It seems unlikely for a plane to just drop off the map, given today's tracking technology. But this year, two flights have disappeared while travelling over southeast Asia.

AirAsia Flight QZ8501 fell off the radar during the morning of Dec. 28, while flying over the Java Sea. The Airbus A320-200 is believed to have crashed into the sea. Rescuers began recovering floating debris and bodies on Tuesday, but the plane itself has yet to be found.

In March, Malaysia Airlines Flight MH370 disappeared while flying from Kuala Lumpur to Beijing. No debris from that plane has been found, but officials believe it crashed in the Indian Ocean after someone on board deliberately diverted the aircraft from its route.

To get a better understanding of how these planes could have vanished, CBC News spoke to aviation experts about how air traffic controllers keep track of aircraft.

Primary radar

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Radar was first widely adopted by air traffic controllers in the 1950s and is still the mainstay of most air traffic control systems around the world today.

There are two types of radar: primary and secondary. Primary radar sends out electromagnetic waves that are bounced off any object in their path — in this case, an airplane — and does not rely on the plane's transponder having to send any signals back.

"This primary radar can see everything no matter if the transponder is on or off, but the primary radar can't identify the object. It can just see a point on the screen," says Mikael Robertsson, co-founder of Flightradar24.com, a flight-tracking website based out of Sweden that gets about six million visitors a week.

Primary radar is generally used more for military air defence than civil aviation, which relies on secondary radar. While the transponder on Flight MH370 stopped transmitting, the Boeing 77 should have remained visible to any military primary radar that was scanning the area at the time. It's believed the radar saw the flight change course and head west toward the Andaman Sea.

It's unknown if any military primary radar picked up Flight QZ8501.

Secondary radar

Air traffic controllers who manage commercial air traffic rely on secondary radar, which also sends out electromagnetic waves, but when the plane picks them up, its transponder sends back a signal identifying the plane and giving its altitude, speed and bearing.

This signal contains a unique four-digit code, called a squawk, that corresponds to that specific flight. The code is assigned to the plane by air traffic control and entered into the transponder by the pilot.

For a plane to be detected by secondary radar, there needs to be a radar station within about 300 kilometres, and since these stations need to be on land, radar coverage is limited over large bodies of water and is also affected by geography, the curvature of the Earth and a plane's altitude.

In places such as North America and Europe, there are enough radar stations spread across the land mass that coverage overlaps, and little territory is left "off the radar," but that is not the case in the middle of the Atlantic Ocean, for example.

"Unless you have that overlap and beyond that 200-mile range, you don't have radar coverage," said Sid McGuirk, a former air traffic controller with the Federal Aviation Administration in the U.S. who teaches air traffic management at Embry-Riddle Aeronautical University in Florida.

In the case of Flight MH370, the plane was still in range of ground-based radar stations when its transponder stopped transmitting over the Gulf of Thailand, rendering it invisible to secondary radar.

It's still not clear how that flight's transponder became disabled, but pilot Patrick Smith of the website AskthePilot.com points out that pilots need to have the ability to turn off transponders.

"In the interest of safety — namely, fire and electrical system protection — it's important to have the ability to isolate a piece of equipment," he says on his site. "Also, transponders will occasionally malfunction and transmit erroneous or incomplete data, at which point a crew will recycle the device — switching it off, then on — or swap to another unit."

Planes like the Boeing 777 usually have two secondary-radar transponders on board, with one serving as back-up.

ADS-B

The ADS-B system of tracking planes uses signals sent from GPS satellites and plane transponders to relay information such as a plane's location, speed and flight number to radio receivers on the ground. (Courtesy of Flightradar24.com) A third type of flight surveillance system is known as ADS-B , or automatic dependent surveillance-broadcast. ADS-B relies on radio waves being emitted by another type of transponder, which is usually attached to the bottom of the plane and controlled from the cockpit.

The ADS-B transponder sends out radio waves containing all kinds of information about the airplane, including GPS data about the plane's location relayed by navigational satellites, but also the flight number, speed and vertical velocity, which indicates whether the plane is climbing.

Anybody can pick up these radio waves using a cheap receiver similar to that used in car radios, says Robertsson.

