Bumpology for pilot
| Figure 1: Winds blowing in different directions close together is one cause of wind shear. |
| Figure 2: This diagram shows a recipe for a gust front, a condition pilots want to avoid. |
| Figure 3: This is the National Weather Service radar image for around the time of the photo in Figure 2. Red and yellow indicate the thunderstorm's strongest precipitation; the blue arc near the bottom is the gust front. |
For your first two lessons as a student pilot you flew early in the morning when the air was nice and calm. But your third flight is on a warm afternoon, with light but gusty winds.
By the time you landed, you were wondering whether you're meant to be a pilot. The airplane bounced around, not much but enough to make you uneasy. Not frightened--but uncomfortable. A couple of times you felt a bump, it was like being in a car that hadn't slowed down enough before hitting a speed bump. Even more troubling, you weren't always sure whether the airplane was banking or its nose was pitching up or down as a result of your control inputs, or because of the wind.
Getting used to turbulence is a part of becoming a pilot. Airplanes move though the air the way fish move through water. The air is usually moving and turbulence is a result of air's movement. Becoming comfortable with turbulence, in fact, is needed to become not only a pilot, but also a relaxed airline passenger. Courses, books, and Web sites for people who are trying to master a fear of flying all mention turbulence. For example, one such Web site, Airfraid.com, says, "The important thing to remember about turbulence is that although it can be uncomfortable it is not dangerous."
This isn't quite true, even for passengers. Turbulence that hits airliners in cruise flight regularly injures passengers who don't follow instructions to keep their seat belts fastened when they're seated, and flight attendants whose jobs require them to move about the cabin.
The real point the Web site is making for fretful passengers is that turbulence isn't going to rip an airplane apart. That's true as long as a pilot doesn't fly the airplane into the kind of rare, extreme turbulence found aloft in strong thunderstorms and occasionally near the ground when thunderstorms blast wind down to the ground.
Avoiding thunderstorm turbulence aloft is simple for pilots flying under visual flight rules: stay at least 20 miles away from thunderstorms. Avoiding wind shifts and the resulting milder turbulence that can endanger takeoffs and landings is more complicated.
You can begin to understand wind and turbulence by thinking of the air as being like water that's sometimes still, sometimes flowing. Like flowing water, moving air is sometimes smooth, sometimes turbulent. Imagine the water in a white-water stream. When the water flows over a rock it first goes up, then comes down, creating a wave downstream from the rock.
In a similar way, wind flowing over a mountain, the top of a thunderstorm--or even a line of trees adjacent to the runway you're landing on--can create waves in the air. If you happen to be landing when the part of the wind's wave that's moving up hits your airplane, you'll land farther down the runway than expected. If the down-moving part of the wave hits when your airplane's wheels are inches above the runway, you'll thump down, maybe bounce.
The lesson: In addition to being aware of the wind direction, you need to imagine how any waves it's creating could affect your airplane. And these waves aren't always up and down. Wind flowing between two hangars on one side of the runway can create turbulence as well as intermittent crosswinds. Turbulence you feel while you fly the pattern and turn final could alert you to waves the wind is creating, but you also need to watch the windsock, trees, or blowing dust around the airport to create quick pictures of the wind and how it could affect you.
Wind can create turbulence in a number of ways. The most common is wind shear. Figure 1 illustrates one cause of wind shear: winds blowing in different directions close together. In figure 1 the winds would also create shear if they were blowing in the same direction, but one was much faster than the other. Finally, winds blowing roughly the same speed don't have to be blowing in opposite directions to create shear, but can be blowing at an angle to each other--imagine the wind at the top of Figure 1 blowing out from the page while wind at the bottom of the image continues to blow toward the left.
The mathematics of wind shear can become complicated because differences in both speed and direction have to be taken into account. One rule of thumb that pilots can use when planning cross-country flights is to check the differences in speeds and directions of the winds aloft, which are given for each 3,000 feet. If the difference in wind speed between two reported levels, such as for 6,000 feet and 9,000 feet, is more than 18 knots, wind shear could make a flight between those altitudes bumpy. Wind shear, by the way, can be either horizontal or vertical and can occur at any altitude.
