Originally published on March 12, 2014

As the search for a missing Malaysia Airlines flight enters its fifth day, the plane is still nowhere to be found. The hunt has grown increasingly difficult in part because the aircraft's transponder stopped transmitting signals indicating its altitude, speed and direction.

Air traffic controllers rely on two types of radar to keep track of airliners: primary surveillance radar and secondary surveillance radar. The former is passive in the sense that it locates a plane by analysing signals that bounce off an aircraft. Secondary surveillance radar acts as a complement: it requests information from aircraft, which respond using a transponder.

Pilots communicate with air traffic control by punching in four-digit codes on the transponder, a process known as "squawking". Aside from the aircraft's altitude, speed and direction, the transponder can also inform air traffic control of special flight problems. The code 7500, for example, means a hijacking is taking place; 7600 implies communication failure; and 7700 indicates an emergency.

The transponder is switched off in certain situations. Air traffic controllers may request pilots turn off the transponder when planes get close to each other; pilots may shut it off if the device disseminates faulty data.

Some are speculating that the transponder had been switched off intentionally before the plane's disappearance.