WINDSHEAR

1. WINDSHEAR Lt Col Wally Emerson 15 April 2007

2. WHY STUDY WINDSHEAR? 744 Dead 287 Injured CONUS Only Since 1964

3. OBJECTIVE Emphasize that the best defense against the hazards of low altitude windshear is avoidance. Give you key information to assist in recognizing and avoiding windshear. Discuss some precautions and techniques for improving chances of surviving an inadvertent windshear encounter.

4. OBJECTIVE Emphasize that the best defense against the hazards of low altitude windshear is avoidance. Give you key information to assist in recognizing and avoiding windshear. Discuss some precautions and techniques for improving chances of surviving an inadvertent windshear encounter.

5. OBJECTIVE Emphasize that the best defense against the hazards of low altitude windshear is avoidance. Give you key information to assist in recognizing and avoiding windshear. Discuss some precautions and techniques for improving chances of surviving an inadvertent windshear encounter.

6. OVERVIEW 1) Windshear weather, particularly microbursts, and clues which may indicate its presence, 2) Effects of windshear on airplanes 3) Windshear recognition cockpit, and avoidance, 4) Precautions to take when windshear is suspected, 5) Standard operating techniques related to windshear 6) Recovery techniques to an inadvertent windshear encounter.

7. OVERVIEW 1) Windshear weather, particularly microbursts, and clues which may indicate its presence, 2) Effects of windshear on airplanes 3) Windshear recognition cockpit, and avoidance, 4) Precautions to take when windshear is suspected, 5) Standard operating techniques related to windshear 6) Recovery techniques to an inadvertent windshear encounter.

8. OVERVIEW 1) Windshear weather, particularly microbursts, and clues which may indicate its presence, 2) Effects of windshear on airplanes 3) Windshear recognition cockpit, and avoidance, 4) Precautions to take when windshear is suspected, 5) Standard operating techniques related to windshear 6) Recovery techniques to an inadvertent windshear encounter.

9. OVERVIEW 1) Windshear weather, particularly microbursts, and clues which may indicate its presence, 2) Effects of windshear on airplanes 3) Windshear recognition cockpit, and avoidance, 4) Precautions to take when windshear is suspected, 5) Standard operating techniques related to windshear 6) Recovery techniques to an inadvertent windshear encounter.

10. OVERVIEW 1) Windshear weather, particularly microbursts, and clues which may indicate its presence, 2) Effects of windshear on airplanes 3) Windshear recognition cockpit, and avoidance, 4) Precautions to take when windshear is suspected, 5) Standard operating techniques related to windshear 6) Recovery techniques to an inadvertent windshear encounter.

11. OVERVIEW 1) Windshear weather, particularly microbursts, and clues which may indicate its presence, 2) Effects of windshear on airplanes 3) Windshear recognition cockpit, and avoidance, 4) Precautions to take when windshear is suspected, 5) Standard operating techniques related to windshear 6) Recovery techniques to an inadvertent windshear encounter.

12. WINDSHEARDefinitions

13. WINDSHEAR Windshear is a generic term referring to any rapidly changing wind currents. GP: WIND SHEAR - A change in wind speed and/or wind direction in a short distance, resulting in a tearing or shearing effect. It can exist in a horizontal or vertical direction and occasionally in both.

14. WINDSHEAR • WIND SHEAR • Vertical wind shear is the rate of change of the wind with respect to altitude. • Horizontal wind shear is the rate of change on a horizontal plane.

15. WINDSHEAR • WIND SHEAR • AC 00-54 • Severe Windshear - A rapid change in • wind direction or velocity causing • airspeed changes greater than 15 knots • or vertical speed changes greater than • 500 feet per minute.

16. WINDSHEAR Wind shear can be encountered when flying through a temperature inversion layer. However, the dangerous wind shear we're interested in is generally associated with a frontal system, especially one which includes a thunderstorm.

17. WINDSHEARWeather to look out for

18. WINDSHEAR During a thunderstorm, a large column of cold, dense air rapidly descends to the surface of the earth. This fast moving column can be described as a downburst. Once the downburst reaches the surface of the earth, it expands horizontally in all directions.

19. WINDSHEAR Once the downburts spreads out horizontally, it undercuts the warmer air outside the storm. This mixing of air produces a rolling vortex of wind. The vortex then causes high velocity winds to surge in opposing directions. These strong winds which makes up the gust front are also known as horizontal wind shear.

20. WINDSHEAR "Valley wind shear" is a name for another natural cause of wind shear which results from a temperature inversion. This phenomena begins by the cooling of the air in a valley. This cooling results in a stable air mass on the valley floor. The wind blowing across the top of the mountain pushes air down the mountain slope.

21. WINDSHEAR The wind blowing across the top of the mountain pushes air down the mountain slope. The air experiences heating during this descent. However, when this air encounters the stable air mass on the valley floor, it cannot penetrate it and flows over top of it. This results in a layer of warmer air being pushed out over a layer of colder air, a temperature inversion. Thus a wind shear is developed due to the air flowing over the inversion, and stable air below it.

22. WINDSHEAR As the valley floor air is warmed the next morning, it begins to rise weakly. Air begins moving down the slope of the mountain downwind to replace this rising air. As this continues and increases in strength, it results in a rotary motion. This rotary motion is another form of wind shear.

