Course 5 - Section 12 - Turbulence Flashcards
Turbulence Definition
Turbulence is an irregular motion of the air from the formation of eddies or vertical currents in the air
It can be a major hazard to aviation
The degree of turbulence depends on the type of aircraft. For example, turbulence considered light for a 747 may be severe for a cessna 172
List the main causes of turbulence
Convective (or thermal) currents
mechanical turbulence
wind shear
wake turbulence
Turbulence: Convective (or thermal) currents
Convective (or thermal) currents refer to disturbed airflow resulting from air moving vertically in convective currents
Turbulence: Mechanical Turbulence
Mechanical turbulence refers to disturbed airflow resulting from air moving past or over obstructions, such as irregular terrain
Mechanical turubulence is a type of friction turbulence resulting of friction between the air and the ground
Turbulence: Wind shear
Wind shear is a drastic change in wind speed, direction, or both over a relatively short distance
Turbulence: Wake Turbulence
Wake turbulence is the turbulent air behind an aircraft that occurs due to various causes, including wingtip vortices
Stable Air
generally means smooth and stable flying conditions since the air will resist upward or downward displacement. Stable air tends to return to its original horizontal level
Since stable air resists vertical motion, stable air that is lifted will return to its original position. This is because the lifted air becomes colder and denser than the surrounding air and therefore sinks
Unstable Air
Unstable air is lifted and continues to rise on its own, since the lifted air is buoyant and becomes warmer and lighter than the surrounding air
if warm air is lifted and becomes surrounded by colder air, it will continue to rise. The colder air aloft will then descend causing an up and down movement in the atmosphere making flight bumpy and unstable
Warm air is less dense and tends to rise. Cold air is denser and tends to descend.
The warmer the air, the more moisture it can hold. As it rises, warm air is cooled by the surrounding air.
A rising water vapour molecule cools at a rate of approximately 1.98 degrees C per 1000 feet.
Rising air eventually reaches a level where its temperature is equal to its dew point and condensation occurs. This leads to the formation of fog and clouds
Indicators of stable air
-Startus-type cloud or fog
-low visibility when dust, smoke haze or fog are present
-steady precipitation
-Consistent, steady winds
-IFR conditions for ceiling and visibility
Indicators of unstable air
-Cumuliform clouds
-Good visibility
-gusty winds
-showery precipitation
-thunderstorms
List and describe the two main categories of turbulence
Friction
- friction turbulence is caused by the restriction of the movement of wind
Thermal
- Thermal turbulence is caused by a rapid change of temperature in the atmosphere
List the types of friction turbulence
Mechanical Turbulence
Shear Turbulence
Frontal Turbulence
List and describe the three factors that affect mechanical turublence
- Stability of the air
The more unstable the air the higher the turbulence will extend
-Roughness of the ground
The rougher the surface, the greater the friction and the greater degree of turbulence
Strength of the wind
- The stronger the wind, the greater the friction and the greater the degree of turbulence
Shear Turbulence
Shear turbulence is the result of friction between opposing air currents
This type of turbulence occurs when there is a strong WIND SHEAR, which is a drastic change in wind speed, direction, or both over a relatively short distance
Frontal Turbulence
Frontal turbulence is the result of friction between the two opposing air masses near the frontal surface
This turbulence is most noticeable hen the lifted warm air is moist and unstable. It becomes severe if thunderstorms develop
Recall that turbulence is more commonly associated with cold fronts, but it may be present to a lesser degree in warm fronts as well
Thermal Turbulence: List two types
Daytime convection turbulence
Cold air advection turbulence
Thermal turbulence: Daytime Convection Turbulence
What is it? And what factors contribute to its strength
Daytime convection turbulence is the result of vertical currents cause by unequal heating of the earth’s surface
The strength, extent, and distribution of convection currents are affected by:
- The stability of the air and the degree of difference in unequal heating
- Variations in heating throughout the day and across seasons
THERMAL TURBULENCE IS MOST PRONOUNCED IN THE SUMMER AND IN THE AFTERNOON
Variations in the composition of the surface result in uneven heating of the air near the ground. Eg, land heats up faster than water.
