March 25, 2013
BY JACK WILLIAMS (From Flight Training, June 1997.)
Anyone who's made more than a few flights has almost surely had at least one bumpy ride. To pilots and passengers, the bumps are the result of "turbulence." To an atmospheric scientist, turbulence is "a state of fluid flow in which the instantaneous velocities exhibit irregular and apparently random fluctuations."
Those "irregular fluctuations" of the air create the bumps. Flights through turbulent air can be fatiguing. Turbulence can also be a psychological barrier to students who fear they'll lose control when the airplane seems to take on a mind of its own. They might even fear the bumps will damage the aircraft.
As with most fears, knowledge is the antidote. Only "extreme" turbulence is likely to cause a pilot to lose control or to damage an aircraft. Fortunately, extreme turbulence is rare, and pilots with a basic knowledge of what causes turbulence can avoid it.
The best way to understand turbulence is to visualize what the air is doing. You can't see air, but Larry Cornman, a turbulence researcher at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, says you can "see" turbulence by watching (or imagining) a mountain stream. "I think of a big boulder in the middle of a stream," he says. "When someone opens the flood gates upstream, and the speed of the river increases two or three fold, we see a wave coming over the top of the boulder, breaking and frothing."
To "see" the turbulence a jet stream or wind can create blowing through a mountain pass, settle into a Jacuzzi, then turn the water jets on. "You have a very strong discontinuity in velocity,"" Cornman says. "It generates a lot of turbulence. It feels good on your back, but it wouldn't feel good if it were the air around your airplane."
Turbulence in air is more violent than it is in water, Cornman says. Forces causing turbulence affect air more because the air is lighter and has lower viscosity than water. If air had a higher viscosity -- say like oil -- flights would be a lot less bumpy.
You may encounter several different causes of turbulence. "Mechanical" turbulence is common near the ground as wind blowing over or around buildings creates eddies. The faster the wind, the stronger the turbulence. Most turbulence involves eddies. These are examples of the random fluctuations in instantaneous velocities in the scientific definition.
Beautiful, sunny days with calm winds can create annoying turbulence as bubbles of warm air begin rising, creating thermals. Glider pilots love thermals because the rising air keeps them aloft, but they create bumpy rides for power pilots. If there's enough moisture in the air, puffy, fair-weather cumulus clouds might cap the tops of the thermals. The flying is smoother above these clouds.
Flying early in the morning before the sun heats the ground, or last thing in the afternoon as it's cooling off will also be smoother. Thermals over deserts can be especially high and violent, which is why early morning flights are recommended in these areas.
If conditions are right, thermals blossoming early in the day can grow into thunderstorms by afternoon. All pilots learn early in their training to avoid thunderstorms because they have extreme turbulence. Thunderstorms also create turbulence in the clear air around them, often with no visible sign of what's going on. A good rule of thumb is to stay at least 20 miles away from any thunderstorm.
A thunderstorm acts like a solid object with winds blowing over or around it. Like a boulder in a stream, it can create waves in winds flowing over it. At lower levels, a thunderstorm can create eddies as winds flow around it. Cornman says this is why the most violent winds are likely to be in clear air downwind from a thunderstorm.
Mountains create some dangerous turbulence, too, such as waves that can reach high into the atmosphere and 200 miles or more downstream. Extremely dangerous "rotors" also can form on the lee side of mountains.
A rotor might have been a partial cause of the crash of a Boeing 737 on approach to Colorado Springs, Colorado, on March 3, 1991. The National Transportation Safety Board was never able to determine a cause, but its report said a rotor was a possibility. Conditions were right for such winds, but today's regular weather observations, including weather radar, don't see rotors in clear air.
Wind shear -- a large change in wind speed or direction over a short distance - is the final cause of turbulence. This term can be confusing because the news media often uses it to refer to the particular kind of wind shear caused by microbursts, which are winds that blast down from showers or thunderstorms and have caused several airline crashes.
Wind shear occurs at all altitudes, and it can be horizontal or vertical. At high altitudes, an airplane can encounter a shear when it flies into a jet stream, where the wind speed increases from less than 50 mph to maybe 150 mph over a few miles.
Most general aviation pilots don't have to worry about high-altitude jet streams, but low-level jets that form at night can surprise pilots who climb from calm air on the ground into winds that might be blowing 80 mph only 1,000 feet up.
Weather forecasts try to alert pilots to turbulence, but meteorologists often can give only very general forecasts. Pinning down exact locations of the worst turbulence is difficult, but many researchers are focusing on improving turbulence predictions. As the computer models used for forecasting improve, they should give forecasters better indications of where and when the worst turbulence is likely.
Also, the FAA's Weather Research Program is funding intensive turbulence studies at Colorado Springs and Juneau, Alaska, with the aim of setting up turbulence detection and warning systems. They would be like the one NCAR developed for Hong Kong's new Chek Lap Kok Airport, scheduled to open in fall 1998. The airport is on an island with nearby hills that create complicated wind flow patterns - and turbulence.
The Hong Kong, Colorado Springs, and Juneau studies include a network of anemometers on hills or mountains near the airports to keep constant track of the winds. Researchers profile the wind with Doppler radars that point upward and read wind speeds and directions. They also use Doppler lidars, which use laser light instead of radio waves. A research aircraft, such as a Beech King Air, then makes approaches while measuring the turbulence encountered.
By understanding what kind of turbulence various patterns of wind speed and direction create, the scientists will be able to develop computer programs to collect data continually from the various sensors and produce clear-cut alerts for controllers to pass on to pilots.
Cornman, who was part of the scientific team that developed today's microburst warning system, says about 90 percent of the work to develop such a system goes into minimizing the false alarm rate. "You've got to have a system that pilots will believe," he says. "When the controller says 'severe turbulence on the approach to Runway 10,' the pilot can't see anything; he has to have faith that the alarm is real."
The new systems will be an extension of the lore that among pilots who fly regularly at same airports builds up over the years. At an airport near mountains, for example, the lore might say, "When the wind blows directly over the ridge to the west at more than 25 knots, you don't want to even think about flying a light trainer."
Cornman and other scientists collect such lore from local pilots because it helps guide their research, and it helps them establish ties with those who'll be using the turbulence-detection systems they develop.
Cornman advises pilots to collect and study such local lore for the airports they use. If you're flying to a new airport near mountains, a call to a local fight instructor could give you some ideas about what to watch out for. Flight instructors could add to such lore for their own airports by noting the weather and winds around the airport and whether the rides are bumpy or smooth. Enlisting students in such exercises could be a way to help them understand, and overcome, fears about turbulence.
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