Wake turbulence

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 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.

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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)