Turbulence

Introduction:

  • Aircraft experience turbulence in varying intensities due to the irregular motion of an aircraft in flight, especially when characterized by rapid up-and-down motion caused by a rapid variation of atmospheric wind velocities
    • Turbulence is caused by convective currents (called convective turbulence), obstructions in the wind flow (called mechanical turbulence), and wind shear
  • Turbulence varies from annoying bumpiness to severe jolts, which cause structural damage to an aircraft and/or injury to its passengers
    • The effect of turbulence varies based on the size of the aircraft

Turbulence Intensities:

  • Turbulence is classified by its intensity:

    • Light Turbulence:

      • Causes slight, erratic changes in altitude and/or attitude (pitch, roll, or yaw). Report as Light Turbulence. Or causes slight, rapid, and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude. Report as Light Chop

    • Moderate Turbulence:

      • Similar to Light but of greater intensity. Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed. Report as Moderate Turbulence. Or turbulence that is similar to Light Chop but of greater intensity. It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude. Report as Moderate Chop

    • Severe Turbulence:

      • Causes large, abrupt changes in altitude and/or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control

    • Extreme Turbulence:

      • The aircraft is violently tossed about and is practically impossible to control. It may cause structural damage

Convective Turbulence:

  • Convective Turbulence
    Convective Turbulence
  • Thermals
    Thermals
  • Convective turbulence is turbulent vertical motions that result from convective currents and the subsequent rising and sinking of air
  • Low altitude, with updrafts 200-2,000 FPM
  • For every rising current, there is a compensating downward current
    • The downward currents frequently occur over broader areas than do the upward currents; therefore, they have a slower vertical speed than do the rising currents
  • Convective currents are most active on warm summer afternoons when winds are light
    • Heated air at the surface creates a shallow, absolutely unstable layer within which bubbles of warm air rise upward
    • Convection increases in strength and to greater heights as surface heating increases
    • Barren surfaces such as sandy or rocky wastelands and plowed fields become hotter than open water or ground covered by vegetation
      • Thus, air at and near the surface heats unevenly
    • Because of uneven heating, the strength of convective currents can vary considerably within short distances
  • Typically short distance, with possibly severe turbulence below the clouds
  • Varies widely with terrain
  • As air moves upward, it cools by expansion
    • A convective current continues upward until it reaches a level where its temperature cools to the same as that of the surrounding air
    • If it cools to saturation, a cumuliform cloud forms
  • Billowy cumuliform clouds, usually seen over land during sunny afternoons, are signposts in the sky indicating convective turbulence [Figure 7]
    • The cloud top usually marks the approximate upper limit of the convective current
    • A pilot can expect to encounter turbulence beneath or in the clouds, while above the clouds, air generally is smooth
    • When convection extends to great heights, it develops larger towering cumulus clouds and cumulonimbus with anvil-like tops
    • The cumulonimbus gives visual warning of violent convective turbulence
  • When the air is too dry for cumuliform clouds to form, convective currents can still be active
    • This is called dry convection, or thermals [Figure 8]
    • A pilot has little or no indication of their presence until encountering the turbulence
  • Convective Turbulence
    Convective Turbulence
  • Thermals
    Thermals

Mechanical Turbulence:

  • Mechanical turbulence is turbulence caused by obstructions to the wind flow, such as trees, buildings, mountains, and so on
    • Obstructions to the wind flow disrupt smooth wind flow into a complex snarl of eddies
  • Can form in stable or unstable air
  • Presents landing hazards
  • The intensity of mechanical turbulence depends on wind speed and roughness of the obstructions
  • The wind carries the turbulent eddies downstream. How far depends on wind speed and stability of the air
    • Unstable air allows larger eddies to form than those that form in stable air; but the instability breaks up the eddies quickly, while in stable air they dissipate slowly

Mountain Wave:

