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
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
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
Evolution of microbursts over time
Windsheer: Directional Sheer
Windsheer: Speed Sheer
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)
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
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)
