Cause of Vortices
The wing produces a pressure differential between its top and bottom surfaces as it moves through the air. As a result, the high pressure air from below the wing tries to move toward the low pressure area above the wing. The wing gets in the way, and lift is produced. At the wing tips, however, the high pressure air from below the wing spills outward, upward, and around the top of the wing. A pair of wingtip vortices are produced, which trail behind the airplane as it flies. If a wing is producing lift, it is also producing wingtip vortices.
Vortex Strength
The strength of an airplane’s wingtip vortices depends on the amount of lift being produced by the wing. Vortex intensity varies with an airplane’s weight, speed, wing design, and configuration. Wingtip vortices will be the strongest when the producing airplane is heavy, slow, and in a clean configuration, such as during takeoff.
Vortex Behavior
Wingtip vortices will sink at about 400 to 500 feet per minute, slowing their rate of descent with time. They tend to level off about 900 feet below the altitude at which they were produced, and then dissipate over time. If they are generated near the ground, they will usually stop descending within 200 feet of the ground, and begin to move apart from each other, until they dissipate. In this case, their lateral movement tends to be at a slow speed of only a few knots.
Wake Turbulence Avoidance
An encounter with another airplane’s wake can cause your airplane to roll uncontrollably. This lack of control can be very dangerous, especially near the ground, when the pilot has minimum time to recover from such an upset. And at any altitude, it would be possible for a wake turbulence encounter to damage the airplane. It can’t be seen, so pilots must learn to visualize and avoid wake turbulence.
Since wake turbulence sinks and dissipates, wake turbulence avoidance usually comes down to staying above the flight path of the generating airplane. If you were landing behind a larger airplane, staying above that airplane’s flight path all the way to landing would mean a landing at or beyond its touchdown point. A common mistake is to stay above the other airplane’s path until the last few hundred feet, and forgetting about the wake turbulence to concentrate on the landing. However, the other airplane produces wake turbulence all the way to touchdown. Make sure to land beyond its touchdown point, or go around if you are not comfortable with the situation.
If you were departing behind a larger aircraft, you should note its rotation point. It will start generating vortices as soon as its nose wheel comes off the ground. Staying above its flight path means lifting off prior to its rotation point, and climbing out so as to remain above its climb path, or turning to diverge from its flight path.
Wake turbulence moves with the wind. Since it drifts at a few knots along the ground, this means a few knots of crosswind can hold the upwind vortex in place, or carry the downwind vortex to a parallel runway. When landing on a downwind, parallel runway, be cautious of wake turbulence from the other runway, which could drift over your runway with the wind. If landing with a crosswind of a few knots, be cautious of the upwind vortex, which is likely sitting in place, right over the runway. Stay above the flight path of the previous arrival, and land beyond its touchdown point. A light quartering tailwind is particularly hazardous with regards to wake turbulence, since it brings the upwind vortex down the runway, and holds it in place over the runway.
Wake turbulence must also be visualized in relation to all operations near larger aircraft, not just when arriving or departing behind one. Maybe another airplane departs or lands via a runway which crosses the runway you will be landing or departing on. If that airplane’s nose wheel is in the air when it crosses your runway, it just showered that runway intersection with wake turbulence, and that wake will drift with the wind. If an airplane executes a low approach, missed approach, or touch-and-go landing, then the runway they just overflew will have wake turbulence descending on it, which drifts with the wind, for the next several minutes. Visualize where the wake turbulence is, stay above it, avoid it laterally, or wait a few minutes while it dissipates.
Wake turbulence can also be encountered en route or in the traffic pattern. usually, 1000 feet below the other airplane is sufficient to avoid wake turbulence.
Remember to be extra alert if another airplane’s vortices might:
Wake Turbulence Responsibilities
Depending on the circumstances, air traffic control may inform you of traffic you are following at a proximity, such that wake turbulence is a factor. ATC will use the phrase, “Caution, Wake Turbulence”, and advise you of this traffic. ATC is not responsible for your wake turbulence avoidance. The controller will expect you to adjust your flight path as necessary to avoid wake turbulence, while conforming with your air traffic control clearance or instructions. If you are unsure about where the wake turbulence is, or you are uncomfortable, do not hesitate to take appropriate action, such as performing a go-around and reporting to tower you were not comfortable with the wake turbulence. Often times, taking a delay of only a few minutes will allow the wake turbulence to dissipate.
Jet Stream Turbulence
Pilots of light aircraft also need to be cautious of jet wash, prop wash, and rotor wash. Make sure adequate space exists between aircraft and helicopters you taxi behind or around. For jet airplanes, this distance might be 500 or 1000 feet. For helicopters, within three times the rotor diameter is generally considered to be where the majority of the rotor wash exists.
Rotor Wash
When a helicopter is in forward flight, it generates a trail of rotor wash that behaves similarly to the wingtip vortices of an airplane.