Wake turbulence
Encyclopedia
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
that forms behind an aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
as it passes through the air. This turbulence includes various components, the most important of which are wing vorticies and jetwash. Jetwash refers simply to the rapidly moving gases expelled from a jet engine; it is extremely turbulent, but of short duration. The phenomenon that creates wake turbulence results from the forces that lift the aircraft. High pressure air from the lower surface of the wings flows around the wingtips to the lower pressure region above the wings. A
pair of counter-rotating vortices are thus shed from the wings, the right wing vortex rotates counterclockwise, and the left wing vortex rotates clockwise. This region of rotating air behind the aircraft is where wake turbulence occurs. The strength of the turbulence is predominantly determined by the weight, wingspan, flap settings, and speed of the aircraft. Wing vortices, on the other hand, are much more stable and can remain in the air for up to three minutes after the passage of an aircraft. Wingtip vortices
Wingtip vortices
Wingtip vortices are tubes of circulating air that are left behind a 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 are regions of very low pressure...
make up the primary and most dangerous component of wake turbulence.
Wake turbulence is especially hazardous during the landing
Landing
thumb|A [[Mute Swan]] alighting. Note the ruffled feathers on top of the wings indicate that the swan is flying at the [[Stall |stall]]ing speed...
and take off
Takeoff
Takeoff is the phase of flight in which an aerospace vehicle goes from the ground to flying in the air.For horizontal takeoff aircraft this usually involves starting with a transition from moving along the ground on a runway. For balloons, helicopters and some specialized fixed-wing aircraft , no...
phases of flight, for three reasons. The first is that during take-off and landing, aircraft operate at low speeds and high angle of attack
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
. This flight attitude maximizes the formation of dangerous 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. Thirdly, these phases of flight put aircraft closest together and along the same flightpath, maximizing the risk of encountering the phenomenon.
Fixed wing – level flight
At altitude, vortices sink at a rate of 90 to 150 metreMetre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
s per minute and stabilize about 150 to 270 metres below the flight level
Flight level
A Flight Level is a standard nominal altitude of an aircraft, in hundreds of feet. This altitude is calculated from the International standard pressure datum of 1013.25 hPa , the average sea-level pressure, and therefore is not necessarily the same as the aircraft's true altitude either...
of the generating aircraft. For this reason, aircraft operating greater than 600 metres above the terrain are not considered at risk.
Helicopters
HelicopterHelicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...
s 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 to 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 runwayRunway
According to ICAO a runway is a "defined rectangular area on a land aerodrome prepared for the landing and take-off of aircraft." Runways may be a man-made surface or a natural surface .- Orientation and dimensions :Runways are named by a number between 01 and 36, which is generally one tenth...
. Since the wingtip vortices exist at the outer edge of an airplane's wake, this can be dangerous.
Wake vortex separation
ICAOInternational Civil Aviation Organization
The International Civil Aviation Organization , pronounced , , is a specialized agency of the United Nations. It codifies the principles and techniques of international air navigation and fosters the planning and development of international air transport to ensure safe and orderly growth...
mandates separation
Separation (air traffic control)
In air traffic control, separation is the name for the concept of keeping an aircraft outside a minimum distance from another aircraft to reduce the risk of those aircraft colliding, as well as prevent accidents due to wake turbulence....
minima based upon wake vortex categories that are, in turn, based upon the Maximum Take Off Mass (MTOW|MTOM) of the aircraft.
These minima are categorised are as follows:
- Light – MTOW of 7000 kilograms (15,432.4 lb) or less;
- Medium – MTOW of greater than 7,000 kilograms, but less than 136000 kilograms (299,828.7 lb);
- Heavy – MTOW of 136,000 kilograms 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 Controller
Air traffic controller
Air traffic controllers are the people who expedite and maintain a safe and orderly flow of air traffic in the global air traffic control system. The position of the air traffic controller is one that requires highly specialized skills...
s will sequence aircraft making instrument approach
Instrument approach
For aircraft operating under instrument flight rules , an instrument approach or instrument approach procedure is a series of predetermined maneuvers for the orderly transfer of an aircraft under instrument flight conditions from the beginning of the initial approach to a landing, or to a point...
es 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.
