Ch 12 - Turbulence

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Ch 12 - Turbulence
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Introduction

• A characteristic of most naturally occurring
  fluids is that they contain some degree of
  turbulence
• This means that you can usually find some part of
  the fluid where the velocities are fluctuating in
  a chaotic manner.
• The atmosphere is one of those fluids
• The velocity fluctuations found within the
  atmosphere are often weak and barely noticeable
  in flight.
• Occasionally, however, atmospheric turbulence is
  so strong that passengers and crew are injured
  and the aircraft is damaged or destroyed

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Introduction

• The purpose of this chapter is to provide
  information that will help you avoid or at least
  minimize the effects of turbulence on your
  flight.
• When you complete this chapter, you will
  understand the basic types of turbulence and
  their causes, and you will know the large-scale
  conditions under which turbulence occurs.
• Also, you will have learned some rules of thumb
  that will help you anticipate and deal with the
  turbulence problem.

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Ch 12 - Turbulence

• Section A Turbulence Defined
• Aircraft and Pilot Response
• Turbulence Measures
• Section B Turbulence Causes Types
• Low-Level Turbulence (LLT)
• Mechanical Turbulence
• Thermal Turbulence
• Turbulence in Fronts
• Wake Turbulence

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Ch 12 - Turbulence

• Turbulence in and Near Thunderstorms (TNT)
• Turbulence Within Thunderstorms
• Turbulence Below Thunderstorms
• Turbulence Around Thunderstorms
• Clear Air Turbulence (CAT)
• Mountain Wave Turbulence (MWT)
• Lee Wave Region
• Lower Turbulent Zone

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Ch 12 - Turbulence

• Section A Turbulence Defined
• Aviation turbulence Based on descriptions from
  pilots, crew, and passengers, aviation turbulence
  is best defined simply as bumpiness in flight.
  
• This definition is based on the response of the
  aircraft rather than the state of the atmosphere.

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Ch 12 - Turbulence

• Aircraft and Pilot Response
• Turbulent gusts Atmospheric motions produced by
  turbulent eddies are often referred to as
  turbulent gusts
• Maneuvering If the pilot (or autopilot)
  overreacts, control inputs may actually add to
  the intensity of bumpiness.

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Ch 12 - Turbulence

• Turbulence Measures
• Turbulence reporting criteria Turbulence
  intensity varies from light, moderate, severe to
  extreme and is related to aircraft and crew
  reaction and to movement of unsecured objects
  about the cabin.
• G-load Also known as gust load, this force
  arises because of the influence of gravity.

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Ch 12 - Turbulence

• Section B Turbulence Causes and Types
• Low-Level Turbulence Defined as that turbulence
  which occurs primarily within the atmospheric
  boundary layer.
• The boundary layer is the lowest few thousand
  feet of the atmosphere
• that is, where surface heating and friction
  influences are significant.
• Mechanical Turbulence Over flat ground,
  significant LLT occurs when surface winds are
  strong.

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Ch 12 - Turbulence

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

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Ch 12 - Turbulence

• Turbulent wake Typically, a trail of turbulent
  eddies is produced downwind of an obstacle with a
  sheared layer between the ground-based turbulent
  region and smooth flow aloft.
• Funneling effect Similar to the increase in the
  speed of the current of a river where it narrows,
  strong local winds with substantial LLT and wind
  shear are created when a broad air stream is
  forced to flow through a narrow mountain pass.
• Strong winds due to this funneling effect may
  extend well downstream of the pass.

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Ch 12 - Turbulence

• Thermal Turbulence
• Thermal turbulence Thermal turbulence is LLT
  produced by dry convection in the boundary layer.
• It is typically a daytime phenomenon that occurs
  over land under fair weather conditions.

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Ch 12 - Turbulence

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

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Ch 12 - Turbulence

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

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Ch 12 - Turbulence

• Turbulence in Fronts
• Wake turbulence The term wake turbulence is
  applied to the vortices that form behind an
  aircraft that is generating lift.

