While the atmosphere isn't up to any of the tricks that you've been learning about, you can't forget the "weather" - especially if you extend the term weather to mean any movements of the atmosphere, no matter what causes them.
That Boeing 737 that you just heard the tower clear to land is creating some of the worst "weather" that the pilot of a smaller plane could ever encounter. And, this potentially deadly weather is almost always completely invisible.
Fortunately the twin, tornado-like vortices that the Boeing 737 is trailing from the tips of its wings, creating wake turbulence, don't last too long - maybe four or five minutes. Also, the job of the controller who cleared the airliner to land also includes helping you to stay out of the way of the larger airplane's wake turbulence.
Still, anyone flying any kind of aircraft needs to know about wake turbulence and how to avoid it. You don't want to count on a controller not making a mistake, and there's a chance you could encounter dangerous wake turbulence at a nontowered airport or even in cruise flight. Any wing that's developing lift creates vortices that trail off the tips of the wings.
As student pilots learn in their basic aeronautics studies, a wing needs to create lower air pressure on top of the wing than below the wing to produce lift. One of the basic things that nature always tries to do is to equalize unequal air pressures - that's how winds are created. In the case of a wing, the higher air pressure below the wing pushes air around the end of the wing toward the lower pressure on top. But, by the time any particular molecule of air reaches the "top," the wing has moved on, and the air is left swirling behind the wing tips.
In other words, the flight crew of a Boeing 747 does not have to worry about the vortices created by a Cessna 152 that takes off ahead of them. The Cessna pilot, however, absolutely must be concerned about the wake vortices from the jumbo jet.
While the weight of an airplane is the main factor in the vigor of its wake vortices, other factors come into play. Wake vortices are strongest when an airplane is flying slowly at a high angle of attack with the flaps up. When flaps are down they can create turbulence of their own that can tend to weaken wake vortices.
Wake turbulence offers the greatest danger during takeoffs and landings, but it can occur at altitude. Many pilots of smaller jets flying at altitudes above 20,000 feet have reported scary encounters with wake turbulence caused by larger jets.
After forming, the vortices coming off the wing tips tend to sink. If they are near the ground and the wind isn't blowing, they tend to move away from the airplane that created them, which means pilots landing or taking off on parallel runways have to watch out.
If a light wind is blowing, it will carry the vortices with it, maybe to a downwind parallel runway. But, a light wind can also push the vortex that is trying to drift upwind back to the runway, making it linger there.
The key to avoiding the wake turbulence from another airplane is to visualize what the vortices are doing and stay out of their way. The best way to stay out of the way is to allow plenty of time, maybe four or five minutes to be extra safe, before taking off or landing behind a large aircraft.
Even with time, you want to avoid places where wake vortices could be lurking. For instance, if you are taking off behind a large airplane, note where it rotates and be sure to rotate before you get there. It's also a good idea to tell the controller you'd like to make an upward turn soon after takeoff to avoid wake turbulence.
If you are landing behind a large airplane, you should note where it touches down and make sure to stay above its glide path all of the way down and land beyond the larger airplane's touchdown point on the runway.
Researchers are looking for ways to spot wake turbulence - some sort of sensor that would show controllers and maybe pilots where wake turbulence is located and give some idea of its strength. Other research projects are trying to find ways to reduce the strength of the wake vortices created by large aircraft, but the big hurdle is finding a method that wouldn't rob the aircraft of lift or efficiency. Both of these lines of research, however, seem to be a long way from success.
This means that pilots will have to continue to understand how wake turbulence acts, be able to visualize which way the wind is pushing any turbulence, and not allow pressure from a controller to force you to fly where you think a wake vortex or two might be waiting.
It's also why you need to be extra careful on those perfect days. Strong, gusty winds tend to quickly break up wake vortices. Silky smooth air is where a vortex is most likely to be lurking in the wake of a large airplane.
Jack Williams is the weather editor of USAToday.com. An instrument-rated private pilot, he is the author of The USA Today Weather Book and co-author with Dr. Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
