Ch 12 - Turbulence Introduction A characteristic of most naturally occurring fluids is that they contain some degree of turbulence A characteristic of

Ch 12 - TurbulenceCh 12 - Turbulence

IntroductionIntroduction

• A characteristic of most naturally occurring fluids is A characteristic of most naturally occurring fluids is that they contain some degree of turbulencethat they contain some degree of turbulence

• This means that you can usually find some part of This means that you can usually find some part of the fluid where the velocities are fluctuating in a the fluid where the velocities are fluctuating in a chaotic manner.chaotic manner.

• The atmosphere is one of those fluidsThe atmosphere is one of those fluids• The velocity fluctuations found within the The velocity fluctuations found within the

atmosphere are often weak and barely noticeable atmosphere are often weak and barely noticeable in flight.in flight.

• Occasionally, however, atmospheric turbulence is Occasionally, however, atmospheric turbulence is so strong that passengers and crew are injured and so strong that passengers and crew are injured and the aircraft is damaged or destroyed the aircraft is damaged or destroyed

IntroductionIntroduction

• The purpose of this chapter is to provide The purpose of this chapter is to provide information that will help you avoid or at least information that will help you avoid or at least minimize the effects of turbulence on your flight.minimize the effects of turbulence on your flight.

• When you complete this chapter, you will When you complete this chapter, you will understand the basic types of turbulence and understand the basic types of turbulence and their causes, and you will know the large-scale their causes, and you will know the large-scale conditions under which turbulence occurs.conditions under which turbulence occurs.

• Also, you will have learned some rules of thumb Also, you will have learned some rules of thumb that will help you anticipate and deal with the that will help you anticipate and deal with the turbulence problem.turbulence problem.


• Section A – Turbulence DefinedSection A – Turbulence Defined– Aircraft and Pilot ResponseAircraft and Pilot Response– Turbulence MeasuresTurbulence Measures

• Section B – Turbulence Causes & TypesSection B – Turbulence Causes & Types– Low-Level Turbulence (LLT)Low-Level Turbulence (LLT)

•Mechanical TurbulenceMechanical Turbulence

•Thermal TurbulenceThermal Turbulence

•Turbulence in FrontsTurbulence in Fronts

•Wake TurbulenceWake Turbulence


– Turbulence in and Near Thunderstorms (TNT)Turbulence in and Near Thunderstorms (TNT)

•Turbulence Within ThunderstormsTurbulence Within Thunderstorms

•Turbulence Below ThunderstormsTurbulence Below Thunderstorms

•Turbulence Around ThunderstormsTurbulence Around Thunderstorms– Clear Air Turbulence (CAT)Clear Air Turbulence (CAT)– Mountain Wave Turbulence (MWT)Mountain Wave Turbulence (MWT)

•Lee Wave RegionLee Wave Region

•Lower Turbulent ZoneLower Turbulent Zone


• Section A: Turbulence DefinedSection A: Turbulence Defined– Aviation turbulence – Aviation turbulence – Based on descriptions Based on descriptions

from pilots, crew, and passengers, aviation from pilots, crew, and passengers, aviation turbulence is best defined simply as turbulence is best defined simply as “bumpiness in flight.” “bumpiness in flight.”

•This definition is based on the response of This definition is based on the response of the aircraft rather than the state of the the aircraft rather than the state of the atmosphere.atmosphere.


• Aircraft and Pilot ResponseAircraft and Pilot Response– Turbulent gusts – Turbulent gusts – Atmospheric motions Atmospheric motions

produced by turbulent eddies are often produced by turbulent eddies are often referred to as turbulent gustsreferred to as turbulent gusts

– Maneuvering – Maneuvering – If the pilot (or autopilot) If the pilot (or autopilot) overreacts, control inputs may actually add to overreacts, control inputs may actually add to the intensity of bumpiness.the intensity of bumpiness.


• Turbulence MeasuresTurbulence Measures– Turbulence reporting criteria – Turbulence reporting criteria – Turbulence Turbulence

intensity varies from light, moderate, severe to intensity varies from light, moderate, severe to extreme and is related to aircraft and crew extreme and is related to aircraft and crew reaction and to movement of unsecured reaction and to movement of unsecured objects about the cabin.objects about the cabin.

– G-load – G-load – Also known as gust load, this force Also known as gust load, this force arises because of the influence of gravity.arises because of the influence of gravity.


• Section B: Turbulence Causes and TypesSection B: Turbulence Causes and Types– Low-Level TurbulenceLow-Level Turbulence – Defined as that – Defined as that

turbulence which occurs primarily within the turbulence which occurs primarily within the atmospheric boundary layer. atmospheric boundary layer.

•The boundary layer is the lowest few thousand The boundary layer is the lowest few thousand feet of the atmosphere; feet of the atmosphere;

– that is, where surface heating and friction that is, where surface heating and friction influences are significant.influences are significant.

– Mechanical Turbulence – Mechanical Turbulence – Over flat ground, Over flat ground, significant LLT occurs when surface winds are significant LLT occurs when surface winds are strong.strong.


