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Wake turbulence is turbulence that forms behind an aircraft as it passes through the air. This turbulence includes various components, the most important of which are wingtip vortices (see below) and jetwash. Jetwash refers simply to the rapidly moving gasses expelled from a jet engine; it is extremely turbulent, but of short duration. Wingtip vortices, on the other hand, are much more stable and can remain in the air for up to two minutes after the passage of an aircraft. Wingtip vortices make up the primary and most dangerous component of wake turbulence.

Wake turbulence is especially hazardous during the landing and take off phases of flight, for two reasons. The first is that during take-off and landing, aircraft operate at low speeds and high angle of attack. This flight attitude maximizes the formation of dangerous wingtip vortices. Secondly, takeoff and landing are the times when a plane is operating closest to its stall speed and to the ground - meaning there is little margin for recovery in the event of encountering another aircraft's wake turbulence.

This picture from a NASA study on wingtip vortices clearly illustrates the power of this wake turbulence component.

Fixed wing - Level flight

At altitude, vortices sink at a rate of 91 to 152 m per minute and stabilize about 152 to 274 m below the flight level of the generating aircraft. For this reason, aircraft operating greater than 610 m above the terrain are not considered at risk.

Helicopters

Helicopters also produce wake turbulence. Helicopter wakes may be of significantly greater strength than those from a fixed wing aircraft of the same weight. The strongest wake can occur when the helicopter is operating at lower speeds (20 - 50 knots). Some mid-size or executive class helicopters produce wake as strong as that of heavier helicopters. This is because two blade main rotor systems, typical of lighter helicopters, produce stronger wake than rotor systems with more blades.

Parallel or crossing runways

During takeoff and landing, an aircraft's wake sinks toward the ground and moves laterally away from the runway when the wind is calm. A 3 to 5 knot crosswind will tend to keep the upwind side of the wake in the runway area and may cause the downwind side to drift toward another runway. Since the wingtip vortices exist at the outer edge of an airplane's wake, this can be dangerous.

Hazard avoidance

Wake vortex separation

ICAO mandates separation minima based upon wake vortex categories that are, in turn, based upon the Maximum Take Off Mass (MTOM) of the aircraft.

These minima are categorised are as follows:

• Light - MTOM of 7,000 kg or less;
• Medium - MTOM of greater than 7,000 kg, but less than 136,000 kg;
• Heavy - MTOM of 136,000 kg or greater.

There are a number of separation criteria for take-off, landing and en-route phases of flight based upon these categories. Air Traffic Controllers will sequence aircraft making instrument approaches with regard to these minima. Aircraft making a visual approach are advised of the relevant recommended spacing and are expected to maintain their own separation.

Common minima are:

Take-off

An aircraft of a lower wake vortex category must not be allowed to take off less than two minutes behind an aircraft of a higher wake vortex category. If the following aircraft does not start its take off roll from the same point as the preceding aircraft, this is increased to three minutes.

Landing

Staying on or above leader's glide path

Incident data shows that the greatest potential for a wake vortex incident occurs when a light aircraft is turning from base to final behind a heavy aircraft flying a straight-in approach. Light aircraft pilots must use extreme caution and intercept their final approach path above or well behind the heavier aircraft's path. When a visual approach following a preceding aircraft is issued and accepted, the pilot is required to establish a safe landing interval behind the aircraft s/he was instructed to follow. The pilot is responsible for wake turbulence separation. Pilots must not decrease the separation that existed when the visual approach was issued unless they can remain on or above the flight path of the preceding aircraft.

Warning signs

Any uncommanded aircraft movements (e.g., wing rocking) may be caused by wake. This is why maintaining situation awareness is so critical. Ordinary turbulence is not unusual, particularly in the approach phase. A pilot who suspects wake turbulence is affecting his or her aircraft should get away from the wake, execute a missed approach or go-around and be prepared for a stronger wake encounter. The onset of wake can be insidious and even surprisingly gentle. There have been serious accidents where pilots have attempted to salvage a landing after encountering moderate wake only to encounter severe wake turbulence that they were unable to overcome. Pilots should not depend on any aerodynamic warning, but if the onset of wake is occurring, immediate evasive action is vital.

