Types of Turbulence

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Types of Turbulence

• Mechanical
• Convective
• Downburst
• Clear Air Turbulence
• Aircraft-Produced

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Turbulence
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Mechanical Boundary Layer Turbulence

• Determined by terrain, stability of the
  atmosphere, and wind speed.

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Mechanical Turbulence

• Mechanical turbulence is caused by mountains,
  hill, buildings, or other large obstructions.
  When winds above 15 knots are bent around the
  objects, this creates the turbulence.
• Usually this isnt a major problem for most
  aviation, but when they winds become above 25
  knots and pass over a large enough obstacle they
  can create turbulent areas as high as
  5,000-10,000 feet.

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Mountain Wave Turbulence
Strong winds are forced over and around large
terrain features Turbulence often is
severe Characteristic cloud patterns
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Scales of Turbulence and Eddies
Whirling eddies of wind are generated at various
scales and orientations. Here are horizontal
whirls leeward of an obstructing mountain and
vertical rotors, or roll eddies, that extend for
many kilometers.
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Billow Cloud

• Billow cloud caused by wind shear (where there
  was sufficient moisture to show eddy).

Courtesy of Dr Moser, ERAU Dept of Aviation
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Convective Turbulence

• Daytime surface is hot, air above surface is
  cooler
• In lowest 1 km, when atmospheric lapse rate is
  greater than the dry adiabatic rate, a rising
  parcel is warmer than its surroundings, and will
  keep rising
• These warm volumes of air carry thermal energy
  upwards convective heat transport

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Convection Effects

• Some surfaces absorb and radiate energy at a much
  higher rate than others.
• This creates a light form of Convective
  turbulence.
• Common when two areas of land heat at different
  rates (such as a large parking lot next to a
  field)
• When an aircraft flies into this it may
  experience vertical shifts as it moves over the
  two areas boundaries.
• This is also known as Thermal Turbulence.

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Convective Turbulence

• A more severe type of convective turbulence is
  caused when the convective currents form cumulus
  clouds.
• Inside the clouds the air is unstable
• Frequently aircraft come out at much higher level
  or lower levels then they entered.
• The downdrafts and updrafts can be strong enough
  to cause structural damage because the aircraft
  is moved vertically at such a fast rate.

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Thunderstorm Downbursts

• Strong downward rush of winds from a mature
  thunderstorm.
• Causes serious takeoff and landing hazards
• When aircraft is most vulnerableclose to the
  ground and operating near stall speed

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Downburst Winds
Virga
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Microburst Simulation
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Microburst Takeoff and Landing
-3 factors vital to aircraft survival Altitude
at which shear is experienced The pilots
experience Type of Aircraft
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Air Force One Microburst
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Onboard Warning Systems
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Delta Flight 191 Downburst
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Clear Air Turbulence

• Fast winds over slow winds in neutral or unstable
  environment will cause turbulent eddies within
  clear atmosphere (no cloud signs).
• Often found in the vicinity of the jet stream
  core
• At least, an inconvenience
• At worst, cause significant problems
• Strength depends on vertical wind shear and
  vertical temperature profile (stability).
• ADDS Turbulence

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CATAn Aviation Hazard

• In December of 1992, a DC-8 cargo plane westbound
  out of Denver encountered severe clear air
  turbulence. Despite losing an engine (far right
  side) and 6 meters of wing, the crew managed to
  land the plane safely.

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Wake Turbulence

• All aircraft produce wake turbulence.
• Wake vortices are formed any time an airfoil is
  producing lift. The pressure differential across
  the wing triggers the rollup of the airflow aft
  of the wing resulting in swirling air masses
  trailing downstream of the wingtips.
• Viewed from behind the generating aircraft, the
  left vortex rotates clockwise and the right
  vortex rotates counterclockwise.
• The strength of the vortex depends primarily on
  aircraft weight and configuration.
• The strongest vortices are produced by heavy
  aircraft, flying slowly, in a clean
  configuration.
• While there have been rare instances where wake
  turbulence caused structural damage, the greatest
  hazard is induced roll and yaw.
• This is especially dangerous during takeoff and
  landing when there is little altitude for
  recovery.
• Helicopters also make wake turbulence
• Tends to be much stronger than fixed wing
  aircraft of the relatively same size.

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Wake Vortex
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Low Level Wind Shear and CrosswindsBad
weathergood pilot!
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Turbulence Categories

• Light turbulence - briefly causes slight, erratic
  changes in altitude and/or attitude.
• Light chop - slight, rapid and somewhat rhythmic
  bumpiness without noticeable changes in altitude
  or attitude.
• Moderate turbulence - similar to light
  turbulence, but greater intensity. Changes in
  altitude/attitude occur. Aircraft remains in
  control at all times. Variation in indicated air
  speed.
• Moderate chop - similar to light chop, but
  greater intensity. Rapid jolts without obvious
  changes in altitude or attitude.
• Severe turbulence - large, abrupt changes in
  altitude/attitude. Large variation in indicated
  airspeed. Aircraft may be temporarily out of
  control.
• Extreme turbulence - aircraft is violently tossed
  about and is impossible to control. May cause
  structural damage.