Hushing The Roar Of Air Traffic Growth

Higher, faster, and farther have long been the goals of aircraft designers; less obvious has been the quest for quiet.


Craig says Boeing is in discussions with engine companies and other potential partners to conduct a demonstration that would be beyond QTD2 and be applicable to a 737 replacement or other aircraft down the line.

Noise is just as important for General Electric, a key player in the QTD2 study.

"It's certainly driving our engines today," says Steve Petersen, manager of the acoustics and installation aero teams at GE. "It's right up there with fuel burn, emissions. It's a key requirement now in sizing engines."

GE has been applying noise-damping treatments to its CF-56 and CF-6 engines since the 1970s. The kind of chevrons tested on the 777 during QTD2 are already in service on CF34 series engines on Bombardier and Embraer regional jets, as well as the CFM-56-5B on the Airbus A321.

Petersen says the company is actively studying higher bypass ratios, and more advanced chevrons and noise-absorption coatings, among other things.

"It's being quieter without sacrificing fuel burn, which is really the key airplane metric," says Darin Ditommaso, manager of systems technologies at GE.

For the future, GE says the GP7200 being built for the A380 already meets the noise requirements at Heathrow that will not go into effect for years.

In fact, says Petersen, the engines being built for the 747-8 and A380 will be both more powerful and quieter than current, comparable-sized engines. This provides a tangible benefit to operators using airports like Heathrow, where a point system is used to restrict the number of takeoffs allowed to carriers.   Continuous descent

New technology is only one approach to reducing noise levels; much could also be accomplished by adjusting arrival procedures. A concept known as the Continuous Descent Approach concentrates on an airliner's flight path from about 12,000 ft above the ground to the surface. The idea is to keep engine power at idle thrust and let gravity pull the plane downhill.

For all intents and purposes, it is a normal approach, says Lewis, NASA's principal investigator for low-noise guidance flight validation. The only thing passengers would notice would be less noise from the engines.

In a perfect world, an airplane on a long-distance trip would take off, climb immediately to its optimal cruising altitude, and stay up there as long as possible before beginning a constant, engines-idle descent that would end when the wheels touched the runway. In the real world, airplanes have to mesh into the air traffic control system, which usually interrupts climbs and descents with level-offs and turns that force airliners to spend more time at lower altitudes, where they burn more fuel and make noise that can be heard on the ground.

A Continuous Descent Approach eliminates level flight at low altitudes where pilots must step on the gas to keep the plane moving.

"You'd have a higher average altitude throughout the approach, and thereby better noise attenuation," Lewis says.

To obtain vertical guidance to a runway, an aircraft following this flight profile would intercept the electronic glide slope farther away from the airport than it normally would today, and a little higher above the ground. Once the glide slope was intercepted, the low-noise approach would become like any other.

NASA has developed avionics that enhance the vertical navigation equipment that already guides modern aircraft in climbs and descents. The new low-noise guidance system calculates a quieter trajectory for whatever lateral route is programmed into the aircraft's flight management system. The system provides the pilots with an indication of the aircraft's estimated energy. If controllers ask the pilots to deviate, either laterally or by assigning a level altitude that requires a power change, as often happens, the system recalculates the aircraft's estimated energy and vertical trajectory. The goal is for low-noise guidance avionics to be able to deal with speed, route, and altitude adjustments issued by individual controllers.

The system is simple to interpret and use. Lewis notes that pilots who have evaluated the new avionics in a simulator say it is a "no brainer." This fall the avionics are scheduled to fly in a validation test, which will include microphone arrays on the ground to measure noise directly.

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