All aircraft produce wake turbulence, which
consists of wake vortices formed any time an aerofoil is producing lift.
Lift is generated by
the creation of a pressure differential over the wing surfaces. The
lowest pressure occurs over the upper surface and the highest pressure
under the wing. Air will want to move towards the area of lower pressure.
This causes the air to move outwards under the wing and curl up and over
the upper surface of the wing. This starts the wake vortex. The pressure
differential also causes the air to move inwards over the wing. Small
trailing edge vortices, formed by outward and inward moving streams of
air meeting at the trailing edge, move outwards to the wingtip and join
the large wingtip vortex. Swirling air masses trail downstream of the
wingtips. Viewed from behind the left vortex rotates clockwise and the
right vortex rotates counter- clockwise. They spread laterally away from
the aircraft and descend 500 to 900 feet at distances of up to five miles
behind it. These vortices tend to descend 300 to 500 feet per minute
during the first 30 seconds. Light crosswinds may cause the vortices to
drift, and crosswinds in excess of five knots tend to cause them to break
up behind the aircraft. Atmospheric turbulence generally causes them to
break up more rapidly. The intensity or strength of the vortex is
primarily a function of aircraft weight, wingspan and configuration (flap
setting, etc). The strongest vortices are produced by heavy aircraft
flying slowly in a clean configuration. For example, a large or heavy
aircraft, which must reduce its speed to 250 knots below 10,000 feet,
while flying in a clean configuration is producing very strong wake
vortices while it descends.
Viewed from behind the generating
aircraft, the left vortex rotates clockwise and the right vortex rotates
counter-clockwise. They spread laterally away from the aircraft and
descend 500 to 900 feet at distances of up to five miles behind it.
Vortices tend to descend 300 to 500 feet per minute in the first 30
produce wake turbulence. Helicopter wakes may be of significantly greater
strength than those from fixed-wing aircraft of the same weight. The
strongest wake turbulence can occur when the helicopter is operating at
lower speeds (20 to 50 knots). Some mid-size or executive class
helicopters produce wake turbulence as strong as that of heavier
helicopters. Two- blade main rotor systems produce stronger wake
turbulence than rotor systems with more blades.
Wake Turbulence During Takeoff and Landing
While there have been instances where wake
turbulence caused structural damage, the greatest hazard is induced roll
and yaw. This is especially dangerous during takeoff and landing, when
there is little height for recovery. Wake turbulence-induced roll rates
can be extreme. Countering roll rates may be difficult or impossible,
even in high performance aircraft with excellent roll control authority.
In fixed-wing aircraft, wake vortices begin as the nose is rotated for
takeoff and continue throughout flight until the nosewheel touches down
on the runway once again. The vortices can cause problems for aircraft
crossing behind or below leading aircraft. Low approaches, touch-and-
goes and go-arounds can also cause problems for taxiing or departing
aircraft. During takeoff and landing, the vortices sink toward the ground
and move laterally away from the runway when the wind is calm. A
crosswind of 3 to 5 knots will tend to keep the upwind vortex in the
runway area and may cause the downwind vortex to drift toward another
runway. Wake vortices sometimes bounce, diverge and dissipate more
rapidly in ground effect. Wake turbulence separation is provided by Air
Traffic Control (ATC) to all aircraft which may be affected by wake
turbulence, except in the case of IFR aircraft making a visual approach
or VFR aircraft arrivals. In these cases it is the pilot’s responsibility
to provide adequate spacing from preceding, arriving or departing
aircraft. Pilots should follow the guidelines below and ATC will make
allowance when sequencing. Wherever practicable, aerodrome controllers
will advise pilots of the likelihood of wake turbulence by using the
phrase, “Caution – wake turbulence”.
For the purpose of assessing wake
turbulence separation, aircraft are divided into the following categories
of Maximum Certificated Takeoff Weight (MCTOW):
All aircraft types of 136,000 kg MCTOW or more. Some examples of these
are: Boeing B777, B767, B747, McDonnell Douglas DC–8, MD–11, and DC–10.
Aircraft types of more than 7,000 kg and less than 136,000 kg MCTOW. Some
examples of these are: Boeing B727, B737 and B757*, Fokker Friendship,
Metro 4 , BAe–146, Dash 8, ATR–72, Hercules, DC–3 and Saab 340.
* B757 aircraft are categorised
as ‘heavy’ (H) aircraft for the purpose of assessing wake turbulence
experienced by following aircraft.
Aircraft types of less than 7,000 kg MCTOW. Some of the heavier examples
of these are: Bandeirante,
3 , Cessna 402 and 421, Islander, Nomad, Piper Navajo and Beech 99.
*Depending on which model of Metro, its modification status, and its
operating weight on the day, it can sometimes fall into the medium
category of over 7,000 kg MCTOW. This would appear to make little
difference to procedural separations, but all pilots should be aware that
Metro wake turbulence can have a bigger bite than might be suspected from
having the type listed in the light category.
After takeoff, avoid headings which cross below
nd behind the path of larger aircraft.
Wake Turbulence Separations
ATC applies differing separations
depending on the wake turbulence category of the leading aircraft and the
equipment available to them to provide separation eg, radar. The tables
given below are issued by ICAO. The UK has slightly different values.
* The B757 is categorised as ‘heavy’ when applying following distances.
Non-radar separation standards for arriving or departing flights for
aircraft using the same (or close parallel) runway are as follows:
* 3 mins if taking off from an intermediate position.
These are elaborated on, and there are further standards listed, in
the AIP Planning Manual - such as opposite direction runway operations
and crossing runways.
