Wind Shear: Types and Causes Flashcards
Wind Shear
● Wind Shear:
○ A sudden wind speed and/or wind direction change over a short distance.
○ Wind Shear can be both vertical and horizontal.
● Several significant aviation accidents have been caused by low-level wind shear.
○ This is because wind shear induces sudden changes in the angle of attack and/or the airspeed of the aircraft.
○ In turn, this causes a temporary change in the amount of lift that we are generating
● Although windshear has an effect on aircraft of all sizes, in some situations, large aircraft may be more susceptible!
○ As you will see, this is because they have more inertia, and they are unable to respond as quickly to the new zone of air that they are encountering.
● Generally, a shear of 30 knots or greater is impossible for a small general aviation aircraft to handle.
○ A shear of 45 knots is considered the limit for large commercial aircraft.
● Wind shear is most dangerous to a pilot when it happens at low altitudes.
○ Aircraft are particularly vulnerable when they are in the approach, landing, and takeoff phases of flying.
Wind Shear: Two Basic Types
● That being the case, it is helpful to think of them as coming in one of two basic types:
● Speed shear
○ Wind that is blowing at different speeds will create a zone of wind shear.
● Directional shear
○ Wind that is blowing in different directions also creates shear
● It is important to remember that the two types of shear discussed in the previous slide can happen in either the vertical or horizontal components.
● In many cases, it may be occurring in both simultaneously!
● In the aerodynamic discussions that follow, it will often be helpful to simplify these effects by considering a shear encounter as mostly–or only–occurring in either the horizontal or vertical direction
Wind Shear: Turbulence
● Since shear will often create eddies between these differing zones of wind speed and/or wind direction, we sometimes find that turbulence is associated with it.
○ However, it is important to remember that turbulence is not always associated with an area of shear!
○ Some types of shear–such as low level jets–are very good at maintaining a smooth flow throughout the shear layer.
○ It may be surprising to learn that extremely stable air can keep the flow fairly laminar–and of course, that would mean free of turbulence.
● Turbulence is not always a reliable indicator in telling us about the presence of shear
Effects: Downdrafts / Updrafts
● Wind shear can be due to downdrafts and updrafts or sharp change of wind speed and direction in the horizontal plane.
○ This of course, would be in the case of Horizontal Wind Shear.
● Shear like this will mostly influence the angle of attack.
○ The lift equation tells us that if we change our angle of attack, we change the amount of lift.
● During a downdraft:
○ Our angle of attack will decrease
○ Lift will decrease
● During an updraft:
○ Our angle of attack will increase
○ Lift will increase
○ In extreme cases, this could result in a stall
Wide Shear effects
● Wind speed often varies with the height above ground.
○ Generally, the higher we go, the faster the flow.
○ With sharp changes in the wind speed and direction, this would be Vertical Wind Shear.
● Wind shear has the greatest effect on our airspeed.
○ It will suddenly–though temporarily–change.
● Once again, if we look to the lift equation L=.5CLp(V^2)S, we are reminded that the amount of lift that we generate is related to our airspeed.
Wind Shear: Increasing Performance
● IAS will increase because the speed of the air will temporarily have a greater dynamic pressure within the pitot tube
● This happens during an increasing headwind or a decreasing tailwind or updraft
● Lift will increase
● The aircraft will likely pitch up–at least initially
● If it is encountered on final, the aircraft may rise above glide path and a long landing or overrun may occur
● Meanwhile, the ground speed will become slower.
○ This increased performance leads to an increase in gradient of climb out over obstacles on the ground
Wind Shear: Decreasing Performance
● IAS will decrease
● This is due to a decreasing headwind or increasing tailwind
● Lift will decrease
● Aircraft will pitch down–at least initially
● A decrease in headwind (or increase in tailwind) on approach will cause the airspeed to decrease causing the aircraft to experience a loss of performance and tend to sink and undershoot the aiming point.
● If this shear is encountered on final aircraft, the aircraft may drop below the glide path and a short landing may occur
Shear and Aerodynamics
● In addition, the sudden change in direction will have aerodynamic effects.
● The airplane will want to weathervane into the “new” wind.
● We may also be suddenly pushed one way or the other
Low Level Wind Shear
● Causes of Low Level Windshear:
○ Nocturnal Inversions
○ Low Level Jet Streams
○ Frontal Systems
○ Coastal Shear
○ Mountains
■ Ridge Crests
■ Lee Side Vertical Shear
■ Rotors
○ Thunderstorms
■ Updrafts
■ Downbursts
■ Microbursts
High Level Wind Shear
● Causes of High Level Windshear:
○ Jetstreams
○ Mountain Waves
Wind Shear－Low Level
The following guidelines are used to establish, report, and forecast significant non-convective wind shear:
○ A wind speed change exceeding 25 KT within 500 feet AGL;
○ A wind speed change exceeding 40 KT within 1000 feet AGL;
○ Wind changing by 50 KT within 1500 feet AGL;
○ A pilot reporting a loss or gain of IAS of 20 KT or more within 1500 feet AGL
Low Level Nocturnal Inversion
● As night falls winds aloft become uncoupled from the winds at the surface due to a reduction in vertical currents.
