Ch 9 - Turbulence And Windshear Flashcards
Turbulence
Disturbed air whose sudden changes in velocity causes air to move abruptly. Generally, it only affects an aircrafts attitude, not altitude
Better and safer to allow the AC to be disturbed around its mean altitude than trying to fight the constant attitude changes,
Windshear
A serious phenomenon. Substantial changes in wind velocity in the vertical or horizontal plane (can be more than 60kt change in a few hundred metres)
Windshear can abruptly displace an aircraft from its flight path or even put it straight into a stall.
Requires urgent and substantial control inputs to control or regain control of the aircraft.
Most modern transport AC have Windshear alert systems in place.
Turbulence Criteria Table
Light - 5-15kts change in IAS, can continue with food and walking, <0.5g changes experienced at CG, described as light chop
Moderate - 15-25kts change in IAS, can make walking and serving difficult, 0.5-1g experienced at the CG, described as Mod chop, must be reported if found below FL150
Severe - >25kt change in IAS, everyone must be seated and very uncomfortable, can cause temporary loss of control, >1g change experienced at CG, described as only severe turb., also must be reported below FL150
2 Main Types of Turbulence
Mechanical - caused by physical obstructions such as hills, mountains, buildings, trees
- The more uneven the surface and/or the stronger the wind, the deeper and stronger the turbulent layer will be
Thermal - caused by either air rising due to surface heating or in and around cloud formations
- usually experienced on warm days with clear sky’s, indicated by cumulus type clouds which are vertically developed, within and around these, mod-sev turb can be expected
The Frictional Layer and Low Level Turbulence
Most low level turbulence is caused by the interaction between the wind and the earths surface. - The friction/boundary layer (top of this called the friction level)
- Depth depends on thermal and mechanical turbulence
Hot days/ in areas of uneven surface, friction layer will be higher.
Usually 1km above the surface
Wind direction changes result in mild Windshear
Mountain Waves (standing waves / lee waves)
Turbulent undulating waves of air which can form above and downwind of a mountain range.
Can extend 50-100nm downwind and all the way up to the tropopause
Upward and downward movement can exceed 2000ft/min
Europe - over alps
America - over rockies
The conditions which lead to Mountain Waves
Wind speed at mountain height must be at least 15-20kt, increasing with height
Wind direction must be writhing 30’ perpendicular to the range of hills/mountains
There must be a region of marked stability (in inversion/isothermal layer) at mountain top height with less stable air above and below it
Hazards of mountain waves
Most severe turbulence occurs in the rotor zone lying just beneath the crests of the waves (roll clouds) - most powerful beneath the first wave crest
Sever turbulence occurs where the wave breaks - area must be avoided at all costs - particular danger on app road
Mountain waves can extend right up to the tropopause (decreasing severity with alt)
Windshear
A large change in wind direction and/or speed including up draughts and down draughts.
Variations in wind vector along flight path
Intensity and duration will displace an AC abruptly from its intended lifts path - must be corrected
Low Altitude Windshear
Windshear along the final approach path or along the runway and along the TO and initial climb out path
Vertical Windshear
The change of horizontal wind vector with height, typically measured in knots per 100ft
Determined by 2 or more anemometers at different heights
Horizontal Windshear
The change of the horizontal wind vector with horizontal distance. Typically measured in knots per 1000ft
determined by 2 or more anemometers mounted at same heigh along a RW
Updraught or Downdraught Shear
The changes in the vertical component of wind vector with horizontal distance.
Updraughts in TS can reach 10,000ft/min
Countermeasure Against Windshear
AVOID ENTERING IT
Causes of Windshear
Most dangerous; Thunderstorms (anyside around)
Passage of a front
Strong temperature inversion
Strong low-level wind
Turbulent boundary layer
Terrain and buildings can also create localised windshear (hangers near a RW) (TOPOGRAPHICAL)
Frontal Passage; Windshear
Well developed active fronts with narrow surface frontal zones and marked temperature differences carry the risk of windshear
Warning signs; sharp change in wind direction indicated on surface charts
- Temperature difference of 5C or more across the frontal zone
- Frontal movement exceeding 30kt or more
Inversions: Windshear
MTI’s (marked temperature inversions) - more than 10C per 1000ft
- Commonly associated with windshear
- Easy to spot
- Turbulence and Windshear experienced when flying through the boundary level
- Can be a hazard for AC Taking off or landing
The Effects of Windshear on an AC in flight
From 40kt -> 5kt decreases wind speed over wing by 35 kts which means that lift decreases and there fore you may drop in alt
Reverse is true
From 4kt ->40kt - increase in 35kt which increases lift produced which may cause you to rise above where you were before
Downdraughts
Can cause really violent, substantial windshear which causes the RAF over the wing to change;
- Airspeed changes
- Wings angle of attack changes
Downdraught = sink
Updraught = rise
Microbursts
Associated with intense thunderstorms. If flying through, you are likely to experience all forms of Windshear
If warned, abort your approach/TO (delay)
Jet Stream Turbulence and Windshear
Jet streams have eddies and shear lines forming around them, shear lines are created when there is a marked change in wind speed and direction - worst adjacent to the cold air - where the greatest CAT (clear air turbulence) is
Turbulence is most severe; with stronger winds, curved jet streams, above and to the lee of very high mountain ranges, in the primary area for CAT (cold side, at or below), with developing and rapidly moving jets
Countermeasures
Approach - increase speed, raise nose to check descent, co-ordinate power and pitch, be prepared to carry out a missed approach
TO - Delay until the possibility has diminished
- Recognise threat quickly and commit to the appropriate action, follow ops manual or AC flight manual techniques
- Use max power ASAP
- Adopts appropriate pitch angle and try and hold it, do not chase the airspeed
- Be guided by stall warners
