Air traffic controllers have multiple radar systems in place to track flights over land and sea, so when a plane disappears from radar, it's usually a sign that it's lost power, switched off its transmitters or dropped below cruising altitude.
Air Algerie Flight 5017 that disappeared from radar on Thursday was flying entirely over land, where radar coverage is more thorough than over the ocean.
However, radar systems are designed to track flights at cruising altitude. They don't monitor the skies below safe cruising altitude because terrain and buildings start to get in the way of the radar signal. Therefore, if a plane drops below that minimum altitude, it's no longer high enough for radar to see it.
Here's how air traffic control keeps track of commercial flights over land and sea.
Radar coverage over land
Flights over land are much easier to track than flights over water because air traffic controllers have on-the-ground means to send their own radio signals to the planes overhead.
Airports track flights using two radar methods, called primary and secondary radar.
Primary radar bounces radio signals off the plane and judges its location based on the signal it receives back. Primary radar does not require the object to engage with the signal, so even if a plane is not communicating with ground control, it will still be visible on primary radar. However, primary radar does not "talk" to the aircraft to determine its flight information.
Secondary surveillance radar is more complex and specific. This more advanced system has a longer range than primary radar, so it can extend coverage farther out over the ocean. It works using a series of periodic "handshakes," repeatedly sending query signals to the plane and receiving answers back that verify the target's location, identity and altitude. Essentially, the query signal asks, "Is that you, where are you?" and the plane responds, "Yes, it's me, and here's how fast and high I'm flying." Secondary radar is automatic and can be switched off or disabled by loss of power. It's also encrypted so it cannot be faked.
When terrorists hijacked the planes that crashed on Sept. 11, 2001, they switched off the secondary surveillance radar on their flights.
In the case of Malaysia Airlines Flight MH370, ground controllers were tracking the aircraft by secondary radar when it disappeared from their monitors.
