Safety Alarms May Prevent Jet Crashes Into Mountains
|
|
A new kind of altitude warning system that might have saved the Korean Air Lines jumbo jet that crashed on Guam is being installed in hundreds of American jetliners and may eventually be required on all airliners by the Federal Aviation Administration. The system, already being placed in some passenger jets by American, Alaska and United Airlines, Lufthansa and British Airways, learns the aircraft’s location, altitude and course from existing navigation equipment on board. It then compares that information with a computerized map of all the world’s mountains and predicts the length of time before an impact would occur. It sounds a warning at 60 seconds before impact and again, more sharply, at 30 seconds. Conventional systems sometimes fail to give any warning at all. Those devices, known as ground proximity warning systems, became standard equipment on airplanes in the early 1970’s. But they have no ability to assess the terrain ahead of an airplane. Instead, they use a radar altimeter to measure the distance between the plane and the ground directly below it; they sound an alarm if the distance is narrowing too fast. The problem with such systems was demonstrated by the crash last week in Guam and by two crashes in 1996, one in which an American Airlines jet hit a mountain near Cali, Colombia, and another in which an Air Force plane carrying the Commerce Secretary, Ronald H. Brown, crashed near Dubrovnik, Croatia. The existing system works well enough over flat terrain. But when a plane is approaching a mountain, its height over the ground can be reduced to zero so quickly that the system cannot provide adequate warning. If the slope of the mountain is steep enough and the plane is moving fast enough, the device may not provide any warning at all. In aviation jargon, crashes like the ones in Guam, Cali and Dubrovnik are known as controlled flights into terrain, or C.F.I.T.’s. The plane experiences no mechanical problem, but the crew, usually because of poor visibility and confusion, flies into a mountain. Experts say that the new instrument, called an enhanced ground proximity warning system, has the potential to make those kinds of accidents extremely rare. Such crashes are ”the single greatest cause of death in aviation accidents,” said Jack Thompson, the deputy director of technical projects at the Flight Safety Foundation, a nonprofit group in Alexandria, Va. ”The adoption of enhanced ground proximity warning systems could virtually eliminate C.F.I.T. accidents,” Mr. Thompson added. At the Federal Aviation Administration, Thomas E. McSweeney, the director of aircraft certification, said that next year his agency would publish a proposed rule to make the enhanced systems mandatory. Industry executives say the only question is the installation schedule that the F.A.A. will seek to require. Engineers have yet to figure out how to adapt the devices for use in all planes. ”The really encouraging thing,” Mr. McSweeney said, ”is the airlines are already recognizing the advantages of it and are going their own way, without a rule, to purchase the equipment.” Mr. McSweeney and others said the existing systems had cut accidents sharply but had weaknesses. One is that they can be easily lulled into staying silent because of the way they are set up to avoid issuing false alarms, especially as the plane is approaching landing — studies show that pilots tend to ignore equipment that cries wolf. Conventional ground proximity systems of different vintages have different criteria for sounding alarms, but on most models, when the plane’s landing gear and flaps are extended, the warning system senses this and does not sound an alarm unless the distance to the ground is dropping extremely fast. After the Dubrovnik crash, which occurred on the approach for landing, engineers said the warning system would have sounded only a second or two before the impact, if at all. In the Guam case, the gear and flaps were also set for landing. More : query.nytimes.com |