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Turbulence that follows behind aircraft traveling through air From Wikipedia, the free encyclopedia
Wake turbulence is a disturbance in the atmosphere that forms behind an aircraft as it passes through the air. It includes several components, the most significant of which are wingtip vortices and jet-wash, the rapidly moving gases expelled from a jet engine.
Wake turbulence is especially hazardous in the region behind an aircraft in the takeoff or landing phases of flight. During take-off and landing, an aircraft operates at a high angle of attack. This flight attitude maximizes the formation of strong vortices. In the vicinity of an airport, there can be multiple aircraft, all operating at low speed and low altitude; this provides an extra risk of wake turbulence with a reduced height from which to recover from any upset.[1]
Wake turbulence is a type of clear-air turbulence. In the case of wake turbulence created by the wings of a heavy aircraft, the rotating vortex-pair lingers for a significant amount of time after the passage of the aircraft, sometimes more than a minute. One of these rotating vortices can seriously upset or even invert a smaller aircraft that encounters it, either in the air or on the ground.[citation needed]
The vortex circulation is outward, upward, and around the wingtips when viewed from either ahead or behind the aircraft. Tests with large aircraft have shown that vortices remain spaced less than a wingspan apart, drifting with the wind, at altitudes greater than a wingspan from the ground. Tests have also shown that the vortices sink at a rate of several hundred feet per minute, slowing their descent and diminishing in strength with time and distance behind the generating aircraft.[2]
At altitude, vortices sink at a rate of 90–150 m (300–490 ft) per minute and stabilize about 150–270 m (490–890 ft) below the flight level of the generating aircraft. Therefore, aircraft operating at altitudes greater than 600 m (2,000 ft) are considered to be at less risk.[3]
When the vortices of larger aircraft sink close to the ground — within 100–200 ft (30–61 m) — they tend to move laterally over the ground at a speed of 2–3 kn (3.7–5.6 km/h; 2.3–3.5 mph). A crosswind decreases the lateral movement of the upwind vortex and increases the movement of the downwind vortex.[4]
Helicopters also produce wake turbulence. Helicopter wakes may be significantly stronger than those of a fixed-wing aircraft of the same weight. The strongest wake will occur when the helicopter is operating at slower speeds (20 to 50 knots). Light helicopters with two-blade rotor systems produce a wake as strong as heavier helicopters with more than two blades. The strong rotor wake of the Bell Boeing V-22 Osprey tiltrotor can extend further and has contributed to a crash.[5]
Wingtip devices may slightly lessen the power of wingtip vortices. However, such changes are not significant enough to change the distances or times at which it is safe to follow other aircraft.[6]
ICAO mandates wake turbulence categories based upon the maximum takeoff weight (MTOW) of the aircraft. These are used for separation of aircraft during take-off and landing.
There are a number of separation criteria for take-off, landing, and en-route phases of flight based upon wake turbulence categories. Air Traffic Controllers will sequence aircraft making instrument approaches with regard to these criteria. The aircraft making a visual approach is advised of the relevant recommended spacing and are expected to maintain their separation.[7]: 9
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 three-to-five-knot (3–6 mph; 6–9 km/h) 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. [7]: 10
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Uncommanded aircraft movements (such as wing rocking) may be caused by wake. This is why maintaining situational awareness is 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 subtle and even surprisingly gentle. There have been serious accidents (see the next section) 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.
In 2020, researchers looked into installing "plate lines" near the runway threshold to induce secondary vortices and shorten the vortex duration. In the trial installation at Vienna International Airport, they reported a 22%-37% vortex reduction.[9][10]
| This section needs additional citations for verification. (June 2016) |
Wake turbulence can be measured using several techniques. Currently, ICAO recognizes two methods of measurement, sound tomography, and a high-resolution technique, the 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.
Wake turbulence can occasionally, under the right conditions, be heard by ground observers.[25] On a still day, the wake turbulence from heavy jets on landing approach can be heard as a dull roar or whistle. This is the strong core of the vortex. If the aircraft produces a weaker vortex, the breakup will sound like tearing a piece of paper. Often, it is first noticed some seconds after the direct noise of the passing aircraft has diminished. The sound then gets louder. 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.
 | This Section possibly contains original research. (September 2010) |
In the 1986 film Top Gun, Lieutenant Pete "Maverick" Mitchell, played by Tom Cruise, suffers two flameouts caused by passing through the jetwash of another aircraft, piloted by fellow aviator Tom "Ice Man" Kazansky (played by Val Kilmer). As a result, he is put into an unrecoverable spin and is forced to eject, killing his RIO Nick "Goose" Bradshaw.[26] In a subsequent incident, he is caught in an enemy fighter's jetwash, but manages to recover safely.
In the movie Pushing Tin, air traffic controllers stand just off the threshold of a runway while an aircraft lands in order to experience wake turbulence firsthand. However, the film dramatically exaggerates the effect of turbulence on persons standing on the ground, showing the protagonists being blown about by the passing aircraft. In reality, the turbulence behind and below a landing aircraft is too gentle to knock over a person standing on the ground. (In contrast, jet blast from an aircraft taking off can be extremely dangerous to people standing behind the aircraft.)
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