Team designs a quieter way to land jets

The team, led by a scientist at the Massachusetts Institute of Technology, has designed a new way for planes to land that keeps their engines quieter and farther from homes. Termed a "continuous descent approach," the technique calls for an approaching plane to stay high for longer than is done now, and then glide toward the runway with its engines idling, reducing the noise it emanates.

In a test of the system in Louisville, Ky., the first in the United States, the noise on the ground decreased by up to 6 decibels -- the equivalent of shutting down three of a plane's four engines.

"You get a significant reduction in noise," said Ralph Dormitzer, who was not involved in the research and is cochair of a citizens group that advises Logan Airport on noise issues. "We would like to see this done here."

Before the technique can be widely adopted, the team has to win FAA approval for the approach in Louisville and then solve technical issues that currently prevent it from being used in a heavy-traffic airport such as Logan. The system can't be used for aircraft that are taking off -- only those that are landing.

But the system represents an important new way to help solve a problem that has plagued communities and held up the construction of new runways, contributing to congestion and delays in air travel around the country. The test showed the new approach can bring large reductions in noise without the need for new technology, and that it might even save money and reduce pollution by using less fuel than the approach used today.

"You get a lot of bang for the buck," said John-Paul Clarke, who is leading the research and is an associate professor of aeronautics and astronautics at MIT.'

When an airplane approaches an airport now, air traffic controllers typically direct the plane to reduce its altitude in steps, descending a bit and then flying level. When the plane is close to the airport, it intercepts a radio beam and follows the beam down for the final approach to the runway.

This means that jets come in low and with their engines running loudly. Jets land this way partly to ensure that they will lock onto the correct radio beam for their final approach, Clarke said.

The new method makes use of the sophisticated avionics on modern jets, which allow planes to pinpoint their position three-dimensionally. Using the flight computer, a pilot can direct the plane to the exact point in the air where it is sure to pick up the correct radio beam. The plane can then follow the beam in, on a slow glide, to the airport.

For the test at the Louisville International Airport last year, the team asked one plane from UPS to land using the traditional method, and one plane to land using the new method. Placing microphones along the flight path, the team measured noise levels during the landing, and found the new technique reduced noise between about 4 and 6 decibels, according to an unpublished paper describing the experiment.

The landing would not make any difference to the airport's immediate neighbors, Clarke said, since within about 5 miles of the airport there would be no difference in altitude. It would make the biggest difference at about 7.5 to 15 miles out. For approaches to runway 33L at Logan, the technique would benefit Hull, Cohasset and other places on the South Shore.

The technique would add to an array of methods now used to fight noise pollution, including changing the design of engines, flying over less-populated areas, using less engine power while climbing away from airports, and soundproofing homes. But using continuous descents is especially welcome because there is no "silver bullet" that can make engines dramatically quieter than they are now, said Carl Burleson, director of the FAA's Office of Environment and Energy.

The team, a consortium that includes UPS, the FAA, NASA, and Boeing Commercial Airplanes, is designing a continuous descent procedure that can be used for traffic landing at Louisville by next September, and then plans to seek the FAA's approval. A similar technique has been successfully tested in Europe.

As a part of Logan's effort to build a new runway, MassPort is looking for creative ways to reduce noise. The continuous descent approach was one of the ideas considered at a meeting this month, said Dormitzer, of the citizens' group. But he hopes the airport will consider other proposals, too, including pushing more of the low air traffic out over the water.

Gareth Cook can be reached at cook@globe.com.