• ***The type of approach and landing ***The type of approach and landing recommended during gusty wind conditions recommended during gusty wind conditions is a power-on approach and a power-on is a power-on approach and a power-on landinglanding


– Turbulent wake – Turbulent wake – Typically, a trail of turbulent Typically, a trail of turbulent eddies is produced downwind of an obstacle with eddies is produced downwind of an obstacle with a sheared layer between the ground-based a sheared layer between the ground-based turbulent region and smooth flow aloft.turbulent region and smooth flow aloft.

– Funneling effect – Funneling effect – Similar to the increase in the Similar to the increase in the speed of the current of a river where it narrows, speed of the current of a river where it narrows, strong local winds with substantial LLT and wind strong local winds with substantial LLT and wind shear are created when a broad air stream is shear are created when a broad air stream is forced to flow through a narrow mountain pass. forced to flow through a narrow mountain pass. •Strong winds due to this funneling effect may Strong winds due to this funneling effect may

extend well downstream of the pass.extend well downstream of the pass.


– Thermal TurbulenceThermal Turbulence

•Thermal turbulence – Thermal turbulence – Thermal turbulence Thermal turbulence is LLT produced by dry convection in the is LLT produced by dry convection in the boundary layer.boundary layer.

– It is typically a daytime phenomenon that It is typically a daytime phenomenon that occurs over land under fair weather occurs over land under fair weather conditions.conditions.


• ***The characteristics of an unstable cold ***The characteristics of an unstable cold air mass moving over a warm surface are air mass moving over a warm surface are cumuliform clouds, turbulence, and good cumuliform clouds, turbulence, and good visibility. visibility. – A stable air mass is most likely to have A stable air mass is most likely to have

smooth air.smooth air.


• Capping stable layer – Capping stable layer – This layer is caused by a This layer is caused by a very slowly sinking motion aloft; very slowly sinking motion aloft; – typically associated with a macro scale high typically associated with a macro scale high

pressure region. pressure region.


• Turbulence in FrontsTurbulence in Fronts– Wake turbulence – Wake turbulence – The term wake The term wake

turbulence is applied to the vortices that form turbulence is applied to the vortices that form behind an aircraft that is generating lift.behind an aircraft that is generating lift.


• ***The greatest vortex strength occurs ***The greatest vortex strength occurs when the generating aircraft is heavy, clean when the generating aircraft is heavy, clean and slow. and slow. – Wake turbulence is near maximum behind Wake turbulence is near maximum behind

a jet transport just after takeoff because a jet transport just after takeoff because of the high angle of attack and high gross of the high angle of attack and high gross weight.weight.

• ***The wind condition that prolongs the ***The wind condition that prolongs the hazards of wake turbulence on a landing hazards of wake turbulence on a landing runway for the longest period of time is a runway for the longest period of time is a light quartering tailwindlight quartering tailwind


• Turbulence in and near ThunderstormsTurbulence in and near Thunderstorms– Turbulence in and near Thunderstorms Turbulence in and near Thunderstorms

(TNT) – (TNT) – Turbulence which occurs within Turbulence which occurs within developing convective clouds and developing convective clouds and thunderstorms, in the vicinity of the thunderstorms, in the vicinity of the thunderstorm tops and wakes, in downbursts, thunderstorm tops and wakes, in downbursts, and in gust fronts.and in gust fronts.


• ***When landing behind a large aircraft, the ***When landing behind a large aircraft, the pilot should avoid wake turbulence by pilot should avoid wake turbulence by staying above the large aircraft’s final staying above the large aircraft’s final approach path and landing beyond the large approach path and landing beyond the large aircraft’s touchdown point. aircraft’s touchdown point. – When departing behind a heavy aircraft, When departing behind a heavy aircraft,

the pilot should avoid wake turbulence by the pilot should avoid wake turbulence by maneuvering the aircraft above and maneuvering the aircraft above and upwind from the heavy aircraft.upwind from the heavy aircraft.


• Turbulence within ThunderstormsTurbulence within Thunderstorms– Overshooting tops – Overshooting tops – Although updrafts weaken Although updrafts weaken

above the equilibrium level, in intense above the equilibrium level, in intense thunderstorms, they may penetrate several thunderstorms, they may penetrate several thousand feet into the stratosphere before they are thousand feet into the stratosphere before they are overcome by the stability. overcome by the stability.

•The strongest updrafts can often be identified by The strongest updrafts can often be identified by cumuliform bulges that extend above the other-cumuliform bulges that extend above the other-wise smooth anvil top of the thunderstorm. wise smooth anvil top of the thunderstorm.

– These are called overshooting tops and they These are called overshooting tops and they are evidence of very strong thunderstorms and are evidence of very strong thunderstorms and turbulence.turbulence.


• Turbulence below thunderstormsTurbulence below thunderstorms– Turbulence below thunderstorms – Turbulence below thunderstorms – The The

downdrafts, downbursts and micro bursts define downdrafts, downbursts and micro bursts define the primary turbulent areas below the the primary turbulent areas below the thunderstorm. thunderstorm.

•These phenomena produce intense turbulence These phenomena produce intense turbulence as well as wind shear. as well as wind shear.

