Turbulence: the basic & the dangers

Boeing 777 wake turbulence and vortex effect

The Basic
Surprising as it is to most people, an airplane moves through air that is itself moving. Moving air flows in ways that are quite similar to moving water, only we cannot see the air currents.

Air currents vary, and flying rapidly from one current to another is what leads to the feeling of turbulence. Some people call this “hitting an air pocket,” but this description is a misnomer. The jolt comes not from falling into a “hole” in the air but from crossing a barrier between different currents.

There are several causes of turbulence:
•    Convective currents result from the sun heating the ground, causing air to rise. As the air rises, it cools and forms clouds—those pretty, white, fluffy cumulus clouds that look so nice and soft on the outside and are boiling with activity inside. Hence pilots are always looking for smooth air above the clouds where the convection stops.
•    After sunset the air is generally much smoother because of a lack of convective activity. But other forms of turbulence can occur at any time, even at night:
•    Obstructions to wind flow cause all kinds of eddies and currents. On a city street you may have seen papers blowing around in circles between buildings; in an airplane you will notice this kind of turbulence when flying over mountains; e.g., especially on the eastern side of the Rockies in the US.
•    Wind shear occurs at the boundary between winds that differ in speed or direction, or both. Common near temperature inversions and at the border between weather fronts, this kind of turbulence is most likely encountered in a large aircraft at high altitudes near the jet stream.

The Dangers
Although turbulence is generally not dangerous, it would be a lie to say that it is never dangerous. So let’s sort out the dangerous aspects of it.

Commercial aircraft prefer to fly “straight and level” (at a constant heading and altitude) because it is convenient. First of all, air traffic controllers have to keep aircraft from flying into each other, and having pilots fly at predictable paths during each segment of a flight makes a controller’s job possible. Second, people tend to get airsick when an airplane moves erratically, so straight and level flight makes things more comfortable for the passengers and crew. And third, because the shortest distance between two points is a straight line, straight and level flight makes for economy. (Actually, since the earth’s surface is curved, aircraft routes are always curves, not straight lines—but that doesn’t matter to this discussion.)

When an aircraft flies through turbulent air, though, it will tend to rock its wings and dip and bob, all because the air in which it is flying is moving every which way. There is nothing dangerous about this because there is no physical law that says an airplane has to fly in a straight line at a constant altitude and at a constant airspeed. If you watch an aerobatic airshow sometime you will realize that airplanes can fly in all sorts of positions—even backwards, for a short time—and still be safe. So, even though level flight may be preferred, if an airplane enters turbulent air, its erratic flight poses no real safety issue to the airplane itself. Usually.

First Danger: Structural Failure
According to FAA regulations, all aircraft are designed and built to withstand far more stress than occurs in normal flight, including ordinary turbulence. But the turbulent air in severe thunderstorms can be so powerful that it can literally rip an airplane to pieces.

Now, I’m talking here about the danger of flying right into the middle of the biggest and meanest thunderclouds there are. No competent pilot would ever do that deliberately.
It is true that small, general aviation airplanes have often gotten destroyed in thunderstorms, all because the pilot was flying in the clouds and, not having on-board radar to distinguish a thunderstorm from the surrounding clouds, inadvertently flew right into a big thundercloud.

But commercial aviation has a far happier history. Dispatchers who plan the flights will route flights away from thunderstorms. Sometimes they will even cancel flights because of thunderstorms. Pilots of commercial aircraft also have on-board radar to spot and avoid thunderstorms, and they will often request a course change to avoid bad weather. So, if your flight is delayed or cancelled because of weather, be grateful, not angry.

Second Danger: Passenger Injury
When an airplane flies into downward-moving air, the airplane will drop with the air. But anything not securely attached to the airplane itself—such as passengers who are not strapped to their seats—can get thrown around the cabin.
You should be aware that turbulence can be forecast by aviation weather services, so pilots are likely to be aware of it in advance and will try to avoid it. This explains why the “Fasten Seat Belts” sign comes on well before the bumps start. Occasionally, though, turbulence (such as “clear air turbulence” which doesn’t have any clouds around it to give a visual warning) can be unexpected.

Many passengers who get injured because of turbulence are those who, unlike more experienced travelers, do not keep their seat belts loosely fastened at all times.

To be safe, you might want to remember that any time you unfasten your seat belt, whether to feel more “comfortable” or to get up from your seat—regardless of whether or not the seat belt sign is off—you’re risking injury from sudden turbulence. Although injury from turbulence is rare, you should realize that walking around an airplane cabin is not as simple and safe as walking around your living room.

