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The DC-8 lost its left outboard engine and 19 feet of wing and fell 500 feet in 10 seconds, but landed safely. (NCAR)

The Calculators of Calm

Just how far out of their way will airlines go to give you a smooth ride?

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.”

NOAA, in conjunction with the National Weather Service, regularly issues clear-air turbulence forecasts based on calculations of things like temperature, wind speed, and wind direction in different parts of the country and at different altitudes. But some in the industry find the forecasts too conservative. So a few airlines, like United, have invested in their own meteorology departments.

“To put out a CAT forecast that we’re confident in is very labor-intensive,” Goldman says. “We need to be evaluating a lot of things. Even then, we wait until we start to get verification from flights in the area. And if their reports correspond to what the data are telling us, we then put out a warning.” Otherwise United might impose unnecessary deviations, resulting in extra costs. Exactly what those extra costs are is difficult to say, but United’s willingness to spend more than $2 million every year just to have a meteorology department is some indication.

Some parts of the country are known for having either convective or clear air turbulence almost all the time, such as above and just east of the Rocky Mountains, particularly around Denver. (United refers to a circle defined by a 50-mile radius around the city and up to 25,000 feet in altitude as “the Denver cylinder,” in which pilots can always expect rough rides.) Other usually turbulent areas include the Gulf Coast, southern Florida, the area around Cape Cod, Massachusetts, and the Montana-Canada border.

Along with the National Center for Atmospheric Research and other organizations, NASA’s Aviation and Security Program is currently working on what it calls a Turbulence Prediction and Warning System, which is intended to help reduce turbulence-related injuries. According to Jim Watson, the TPAWS project manager, two very promising technologies are being developed to help detect turbulence. The first is a software upgrade of the Doppler radar systems that many airliners already use to detect wind shear. With new algorithms, these radars will be able to look for and process weak radar reflections of moisture or ice crystals typically found in convective turbulence far away from storm clouds.

Laser-based radar, or lidar, is the second TPAWS technology under development (see “How Lidar Detects Turbulence,” next page). Much more sensitive than Doppler radar, lidar can show air motion at a very high resolution, a capacity that gives Watson and his colleagues hope that—pending more refinement and development—it will be able to reflect off minute dust particles blown around by clear air turbulence. But after the attacks of September 11, 2001, as airlines lost passengers and were forced to spend money on security, it became clear that developing an expensive technology like lidar would be difficult. “One of the challenges for a laser-based system is to make it more affordable,” says Steve Hannon, chief scientist of CLR Photonics, a Colorado firm that is working with NASA to develop turbulence warning sensors.

According to Hannon, most research has focused on 10- to 20-centimeter-diameter “pencil beam” lidar, which stays fixed straight ahead of the aircraft and merely alerts crews to the presence of turbulence, rather than more expensive scanning lidar, which sweeps across the path of the aircraft and returns an image of the air. It’s still unclear whether pilots need to know the shape of turbulence, as opposed to just the fact that it is looming ahead.

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