Free flight, which would cut travel time by routing air traffic directly between airports, might be just as safe as the current air traffic control system that holds airplanes to grid paths, according to a preliminary study published in a journal of the Institute for Operations Research and the Management Sciences (INFORMS®).
Shortened flight paths could also translate to reduced costs for the nation's airlines. "The changed geometry of flight paths after a transition to free-flight tends in itself to diminish mid-air collision risk," says Arnold I. Barnett, George Eastman Professor of Management Science, the Sloan School, Massachusetts Institute of Technology.
"But there is an important caveat: Free-flight would also replace the orderly patterns that now appear on an air traffic controller's screen with something closer to the random scatter of gas molecules. Computer aids are supposed to prevent confusion, but their success must be demonstrated before we can be sure that they can preserve the extraordinary safety of the present system."
The study, "Free-Flight and En Route Air Safety: A First-Order Analysis," appears in the current issue of Operations Research, an INFORMS publication.
In U.S. airspace, planes generally cannot travel the straight-line paths from their origins to their destinations. Rather, they usually are restricted to a series of fixed routes that are the aerial equivalent of the interstate highway system. The specific paths were selected more than half a century ago, and radar beacons for navigation were placed along them.
Recent advances in technology, however, raise the possibility that present arrangements can be replaced by a "free-flight "regime, under which planes generally could travel directly from one city to another. Among these advances are global positioning satellites, new ground/air communication links, enhanced collision-avoidance systems aboard planes, and powerful automation aids to air-traffic controllers. The benefits of free-flight include shorter trip time, lesser fuel consumption, and additional flights. Prof. Barnett applied operations research models to the geometry of flying in and out of a hypothetical square air sector.
"My basic view," he says, "is that the geometric consequences of free-flight - reduced path crossings and lesser angles at which crossings occur - might in themselves be expected to increase safety." His results showed a "safety dividend" as high as 69% under free flight. The geometric safety dividend reflects fewer flight-path crossings under the proposed system, as well as changes in crossing angles which tend to allow more time to avert disaster when two planes are on a collision course. Free flight reduced mean transit distance by 11% in the initial case Prof. Barnett studied.
When the research was repeated with data from a representative air sector above Albany, New York the results were more modest but still significant. In the Albany example, mean transit distance improved by 2%, the limited improvement largely a tribute to the skill with which the original aerial "highways" were chosen to fit demand patterns. The safety dividend based on changed geometry improved by 58%. The results could mean savings for airliners, says Prof. Barnett. A 2% reduction in average flight paths - barring erosion of this effect in the air terminal areas - would imply a 2% drop in en route fuel consumption and the shortening of typical flights by a few minutes. Given annual fuel costs of about $50 billion for US domestic jet carriers, savings could come to $1 billion a year, roughly one-fifth of the industry's total annual profit in the late 1990s.
The results also imply that free flight might allow air carriers to double the number of flights through an air sector while maintaining current safety figures.
The Albany data analyzed in the study represented traffic on July 22, 1994; it was supplied by the Federal Aviation Administration's Boston center.
Prof. Barnett cautions that actual savings could be less if congestion around the nation's airports leads to airplanes using additional fuel before landing. He notes that results in a single sector may not generalize to the totality of air sectors around the country because of differences in flight patterns in different sectors.
Prof. Barnett points out that his research involves a number of idealized assumptions. He assumes free-flight in latitude and longitude but not altitude; thus, planes could not simply announce that they would travel at, say, 37,000 feet but would be required to fly at a mandated altitude. (Prof. Barnett suggests that two-dimensional free flight might be considerably more prudent than a three-dimensional version.)He only examines planes that have achieved a fixed high altitude and are en route, rather than taking off or landing. He also generally assumed that all en-route aircraft cruise at the same speed (though he relaxed that assumption in one exercise).
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