Third Danger: Altitude Loss Near the Ground.
Sudden altitude changes can also be a consequence of flying in turbulent air. When you are thousands of feet above the ground, a few hundred feet of altitude loss doesn’t count for much. But if the airplane is only a few hundred feet above the ground, as when it is in the process of taking off or landing, then a few hundred feet of altitude loss can make all the difference in the world.

Severe turbulence near the ground is usually the result of one of two things:
 1. Wake turbulence occurs when an aircraft leaves a trail of disturbed air behind it simply as a result of its flying through the air. This turbulence is greatest when it is flying slowly during take-off or landing. The turbulence poses no danger to the aircraft itself, but any other aircraft following too closely behind it can fly into the turbulent air and lose control. For this reason, air traffic controllers maintain strict limits of spacing between aircraft, both on arrival and departure. This concern for safety can cause traffic delays, but they are well worth the safety advantage.

 2. Thunderstorms, as I mentioned above, can cause big problems for airplanes flying near them, especially because the storms can produce strong, unexpected downdrafts. And there have been planes that crashed while landing all because the plane dropped onto the ground before it could recover from a sudden downdraft. Thankfully, these accidents of the past have only made pilots more aware of the problem of sudden wind shifts during take-off and landing. In addition, many airports now have special detectors to warn pilots of unusual wind behavior in the vicinity of the airport.

Fourth Danger: Wind Changes Near the Ground.
Turbulent air aloft is not a problem in regard to an aircraft’s airspeed, because no matter how fast the wind is “blowing,” and no matter whether the aircraft is flying “with” or “against” the wind, all that matters aerodynamically is that the aircraft be moving sufficiently fast relative to the air around it to generate the lift necessary to keep flying.

For example, if an aircraft’s airspeed is 300 knots that means it is moving through the air mass around it at 300 knots. If that same mass of air is also moving (relative to the ground) at 300 knots opposite to the direction the aircraft is flying, the aircraft’s airspeed is still 300 knots. Even though we might think that the aircraft is flying “against” the wind, it is really flying quite safely within a moving air mass.

In the above example, although the aircraft has an airspeed of 300 knots, its groundspeed is 0 knots. 

Technically, it is hovering over the ground because the air is moving it backwards (relative to the ground) at the same rate as the aircraft is flying forwards (relative to the ground). Of course, my example of wind blowing at 300 knots is highly exaggerated, even in the jet stream, and I use the example just to make the point about groundspeed easier to comprehend. More realistically, though, if you watch gliding birds such as hawks and seagulls, you can occasionally see them hover over one spot on the ground just by pointing themselves into the wind and matching their airspeed with the speed of the wind.

On the other hand, if the aircraft flies in an air mass moving in the same direction as the aircraft, the aircraft’s groundspeed will be increased, sometimes dramatically. In fact, air travel across the US from the west coast to the east coast can be greatly facilitated by flying in the west-to-east jet stream. (Flying east-to-west, of course, airplanes avoid the jet stream as best as possible—otherwise, they might end up hovering above the ground, and that’s not an effective way to travel anywhere.

OK. So understanding this much about airspeed and groundspeed, you can now grasp the safety problem in regard to wind changes when the aircraft is near the ground. If the aircraft is just about to land and suddenly the wind changes to a tail wind, the plane can actually get “blown” right off the end of the runway. In fact, several aircraft accidents have happened like this. In trying to land in the vicinity of a thunderstorm, pilots under pressure to land, rather than divert to another airport, have been surprised by strong, erratic winds and, on touchdown, have landed too far down the runway, lost control, and slid right off the runway.

Please note that sudden, unpredictable wind changes near the ground usually happen because of a thunderstorm in the vicinity of the airport. Wind changes can happen because of a larger weather system such as a front, but such wind changes are usually a matter of changes in wind speed, not radical changes in wind direction. Therefore, strong winds in themselves do not usually cause problems for commercial aviation.
If flights are cancelled because of strong winds, it’s usually for a very special reason. Large airports have multiple runways in use at the same time, under the direction of Air Traffic Control, and, quite often, various runways are oriented in different directions. If winds are especially strong, the airport may, for safety reasons, close its crosswind runways—that is, the runways the wind is blowing across, rather than more-or-less straight down—because (a) it can be difficult for an airplane to accelerate or decelerate straight down a runway when a very strong wind is blowing it sideways, and (b) takeoff and landing distances are increased whenever the airplane is not headed directly into the prevailing wind. And so, with several runways closed, many flights can get delayed.