Accidents/incidents due to wake turbulence

• On June 8, 1966 an XB-70 collided with an F-104. Though the true cause of the collision is unknown, it is believed that due to the XB-70 being designed to have an enhanced wake turbulence to increase lift, the F-104 moved too close, therefore getting caught in the vortex and colliding the wing (see main article).
• Delta Air Lines Flight 9570 crashed at the Greater Southwest International Airport on 30 May 1972 while performing "touch and go" landings behind a DC-10. This crash prompted the FAA to create new rules for minimum following separation from "heavy" aircraft.
• A chartered aircraft with 5 onboard, including In-N-Out Burger's president, Rich Snyder, crashed at John Wayne International Airport on December 15, 1993. The aircraft followed in a Boeing 757 for landing, became caught in its wake turbulence, rolled into a deep descent and crashed.
• USAir Flight 427 crashed near Pittsburgh, Pennsylvania in 1994. This accident was believed to involve wake turbulence, though the primary cause was a defective rudder control component (see main article).
• American Airlines Flight 587 crashed into the Belle Harbor neighborhood of Queens, New York shortly after takeoff from John F. Kennedy International Airport on November 12, 2001. This accident was attributed to pilot error in the presence of wake turbulence from a Japan Airlines Boeing 747 that resulted in rudder failure and subsequent separation of the vertical stabilizer.

Measurement

Wake turbulence can be measured using several techniques. A high-resolution technique is doppler lidar, a solution now commercially available. Techniques using optics can use the effect of turbulence on refractive index (optical turbulence) to measure the distortion of light that passes through the turbulent area and indicate the strength of that turbulence.

Audibility

Wake turbulence can occasionally, under the right conditions, be heard by ground observers. On a still day, heavy jets flying low and slow on landing approach may produce wake turbulence that is heard as a dull roar/whistle. Often, it is first noticed some seconds after the direct noise of the passing aircraft has diminished. The sound then gets louder, sometimes becoming as loud as was the original direct sound of the aircraft. Nevertheless, being highly directional, wake turbulence sound is easily perceived as originating a considerable distance behind the aircraft, its apparent source moving across the sky just as the aircraft did. It can persist for 30 seconds or more, continually changing timbre, sometimes with swishing and cracking notes, until it finally dies away.

In Popular Culture

In the movie Top Gun, Lieutenant Pete "Maverick" Mitchell, played by Tom Cruise, suffers two flameouts caused by passing through the jet wash of another aircraft. During a training mission Maverick is caught in Tom Kazansky's (played by Val Kilmer) jet wash. Maverick enters a flat spin as a result of an engine flameout, and loses his RIO and best friend "Goose" as they eject out of the plane. In the second incident, he is with "Merlin" and they are caught in a bogey's jet wash. Maverick recovers from the flameout but is shaken up.

In the movie Pushing Tin, air traffic controllers stand at the start of a runway while an airplane lands in order to experience wake turbulence firsthand, although they are more likely being exposed to jet blast.

See also

• Wake (of boats)

External links

• Captain Meryl Getline explains "Heavy"
• The Airman's Information Manual on Wake Turbulence

Condensation in the cores of wingtip vortices from an F-15E as it disengages from a KC-10 Extender following midair refueling.

Wingtip vortices are tubes of circulating air which are left behind by the wing as it generates lift. One wingtip vortex trails from the tip of each wing. The cores of vortices spin at very high speed and they are regions of very low pressure. The cores of wingtip vortices are sometimes visible due to condensation of water vapour in the very low pressure.

Wingtip vortices are associated with induced drag, an essentially unavoidable side-effect of the wing generating lift. Managing induced drag and wingtip vortices by selecting the best wing planform for the mission is critically important in aerospace engineering.

Wingtip vortices form the major component of wake turbulence.

Migratory birds take advantage of each other's wingtip vortices by flying in a V formation so all but the leader are flying in the upwash from the wing of the bird ahead. A little upwash makes it a little easier for the bird to support its own weight.

Many technical writers use the alternative expression "trailing vortices" because these vortices do not trail only from the wing tips. They also trail from the outboard tip of the wing flaps and other abrupt changes in wing planform.

