Technology: Danger in the wake of an aeroplane

时间:2019-02-28 02:14:00166网络整理admin

By JONATHAN BEARD in WASHINGTON DC Earlier this year, a small Cessna 310 aircraft was descending on its final approach to Montpellier airport in France when a powerful whirlpool of air seized the plane’s wings and flung it through a complete corkscrew roll. The pilot, who had training in aerobatics, was able to regain control and make a safe landing. The whirling air that almost killed him was not a freak of nature. It was the wake vortex from an Airbus airliner flying 500 feet above the Cessna. The sinking vortex came into the Cessna’s path almost a minute after the airliner had passed overhead. According to Claude Stouff of France’s Direction Generale de L’Aviation Civile, the Cessna-Airbus encounter is typical of wake vortex accidents: a small plane, making an approach to land, runs into the vortex created by a much larger jet which passed nearby as much as 3 minutes before. Speaking at an international symposium on wake vortices in Washington DC recently, Stouff said that the other four accidents reported since 1989 in France due to wake vortexes all resulted in fatal crashes. Every aeroplane creates vortexes as it flies; they are an inevitable by-product of the lift generated by its wings. The vortexes are like two horizontal tornadoes beginning at the wingtips and streaming back in the plane’s wake. Depending on atmospheric conditions and crosswinds, vortexes can persist for several minutes. When an aeroplane encounters a vortex with a diameter that is roughly the same as its wingspan, it can be tossed suddenly into a violent roll. During the past 35 years, aviation authorities have developed weight categories for aircraft and rules specifying the separation between aircraft for each class. Britain’s Civil Aviation Authority (CAA) specifies that when a Boeing 747 airliner (a 360-tonne ‘heavy’) is on its approach, the controllers try to keep at least 4 nautical miles between it and a 747 following it (1 nautical mile = 1.85 kilometres). If the follower is a ‘medium’, such as a Boeing 727 at 90 tonnes, the distance must be 5 miles. For small aircraft around 19 tonnes the separation is 6 miles, and executive jets – ‘light’ planes – have to stay 8 miles behind a heavy leader. These separations have prevented serious accidents, but they cause huge delays at busy airports which cost airlines and the airports millions of dollars every year. ‘If we could get one additional landing per hour at Frankfurt, over a year it would mean 10 million deutschmarks savings in delays,’ said Heinz Winter of Germany’s Aerospace Research Establishment. Airlines and airports want to reduce the separation distances between planes while maintaining safety. Present standards, they say, are based on ‘worst case’ scenarios, and seem to offer a generous margin of error. In clear weather, 747s often close up and follow one another with as little as 2.5 miles between them. But the wide range of planes in the air makes reducing separations difficult. Researchers are looking into several methods of dealing with the vortex problem, such as an instrument that could detect and track vortexes. Jim Evans of the Lincoln Laboratory at the Massachusetts Institute of Technology has experimented with a variant of radar, called lidar, to locate and measure the velocity of wake vortexes. With lidar, Evans reflects a light beam from a carbon dioxide gas laser off the fast-moving air in a vortex. A receiver is able to detect the reflected light and track the vortex and the surrounding air in order to predict its path. Evans believes that a set of coarser resolution lidars might be suitable as an advisory system at an airport. Alain Donzier of Remtech, a company in Velizy, France, has carried out tests of sodar – sound detection and ranging – at Charles de Gaulle airport in Paris. He bounces sound waves off high-speed air and listens for their echoes. The sodar system was originally designed to detect wind shear, a weather phenomenon which produces violent downdraughts near the ground, and Donzier says that it will have to be improved to be able to react fast enough to detect vortexes, which are much smaller phenomena. In Britain, Trevor Gilpin of the Chief Scientist’s Division of the CAA says the authority has sponsored work at the Royal Signals and Radar Establishment (now part of the Defence Research Agency) to test a laser system at Heathrow. He said the CAA is also looking at Doppler radar, which measures the differing echoes from air moving in different directions, to see if it could be used to detect and track vortexes. So far, he says ‘the results have not been conclusive enough to indicate whether lasers or radar would be used’. All such groups at the symposium reported some success in detecting vortexes, yet none of the devices is yet in regular service at an airport. All of the instruments had trouble picking up their small targets – the turbulent core of a vortex is less than 10 metres across – at long ranges. Also, no one is sure that there is a practical way of presenting this information to air traffic controllers who are already very busy watching radar screens and talking to pilots. Eric Stewart of NASA’s Langley Research Center reported tests of a wake-vortex detector mounted on aircraft, but it had similar limitations. Patrick Curran, an engineer from Sundstrand Aerospace in Rockford, Illinois, reported on a method of reducing vortexes at their source with small turbines positioned at an aircraft’s wingtips. The company fitted unpowered turbines on a single-engined test aircraft belonging to NASA. With the turbine installed, Curran says ‘the typical vortex is no longer present, its energetic core is gone, replaced by random turbulent flow’. The turbines can also be connected to a generator to produce electricity for use on the aircraft. This and the fuel savings due to reduction in drag would pay for the turbines over a short time,