The Calculators of Calm
Just how far out of their way will airlines go to give you a smooth ride?
- By Willilam Triplett
- Air & Space magazine, March 2005
(Page 3 of 6)
Garry Hinds, manager of United’s meteorology department, likens the jet stream to a stream of water. “If the stream is straight and moving quickly, you can get in it and there’s really no problem,” he says. “But put rocks in that stream, causing white water, and that’s what mountain wave is—the atmosphere running into the rocks and getting pummeled and spun—and you get wind shear, which is simply a change in wind speed and direction. That generates breaking waves of air, just like breaking waves of water. The difference between the waves in the atmosphere and the waves in the ocean is you can’t see waves in atmosphere.” Well, for the most part.
“ ‘Clear air’ is kind of a misnomer,” says scientist Larry Cornman of the National Center for Atmospheric Research in Boulder, Colorado. “People use that because pilots fly around and get hit by something they don’t see. You can have a mountain wave, which has water vapor that condenses, and so you can actually see the lower part of the wave structure.”
Avoiding those waves is one primary job of the dispatcher, who begins working on finding a route even before passengers arrive at the airport. When U.S. Airway’s Andelmo starts a shift at the airline’s operations center in Pittsburgh, the dispatcher who is about to go off duty briefs him on air traffic control issues, the weather in general, and turbulence in particular. Before planning routes, Andelmo reviews weather information provided over the Internet by the National Weather Service, Weather Services International, and the National Oceanic and Atmospheric Administration (forecasts, turbulence alerts, and pilot reports—“pireps”—can be seen at adds.aviationweather.gov).
These sources show thunderstorms and provide the dispatcher with a map of all recent pilot reports of moderate or worse clear-air turbulence. Trouble is, CAT is so mercurial that pireps may be obsolete after only 20 minutes. “Turbulence is like a secondary atmospheric effect,” says United meteorologist John Goldman. “You can forecast wind shear, you can forecast atmospheric stability, but it’s a combination of those things that causes turbulence, and within an area where there is turbulence, it’s not going to be observed at all locations. It’s very random.”
Indeed, flights often smack into turbulence in areas that pireps had said were smooth. In 2000, a U.S. Airways flight sailing uneventfully over Chattanooga, Tennessee, at 35,000 feet got banged hard by an encounter with turbulence. “People’s heads hit the [ceiling] and cracked it,” Andelmo says. “We had some serious injuries.”
The airlines and the government also work together to prevent rough rides. “We have regular conference calls throughout the day with the FAA and the other airlines where we share any information we have,” says United spokesman Jeff Green. “In the case that our dispatchers need to get or share any information, we do have the ability to contact the operations control centers at other airlines.”
The airlines and the FAA characterize turbulence as light, moderate, severe, or extreme. Respectively, the categories are defined as (1) causing slight, erratic changes in altitude or attitude and rhythmic bumpiness; (2) same characteristics but greater intensity and rapid bumps and jolts, with passengers straining against seat belts; (3) large, abrupt changes in altitude or attitude and large variations of airspeed, with aircraft temporarily out of control; and (4) violent jolts making control of aircraft nearly impossible and structural damage possible.
But what feels like light turbulence in a 747 might seem severe in a 737. Turbulence even varies along the length of an aircraft: Pilots feel some bouncing, but the center of the aircraft shakes more than the cockpit, and the rear sections sway back and forth more than the center. Says Goldman, “Turbulence is the only meteorological parameter that’s a subjective report.”