The FAA does not use the ICAO categories for wake turbulence separation, instead using the following:
Edit 11/26/10 from http://www.faa.gov/documentLibrary/media/Order/ATC.pdf pdf page 616 document page BG-8
"Appendix A is being revised to harmonize FAA weight category standards with those of the
International Civil Aviation Organization (ICAO). All aircraft with a maximum certificated takeoff weight of more than
41,000 pounds but less than 300,000 pounds maximum certificated takeoff weight will now be classified as a “Large” aircraft
according to FAA standards. Aircraft with a maximum certificated takeoff weight of 300,000 pounds or more will now be
classified as a “Heavy” aircraft according to FAA and ICAO weight classification standards.
This change reclassifies all B757 aircraft as “Large” aircraft; however, controllers are required to apply the special wake
turbulence separation criteria as specified in paragraph 5-5-4. This change cancels and incorporates N JO 7110.525,
Appendix A, Aircraft Information Fixed-Wing Aircraft, effective April 8, 2010."
- Super - A separate designation that currently only refers to the AirbusAirbusAirbus SAS is an aircraft manufacturing subsidiary of EADS, a European aerospace company. Based in Blagnac, France, surburb of Toulouse, and with significant activity across Europe, the company produces around half of the world's jet airliners....
A380Airbus A380The Airbus A380 is a double-deck, wide-body, four-engine jet airliner manufactured by the European corporation Airbus, a subsidiary of EADS. It is the largest passenger airliner in the world. Due to its size, many airports had to modify and improve facilities to accommodate it... - Heavy - Aircraft capable of takeoff weights of 300000 pounds (136,077.7 kg) or more whether or not they are operating at this weight during a particular phase of flight.
- Large - Aircraft of more than 41000 pounds (18,597.3 kg), maximum certificated takeoff weight, up to but not including 300000 pounds (136,077.7 kg) .
- Small – Aircraft of 41,000 pounds or less maximum certificated takeoff weight.
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
| Preceding aircraft | Following aircraft | Minimum radar separation |
|---|---|---|
| Super | Super | 4 NM Nautical mile The nautical mile is a unit of length that is about one minute of arc of latitude along any meridian, but is approximately one minute of arc of longitude only at the equator... |
| Heavy | 6 NM | |
| Large | 7 NM | |
| Small | 8 NM | |
| Heavy | Heavy | 4 NM |
| Large | 5 NM | |
| Small | 5 NM | |
| Large | Small | 4 NM |
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 finalAirfield traffic pattern
An airfield traffic pattern is a standard path followed by aircraft when taking off or landing, while maintaining visual contact with the airfield....
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 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 (such as wing rocking) may be caused by wake. This is why maintaining situational 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 approachMissed approach
Missed approach is an instrument flight rules procedure which is a standard component segment of an instrument approach. Generally, if the pilot flying or the pilot in command determines by the time the aircraft is at the decision height or missed approach point , that the runway or its...
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.
Incidents involving wake turbulence
- 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 with the wing (see main article).
- May 30, 1972 - Delta Air Lines Flight 9570Delta Air Lines Flight 9570Delta Air Lines Flight 9570 crashed on 30 May 1972 at the Greater Southwest International Airport in Fort Worth, Texas while conducting a training flight. The aircraft, a McDonnell Douglas DC-9-14, began to oscillate about the roll axis after crossing the runway threshold during a landing...
crashed at the Greater Southwest International AirportGreater Southwest International AirportGreater Southwest International Airport was the commercial airport serving Fort Worth, Texas from 1953 until 1972. Dallas-Fort Worth International Airport opened in 1974 on property adjacent to the north side of the airport.-Early history:...
while performing "touch and go" landings behind a DC-10McDonnell Douglas DC-10The McDonnell Douglas DC-10 is a three-engine widebody jet airliner manufactured by McDonnell Douglas. The DC-10 has range for medium- to long-haul flights, capable of carrying a maximum 380 passengers. Its most distinguishing feature is the two turbofan engines mounted on underwing pylons and a...
. This crash prompted the FAA to create new rules for minimum following separation from "heavy" aircraft. - December 15, 1993 - a chartered aircraft with five people on-board, including In-N-Out Burger'sIn-N-Out BurgerIn-N-Out Burger is a regional chain of fast food restaurants with locations in the western United States. Founded in 1948 by Harry Snyder and his wife Esther, establishing the first In-N-Out burger in Baldwin Park and headquartered in Irvine, California, In-N-Out Burger has since expanded outside...
president, Rich Snyder, crashed several miles before John Wayne AirportJohn Wayne AirportJohn Wayne Airport is an airport in an unincorporated area in Orange County, California, with its mailing address in the city of Santa Ana, which is also the county seat, hence the International Air Transport Association airport code. The main entrance to the airport is off of MacArthur Blvd in...