For pilots, the most dangerous wind shear is the low-level shear created by microbursts. When wind blasts down from a thunderstorm or shower, it not only tends to push an airplane down, it also creates sudden changes in wind speed and direction when it hits the ground and spreads out. To be considered a microburst, the descending air must be concentrated in an area of about 2.5 miles in diameter or less (see "The Weather Never Sleeps: Menacing Microbursts," July 2007 AOPA Flight Training).
When the air descending from a shower or thunderstorm isn't concentrated enough to be a microburst, it can still be problematic. If you've been on the ground when thunderstorms are in the neighborhood, you've probably felt a cool breeze, maybe a breeze that changes the wind's direction. This is the gust front created by a downburst and when you're on the ground on a muggy day, you appreciate the cooling effect. If you're on final approach or just taking off when a gust front hits, you might wish you were on the ground, at least for a few moments.
Meteorologists call gust fronts outflow boundaries. They can create conditions that a pilot wouldn't want to tangle with. Figure 2 is a National Weather Service photo with a diagram showing what's going on to form the cloud that an outflow boundary created near the Hastings, Nebraska, NWS office the morning of August 26, 2002.
The red arrows on the photo show that warm winds of less than 10 knots were blowing over the region. The "cold north wind" was the air that had descended from the thunderstorm and was gusting up to 30 knots as it blew toward the south. Imagine that the road across the bottom of the photo is a runway and an airplane was taking off toward the south (into the existing southerly wind) when the gust front hit with a 30-knot gust of wind from the north.
If the airplane's airspeed indicator were showing 52 knots--meaning that air was flowing across the airplane's wings from front to rear at 52 knots--a gust from the north would have instantly cut the airspeed to 30 knots, which is not nearly enough to produce the needed lift. If the airplane is a few hundred feet above the ground when this happens, the wing will stall. An alert pilot, with enough altitude, will recover from the stall and fly away with a resolve to pay closer attention to the weather.
As the diagramed photo shows, the cool outflow air (the blue arrows) is shoving under the less dense warm air, forcing it up to form the smooth arcus. This morning the atmosphere was very stable, which means the warm air rose only as long as it was being shoved. If the atmosphere had been unstable, the warm, humid air would have continued rising to develop billowy, cumulus clouds that could have grown into new thunderstorms. In fact, outflow boundaries are a common trigger for thunderstorms.
Figure 3 is the NWS weather radar image around the time of the photo in Figure 2. The red and yellow parts of the image locate the thunderstorm's strongest precipitation. The white arrow on the right side of the image shows that the thunderstorm is moving toward the east-southeast (north is the top of the image). The blue arc at the bottom of the image is the gust front. You probably think of weather radar as showing precipitation, but the blue arc here isn't rain. The radar is detecting insects and dust that the gust front is pushing up into the air.
Thunderstorm downdrafts illustrate another aspect of air movements that can cause turbulence: up and down air movements. When the ground warms up and heats the air next to it, the air becomes less dense and begins to rise. Depending on several factors such as the temperature of the air at different heights above the ground, the air might rise a couple of thousand feet or more than 30,000 feet. Air that rises to 30,000 feet will create thunderstorms.
Streams of air rising from the ground are called thermals, and are the friends of glider pilots, who use them to stay aloft without the benefit of engines. Pilots of powered airplanes usually find thermals annoying. If the ride is bumpy and the sky is dotted with small cumulus clouds that aren't growing taller, you should climb above the clouds for a smoother ride. The tops of the clouds are also the tops of the thermals.
Learning what causes air to move in the ways it does will help you avoid dangerous up-and-down and horizontal movements of the air. This knowledge and time spent at the controls of airplanes will make you comfortable with turbulence.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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