"Noise is still number one," says Carl Burleson, director of the office of environment and energy at the Federal Aviation Administration. It is the "front-page issue" for most communities, although concern about air quality -- emissions, particulate matter, and the like -- is rising rapidly, he says.
Weather remains the major reason for air traffic snarls and stranded passengers. But the biggest obstacles to building new airports and expanding runway capacity are environmental concerns -- with noise at the forefront.
Today, it typically takes 10 years to build a new runway in the U.S., says Burleson, noting that environmental issues will restrain the growth of our air traffic system before technological issues do.
"We really have a set of emerging environmental issues that could potentially put constraints on the ability of the industry to grow, and could fundamentally change access to mobility for the American public," he says. "That's why environment is such a critical issue."
Noise is primarily a political problem, says Kevin Shepherd, a NASA expert on how aircraft noise affects communities. "Congress hears about it, airports hear about it; it's used a lot to block airport expansion," he says.
Loss of sleep and difficulty hearing conversations are some common complaints resulting from aircraft noise. But the problem can get so bad in certain places around some airports that the FAA winds up paying damages. When noise levels under flight paths go above mandated limits, homeowners are entitled to sound insulation and even buyouts, all at government expense.
With the U.S. population growing and the number of aircraft needed to satisfy the demand for travel growing along with it, the federal government is searching for ways to increase the capacity of the national airspace system. Obviously, airplane noise complicates that.
"It's not just an impediment to expansion of airports; it can be an impediment to expansion of the traffic," says Tod Lewis, another NASA aircraft noise expert.
Anyone who remembers the racket unleashed by the early straightpipe turbojet engines knows that modern jet aircraft have actually gotten quieter, thanks in large part to the advent of high-bypass turbofans, which today power all jet transports. A turbofan pushes a lot of air at slower speeds, yielding more thrust and greater fuel efficiency than a turbojet, which moves much less air at higher speed -- and screams like a rocket.
"That was one of the best things that ever happened for aircraft noise, and it was not actually designed for [noise reduction]," says Charlotte Whitfield, an aeroacoustics specialist at NASA. "It's about the only time that performance benefit and noise benefit have gone hand in hand."
But it is not enough, so aircraft and engine makers as well as people in academia and government are constantly working on ways to make airplanes even quieter. Their efforts cover a range of initiatives focusing on engines, airframes, and procedures. NASA is heavily involved. Its Quiet Aircraft Technology program, which provides research and funding, aims to reduce perceived aircraft noise by 50% within 10 years and by 75% within 25 years, relative to 1997 levels.