Radar can only track a plane for so long.
This weekend, a Malaysian Airlines flight apparently disappeared from the skies. The airplane in question was an 11-year-old Boeing 777 powered by Rolls-Royce Trent engines. Flight MH370 took off from Kuala Lumpur International Airport at 12:41 am Saturday morning in clear conditions, climbed to 35,000 feet, then vanished from radar screens.
Commercial flights aren’t as closely tracked as we think. There are dead zones over bodies of water and different technology and operational guidelines in different countries. And it’s not unheard of for planes to disappear mid-flight. It’s happened before, most notably in the 2009 Air France disaster.
In the US, there are multiple systems that monitor commercial flights. The Air Traffic Organization (ATO) is the operational arm of the Federal Aviation Association (FAA). As they say on their website, this branch is “responsible for providing safe and efficient air navigation services to 30.2 million square miles of airspace.”
You can think about the ATO activities as being divided into branches or sections. The stations we’re most familiar with are the Airport Traffic Control Towers, the towers at airports that monitor incoming and outgoing flights to make sure none cross on runways or in the air. The branches we’re less familiar with are the Air Route Traffic Control Centers and the Terminal Radar Approach Control Facilities. Terminal Radar Approach Control Facilities are typically, though not exclusively, associated with airport towers and help bring planes in for landings, so there are a lot of these stations throughout the country. But there are only 22 Air Route Traffic Control Centres in the United States.
As a plane flies from its origin to its destination, it passes from one air traffic station to another. But for the most part these flights are always on a radar screen somewhere, a moving spot with associated flight information like airline and flight number, altitude, and heading. This information also helps controllers spot planes that are flying too close to one another, ideally avoiding any mid-air collisions. Any incident that does happen in this closely monitored airspace usually registers as a dot disappearing from a radar screen.
Things are different in airspace that’s less closely watched, like over the Atlantic Ocean, for example. Radar can only track a plane for so long; eventually it moves out of radar range and flies off the controller’s screen. There are other monitoring methods in place like satellite navigation systems and air-ground data links, advances that have led to more thorough tracking methods in more recent years. One of these systems is so-called “dependent monitoring” that uses the aircraft’s known position from its onboard navigational system. The system, called Automatic Dependent Surveillance or ADS, exists on both Boeing and Airbus airplanes.
But for the most part, transatlantic flights follow preset routes similar to those in the sky over the United States. These defined corridors are designed to keep aircraft flying safely around one another between 28,500 and 41,000 feet. Planes on these routes aren’t tracked like little blimps with identifying marks, heading, and altitude information. They fly under radar control to a certain point, then pass a way point after which their next communication point is determined based on its speed, heading, and flight plan. So if a plane is struck by disaster between these way points, it won’t drop off a radar screen; it will simply fail to show up on a radar screen when it’s expected to.
This is what happened to Air France flight 447 on June 1, 2009. This plane, an Airbus, left Rio de Janeiro, Brazil for Paris, France, on June 1, 2009. It never arrived in French airspace when expected. Its arrival time came and went, and Air France executives knew the plane would have run out of fuel. And because it was in a radar dead zone when it disappeared, finding it and figuring out what brought down such a sophisticated plane was a huge undertaking.
Accident investigators had almost nothing to go on when they started looking into the mystery of Air France 447’s disappearance. The most useful piece of evidence they had was a maintenance report, an automatically generated report sent from the plane to Air France at 10 minute intervals and designed to help ground crews prepare for any maintenance before the plane’s next takeoff. One of these Aircraft Communication Addressing and Reporting System reports gave investigators a rough idea of the airplane’s last position, but it still took five days before any debris was found. And even then it wasn't a full airplane, just pieces of fuselage and a handful of the 239 people that were on board.
It took almost two years of searching and help from the team and machines that found the Titanic wreckage, but investigators eventually found the Airbus and, miraculously, its black boxes. Two years after it failed to show up when expected on a radar screen, the mystery was solved. In short: The plane was flying through a storm.
Ice clogged the pitot tubes, the small cylindrical instrument on the side of the plane that measures incoming air to feed the computer airspeed. With airspeed values suddenly gone, the Airbus turned control back to the pilots who unfortunately reacted by trying to climb to increase their airspeed. At the altitude they were flying, putting the plane in a nose up orientation caused an aerodynamic stall. The plane fell like a stone, and by the time the crew realized what was happening it was too late. The Airbus slammed belly down into the Atlantic.
This isn’t to say that the Malaysian Airlines flight will have an explanation akin to the Air France flight—reports of passengers on board flying with fake IDs are raising flags about possible hijacking—but it just goes to show that planes can disappear and the cause can be more complicated than expected.
It’s worth remembering, too, that the systems in place in the US aren’t necessarily standard throughout the world. Standard practices, available technology, and even accepted risks can vary. Unless clear evidence of an explosion turns up, we might be months or years from figuring out why this plane just disappeared from radar screens.