Cause and effects

A wing generates aerodynamic lift by creating a region of lower air pressure above the wing than beneath it. Fluids are forced to flow from high to low pressure and the air below the wing tends to migrate towards the top of the wing, via the wingtips. The air does not escape around the leading or trailing edge of the wing due to airspeed, but it can flow around the tip. Consequently, air flows from below the wing and out around the tip to the top of the wing in a circular fashion. This leakage will raise the pressure on top of the wing and reduce the lift that the wing can generate. It also produces an emergent flow pattern with low pressure in the center surrounded by fast moving air with curved streamlines.

Wingtip vortices only affect the portion of the wing closest to the tip. Thus, the longer the wing, the smaller the affected fraction of it will be. As well, the shorter the chord of the wing, the less opportunity air will have to form vortices. This means that for an aircraft to be most efficient, it should have a very high aspect ratio. This is evident in the design of gliders. It is also evident in long-range airliners where fuel efficiency is of critical importance. However, increasing the wingspan reduces the maneuverability of the aircraft, which is why combat and aerobatic planes usually feature short, stubby wings despite the efficiency losses this causes.

Another method of reducing fuel consumption is use of winglets, as seen on a number of modern airliners such as the Airbus A340. Winglets work by forcing the vortex to move to the very tip of the wing and allowing the entire span to produce lift, thereby effectively increasing the aspect ratio of the wing. Winglets also change the pattern of vorticity in the core of the vortex pattern; spreading it out and reducing the kinetic energy in the circular air flow, which reduces the amount of fuel expended to perform work by the wing upon the spinning air. Winglets can yield very worthwhile economy improvements on long distance flights.

Since the cores of vortices have a very low pressure, when the air is of high humidity, water vapour condenses to form cloud in the vortex cores, allowing wingtip vortices to be seen. This is most common on aircraft flying at high angles of attack, such as fighter aircraft in high g maneuvers, or airliners taking off and landing on humid days.

Hazards

A NASA study on wingtip vortices produced these pictures of smoke in the wake of an aircraft, clearly illustrating the size and power of the vortices produced.

Wingtip vortices can also pose a severe hazard to light aircraft, especially during the landing and take off phases of flight. The intensity or strength of the vortex is a function of aircraft size, speed, and configuration (flap setting, etc.). The strongest vortices are produced by heavy aircraft, flying slowly, with wing flaps extended. Large jet aircraft can generate vortices which are larger than an entire light aircraft. These vortices can persist for several minutes, drifting with the wind. The hazardous aspects of wingtip vortices are most often discussed in the context of wake turbulence. If a light aircraft is immediately preceded by a heavy aircraft, wake turbulence from the heavy aircraft can roll the light aircraft faster than can be resisted by use of ailerons. At low altitudes, particularly during takeoff and landing, this can lead to an upset from which recovery is not possible. Air Traffic Controllers ensure an adequate separate between departing and arriving aircraft, particularly where a heavy aircraft is preceding a light aircraft.

Gallery

Condensation in the cores of wingtip vortices from an F-15E as it disengages from a KC-10 Extender following midair refueling.	An EA-6 Prowler with condensation in the cores of its wingtip vortices and also on the top of its wings.	F/A-18C leaving vapor trails in the low pressure cores of its wingtip vortices.	The core of the vortex trailing from the tip of the flap of a commercial airplane with landing flap extended.
F/A-18C showing condensation in the cores of the vortices trailing from its leading edge extensions	A NASA study on wingtip vortices produced these pictures of smoke in the wake of an agricultural airplane.	Wingtip vortices from a Cessna 182 wind tunnel model.	Canadian geese in V formation to make best use of each bird's wingtip vortices. Source: NASAexplores
Wingtip vortices shown in chaff smoke left behind a C-17 Globemaster III. Also known as smoke angels.

See also

• Vortex
• Horseshoe vortex
• Aspect ratio
• V formation

External links

Wikimedia Commons has media related to:

Wingtip vortices

• Video from NASA's Dryden Flight Research Center tests on wingtip vortices:
  • C-5 Galaxy: [1]
  • Lockheed L-1011: [2]
• Wind prediction for analysis of vortex drift
• Flares released by an air force jet form a "smoke angel"