Chapter 15Turbulence 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

• 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%.

G ust Fa c to r= G usts - LullsM e a n Wind Sp e e d

x 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.

Re ve rse flo w a nd turb ule nc e

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’dStanding 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

IntensityAircraft Reaction

(transport sized aircraft)Reaction Inside Aircraft

LIGHT

Turbulence that momentarily causesslight, erratic changes in altitude and/orattitude (pitch, roll, yaw). IAS fluctuates5-15 kt. (<0.5 g at a/c C. of G.)Report as 'Light Turbulence'

Occupants may feel a slight strain againstseat belts or shoulder straps. Unsecuredobjects may be displaced slightly. Foodservice may be conducted and little or nodifficulty is encountered in walking

orturbulence that causes slight, rapid andsomewhat rythmic bumpiness withoutappreciable changes in altitude oratttude. No IAS fluctuations.Report as "Light Chop'.

MODERATE

Turbulence that is similar to LightTurbulence but of a greater intensity.Changes in altitude and/or attitudeoccur but the aircraft remains in positivecontrol at all times. IAS fluctuates 15-25kt. ( 0.5- 1.0 g at a/c C. of G.)Report as 'Moderate Turbulence'

Occupants feel definite strains against seatbelts or shoulder straps. Unsecuredobjects are dislodged. Food service andwalking are difficult.

orturbulence that is similar to Light Chopbut of greater intensity. It causes rapidbumps or jolts without appreciablechanges in aircraft altitude or attitude.IAS may fluctuate slightly. Report as'Moderate Chop'.

SEVERE

Turbulence that causes large, abruptchanges in altitude and/or attitude.Aircraft may be momentarily out ofcontrol. IAS fluctuates more than 25 kt.(>1.0 g at a/c C. of G.)Report as 'Severe Turbulence'.

Occupants are forced violently againstseat belts or shoulder straps. Unsecuredobjects are tossed about. Food serviceand 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.

Turb u le nt ro to rs- se ve re turb u le nc e

Turb u le nt e d d y

Sta nd ing wa ve s

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

HORIZONTAL WIND SHEAR HERE AT GUST FRONT

DOWN DRAUGHT WINDSHEAR BELOW

BASE OF STORM

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