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Chapter 15
Turbulence CAT and
Wind shear
Definition of Turbulence
• Turbulence:.
– thus may be defined as airflow causing random deviations
from the desired flight path needing immediate corrective
action.
• Gustiness:
Gustiness
– is a fluctuating wind situation where the horizontal wind component is
subject to variations.
– The wind velocity is subject to gusts and lulls about the mean wind velocity.
– The ‘gustiness of the wind is expressed by the percentage ratio between
maximum and minimum horizontal wind speeds to the mean wind speed
recorded in a given period.
– Gusts and lulls are of short term duration lasting seconds rather than
minutes. Gustiness can vary from 25% to 100%.
Gust Factor
Gust Factor =
Gusts - Lulls x 100%
Mean Wind Speed
• If the mean wind speed is 30 KT with gusts up to 45 KT
and lulls down to 15 KT then the range is 30 KT and the
factor is 100%.
Causes of Turbulence
• Friction Layer Turbulence:
– can vary from a few hundred feet on a cold night up to 4000 to 5000 feet
under very turbulent windy conditions where both Mechanical and Thermal
turbulence can occur.
• Mechanical Turbulence:
– is caused by physical obstruction to the air flow by the presence of hills, trees,
mountains, buildings and coast lines and causes eddying and waves to
develop downwind..
Causes of Turbulence
• Thermal Turbulence:
– is caused when convection currents initiated by strong surface heating
interact with the normal horizontal motion of the air as well as mechanical
turbulence
Cloud Turbulence
• Cloud Turbulence:
– Is caused by the vertical motion present in convective clouds.
– It is most severe in thunderstorm clouds where the large scale release
of latent heat within the clouds adds thermal energy to the interior of
the cloud generating strong updraughts.
– Severe turbulence from CB clouds can affect the clear air in the
vicinity of the cloud itself
– May overshoot into the stratosphere.
Orographic Turbulence
• Low hills and buildings cause downstream turbulence
and eddies with reverse flow in the immediate
downstream area of the obstruction.
Reverse flow
and turbulence
Clear Air Turbulence
• Can occur near the tropopause, in the
vicinity of jetstreams
• Standing (Mountain) waves
• Upper level troughs and ridges
• Caused by changes in wind speed/direction
Clear Air Turbulence
• May result in:
– Stalling at low level
– Jet upset at high level, including structural
damage
Clear Air Turbulence, cont’d
Standing Waves
Formation of CAT
•
•
•
•
The shear along the boundary increases,
Waves begin to form which are unstable and which eventually ‘break,’ forming a turbulent eddy.
These may be small or large depending on the amount of shear.
When large waves appear they are frequently referred to as gravity waves and aircraft encountering
these my experience sudden and large vertical displacements known as ‘air pockets’.
• Unsecured objects and people can be thrown violently against the cabin roof on occasions.
Clear Air Turbulence
• Frictional stress caused by rapid changes of windspeed or wind
direction (windshear) causes three dimensional eddies.
– Near the tropopause.
– Close to jet streams, particularly on the cold polar airmass
side.
– Jet stream CAT can be encountered above the tropopause
boundary
Clear Air Turbulence
– Anywhere the vertical change in wind speed exceeds 5 knots/1000
ft or the horizontal change exceeds 20 knots/60 nm.
– In upper troughs and ridges, particularly where they are rapidly
developing.
– In the lee of mountains, under certain conditions - standing waves.
CAT
• Defined as:
– Air turbulence of a type other than that
associated with airflow close to rough ground
or that encountered in or near convective cloud.
– It does not always occur in clear air and may be
associated with thin cirrus.
Wake Turbulence
Incidence:
OCCASIONAL - less than 1/3 of the time
INTERMITTENT - 1/3 to 2/3
CONTINUOUS - more than 2/3
Intensity
Aircraft Reaction
(transport sized aircraft)
Turbulence that momentarily causes
slight, erratic changes in altitude and/or
attitude (pitch, roll, yaw). IAS fluctuates
5-15 kt. (<0.5 g at a/c C. of G.)
Report as 'Light Turbulence'
LIGHT
Reaction Inside Aircraft
Occupants may feel a slight strain against
seat belts or shoulder straps. Unsecured
objects may be displaced slightly. Food
service may be conducted and little or no
difficulty is encountered in walking
or
turbulence that causes slight, rapid and
somewhat rythmic bumpiness without
appreciable changes in altitude or
atttude. No IAS fluctuations.