. The aircraft followed a Boeing 757Boeing 757The Boeing 757 is a mid-size, narrow-body twin-engine jet airliner manufactured by Boeing Commercial Airplanes. Passenger versions of the twinjet have a capacity of 186 to 289 persons and a maximum range of , depending on variant and cabin configuration...
in for landing, became caught in its wake turbulence, rolled into a deep descent and crashed. - September 8, 1994 - USAir Flight 427USAir Flight 427US Air Flight 427 was a scheduled flight from Chicago's O'Hare International Airport to Pittsburgh, with a final destination of West Palm Beach, Florida...
crashed near PittsburghPittsburgh, PennsylvaniaPittsburgh is the second-largest city in the US Commonwealth of Pennsylvania and the county seat of Allegheny County. Regionally, it anchors the largest urban area of Appalachia and the Ohio River Valley, and nationally, it is the 22nd-largest urban area in the United States...
, PennsylvaniaPennsylvaniaThe Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...
in 1994. This accident was believed to involve wake turbulence, though the primary cause was a defective rudder control component. - September 20, 1999 - JAS 39A GripenJAS 39 GripenThe Saab JAS 39 Gripen is a lightweight single-engine multirole fighter manufactured by the Swedish aerospace company Saab. It was designed to replace the Saab 35 Draken and 37 Viggen in the Swedish Air Force...
from Airwing F 7 Såtenäs crashed into Lake VänernVänernVänern is the largest lake in Sweden, the largest lake in the EU and the third largest lake in Europe after Ladoga and Onega in Russia. It is located in the provinces of Västergötland, Dalsland, and Värmland in the southwest of the country.- History :...
in Sweden during an air combat maneuvering exercise. After passing through the wake vortex of the other aircraft, the aircraft abruptly changed course, and the pilot, Capt. Rickard Mattsson, got a highest-severity warning from the ground-collision warning system. He ejected from the aircraft, and landed safely by parachute in the lake. - November 12, 2001 - American Airlines Flight 587American Airlines Flight 587American Airlines Flight 587, an Airbus A300, crashed into the Belle Harbor neighborhood of Queens, a borough of New York City, New York, shortly after takeoff from John F. Kennedy International Airport on November 12, 2001. This is the second deadliest U.S...
crashed into the Belle HarborBelle Harbor, QueensBelle Harbor is an upscale neighborhood in the New York City borough of Queens. It is a tightly-knit, upper class community located on the western half of the Rockaway Peninsula, the southernmost area of the borough. While there are no formal boundaries for the area, Belle Harbor is often used to...
neighborhood of Queens, New YorkNew YorkNew York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
shortly after takeoff from John F. Kennedy International AirportJohn F. Kennedy International AirportJohn F. Kennedy International Airport is an international airport located in the borough of Queens in New York City, about southeast of Lower Manhattan. It is the busiest international air passenger gateway to the United States, handling more international traffic than any other airport in North...
. This accident was attributed to pilot error in the presence of wake turbulence from a Japan AirlinesJapan Airlinesis an airline headquartered in Shinagawa, Tokyo, Japan. It is the flag carrier of Japan and its main hubs are Tokyo's Narita International Airport and Tokyo International Airport , as well as Nagoya's Chūbu Centrair International Airport and Osaka's Kansai International Airport...
Boeing 747Boeing 747The Boeing 747 is a wide-body commercial airliner and cargo transport, often referred to by its original nickname, Jumbo Jet, or Queen of the Skies. It is among the world's most recognizable aircraft, and was the first wide-body ever produced...
that resulted in rudder failure and subsequent separation of the vertical stabilizer. - November 3, 2008 - Airbus A380Airbus A380The Airbus A380 is a double-deck, wide-body, four-engine jet airliner manufactured by the European corporation Airbus, a subsidiary of EADS. It is the largest passenger airliner in the world. Due to its size, many airports had to modify and improve facilities to accommodate it...
wake turbulence event, Sydney Airport, Australia. Wake turbulence of an Airbus A380-800 causing temporary loss of control to a Saab 340Saab 340The Saab 340 is a discontinued Swedish two-engine turboprop aircraft designed and initially produced by a partnership between Saab and Fairchild Aircraft in a 65:35 ratio...
on approach to a parallel runway during high crosswind conditions. - November 4, 2008 - Mexican Government LearJet 45 XC-VMC2008 Mexico City plane crashAn official Mexican Interior Ministry aircraft crashed in central Mexico City at around 18:45 local time on November 4, 2008. Mexican Secretary of the Interior Juan Camilo Mouriño, who was aboard the plane, was killed in the crash, along with the other seven people on board and at least six people...
