Transcript Ch 12

Ch 12 - Turbulence
Introduction
• A characteristic of most naturally occurring fluids is that
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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
Introduction
• The purpose of this chapter is to provide information
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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.
Ch 12 - Turbulence
• Section A – Turbulence Defined
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– Aircraft and Pilot Response
– Turbulence Measures
Section B – Turbulence Causes & Types
– Low-Level Turbulence (LLT)
• Mechanical Turbulence
• Thermal Turbulence
• Turbulence in Fronts
• Wake Turbulence
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
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.
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.
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.
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.
Ch 12 - Turbulence
• ***The type of approach and landing
recommended during gusty wind conditions is a
power-on approach and a power-on landing
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.
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.
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.
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.
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.
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
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.
Ch 12 - Turbulence
• ***When landing behind a large aircraft, the pilot
should avoid wake turbulence by staying above
the large aircraft’s final approach path and
landing beyond the large aircraft’s 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.
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 otherwise smooth anvil top of the thunderstorm.
– These are called overshooting tops and they
are evidence of very strong thunderstorms and
turbulence.
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.
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.
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.
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
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.
Ch 12 - Turbulence
• ***A sharply curving jet stream is associated
with greater turbulence than a straight jet stream
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.
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.
Ch 12 - Turbulence
• Lower Turbulent Zone
– ***One of the most dangerous features of
mountain waves is the turbulent area in and
below rotor clouds
Summary
• Aviation turbulence is caused by a number of different
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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)
Summary
• You now know why and where this turbulence develops.
• You have some useful conceptual models which help you
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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