– Strong winds in the outflow from the Strong winds in the outflow from the downdraft generate mechanical turbulence, downdraft generate mechanical turbulence, which is especially strong along the edge of which is especially strong along the edge of any microburst and/or gust front.any microburst and/or gust front.


• Turbulence around thunderstormsTurbulence around thunderstorms– Overhang – Overhang – A turbulent wake occurs under the A turbulent wake occurs under the

anvil cloud downwind of the thunderstorm. anvil cloud downwind of the thunderstorm.

•This is one of the most hazardous regions This is one of the most hazardous regions outside of the thunderstorm and above its outside of the thunderstorm and above its base. base.

•Sometimes identified as the region under the Sometimes identified as the region under the overhang (anvil), it is an area well known to overhang (anvil), it is an area well known to experienced pilots and is a location of severe experienced pilots and is a location of severe turbulence and possibly hail.turbulence and possibly hail.


• Clear Air TurbulenceClear Air Turbulence– Clear Air Turbulence – Clear Air Turbulence – Turbulence which Turbulence which

occurs in the free atmosphere away from any occurs in the free atmosphere away from any visible convective activity.visible convective activity.

– Billow clouds – Billow clouds – In the clouds that show In the clouds that show evidence of shearing-gravity wave activity, the evidence of shearing-gravity wave activity, the “herring bone” pattern of billow clouds is a “herring bone” pattern of billow clouds is a common feature in high cloud layers subjected common feature in high cloud layers subjected to vertical shear.to vertical shear.


• ***When a pilot enters an area where ***When a pilot enters an area where significant CAT has been reported, an significant CAT has been reported, an appropriate action when the first ripple is appropriate action when the first ripple is encountered is to adjust airspeed to that encountered is to adjust airspeed to that recommended for rough airrecommended for rough air


– Shearing gravity waves – Shearing gravity waves – Short atmospheric Short atmospheric gravity wave disturbances that develop on the gravity wave disturbances that develop on the edges of stable layers in the presence of vertical edges of stable layers in the presence of vertical shears.shears.

– Jet stream front – Jet stream front – In the vicinity of the jet In the vicinity of the jet stream, there are two specific regions where stream, there are two specific regions where CAT occurs most frequently. CAT occurs most frequently.

•One is in the sloping stable layer below the jet One is in the sloping stable layer below the jet core. core.

– This is a high-level frontal zone, also called This is a high-level frontal zone, also called a jet stream front.a jet stream front.


• ***A sharply curving jet stream is ***A sharply curving jet stream is associated with greater turbulence than a associated with greater turbulence than a straight jet streamstraight jet stream


• Mountain Wave Turbulence (MWT)Mountain Wave Turbulence (MWT)– Mountain Wave Turbulence (MWT) – Mountain Wave Turbulence (MWT) –

Turbulence produced in connection with Turbulence produced in connection with mountain lee waves. mountain lee waves.

• It is responsible for some of the most violent It is responsible for some of the most violent turbulence that is encountered away from turbulence that is encountered away from thunderstorms.thunderstorms.


– Lee Wave RegionLee Wave Region

•Lee Wave RegionLee Wave Region – Lee waves are more – Lee waves are more often smooth than turbulent, but if often smooth than turbulent, but if turbulence does occur in the lee wave turbulence does occur in the lee wave region, it is most likely to occur within 5,000 region, it is most likely to occur within 5,000 feet of the tropopause.feet of the tropopause.


• Lower Turbulent ZoneLower Turbulent Zone– ***One of the most dangerous features of ***One of the most dangerous features of

mountain waves is the turbulent area in mountain waves is the turbulent area in and below rotor cloudsand below rotor clouds

SummarySummary

• Aviation turbulence is caused by a number of Aviation turbulence is caused by a number of different atmospheric phenomena.different atmospheric phenomena.

• In this chapter we have considered the four most In this chapter we have considered the four most common types:common types:– Turbulence generated in the boundary layer Turbulence generated in the boundary layer

(LLT)(LLT)– Turbulence caused by strong convection (TNT)Turbulence caused by strong convection (TNT)– Turbulence in the vicinity of the jet stream Turbulence in the vicinity of the jet stream

(CAT)(CAT)– Turbulence caused by mountain waves (MWT)Turbulence caused by mountain waves (MWT)

SummarySummary

• You now know why and where this turbulence You now know why and where this turbulence develops.develops.

• You have some useful conceptual models which You have some useful conceptual models which help you connect the various types of turbulence help you connect the various types of turbulence to the larger scale circulations in which they are to the larger scale circulations in which they are embedded.embedded.

• Finally, you have learned some rules of thumb to Finally, you have learned some rules of thumb to aid you in turbulence avoidance.aid you in turbulence avoidance.

• In Part IV, we will cover some of the aviation In Part IV, we will cover some of the aviation weather products that are available from the NWS weather products that are available from the NWS and the FAA that help you identify turbulence and the FAA that help you identify turbulence areas areas

Documents

Ch 12 - Turbulence Introduction A characteristic of most naturally occurring fluids is that they contain some degree of turbulence A characteristic of