Remember wake turbulence separation is not provided to landing VFR
arrivals, nor to IFR on visual approach. In these cases it is up to the
pilot to provide adequate spacing from preceding arriving or departing
If a pilot considers the wake turbulence separation standards
inadequate, an increased separation may be requested by specifying the
spacing required. Conversely, if pilots indicate that they will take
responsibility for their own wake turbulence separation then they may
request exemption from these separations. This option should be treated
Another hazard to bear in mind, particularly for light
aircraft, is jet blast and propeller slipstream. Beware of passing close
or landing directly behind aircraft with engines running, particularly
large jets. Jet blast and propeller slipstream can produce localised
wind velocities of sufficient strength to cause damage to other aircraft,
vehicles, personnel and buildings. Some years ago a B727 on engine tests
blew in a hangar door - clear testimony to the force which can be
Taking off behind larger
Ensure you can rotate before the preceding aircraft's rotation point. A
climb above its flight path is also necessary, until you can turn clear.
If this is not possible, delay your takeoff.
Avoid flight below and behind larger aircraft's flight paths. If a larger
aircraft is observed less than 1,000 feet above you on the same track
(same or opposite direction) adjust your position laterally, preferably
When planning to take off from an intermediate point behind an aircraft
that has used full length, delay your takeoff.
Issues Impacting Visual Separation
Air traffic controllers may separate departing
aircraft by visual means after considering aircraft performance, wake
turbulence, closure rate, routes of flight and known weather conditions.
Controller visual separation of aircraft should not be applied between
successive departures when departure routes and/or aircraft performance
will not allow the pilots to maintain adequate separation. In the
terminal area it must be day, the air traffic controller must have both
aircraft in sight and must be in radio contact with at least one of them.
The flight crew of the trailing aircraft must see the lead aircraft and
be informed of the lead aircraft's position, its direction of flight and
its crew's intentions. The pilots of the trailing aircraft must
acknowledge sighting the lead aircraft and be instructed to maintain
visual separation. The tower controller will not provide visual
separation between aircraft when wake turbulence separation is required.
In controlled airspace with ATC radar coverage, the controller must
inform the pilot of converging aircraft and VFR traffic. In cruise, when
IFR and VFR aircraft are sometimes separated by as little as 500 feet,
pilots must use appropriate avoidance procedures. Because wake turbulence
is nearly always invisible, pilots need to anticipate where it might be.
Air traffic controllers issue "Caution - wake turbulence" warnings only
and are not responsible for anticipating the existence or effect of the
Landing behind a larger aircraft
1. Same runway
Stay at or above the larger aircraft's final approach flight path. Note
its touchdown point and land beyond it.
2. Parallel runway or vector
Note wind for possible vortex drift on to the landing vector. Stay at or
above the larger aircraft's final approach flight path. Note its
touchdown point and land beyond a point abeam it.
3. Crossing runway
Cross above the larger aircraft's flight path.
The Warning Signs
Any un-commanded aircraft movements, such as wing
rocking, may be caused by wake vortices. This is why maintaining
situational awareness is so critical. Atmospheric turbulence is not
unusual, particularly in the approach phase. Pilots who suspect wake
turbulence is affecting their aircraft should immediately move away from
the wake by executing a missed approach or go- around; then must be
prepared for an even stronger wake vortex encounter. The onset of wake
turbulence 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.
Pilots should not depend on any aerodynamic warning. If the onset of wake
turbulence is occurring, immediate evasive action is a must!
How to Avoid Wake Turbulence
Pilots should remember three basic warnings concerning wake
*Do not get too close to the lead aircraft.
*Do not get below the lead aircraft's flight path.
*Be particularly wary when light wind conditions exist.
The following avoidance procedures should be followed at all times:
you think wake turbulence from the preceding aircraft may be a factor,
wait about 2 or 3 minutes before taking off. Before taking the active
runway, tell the tower that you want to wait. Plan to lift off prior to
the rotation point of the lead aircraft, and use full takeoff power or
Climb. If possible, climb above the lead aircraft's flight path. If
you can't out- climb it, fly slightly upwind and climb parallel to the
lead aircraft's course. Avoid headings that cause you to cross behind and
below the aircraft in front.
Crossing. If you must cross behind the lead
aircraft, try to cross above its flight path or, terrain permitting, at
least 1,000 feet below.
Trailing. Endeavour to stay either on or above the leading aircraft's
flight path, or upwind, or, terrain permitting, at least 1,000 feet
Approach. Maintain a position on or above the lead aircraft's flight
path with adequate lateral separation.
Landing. Ensure that your touchdown point is beyond the lead
aircraft's touchdown point. Land well before a departing aircraft's
Crossing Approaches. When landing behind another aircraft on crossing
approaches, cross above the other aircraft's flight path.
Crosswinds. Remember crosswinds may affect the position of wake
vortices. Adjust takeoff and landing points accordingly.
Helicopters. Remember that their wake vortices may be of
significantly greater strength than those of fixed-wing aircraft of the
same weight. Avoid flying beneath the flight paths of helicopters. When
piloting a small aircraft, avoid taxiing within three wingspans of a
helicopter that is hovering or hover taxiing at slow speed.
Visual Approach. When making a visual approach, do
not assume that the aircraft you are following is on the same or lower
flight path. The flight crew of the lead aeroplane may have flown a steep
approach (typical of cargo operations). Stay above and at least 3 miles
behind the normal flight path (at least 4 miles behind a B757).
Wake turbulence is one of the factors that pilots and air traffic
controllers must avoid to ensure safe flights. It takes co-operation,
awareness and an understanding of each other's requirements to safely
avoid aircraft-generated wake.
It is your
as flight crew or pilot in command to anticipate
the likelihood of encountering wake turbulence and to alter your flight
path accordingly, or, if necessary, request an alternative clearance from
ATC. Do not rely on others to provide warnings.