Trouble From the Ground Up
On August 7, US Airways Flight 623 ran into turbulence 29,000 feet above McConnellsburg, Pennsylvania. Flight attendants were wheeling a breakfast cart through first class when the plane shuddered and dropped steeply.
"It was like someone wanted to roll the aircraft on its back," the pilot would later say. Dan Molloy, a New Jersey machinist on his way to Las Vegas, felt the 737 whip sharply back and forth. "The nose was suddenly forty-five degrees to the right of where it had been a moment before," he says. "And then it went back left. People were flying around."
A flight attendant made her way to the public-address system to tell passengers to return to their seats. As soon as she turned on the microphone, however, she began sobbing. Many of the passengers panicked, including Molloy's wife. "She was crying her eyes out and asking, 'Why is the nose pointing down?' I hadn't noticed it. That's when I thought we were going to crash."
Fortunately for the 133 people on board, Flight 623 managed to land safely in Pittsburgh, where 10 people were treated for minor injuries. But what caused the incident in the first place?According to a preliminary report by the National Transportation Safety Board (NTSB), the wake of a larger DC-10 traveling 10 miles ahead of 623 was the most likely culprit.
While the media and the public focus on tragedies like September's Swissair crash, many industry experts are quietly worried that increases in air traffic over the next few years will cause lesser but more widespread problems, such as wake turbulence. If plane wakes are hazardous at a distance of 10 miles, how will the aviation industry cope with even more planes in the air?
To answer this question, it's important to understand the causes of turbulence. As a plane moves through the air, a small tornado-like vortex forms behind each wingtip. Design experts are currently working on ways to eliminate these dangerous wake vortices, while various organizations, including NASA, are developing sophisticated new tools to detect them (see box). For the moment, however, the only sure way to avoid wake turbulence is for pilots to stay away from other planes. "Anytime you're following closely behind a larger aircraft," says Paul Smith of the National Business Aviation Association (NBAA), "you're going to be caught in its wake. Pilots learn this early on. Once you're in the wake, you're a passenger."
Consequently, in 1994 the Federal Aviation Administration imposed a "separation standard": planes must travel at least four nautical miles apart. In the cramped airspace near an airport, of course, this is difficult—and it is there that wake turbulence has been most deadly. In 1992, eight people died in Billings, Montana, on the approach to the airport when their small plane hit the turbulence from a larger aircraft. Less than a year later, in Santa Ana, California, five people died in a similar accident. Wake turbulence was also cited in the 1994 crash of USAir Flight 427 near Pittsburgh, which killed 132 people. In 1996, the FAA responded by extending the separation standard to five nautical miles.
Until Flight 623 came along, the consensus was that five nautical miles was a perfectly adequate safety standard. With NTSB investigators exploring the causes of Flight 623's troubles, wouldn't 10 miles be better?
Industry sources say no. "Stricter separation standards are not the answer," maintains Boeing spokesperson Mary Jean Olsen. She says planes would have to wait longer to take off if they were spaced farther apart. For airlines, the result would be fewer flights; for consumers, longer delays. An NBAA fact sheet growls that the FAA's 1996 standards hike may already have reduced airport capacity by as much as 30 percent.
Such figures make it seem as if regulators are to blame for airport congestion. Yet separation standards are only one aspect of the problem. There has been a huge increase in the number of fliers: 2000 will be the busiest year ever, with 1.7 billion fliers worldwide, compared to 1.5 billion in 1997. Many airports are at capacity and cannot be expanded.
Some regulators think the airlines share the blame for congestion. "If they want to schedule ten planes at the same time and pack passengers in, they can," one says. "But there are only so many gates at the airport, and airlines still have to abide by safety standards to get from the gate into the air."
Despite the finger-pointing, regulators don't relish the prospect of a fight with airlines over separation standards. "A lot went into our standards," FAA spokesperson Alison Duquette says. "We'd obviously need conclusive proof from the NTSB before we'd look at that."
The real problem with separation standards is the necessary trade-off between safety and airport capacity. There are already Cassandras, such as Norman Mineta, the chairman of the National Civil Aviation Review Commission, who predict more accidents and more congestion. "For airline passengers, every day will seem like traveling over the holidays," Minetta says, "only without the good cheer." He also warns that the number of major crashes each year could double by 2010.
Ultimately, it may take such a disastrous situation to call attention to the wake-turbulence problem. Dan Molloy, who still suffers neck pain from his experience aboard Flight 623, is amazed by how quickly his near-disaster has been forgotten. "We got no free-flight voucher from the airline," he says, "no form letter, nothing." He sighs. "Well, at least they didn't charge us for the headsets."
room to move
new technology may help
The solution to the wake-turbulence problem may be to give up on separation standards altogether. Bob Kelley-Wickemeyer, Boeing's chief engineer of airplane dynamics, proposes an alternative—an ambitious effort to eliminate wake turbulence at the source. There are two promising approaches, still in the preliminary stages, he says. The research of Philip Marcus of the University of California, Berkeley, suggests that there is a way to puff a stream of air out of a plane's wingtip, thus destroying the trailing vortex. "If you pulse it at just the right frequency," Kelley-Wickemeyer explains, "you can cause the vortex core to resonate and 'erupt.' " The Boeing engineer also thinks it's possible to redesign airplanes so that the two vortices behind the wings interfere with each other, canceling out the wake. "Basically, we get the left-hand vortex to cream the right one," he says.
In the short term, the "avoidance approach"—with some help from technology—may be more practical. NASA is working on an aircraft vortex spacing system that can predict wake turbulence. Such technology would probably have prevented the runway accidents in 1992 and 1994, and stands to cut back on separation times, easing congestion. The system should be available to airports within the next 10 years.