Weather

An atmospheric hazard that’s usually invisible can surprise even the most weather-wise pilot on a perfect flying day with good visibility, no threatening storms, and calm or nearly calm winds.

September 8, 2008, was such a day at the Pueblo (Colorado) Memorial Airport with seven-knot winds from the southeast, visibility of 10 miles, and a layer of broken clouds 2,500 feet above the airport when an instructor and student in a Diamond DA20-C1 entered the pattern to land. The tower controller told them to follow a large, four-engine C–130 Hercules and to be cautious of wake turbulence. About a half-mile from the threshold on final approach, the airplane encountered the wake turbulence from the preceding C–130, the National Transportation Safety Board (NTSB) report said.

“The airplane began an uncommanded pitch up, rolled left, and then began descending. Full throttle and opposite flight controls were applied. Despite attempts to regain level flight by both the student pilot and the flight instructor, the airplane descended until it collided with terrain.” The crash seriously injured both the instructor and student. The wake turbulence they flew into was one of the invisible tornado-like vortices trailing from the wing tips of the C–130, like those shown in the illustration (at right).

Looking at the illustration helps you to imagine what happens when another airplane, especially one that's much smaller than the airplane creating the vortices, flies into them. Even an experienced aerobatic pilot could have difficulty recovering if his airplane unexpectedly rolls sharply, as the Diamond in Pueblo did.

Any aircraft that’s producing lift is creating such wing tip vortices. Even the wings of flying birds generate vortices, and large birds such as geese and pelicans take advantage of them when flying in V formations. The trailing birds, which gain lift from the upward-moving side of the vortex from the bird ahead, don’t have to work as hard as the leading bird. When that bird tires it drops back and a trailing bird moves up to take its place.

As all pilots know, to produce lift a wing creates lower air pressure on its top than on the bottom. Nature, however, always tries to equalize air pressures—that’s how winds are created. With 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 air molecule reaches where the top of the wing was, the wing has moved on, and the air is left swirling behind the wing tips.

The resulting vortices sink and weaken, usually lasting only five or so minutes at any particular location. If a light wind is blowing it carries the vortices with it. The turbulence of faster winds breaks up the vortices more quickly than a light wind. If the air is calm the vortices will move out to both sides of a runway when they touch the ground.

For any airplane, the strongest vortices form when both the gear and flaps are up because lowered flaps create localized turbulence that weakens the vortices. This means that wake turbulence is greatest when an airplane is taking off, since the flaps are normally down for landing. Even so, the wake turbulence of a landing airplane can be deadly for an airplane that flies into it.

The rotors of helicopters, of course, also produce wake vortices that sweep around the helicopter, creating a dangerous circle at least three times the diameter of the rotor when the helicopter is hovering and that trail behind the helicopter when it’s moving.

Obviously, the way to avoid wake turbulence is to avoid flying under the flight path of any larger airplane when you are taking off or landing on the same runway as the larger airplane. You also need to be wary of wake turbulence on intersecting runways, or runways with intersecting approach or departure paths. On parallel runways, a crosswind component from the runway being used by a larger airplane can carry a vortex toward your runway. You need to act as you would if the larger airplane were on your runway. If you are landing on the same runway or a downwind parallel runway that a larger airplane has taken off from, you should be firmly on the runway before the larger airplane’s rotation point.

When cleared to take off after a larger departing airplane you should request an upwind turnout as soon as possible after takeoff, since a smaller airplane is not likely to be able to climb as fast as the larger one and you could fly into a vortex if you follow the runway heading. If the controller clears you for immediate takeoff behind a large or heavy aircraft and doesn’t clear you for an immediate upwind turn, tell the controller you’ll take the required three-minute wait before rolling, and wait three minutes. You also need to take off before you reach the larger airplane’s takeoff point.

An airplane doesn’t have to be all that much larger than yours to cause wake turbulence, as a pilot of a Cessna 172 reported to NASA’s Aviation Safety Reporting System. The pilot wrote that he was doing touch and goes on a clear day with five-knot winds when the tower cleared a Cessna Caravan to take off ahead of him. “As I began to climb, at about 400 feet [above the runway] my aircraft entered an uncommanded, approximately 40-degree bank to the right,” he wrote. “I recovered using all flight controls, particularly full left rudder, full left aileron, and lowered nose to reduce angle of attack.”

The pilot wrote that his training had made him aware of the dangers of wake turbulence but the training materials focused on air carrier jets. “I think it would be beneficial to have the training examples depict airplanes with a smaller disparity in size to show that it does not take that much difference in size to cause a wake-turbulence encounter.”

Although wake turbulence creates the greatest danger during takeoffs and landings, it can occur at altitude. Many pilots flying at altitudes above 20,000 feet have reported scary encounters with wake turbulence caused by larger aircraft above them. The key to avoiding the wake turbulence is to visualize what the vortices are doing and avoid them by staying above the flight path of the larger airplane or staying far enough behind the larger airplane to give the vortices time to dissipate.

Under instrument conditions controllers are supposed to keep a “small” airplane (weighing less than 41,000 pounds) at least six miles behind a “heavy” airplane (weighing more than 255,000 pounds), four miles behind a “large” airplane (41,000 to 255,000 pounds), and three miles behind another small airplane.

Flying on a calm day is more fun than on a day when turbulence is bouncing you around. But the next time you fly on a bumpy day enjoy this thought: The bumps that are making you uncomfortable are also tending to weaken wake vortices of larger aircraft. However, even on a bumpy day, you shouldn’t count on turbulence to fully break up wake vortices.