, carrying Secretary of the Interior Juan Camilo MouriñoJuan Camilo MouriñoJuan Camilo Mouriño Terrazo was a Mexican politician affiliated to the National Action Party and the Secretary of the Interior in the cabinet of President Felipe Calderón...
, crashed close to Paseo de la ReformaPaseo de la ReformaPaseo de la Reforma is a wide avenue that runs in a straight line, cutting diagonally across Mexico City. It was designed by Ferdinand von Rosenzweig in the 1860s and modeled after the great boulevards of Europe, such as Vienna's Ringstrasse or the Champs-Élysées in Paris...
avenue before turning for final approach to runway 05R at Mexico City International AirportMexico City International AirportBenito Juárez International Airport , in Venustiano Carranza, one of the sixteen boroughs into which Mexico's Federal District is divided, is a commercial airport that serves Mexico City, the capital of Mexico...
. The airplane was flying behind a Mexicana Airlines 767-300 and above a heavy helicopter. The pilots were not told about the type of plane that was approaching before them, neither did they reduce to minimum approach speed. (This has been confirmed as the official stance by the Mexican Government as stated by Luiz Tellez, the Secretary of Communications of Mexico.)
Measurement
Wake turbulence can be measured using several techniques. Currently, ICAO recognizes 2 methods of measurement, sound tomography, and a high-resolution technique is dopplerDoppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
lidar
LIDAR
LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...
, a solution now commercially available. Techniques using optics
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
can use the effect of turbulence on refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
(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, this is the large core vortex, if the aircraft produces a small core 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, 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 1986 film Top GunTop Gun
Top Gun may refer to:* Top Gun is a 1986 film starring Tom Cruise.**Top Gun , soundtrack to the movie**Top Gun , a number of games based on the movie...
, Lieutenant Pete "Maverick" Mitchell, played by Tom Cruise
Tom Cruise
Thomas Cruise Mapother IV , better known as Tom Cruise, is an American film actor and producer. He has been nominated for three Academy Awards and he has won three Golden Globe Awards....
, suffers two flameout
Flameout
A flameout refers to the failure of a jet engine caused by the extinction of the flame in the combustion chamber. It can be caused by a number of factors, including fuel exhaustion; compressor stall; insufficient oxygen supply; foreign object damage ; severe inclement weather; and mechanical...
s caused by passing through the jetwash of another aircraft. During a training mission Maverick is caught in Tom Kazansky's (played by Val Kilmer
Val Kilmer
Val Edward Kilmer is an American actor. Originally a stage actor, Kilmer became popular in the mid-1980s after a string of appearances in comedy films, starting with Top Secret! , then the cult classic Real Genius , as well as blockbuster action films, including a supporting role in Top Gun and a...
) jet wash. Maverick enters a spin
Spin (flight)
In aviation, a spin is an aggravated stall resulting in autorotation about the spin axis wherein the aircraft follows a corkscrew downward path. Spins can be entered intentionally or unintentionally, from any flight attitude and from practically any airspeed—all that is required is sufficient yaw...
as a result of an engine flameout; his RIO
Naval Flight Officer
A Naval Flight Officer is an aeronautically designated commissioned officer in the United States Navy or United States Marine Corps that specializes in airborne weapons and sensor systems. NFOs are not pilots per se, but they may perform many "co-pilot" functions, depending on the type of aircraft...
and best friend Nick "Goose" Bradshaw is killed as they eject from the aircraft. 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
Pushing Tin
Pushing Tin is a 1999 comedy-drama film directed by Mike Newell. It centers on a cocky air traffic controller who quarrels over proving "who's more of a man" with fellow employee Bell...
, 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
Jet blast
Jet blast is the phenomenon of rapid air movement produced by the jet engines of aircraft, particularly on or before takeoff.A large jet-engined aircraft can produce winds of up to 100 mph up to 200 feet behind it at 40% maximum rated power...
from an aircraft taking off can be extremely dangerous to people standing behind the aircraft.)