Flight 370 Mystery: How Can a Jetliner Drop Off the Radar?

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The mystery surrounding a missing commercial airliner raises a fundamental question: How can a big jet full of people just vanish into thin air in this day and age?

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Malaysia Airlines Flight 370 took off from the Malaysian capital of Kuala Lumpur on Friday afternoon (March 7) U.S. Eastern time, headed for Beijing. But air traffic controllers lost contact with the plane less than an hour later as it was flying over the Gulf of Thailand -- it just disappeared from the radar. The whereabouts of the Boeing 777 jet, which was carrying 227 passengers and 12 crew members, remain unknown.

"This is a very unusual event," Sid McGuirk, an associate professor of air traffic management at Embry-Riddle Aeronautical University in Florida, told Live Science. "It's highly unusual for an aircraft at altitude -- which, at least according to the press, this aircraft was -- to drop off the radar." [The 5 Real Hazards of Air Travel]

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Two radar systems

Air traffic controllers track commercial jets using two types of radar. "Primary" radar determines a plane's position by analyzing signals that bounce back off the aircraft; the "secondary" or "enhanced" type requests information from each plane, which is then sent by a piece of equipment aboard a jet known as a transponder.

Radar facilities are based on land, and each one has a range of about 200 miles (320 kilometers), McGuirk said. So passenger jets on transoceanic flights do go off the radar map for a period of time, but that doesn't mean nobody's keeping tabs on them.

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"The flight crews use combinations of high-frequency (HF) radio, satellite-based voice communication and text-data networks to report to ATC [air traffic control] the exact time, position and flight level when the crossing begins," said Emily McGee of the Flight Safety Foundation, a nonprofit organization headquartered in Alexandria, Va.

"They then update ATC with voice or text progress reports at defined geographic locations and time intervals," McGee told Live Science via email. "Airlines file flight plans, and airplanes are expected to arrive at certain points by certain times. When an airplane crew fails to check in at its next checkpoint, that is when an alarm is raised. This case is an extremely rare event, especially with the highly technologically advanced aircraft in the air today."

Commercial jets can also fall off the map briefly when they fly at low altitudes because radar relies on line-of-sight contact. Mountains and other landforms can block the signals going to and from the closest radar stations, as can the curvature of the Earth.

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As a result, low-flying jets can be tough to track continuously, especially if their transponders are disabled -- a fact that terrorists took advantage of on 9/11. [9/11 Science: 10 Ways Terrorist Attacks Rocked America]

"The first thing that many of the hijackers did [on 9/11] was turn off the transponder," McGuirk said. "Once they turned off the transponders, then they turned the aircraft back toward whatever their target was."

Someone who wanted to steal the Malaysia Airlines jet could theoretically shut off the transponder and dip down to an altitude of 5,000 feet (about 1,520 meters) or so, he added, while cautioning how far-fetched that scenario is.

"Of course, it's kind of hard to hide a 777," McGuirk said. "Wherever it lands, somebody's going to say, 'Hey! There's a Malaysia Air 777. It didn't crash at all -- it was being stolen.'"