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At 1:00 a.m. on a cold October morning in 2002, John-Paul Clarke ‘91, SM ‘92, ScD ‘97, stood in a damp field in Floyds Knobs, IN, listening for two United Parcel Service jets to roar into Louisville International Airport. The first came in flying a traditional stepped landing pattern in which it incrementally decreased altitude several times before reaching the runway. From where Clarke stood, about 650 meters below, the noise was loud enough to wake slumbering Floyds Knobs residents. The other plane followed 20 minutes later flying an experimental pattern the aeronautics and astronautics associate professor had designed: it stayed at a higher altitude longer before it descended into the airport, a procedure that pushes the limits of both onboard flight management systems and air traffic controllers. Even without the data being recorded by the sound equipment beside him, Clarke knew that his procedure was significantly quieter. “You could really hear a difference,” he remembers.

Clarke was just one of seven researchers monitoring aircraft noise levels in Floyds Knobs on that night and several others. When all of the data was analyzed, it confirmed what he had suspected: the experimental landing procedure cut noise on the ground between three and six decibels, more than enough to be noticeable. What’s more, planes flying Clarke’s procedure saved fuel. Since those first tests, Clarke’s team-several MIT aero-astro graduate students and about a dozen researchers from Boeing, NASA Ames Research Center, and NASA Langley Research Center-has refined its design so that air traffic controllers can manage multiple planes flying the procedure in moderately heavy traffic. Meanwhile, Clarke is creating a similar procedure for Gatwick Airport in London as part of the Cambridge-MIT Institute’s Silent Aircraft Initiative. If the Federal Aviation Administration and the U.K.’s National Air Traffic Services approve Clarke’s designs for Louisville and Gatwick, they will provide much-needed noise relief for the residents of Floyds Knobs and London, and will blaze the trail for other airports interested in designing similar quiet approaches.

The Quiet Challenge

As a major parcel-sorting hub for UPS, Louisville has a particularly difficult noise problem. In the wee hours of every morning, more than 90 UPS jets land at the airport. Contending with the noise those planes generate has been expensive; since 1991 the airport authority has moved more than 1,600 families at a cost of more than $180 million. Still, the airport can’t afford to move everyone who’s ever been awakened by incoming 767s. In 2000, James DeLong, then the airport’s general manager, read one of Clarke’s papers on the feasibility of quieter landing approaches. “I was a pilot, and as I read through [Clarke’s paper], I understood what he was trying to accomplish,” remembers DeLong, who has since retired. “Not only did it have great potential for noise [reduction], but it had potential to save fuel, it had potential to increase the capacity of airportsso it was a win-win as I understood his concept.” Two years later, he invited Clarke to design and test such a landing procedure that could be used by UPS’s existing fleet.

“I [was] in the airport business over 30 years and concluded that the most serious single problem facing the airport system-in what represents a threat to the ability to accommodate future demand for air travel-is noise,” says DeLong.

The idea behind the new design, called a continuous-descent approach, is deceptively simple. The standard landing procedure is a steplike approach in which planes alternately descend and level off several times over about 50 kilometers before they reach the runway. Not only does this require planes to fly at a lower altitude longer, where more of their noise filters down to communities, but it also requires additional force from the engines to keep the plane level, which creates even more noise. But Clarke’s plan called for the plane to stay at a higher altitude longer, and then descend into the airport without leveling off.

The team had to tackle a number of challenges while developing the procedure. Clarke had to make sure that it would significantly reduce noise on the ground, that it was safe under different weather conditions, and that air traffic controllers could handle multiple incoming planes flying the new pattern. “You’ve got to make sure you keep all the different objectives and constraints in view at the same time,” says Clarke. “You can’t just try to optimize on one subject, because you’ll wind up violating another.”

During the initial design stage, Clarke used a noise simulation program he developed to help him decide which physical flight path would be the quietest. The program allows researchers to plug in the exact course they want an airplane to fly, along with the topographical characteristics and the population density of the area it’s flying over. Then the program calculates the noise the airplane creates on the ground and the number of people affected by it. After Clarke evaluated several possible paths, he and researchers at Boeing Commercial Airplanes used a Boeing simulator that includes all the cockpit hardware and controls of a real 767 to create a detailed step-by-step pilot procedure. These simulator sessions also verified that the plane could handle the procedure under different wind conditions.

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