WindShear & Microburst
In simple terms wind shear juz refers to a change in wind speed and/or direction over a relatively short distance/height in the atmosphere.
It happens all the time in nature, near fronts, cyclones, Jet Streams, Thunderstorms, within clouds etc.
WindShears pose a very unfavorable experience while flying due to the reason illustrated below:
Flight Path through Wind Shear
Just imagine that the Black arrow in the picture is the flight path of an aircraft. Imagine the effects you would feel within the cockpit. Just before entering the Shear Zone the aircraft experiences a constant headwind providing ample lift. Once the aircraft enters the Shear Zone theres a bit of confusion in the form of Turbulence. However after exiting the Shear Zone the aircraft now experiences a steady Tailwind! You can now picture what will happen to the aircraft, after loosing all that lift due to the steady headwind. Yes! The aircraft would now experience the unexpected Sink!!
We all know when the aircraft would follow the flight path akin to the one showed in the picture – during a descent. As long there is sufficient altitude the effects of WindShear can be countered and the aircraft can be safely brought under control. However imagine the consequences if the Aircraft experiences WindShear while landing! Just hundreds of feet above ground level (AGL)… It could be disastrous! Sure enough many aircrafts have fallen prey to this phenomena which we will look into soon. Its also a similar explanation for aircrafts experiencing WindShear during Take Off! Where the ability to “Lift” off to safety is of utmost importance for a successful flight. The sudden loss of lift during these crucial moments is again – Disastrous!
Now that we have established WindShear occuring very close to ground level in the vicinities of an airfield is of grave danger to Aircrafts, lets move on to when is it more likely to happen.
Thunderstorms! Yes, Thunderstorms bear so many threats to flying that it requires many articles for itself.
Anyways, thats where the Microbursts come in. They are basically powerful gusts of air that rushes downward towards the ground from the base of the clouds. After hitting the ground, and from no where else to go, they basically spread out in all directions causing massive disturbances in the air around the area. Heres something to help visualize them:
There are 2 types of Microbursts: Dry and Wet.
Dry: In simple terms – Its caused by the evaporation of the falling rain when it comes to contact with the dry air underneath, which in turn cools the air (cooling effect of Evaporation). And cool air is heavier than warm air, therefore it rushes down. And its Dry, so hardly anything can be seen except if theres massive dust particles or debris
Wet: This comes with heavy precipitation blowing under the cloud. Its generally visible in the form of a high density globular mass of rain. Its caused by similar process as the Dry one, except that now there is another layer of warm and humid air below the dry air. And when air comes rushing down from the dry process above, it pushes down the warm and humid air further downwards which condenses and causes torrential rains.
Microbursts are further broken down into 3 stages:
The Contact stage: the initial descent of very strong accelerating winds reaching ground within minutes
The Outburst stage: where the winds bursts outwards in all directions after the contact with the ground
The Cushion stage: the curling winds continue to accelerate, while the winds at the ground are cushioned due to surface friction.
Heres some visualization of the different stages:
Heres some illustrations of how they can affect the Flight Path of an Aircraft:
As promised earlier, here are some of the unfortunate events that were caused by this vicious phenomena:
- A Malev Ilyushin II-18, August 28, 1971 @ Copenhagen Airport
- Eastern Air Lines Flight 66, June 24, 1975 @ JFK – Boeing 727-225
- Pan Am Flight 759, July 9, 1982 @ New Orleans International – Boeing 727-235
- Delta Air Lines Flight 191, August 2, 1985 @ DFW International – Lockheed L-1011 TriStar
- Martinair Flight 495, December 21, 1992 @ Faro Airport – DC-10
- USAir Flight 1016, July 2, 1994 @ Charlotte/Douglas International – DC-9
- American Airlines Flight 1420, June 1, 1999 @ Little Rock – MD-82 (only partially due to WindShear), featured on Air Crash Investigation S01E01 – “Racing the Storm”
Lesson Learnt: In the event of ThunderStorms near the Airport, got to be extra cautious! Safety comes first!