● If a strong temperature inversion is thrown into the mix, this can lead to much greater speeds aloft than at the surface
● This presents hazardous wind shear on approach and departure
Wind Shear - Nocturnal Inversion
● Maximum wind varies from about 700 to 2 000 feet AGL
Wind Shear－Low Level Jet
● These can be created by extreme low level nocturnal jets over the prairies.
● They can also be due to extremely fast and long ribbons of cold or warm air that organize in an air mass and move with fronts
● Such a jet will be included on the GFA icing, turbulence and freezing level chart when it is expected to have a peak core speed of 50 kt or more.
● It may also be included at speeds between 35 and 45 kt when significant associated turbulence or shear is expected
Frontal Shear
● Frontal wind shear tends to happen with fronts that have steep wind gradients.
● Fronts with temperature changes of 5 degrees or more may cause this to happen.
● Another mechanism is a front that is moving at high speed, typically 30 knots or more.
● Cold fronts are most likely candidates to create shear conditions on us.
● The effects of cold front windshear shear usually last one to three hours after passage
● Warm Fronts are also capable of creating these!
● Shear from a warm front can happen up to six hours prior to the passage of the front, and can last up to an hour afterwards.
● Warm front slopes can create significant challenges on the approach
○ They are very shallow, so our glideslope, being steeper than them will cross through the frontal boundary.
● Although all pilots should exercise caution, the IFR pilot must be the most aware!
○ Add to this the frequently low ceilings and the poor visibility associated with a warm front, and you have difficulty
Coastal Shear
● If one remembers that the winds are typically much stronger over water (remember–less friction!) than over land, one can see that shear can be a problem for airports near a coastal area.
● Don’t forget, of course, that winds will also change their direction as they encounter land friction.
○ Someone flying from the land to water would see the winds increase and veer
Mountains: Ridge Crests
● As the air flows over a mountain, the venturi effect will cause the winds at the top of the ridge to flow much faster than those below
Mountains: Lee Side Vertical Shear
● As the air flows over the top, and down into the lee side of the mountain, extreme vertical shear similar to a waterfall can develop.
● The size and depth of this shear can depend on the ridge itself.
○ A sharp ridge is also more likely to create turbulence on the lee side
Mountains: Rotor Clouds
● During a mountain wave scenario, rotor clouds may form. The winds aloft can be very different from those at the surface.
● These rotor clouds will also create areas of strong vertical shear
Thunderstorms
● Thunderstorms can create all kinds of shear.
○ First off, there is the gust front.
○ We can also think of the updrafts and downdrafts that they contain.
● There are many, many ways that thunderstorms can create shear.
○ Two of the worst for us during the takeoff and landing phases are the downdraft and the microburst
Downbursts and Microbursts
● Downbursts are induced by rainshafts and/or cold shafts of air that descend rapidly towards the ground.
○ These spread outwards upon reaching the surface causing both vertical and horizontal wind shear
Microbursts
● Microbursts-are smaller scale, but more intense downbursts causing vertical winds up to 6000 ft per min and horizontal winds up to 45kts.
○ They are frequently accompanied by heavy rain.
● At 2000 AGL, they are approximately 1 NM in diameter.
○ This will spread out to approximately 2-2½ NM at the surface
● The life-cycle of a microburst from the initial downburst to dissipation will seldom be longer than 15 minutes.
○ Maximum intensity winds typically last about 2-4 minutes.
● Sometimes microbursts are concentrated into a line structure and under these conditions, activity may continue for as long as an hour.
○ Once microburst activity starts, multiple microbursts in the same general area are common and should be expected
Dry or Virga Induced Microbursts
● Virga is rain that evaporates prior to reaching the ground, causing downdrafts.
○ This falling air will be dragging cold dense air along with it.
○ Should this rainfall fall into a layer of drier air below it, it will evaporate.
● Since the process of evaporation takes away heat from the atmosphere, the cold air that was descending with the rain shaft will cool even further, and become more dense.
○ This will cause the air to accelerate downwards, increasing both the speed and forcefulness of the air
Wind Shear－Jet Streams
● Near a Jet core, the winds change significantly.
● Vertical shear may be 5 knots per 1000 feet, to extremes of 20 knots per 1000 feet.
Reactive Wind Shear Systems
● Aircraft equipped with Reactive Wind Shear Systems (RWSs) can provide pilots with guidance to conduct a wind shear escape manoeuvre.
● These systems sense the changes in the wind velocities surrounding the aircraft, and the airplane’s inertia.
○ They will then issue an alert when the airplane is in a wind shear scenario.
● One disadvantage to these systems is that the shear must be occurring in order for them to register that something is wrong
Predictive Wind Shear Systems
● Aircraft with Predictive Wind Shear Systems (PWSs) may allow pilots to avoid or to minimize the effects of wind shear.
● Predictive Wind Shear systems are based upon doppler radar information that detects the wind speeds ahead of the aircraft.
○ These systems are reliable for altitudes anywhere from 50 feet to 1000 feet above the ground.
○ It will warn the pilot approximately one minute ahead of the encounter that wind shear is immanent.
● Should the conditions either worsen, or the aircraft get closer to the area of shear, a warning will alert the pilot of the shear, and to go around