Similarly, barren surfaces such as sandy or rocky wasteland and ploughed fields heat faster than ground covered by grass or other vegetation
Uneven heating of the air near the ground causes convection currents to vary in strength within short distances
Thermal Turbulence: Cold Air Advection Turbulence
When cold air moves over warmer water or land, heating from below creates unstable conditions that favour convection currents and turbulence
The stronger the contrast between the air and suface temperatures, the more heat will be generated and the stronger the turbulence
List the three intensities of Turbulence
Light
Moderate
Severe
Light turbulence
Light turbulence momentarily causes slight, erratic changes in the altitude and attitude of an aircraft
Occupants of an aircraft may feel a slight strain against their seat belts. Unsecured objects may be slightly displace
Moderate Turbulence
Moderate turbulence is a little more intense than light turbulence. Changes in altitude and attitude occur, but the aircraft remains in control at all times
Moderate turbulence may cause vatiations in indicated airspeed
Occupants will feel a definate strain against their seatbelts and unsecured objects may be noticeably disloged
Severe turbulence
Severe turbulence causes large and abrupt changes in aircraft altitude and attitude as well as large variations in airspeed
The aircraft may be momentarily out of control
Occupants of the aircraft will be forced violently against their seatbelts. Unsecured objects will be thrown around. The aircraft will need to change its heading or its altitude, or both
Factors of intensities of turbulence
Turbulence intensities are dependent on the initiating lifting agent and the degree of stability of the air
UNSTABLE AIR WILL ALWAYS BE MORE TURBULENT THAN STABLE AIR
Dowbursts
A downburst is a severe downward rush of air with outbursts of damaging winds
Downbursts are a major hazard in aviation and have led to many aircraft accidents
KNOW THE TYPES AND VERTICAL AND HORIZONTAL SPEEDS
What are the two classifications of downburst?
Macroburst
Microburst
Macroburst
A macroburst is a large downburst
-horizontal diameter of 4km or more
-horizontal wind speeds up to 140 knots
-damaging winds can last from 5 to 20 minutes
Microburst
Microburst
-a downburst of less than 4km diameter
-while duration is short, typically less than 5 minutes, they can cause horizontal wind speeds more than 140 knots at tree top level
-downdrafts can have a vertical speed approach 6000 feet per minute (60 knots)
-the downdraft shaft can have a diameter of up to 6000 feet or 1 nm
-multiple microbursts are common
When the microburst hits the ground, the wind will go in all directions, just like if you take a garden hose and point it downwards.
The exact nature of wind associated with a particular microburst is virtually impossible to predict
List the two classifications of microbursts
Wet (with precipitation)
Dry (no precipitation)
Wet microburst
A wet microburst occurs when dry air penetrates saturated air leading to rapid evaporation and cooling of the air. Cool air and precipitation descend
Dry microburst
A dry microburst occurs when precipitation evaporates and the air cools and descends
What are the precursors to microbursts?
The presence of VIRGA (precipitation falling but not reaching the ground) associated with TCUs and CBs is often a precursor of microbursts
Virga evaporates below the cloud and, as a result of the heat required for evaporation, the air in these regions becomes colder than its environment, sinking rapidly and accelerating downwards
Evolution of microbursts over time
Wind Shear
Wind shear is characterized as the sudden “tearing” or “shearing” effect encountered along the edge of a zone in which there is a significant change in wind speed, direction, or both, over a short distance
The effect of wind shear will vary depending on the direction and the speed of the wind
The result will be either increased or decreased aircraft performance
Aircraft land and take off with their nose into the wind, so a change in the direction or speed of the wind could require an aircraft to take longer to depart or to land
Wind shear exists in areas of temperature inversions where there is an increase in temperature with height or altitude
It also exists along troughs and lows, and around jet streams.
Wind shear can exist in a horizontal or vertical direction and occassionally in both. It can also occur at low or high altitudes
List the two types of wind shear
Directional Sheer
Speed Sheer
Windsheer: Directional Sheer
Windsheer: Speed Sheer
Where is wind sheer found?
Inflight and near the ground
In flight, wind shear is found….
- in the lower 3000 feet
- at frontal surfaces
- around jet streams
Near the ground, wind shear is found….
- at frontal surface
- with thunderstorms
- when there are temperature inversions
- around physical obstructions like buildings and hills
Wind shear can be classified as….
