WORLD> Europe
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WORLD> Europe
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Missing plane flew well beyond range of radar
(Agencies)
Updated: 2009-06-03 14:49 BRUSSELS -- Before it vanished, Air France Flight 447 was flying hundreds of miles (kilometers) beyond the scope of the nearest radar station, just as scores of commercial flights do every day over the world's oceans. Above those vast waters, pilots follow different rules for navigation and safety because they are so far from land that air traffic controllers may not be able to pinpoint their precise positions.
Much of what happened to Flight 447 is still unknown, largely because the plane was soaring in a remote zone between Brazil and West Africa. Air crews in that region are never out of radio contact with the ground, but radar cannot track them until they draw closer to shore.
Because of this radar-free void, crews aboard many transoceanic flights must observe safety procedures that are significantly different from those for flying over land. Land overflights are normally separated by 5 to 10 miles (8 to 16 kilometers). But long-range oceanic flights are spaced 20 minutes apart, the equivalent of 80 nautical miles (148 kilometers), to minimize the possibility of midair collisions in places far beyond radar. Oceanic flights also use different navigation techniques. A land-based flight typically follows aerial pathways marked with radio beacons that crisscross the continents. But those paths do not exist over water. Instead, flight controllers determine specific flight tracks each day, one eastbound and one westbound, on the basis of weather reports and other information. Thousands of airliners follow each other along these tracks. "It's not possible to track these flights simply because there are no places where you could install the radars needed to cover the entire trans-Atlantic range," said an official of Europe's aviation agency, Eurocontrol, who could not be identified in line with standing regulations. Parts of the Atlantic Ocean are regularly monitored by radar from locations such as Iceland, the Azores, Ireland and Canada's easternmost tip, but those still cover only a small portion of the oceanic routes, the official said. Pilots can still speak with ground controllers, but standard VHF radios do not work on transoceanic routes because of the earth's curvature. Long-haul pilots must use less reliable HF voice communications which are more susceptible to interference. Modern airliners also have a digital datalink that automatically transmits and receives messages between the aircraft and ground stations. Those messages are then relayed to air traffic control centers or the airline's own dispatch center, and are used by controllers to determine the aircraft's approximate position. On Tuesday, Brazilian aircraft located wreckage from the Airbus A330 about 400 miles (640 kilometers) northeast of the Brazilian island of Fernando de Noronha. The plane carrying 228 people vanished Sunday about four hours into its flight from Rio de Janeiro to Paris. The only other clue to the plane's fate was an automated message received by an Air France dispatch center in Paris that reported an electrical failure and loss of cabin pressure. There was no mayday or distress call, but an Air France spokesman has mentioned that a lightning strike in an area of heavy turbulence may have sparked a chain of events that led to disaster. |