  • Mountain Wave
    Mountain Wave
  • Mountain Wave Clouds
    Mountain Wave Clouds
  • A mountain wave is an atmospheric wave disturbance formed when stable air flow passes over a mountain or mountain ridge, producing hazardous wind conditions [Figure 9]
  • The waves may extend 600 miles (1,000 kilometers) or more downwind from the mountain range
    • Mountain waves frequently produce severe to extreme turbulence
    • Location and intensity varies with wave characteristics
  • Mountain waves often produce violent downdrafts on the immediate leeward side of the mountain barrier, sometimes exceeding maximum aircraft climb rates
    • Up/downdrafts increase as wind speed increases
  • A mountain wave cloud is a cloud that forms in the rising branches of mountain waves and occupies the crests of the waves
    • The most distinctive are the sharp-edged, lens-, or almond-shaped lenticular clouds
    • When sufficient moisture is present in the upstream flow, mountain waves produce interesting cloud formations including: cap clouds, cirrocumulus standing lenticular (CCSL), Altocumulus Standing Lenticular (ACSL), and rotor clouds [Figure 10]
    • These clouds provide visual proof that mountain waves exist
    • However, these clouds may be absent if the air is too dry
  • Stable air and wind speeds between 25-40 knots foster turbulence
  • Mountain Wave
    Mountain Wave
  • Mountain Wave Clouds
    Mountain Wave Clouds

Clear Air Turbulence:

  • Typically above 15,000 and in vicinity to the jet steam
  • Drastic changes in wind speed and velocity

Frontal Turbulence:

  • Occurs in a narrow zone, just ahead of a fast-moving cold front
  • Updrafts of 1,000 FPM, with significant turbulence

Turbulence Reports and Forecasts:

Private Pilot - Weather Information Airman Certification Standards:

  • To determine that the applicant exhibits satisfactory knowledge, risk management, and skills associated with weather information for a flight under VFR
  • References: 14 CFR part 91; FAA-H-8083-25; AC 00-6, AC 00-45, AC 00-54; AIM

Weather Information Knowledge:

The applicant must demonstrate an understanding of:

  • PA.I.C.K1:

    Sources of weather data (e.g., National Weather Service, Flight Service) for flight planning purposes

  • PA.I.C.K2:

    Acceptable weather products and resources required for preflight planning, current and forecast weather for departure, en route, and arrival phases of flight

  • PA.I.C.K3:

    Meteorology applicable to the departure, en route, alternate, and destination under VFR in Visual Meteorological Conditions (VMC) to include expected climate and hazardous conditions such as:
    • PA.I.C.K3a:
      Atmospheric composition and stability
    • PA.I.C.K3b:
      Wind (e.g., crosswind, tailwind, windshear, mountain wave, etc.)
    • PA.I.C.K3c:
      Temperature
    • PA.I.C.K3d:
      Moisture/precipitation
    • PA.I.C.K3e:
      Weather system formation, including air masses and fronts
    • PA.I.C.K3f:
      Clouds
    • PA.I.C.K3g:
      Turbulence
    • PA.I.C.K3h:
      Thunderstorms and microbursts
    • PA.I.C.K3i:
      Icing and freezing level information
    • PA.I.C.K3j:
      Fog/mist
    • PA.I.C.K3k:
      Frost
    • PA.I.C.K3l:
      Obstructions to visibility (e.g., smoke, haze, volcanic ash, etc.)

  • PA.I.C.K4:

    Flight deck displays of digital weather and aeronautical information

Weather Information Risk Management:

The applicant demonstrates the ability to identify, assess and mitigate risks, encompassing:

  • PA.I.C.R1:

    Factors involved in making the go/no-go and continue/divert decisions, to include:
    • PA.I.C.R1a:
      Circumstances that would make diversion prudent
    • PA.I.C.R1b:
      Personal weather minimums
    • PA.I.C.R1c:
      Hazardous weather conditions to include known or forecast icing or turbulence aloft

  • PA.I.C.R2:

    Limitations of:

Weather Information Skills:

The applicant demonstrates the ability to:

  • PA.I.C.S1 :

    Use available aviation weather resources to obtain an adequate weather briefing.

  • PA.I.C.S2:

    Analyze the implications of at least three of the conditions listed in K3a through K3l above, using actual weather or weather conditions in a scenario provided by the evaluator

  • PA.IV.C.S3:

    Correlate weather information to make a competent go/no-go decision

Turbulence Case Studies:

Conclusion:


References: