FLIGHT SAFETY :: Wake Turbulence
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 seconds.
Helicopters
also 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”.
Weight
Categories
For
the purpose of assessing wake turbulence separation, aircraft
are divided into the following categories of Maximum Certificated
Takeoff Weight (MCTOW):
Heavy
(H)
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.
Medium (M)
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.
Light (L)
Aircraft
types of less than 7,000 kg MCTOW. Some of the heavier
examples of these are: Bandeirante,
*Metro
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.
Departure
After takeoff, avoid headings
which cross below nd behind the path of larger aircraft.
Wake
Turbulence Separations
Radar 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 Separations
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 aircraft.
Pilot
Options
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
with caution.
Jet
Blast
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 exerted.
Taking
off behind larger aircraft
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.
En
route
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 upwind.
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 condition.
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 turbulence:
*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:
Takeoff. If
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 thrust.
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 below.
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
rotation point.
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
responsibility 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.
|