23. WINDSHEAR Another way that mountains can create wind shear is by turbulence. As the wind blows up one side of a mountain and reaches the top, it can begin to mix turbulently. This turbulence on the lee (downwind) side of the mountain is a form of wind shear. The same effect takes place around the sides of buildings

24. MICROBURSTS “The Granddaddy of all Windshears”

25. Microbursts: Dry or Wet • Wet microbursts are more common in places like the Mississippi valley where thunderstorm bases tend to be much lower. • Dry microbursts occur primarily in the high plains/intermountain's where the temperature/dew point spread is wide (30° to 50°)

26. Downburst/Microburst Definition • A downburst is HEAVY COLD/MOIST air dropping spilling out producing horizontal shears along a damage path of 1-5 miles . • A microburst is a downburst that covers an area less than 3 miles with peak winds that last 2–5 minutes.

27. Why are they important? • Eastern Airlines Flight 66 crashed 24 June 1975 • August 1983 near miss at Andrews AFB • Fast winds with rapidly shifting directions are bad for planes trying to take off or land • There are typically 50–100 downbursts each year during the convective season

28. Downdraft Formation Two main mechanisms: • Evaporation • Cools the air; cold air sinks • Rain that evaporates before reaching the ground is called virga • Cold air can descend as fast as 40–60 m.p.h. • Drag force • Falling precipitation drags air down with it, creating fast descending air • One raindrop is inconsequential but many drops have a large effect on air flow

29. Downburst/Microburst Structure Weak environmental wind field • Downburst is symmetrical • Equal speed/damage on all sides

30. Weak environmental wind field or more stationary the front Aircraft will experience equal headwinds and tailwinds

31. Downburst/Microburst Structure Strong environmental wind field • Asymmetrical • Strongest wind is downwind of stagnation cone • May produce a well-defined “foot” shape to precipitation

32. Strong environmental wind field or fast moving front Aircraft will experience bigger kick on the backside

33. Vortex Ring

34. WINDSHEAR Evolution of a microburst. Microburst winds intensify for about 5 min after ground contact and typically dissipate about 10 to 20 min after ground contact.

35. DRY MICROBURST

36. In this example, air below a cloud base (up to approximately 15,000 feet AGL) is very dry. Precipitation from higher convective clouds falls into low humidity air and evaporates. This evaporative cooling causes the air to plunge downward. As the evaporative cooling process continues, the downdraft accelerates. Pilots are therefore cautioned not to fly beneath convective clouds producing virga conditions.

37. WINDSHEAR • Greatest danger: Takeoff and landing • During landing, the pilot has already reduced engine power and may not have time to increase speed • During takeoff, an aircraft is near stall speed

38. General Windshear, particularly low altitude windshear encounters, are of significant importance because it can place the flight crew in a situation where the maximum performance capability of the aircraft is required. Windshear encounters below 500ft are the most threatening because there is very little time or altitude to respond and recover from an inadvertent encounter.

39. General Knowledge of how windshear affects performance can be essential to a successful recovery maneuver following an inadvertent windshear encounter.

40. General Windshear that improves performance will be first indicated in the cockpit by an increased airspeed. Pilot reaction – reduce thrust. With power back you experience a shear in the reverse sense that will decrease airspeed and degrade vertical flight path performance.

41. General The magnitude of the pitch change is a function of the following  - Aircraft configuration   - Weight   - Speed   - C of G   - Thrust   - Severity of airspeed change

42. General If an attempt is made to regain lost speed by lowering the nose, the combination of decreasing airspeed and decreasing pitch attitude produces a high rate of descent. In some circumstances as little as 5 seconds may be available to recognize and react to a degrading vertical flight path.

43. General • INDICATORS OF WINDSHEAR: • SIGMETS • Visual signs (T-storms, black wall of death, tornados etc..) • Unusual control forces required • Significant changes in airspeed occur • PIREPS

44. General Crew Actions - Take-off Windshear Precautions Maximum thrust should be used If practical the longest suitable runway available should be used, provided it is clear of areas of known windshear.The flight director should not be relied upon during take-off in suspected windshear conditions. The attitude director is the primary reference for pitch attitude.

45. General Crew Actions - Take-off Windshear Precautions Crews should be alert for airspeed fluctuations during take-off and initial climb. Such fluctuations may be the first indication of windshear. Control forces may be different from those expected, especially if airspeed is below the in-trim speed.

46. General Crew Actions - Take-off Windshear Precautions Crew co-ordination and awareness are very important. Close monitoring of the flight instruments is imperative, and the non-handling pilot should be especially aware of these and call out any deviations from normal values.

47. General Crew Actions - Take-off Windshear Precautions The stick shaker must be respected at all times. If it is activated, pitch attitude should be reduced just enough to stop the stick shaker. Flight with intermittent stick shaker operation may be required to maintain a positive rate of climb during a windshear encounter. MAX PERFORMANCE – just in and out of stick shaker

48. Effect on Airplanes Windshear encounter during takeoff after liftoff. • Takeoff initially appears normal. • Windshear encountered just after liftoff. • Airspeed decrease resulted in pitch attitude reduction. • Aircraft crashed off departure end of runway 20 set after liftoff.

49. Effect on Airplanes Windshear encounter during takeoff on runway. • Takeoff initially appeared normal. • Airspeed buildup slowed due to windshear. • Airplane reached VR near end of runway, lifted off but failed to climb. • Airplane contacted obstacle off departure end of runway.

50. General Crew Actions - Approach and Landing Windshear Large thrust reductions or trim changes in response to a sudden airspeed increase should be avoided as these may be followed by a decrease in airspeed.