Report as "Light Chop'.
Turbulence that is similar to Light
Turbulence but of a greater intensity.
Changes in altitude and/or attitude
occur but the aircraft remains in positive
control at all times. IAS fluctuates 15-25
kt. ( 0.5- 1.0 g at a/c C. of G.)
Report as 'Moderate Turbulence'
Occupants feel definite strains against seat
belts or shoulder straps. Unsecured
objects are dislodged. Food service and
walking are difficult.
MODERATE
or
turbulence that is similar to Light Chop
but of greater intensity. It causes rapid
bumps or jolts without appreciable
changes in aircraft altitude or attitude.
IAS may fluctuate slightly. Report as
'Moderate Chop'.
SEVERE
Turbulence that causes large, abrupt
changes in altitude and/or attitude.
Aircraft may be momentarily out of
control. IAS fluctuates more than 25 kt.
(>1.0 g at a/c C. of G.)
Report as 'Severe Turbulence'.
Occupants are forced violently against
seat belts or shoulder straps. Unsecured
objects are tossed about. Food service
and walking impossible.
Mountain Waves
• Substantial terrain such as mountains and the right conditions Standing or
Mountain Waves develop with a characteristic regular downwind wave pattern
in the upper atmosphere but at the height of the terrain downwind rotors form
with severe turbulence which can cause structural damage.
Standing waves
Turbulent rotors
- severe turbulenc e
Turbulent
eddy
Standing Waves
Windshear
Definitions
• Windshear is defined as:
– variations in vector wind along flight path
which,
– causes abrupt displacement from the
intended flight path such that,
– substantial control action is required.
Definitions
• Low Level Windshear is:
– windshear encountered during final
approach,
– along the runway,
– along take-off flight path,
– along the initial climbout flight path.
Types of Windshear
• Vertical Windshear:
– change of horizontal vector wind with vertical
change of aircraft position.
• Horizontal Windshear:
– change of horizontal vector wind with
horizontal change of a/c position.
• Down-draught Windshear:
– change in the vertical component of the wind
with horizontal change in position.
VERTICAL WINDSHEAR
VERTICAL WINDSHEAR
VERTICAL WIND SHEAR HERE
DOWN DRAUGHT
WINDSHEAR BELOW
BASE OF STORM
HORIZONTAL WIND SHEAR
HERE AT GUST FRONT
DOWN DRAUGHT/MICRO-BURST WIND SHEAR
COMBINED EFFECT OF MICRO-BURST WIND SHEAR
Main Causes of Windshear
• CB/TS Activity
• Frontal Surfaces:
– Warning signs are:
• Sharp changes in wind direction.
• Temperature difference of 5°C or more across a
front.
• Frontal speed 30 kt or more.
– Cold fronts windshear is just behind surface
position of front.
– Warm front windshear is just ahead of
surface position of front.
Main Causes of Windshear (cont.)
• Inversions:
– A low level wind maximum occurs just below
the top of a strong night time or frontal
inversion.
– The effect is sometimes to cause a decoupling
of the surface wind from the prevailing
gradient wind.
– Thus air at the surface may be slow moving or
stagnant and fast moving aloft giving a
pronounced windshear across the interface.
Main Causes of Windshear (cont.)
• Turbulent Boundary Layer:
– Strong surface winds with gusts and lulls
produce horizontal wind shear.
– Thermal turbulence in hot countries due to
intense surface heating gives strong up/down
draughts.
• Topographical Windshear:
– “Wind shadow” caused by topographical or
man-made objects,
• ravine winds
• rotors
• lee side wind of hills, tree lines.
Windshear Warnings & Reporting.
• Warnings reported in following ways:
– Met warning,
– ATS warning
– Departure/ Approach ATIS,
– Pilot warning,
– On-board equipment.
• NOTE: Pilots encountering windshear on
approach or climb-out are required to
make a report to ATC as soon as possible
even if previously encountered or f/cast.
WIND SHEAR ALERTING SYSTEM
Windshear Alerts
• Issued when:
– A marked temperature inversion of 10° or
more exists between surface and 1000 ft. or
– Mean surface wind exceeds 20 kt. or
– Vector difference between mean surface wind
and gradient wind at 2000’ exceeds 40 kt. or
– TS or heavy showers are within 5 nm of
airport.
• More info available from CAA - AIC