High-level or low-level
High-level wind sheer (HLWS)
tubulence is encountered at higher altitudes and is mainly associated with the jet stream
If the wind shear—when entering or leaving the jet stream— is quite violent, the turbulence caused by the wind shear is violent
This violent turbulence is known as CLEAR AIR TURBULENCE (CAT)
Clear Air Turbulence (definition)
CAT is sudden, severe turbulence that occurs without any visual warnings (such as clouds)
It is caused by the meeting of air masses moving at very different speeds
Low level wind shear (LLWS)
Low-level wind shear (LLWS) is hazardous because of the proximity of the aircraft to the ground
LLWS is characterized by wind direction changes that could go as far as 180 degrees and speed changes up to 50 knots
It is commonly associated with passing frontal systems, thunderstorms, and temperature inversions with strong upper level winds (greater than 25 knots)
LLWS is not a daily occurence – in fact, it is unusual. That makes it more of a problem. It is a MAJOR cause of accidents during final approach
Effects of LLWS on an Aircraft
Lateral (directional sheer)
aircraft on approach crosses a warm front, wind ahead of front 040 degrees, behind 220 degrees
*** this lateral sheer causes aircraft to drift rapidly to the left, requiring a large correction to the right near the runway
***This is a dangerous situation and could be even more dangerous in conditions of poor visibility
Effects of LLWS on aircraft
Tailwind to Headwind
Aircraft is above frontal surface in tailwind conditions and has developed a rapid ground speed equal to airspeed plus wind speed. The aircraft passes through the frontal surface and experiences headwind or calm conditions
FAILURE TO SUFFICIENTLY REDUCE POWER PUTS THE PLANE AT RISK OF OVERSHOOTING THE RUNWAY
the aircraft’s momentum will prevent it from immediately slowing to the new ground speed
-airspeed will initially increase
-the aircraft will pitch up due to increased lift
-attitude will increase, causing the aircraft to be above the glide path
To correct, the pilot must drop the nose and reduce the thrust to regain the glide path and reduce airspeed. The aircraft may be too high to land safely
HOWEVER!
Very soon after making the initial corrections, the momentum developed during tailwind will fall off
-airspeed will decrease
-lift will decrease
-an immediate increase in power will be required
FAILURE TO CORRECT IN TIME WOULD RESULT IN A HARD LANDING SHORT OF THE RUNWAY OR CRASH
Effects of LLWS on aircraft
Headwind to Tailwind
final approach commences in headwind and a tailwind or calm is encountered when the aircraft passes through the frontal surface
initial effects:
- airspeed decreases
- aircraft nose pitches down
- altitude decreases
aircraft is low and slow (power deficient state) and thrust must be increased initially to resume normal approach speed and regain the glide path
*** IF THE AIRCRAFT IS TOO LOW OR IF THE CORRECTION IS NOT SUFFICIENT OR SOON ENOUGH, A HARD SHORT LANDING OR CRASH COULD RESULT
As the airspeed stabilizes, thrust must be reduced to maintain normal approach speed otherise the aircraft will soon be “high and fast” and may not be able to stop in the available runway length
When a pilot of an arriving aircraft is made aware of wind shear, they may choose to….
- proceed to another destination
- hold and wait for the front or CB to pass
- request approach to another runway or adjacent airports
Low level wind shear, departing aircraft
LLWS may be dangerous to departing aircraft during climb-out
In the image, the turn after takeoff exposes the aircraft to a rapid change from headwind to tailwind conditions. The immediate loss of airspeed and altitude could be fatal
when wind shear is suspected during departure, the pilot should determine the direction for climb-out to avoid unfavourable conditions and request an appropriate ATC clearance (if required). if it is not possible to avoid an encounter with rapidly decreasing headwinds (or worse a tailwind) the pilot may have to delay the flight
Chop vs turbulence
Tubulence causes changes in altitude or attitude, Chop causes slight, rapid, rhythmic bumps and does not cause an appreciable change in altitude or attitude
While chop and turbulence may be reported as light and moderate, only turbulence is reported as severe
(see chart)
Clear Air Trubulence
CAT occurs when a fast-moving jet stream and a slow moving jet stream pass alongside each other. The air between those streams is disturbed, and an aircraft flying through that area will experience very bumpy conditions
When we use the term CAT we normally mean jet-stream wind-shear turbulence however, it is possible for CAT to occur due to other causes like wake turbulence, convection currents and irregular terrain
CAT usually occurs at altitudes of 20 000 to 40 000 feet. At lower altitudes it may occur near mountain ranges. It is most severe near or just above and below the jet stream core
CAT IS ESPECIALLY HAZARDOUS BECAUSE PILOTS CANNOT SEE IT AND RADAR CANNOT DETECT IT
CAT occurs without warning in clear air that is closely associated with the jet stream in a mostly cloudless sky
What you should remember about clear air turbulence:
- it is characterized by a rapid change of wind direction over a short distance
- it occurs in patches
- its area is elongated by the wind
- it usually occurs at altitudes of 20 000 feet to 40 000 feet but can be encountered at altitudes as low as 15 000 feet
- it is associated with a marked change in wind speed over a short distance in the vertical (vertical sheer) or in the horizontal (horizontal sheer)
- there are no clouds present at the altitudes at which it occurs
- it is transitory
- it occurs most frequency during winter and least frequently during summer
What do pilots have to provide in a turbulence report?
POSITION
TIME
ALTITUDE
TYPE OF AIRCRAFT
TYPE OF TURBULENCE AND INTENSITY
(Type of turbulence = turbulence, chop, CAT)