"With ADS-B, you get much more data at lower cost [than secondary radar]," he says.

Although the technology is about eight years old and most major plane manufacturers already outfit their planes with ADS-B transponders, it is not yet the norm in air traffic control — in part, says Robertsson, because it takes years for the aviation industry to be convinced of the safety of a new technique.

"Australia was the first country that started to use ADS-B [for the whole country] in December last year," Robertsson says. "Any change in the aviation industry takes a very long time."

ADS-B is also used in regions of the U.S. such as Alaska and the Gulf of Mexico, as well as in parts of the Middle East and will eventually become the global standard. The FAA is expected to adopt the system by about 2020.

Robertsson's Flightradar24 has 3,200 ADS-B receivers deployed around the world, most hosted by volunteers, and two of them on the east coast of Malaysia detected Flight MH370.

"It was quite far away from our receivers, so in that area, the coverage is limited to about 30,000 feet, and this aircraft was flying at 35,000 feet, so it was within our coverage — until it disappeared," Robertsson says.

It's still unclear if any have detected Flight QZ8501.

The altitude at which ADS-B can detect planes varies by geography and the location of receivers — in some parts of Europe, where Flightradar24 has more receivers, it can be as low as 500 feet, Robertsson says.

Robertsson's organization began as a hobby six years ago and has grown into an unofficial global network that often provides up-to-date flight information to airlines and airports (but not air traffic controllers) and sells flight-tracking apps for smartphones and tablets (about four million to date, says Robertsson).

Pilot communication

Aside from being tracked through radar and ADS-B, planes also stay in contact with air traffic controllers and ground stations using radio communication and the Aircraft Communications Addressing and Reporting System, or ACARS.

Aside from radio communication, pilots can talk to ground control using a text-message like system known as ACARS. (Bullit Marquez/Associated Press)

Pilots can talk to controllers using radio signals transmitted over ultra high frequencies (UHF) or very high frequencies (VHF) — sometimes these communications are relayed through private third parties.

When airlines want to alert pilots to something, they can also use ACARS, which relays simple, short text messages through radio signals and satellites.

"For example, in 9/11, all of the air carriers — through ACARS — sent a message telling their flight crews to secure their cockpits," says McGuirk.

ACARS also periodically transmits diagnostic data about the performance of engines and other equipment directly to manufacturers or the airline to alert them to potential problems or maintenance issues.

The pilots on Flight QZ8501 last spoke with air traffic controllers to ask permission to increase the plane's altitude to avoid bad weather. Their request was denied because of another plane's position. No distress signal was sent.

The ACARS on Flight MH370 have been a subject of speculation, with suggestions that the plane may have continued to send automated pings to the satellites that transmit ACARS data for several hours after contact with the plane was lost.

The aviation news site Flightglobal has reported that the Inmarsat satellite network has confirmed its satellites received routine signals from Flight MH370 but would not comment on when and for how long.

Aviation consultant Robert W. Mann Jr. told the New York Times that such pings can continue even after a plane lands or crashes if the system has a back-up battery.

GPS

Pilots rely on the GPS network of navigational satellites to get information on their location in the same way your car does.

"Your video map happens to show streets and highways; their video map shows airways and land masses and airports, and it has a pretty sophisticated database, so it will give them a visual of where they are," McGuirk says.

But under the current secondary-radar system used to track most large, commercial aircraft, that information is not relayed to air traffic controllers.

Some smaller planes, regional passenger aircraft and helicopters do convey that GPS information to their ground control through a system of transceivers and satellite communications that some aviation companies use to track their fleets.

Victoria-based Latitude Technologies is one of the companies that supplies such satellite tracking services and devices. Its customers, says vice-president of operations Peter Parrish, include a variety of operators around the world whose aircraft is used for everything from regional passenger transport to search and rescue, medical transport and aerial firefighting. One of its customers is the Malaysia Airlines subsidiary MASwings, a regional carrier.

"For some (unknown to us) reason, the major carriers continue to rely exclusively on old technology to track their aircraft when one of our boxes could be tucked into an out-of-the-way spot on the aircraft to report location on a continuous basis, including on an accelerated basis right up to the point of impact in the event of a crash," Parrish said in an email.