As the disappearance of Malaysia Airlines Flight 370 stretches on with still virtually no reliable clues to work with, what began as a bizarre incident is beginning to look even stranger still. Commercial airliners have gone missing before, but only until their wreckage was found. MH370 seems literally to have vanished into thin air. But how could that happen? And what does it mean for a plane to disappear, anyway?
The shock generated by the Boeing 777 mystery is largely a product of how much we’ve come to take for granted the modern superabundance of information. We expect to know where everything is, all the time. If you log on to a flight-tracking website, you can punch in the flight number of any commercial aircraft and see its current location and direction. Seems pretty foolproof.
And indeed, in most cases, airplanes are constantly connected into the global information network by multiple mutually redundant forms of surveillance and communication. There’s very little uncertainty—which is how air traffic controllers like it, since streams of large metal objects moving at hundreds of miles per hour don’t do well moving randomly around the sky.
For air traffic control purposes, the planet is divided into regions called “centers,” each of which is under the authority of a different set of controllers. Every plane begins its travel through the system with the filing of a flight plan, and as it proceeds on course through the air, its information moves along electronically in parallel from one center to the next.
Centers track each flight in a variety of ways. The first is with good old-fashioned radio calls. Controllers call up pilots to give them instructions, inquire about their intentions, and relay information.
The second source of information is radar, of which there are two kinds. Primary radar tells operators where a plane is located. The military uses a version that can also tell at what altitude a plane is flying, but civilian controllers don’t have access to that kind of system. Instead, they use secondary radar, which pings an electronic device called a transponder inside each aircraft, which then transmits its altitude. Controllers will then be able to see on their screen the number of each flight, its location, and its altitude, derived from secondary radar.
Radar and radio have been used since your grandparents’ day. More recently, a third system has started rolling out called ADS-B. The acronym stands for “Automatic dependent surveillance-broadcast.” Aircraft determine their position via satellite navigation, then transmit that information to a base station. They can also receive information from the base station, including weather info and the location of nearby traffic. It’s a better system than secondary radar and someday will supplant it, but for now we’re in the transitional phase, and in most cases pilots and controllers have access to both.
Together, these last two systems provide a robust and interlinking network, but they share the same limitation: They’re limited in range to somewhere between 100 to 200 miles from the nearest ground station, depending on atmospheric conditions.* “In general, once you go far enough out over the water, if you don’t have a satellite link, there’s no way to talk to the ground,” says Rob Thomas, a program engineer with Ohio University’s Avionics Engineering Center.
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