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Ch 12 - Turbulence

• The greatest vortex strength occurs when the
  generating aircraft is heavy, clean and slow.
• Wake turbulence is near maximum behind a jet
  transport just after takeoff because of the high
  angle of attack and high gross weight.
• The wind condition that prolongs the hazards
  of wake turbulence on a landing runway for the
  longest period of time is a light quartering
  tailwind

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Ch 12 - Turbulence

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

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Ch 12 - Turbulence

• When landing behind a large aircraft, the
  pilot should avoid wake turbulence by staying
  above the large aircrafts final approach path
  and landing beyond the large aircrafts touchdown
  point.
• When departing behind a heavy aircraft, the pilot
  should avoid wake turbulence by maneuvering the
  aircraft above and upwind from the heavy aircraft.

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Ch 12 - Turbulence

• Turbulence within Thunderstorms
• Overshooting tops Although updrafts weaken
  above the equilibrium level, in intense
  thunderstorms, they may penetrate several
  thousand feet into the stratosphere before they
  are overcome by the stability.
• The strongest updrafts can often be identified by
  cumuliform bulges that extend above the
  other-wise smooth anvil top of the thunderstorm.
  
• These are called overshooting tops and they are
  evidence of very strong thunderstorms and
  turbulence.

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Ch 12 - Turbulence

• Turbulence below thunderstorms
• Turbulence below thunderstorms The downdrafts,
  downbursts and micro bursts define the primary
  turbulent areas below the thunderstorm.
• These phenomena produce intense turbulence as
  well as wind shear.
• Strong winds in the outflow from the downdraft
  generate mechanical turbulence, which is
  especially strong along the edge of any
  microburst and/or gust front.

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Ch 12 - Turbulence

• Turbulence around thunderstorms
• Overhang A turbulent wake occurs under the
  anvil cloud downwind of the thunderstorm.
• This is one of the most hazardous regions outside
  of the thunderstorm and above its base.
• Sometimes identified as the region under the
  overhang (anvil), it is an area well known to
  experienced pilots and is a location of severe
  turbulence and possibly hail.

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Ch 12 - Turbulence

• Clear Air Turbulence
• Clear Air Turbulence Turbulence which occurs in
  the free atmosphere away from any visible
  convective activity.
• Billow clouds In the clouds that show evidence
  of shearing-gravity wave activity, the herring
  bone pattern of billow clouds is a common
  feature in high cloud layers subjected to
  vertical shear.

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Ch 12 - Turbulence

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

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Ch 12 - Turbulence

• Shearing gravity waves Short atmospheric
  gravity wave disturbances that develop on the
  edges of stable layers in the presence of
  vertical shears.
• Jet stream front In the vicinity of the jet
  stream, there are two specific regions where CAT
  occurs most frequently.
• One is in the sloping stable layer below the jet
  core.
• This is a high-level frontal zone, also called a
  jet stream front.

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Ch 12 - Turbulence

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

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Ch 12 - Turbulence

• Mountain Wave Turbulence (MWT)
• Mountain Wave Turbulence (MWT) Turbulence
  produced in connection with mountain lee waves.
• It is responsible for some of the most violent
  turbulence that is encountered away from
  thunderstorms.

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Ch 12 - Turbulence

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

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Ch 12 - Turbulence

• Lower Turbulent Zone
• One of the most dangerous features of mountain
  waves is the turbulent area in and below rotor
  clouds

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Summary

• Aviation turbulence is caused by a number of
  different atmospheric phenomena.
• In this chapter we have considered the four most
  common types
• Turbulence generated in the boundary layer (LLT)
• Turbulence caused by strong convection (TNT)
• Turbulence in the vicinity of the jet stream
  (CAT)
• Turbulence caused by mountain waves (MWT)

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Summary

• You now know why and where this turbulence
  develops.
• You have some useful conceptual models which help
  you connect the various types of turbulence to
  the larger scale circulations in which they are
  embedded.
• Finally, you have learned some rules of thumb to
  aid you in turbulence avoidance.
• In Part IV, we will cover some of the aviation
  weather products that are available from the NWS
  and the FAA that help you identify turbulence
  areas