such, it can only indicate the approximate position of an aircraft - it cannot identify it.
It is used today mainly as a back-up system to secondary radar.
Officials in Malaysia have suggested that primary radar tracking may yield information about the missing airliner's trajectory, but that the data requires detailed analysis by experts.
Don't planes have GPS?
Yes, but while GPS (Global Positioning System) is a staple of modern life, the world's air traffic control network is still almost entirely radar-based.
Aircraft use GPS to show pilots their position on a map, but this data is not currently shared with air traffic control.
Some of the most modern aircraft are able to 'uplink' GPS data to satellite tracking services, but handling large volumes of flight data is expensive and such systems are usually only used in remote areas with no radar coverage.
The disappearance of Flight MH370 is likely to bring renewed focus on whether mid-air tracking should be improved.
Aviation expert Chris Yates says the ADS-B system (Automatic Dependent Surveillance-Broadcast) is already using GPS data.
"In good chunks of the world, ADS-B operates which provides a 'pseudo-radar' image of aircraft in flight. That's what's used by flight trackers online, fed into a mapping system.
"It relies partly on GPS to generate a position fix."
Aircraft use ADS-B to work out their position via a satellite. The plane then broadcasts its position to other aircraft and to a ground station.
The US will require all aircraft to be equipped with some form of ADS-B by 1 January 2020 and the system is predicted to replace radar-based tracking in the next decade.
The Malaysian aircraft disappeared from flight-tracking websites at the same time it vanished from air traffic control screens, and no GPS data has emerged to shed any light on its fate.
Could the 'ACAR' data system provide clues, as with Air France Flight 447?
When Air France Flight 447 crashed into the mid Atlantic in 2009, its onboard 'Aircraft Communications Addressing and Reporting System' (ACARS) gave investigators vital early clues into what had gone wrong.
ACARS is a data service which essentially allows computers aboard the plane to 'talk' to computers on the ground, relaying in-flight information about the health of its systems.
Messages are transmitted either by radio or digital signals via satellites, and can cover anything from the status of the plane's engines to a faulty toilet.
This provides ground crews with vital diagnostic information, allowing maintenance to be carried out more quickly.
In the Air France case, ACARS highlighted faulty speed readings, which caused the air crew to become disorientated.
But investigators say no data was received from the Malaysian flight.
Don't a plane's 'black box' recorders also transmit signals?
The mystery of Flight MH370 may only ever be solved when the aircraft's 'black box' flight recorders are recovered.
However, when a plane is likely to have crashed into the sea, recovering them is not easy.
In the case of Air France Flight 447, it took nearly two years.
If under water, the black boxes themselves emit ultrasonic signals - but these signals have a limited range, and search crews may not detect them unless close to the actual crash site.
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