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 gases 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 three minutes after the passage of an
aircraft. Wingtip vortices occur when a wing is
generating lift. Air from below the wing is drawn around the wingtip into the
region above the wing by the lower pressure above the wing, causing a
vortex to trail from each wingtip. Wake turbulence exists in the vortex flow
behind the wing. The strength of wingtip vortices is determined primarily by the
weight and airspeed of the aircraft. Wingtip vortices make up the primary and
most dangerous component of wake turbulence.
Lift is generated by high pressure below the wing and low pressure above the
wing. As the high-pressure air moves around the wingtip to the low pressure,
the air rotates, or creates a horizontal "tornado" behind the wings. This tornado
sinks lower and lower until it dissipates.
Wake turbulence is especially hazardous in the region behind an aircraft in the
takeoff or landing phases of flight. During take-off and landing, aircraft
operate at 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 height, and this provides extra risk
of wake turbulence with reduced height from which to recover from any upset.
Fixed wing – level flight At altitude, vortices sink at a rate of
90 to 150 metres per minute and stabilize about 150 to 270 metres below
the flight level of the generating aircraft. For this reason, aircraft
operating greater than 600 metres above the terrain are considered to be at less
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. 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. The strong rotor wake of the Bell Boeing
V-22 Osprey tiltrotor can extend beyond the description in the manual, which
contributed to a crash. 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 of the aircraft.
These minima are typically categorised as follows:
Light – MTOW of 7,000 kilograms or less; Medium – MTOW of greater than 7,000
kilograms, but less than 136,000 kilograms;
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
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. The FAA does not use the ICAO categories
for wake turbulence separation, instead using the following:
Super - A separate designation that currently only refers to the Airbus A380
Heavy - Aircraft capable of takeoff weights of 300,000 pounds or more
whether or not they are operating at this weight during a particular phase of
flight. Large - Aircraft of more than 41,000
pounds, maximum certificated takeoff weight, up to but not including 300,000
pounds . 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 = Wake Turbulence Recategorization
Separation Standards= In 2012 the FAA authorized Memphis,
Tennessee air traffic controllers to begin applying revised criteria, which
retained the previous weight categories but also addressed differences in
approach speeds and wing configuration. This resulted in six categories of
aircraft, and the revised spacing allowed among these categories was soon
shown to increase airport capacity. With the largest global wake database,
EUROCONTROL has developed advanced wake metrics to set up the European six
category wake turbulence separation minima, RECAT-EU, as an alternative to
the long established ICAO PANS-ATM categories, in order to safely support
an increase in runway throughput at airports in Europe. RECAT-EU also
integrates a Super Heavy category for the Airbus A380 bringing runway capacity
benefits of up to 8% or more during peak traffic periods. As part of the wake
turbulence recategorisation separation review, SESAR partners EUROCONTROL and
NATS have developed RECAT-EU from the long understood concept of Time based
separation. Following approval by the European
Aviation Safety Agency, RECAT-EU is to be initially deployed at Paris Charles
de Gaulle airport by end-2015. Eurocontrol plans to move beyond RECAT
EU onto a more granular separation matrix whereby precise separations for
each of an initial 115 common commercial aircraft are defined by model in a 'Pair
Wise Separation' system. These separation matrices known as Recat
2 and Recat 3 are to be deployed in European airports towards 2020 and 2022
respectively. = 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 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 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 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
8 June 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.
30 May 1972 - A DC-9 crashed at the Greater Southwest International Airport
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. 15 December 1993 - a chartered aircraft
with five people on board, including In-N-Out Burger's president, Rich
Snyder, crashed several miles before John Wayne Airport. The aircraft was
following a Boeing 757 for landing, became caught in its wake turbulence,
rolled into a deep descent and crashed. 8 September 1994 - USAir Flight 427
crashed near Pittsburgh, Pennsylvania. This accident was believed to involve
wake turbulence, though the primary cause was a defective rudder control
component. 20 September 1999 - A JAS 39A Gripen
from Airwing F 7 Såtenäs crashed into Lake Vänern in Sweden during an air
combat maneuvering exercise. After passing through the wake vortex of the
other aircraft, the Gripen abruptly changed course, and 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. 12 November 2001 - An Airbus A300
crashed into the Belle Harbor neighborhood of Queens, New York shortly
after takeoff from John F. Kennedy International Airport. This accident was
attributed to pilot error in the presence of wake turbulence from a
Boeing 747, that resulted in rudder failure and subsequent separation of the
vertical stabilizer. 8 July 2008 - A US Air Force PC-12
trainer crashed at Hurlburt Field, Fla., because the pilot tried to land too
closely behind a larger AC-130U Spooky gunship and got caught in the gunship’s
wake turbulence. Air Force rules require at least a two-minute separation between
slow-moving heavy planes like the AC-130U and small, light planes, but the
PC-12 trailed the gunship by about 40 seconds. As the PC-12 hit the wake
turbulence, it suddenly rolled to the left and began to turn upside down. The
instructor pilot stopped the roll, but before he could get the plane upright,
the left wing struck the ground, sending the plane skidding 669 feet across a
field before stopping on a paved overrun.
3 November 2008 - Airbus A380 wake turbulence event, Sydney Airport,
Australia. Wake turbulence of an Airbus A380-800 causing temporary loss of
control to a Saab 340 on approach to a parallel runway during high crosswind
conditions. 4 November 2008 - Mexican Government
LearJet 45 XC-VMC, carrying Mexican Interior Secretary Juan Camilo Mouriño,
crashed near Paseo de la Reforma Avenue before turning for final approach to
runway 05R at Mexico City International Airport. The airplane was flying behind
a 767-300 and above a heavy helicopter. The pilots were not told about the type
of plane that was approaching before them, nor did they reduce to minimum
approach speed. On 9 September 2012, a Robin DR 400
crashed after rolling 90 degrees in a wake turbulence induced by the preceding
Antonov AN-2, three killed, one severely injured.
On 28 March 2014, Indian Air Force C-130J-30 KC-3803 crashed near Gwalior,
India, killing all 5 personnel aboard. The aircraft was conducting low level
penetration training by flying at around 300 ft when it ran into wake turbulence
from another aircraft in the formation, which caused it to crash.
Measurement Wake turbulence can be measured using
several techniques. Currently, ICAO recognizes 2 methods of measurement,
sound tomography, and a high-resolution technique is Doppler lidar, a solution
now commercially available. Techniques using optics can use the effect of
turbulence on refractive index 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, 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.
In popular culture 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
Kazansky. Maverick enters an unrecoverable spin as a result, and is
forced to eject. In a subsequent incident, he is caught in an enemy
fighter's jet wash, 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. See also
Batchelor vortex Eddy
Wake Wingtip device
References External links
Captain Meryl Getline explains "Heavy" U.S. FAA, The Aeronautical Information
Manual on Wake Turbulence U.S. FAA, Pilot Controller Glossary, see
Aircraft Classes Wake Turbulence, An Invisible Enemy
Photographs of Wake turbulence NASA Dryden - Wake Vortex Research