View Full Version : Wind shear
03-16-2002, 04:01 PM
Hi, can any anybody explain wind shear and any experiance.Thanks,
03-16-2002, 04:43 PM
The air currents in the atmosphere act, usually on a larger scale, much like the water in lakes or oceans. This means that you might have air moving up or down or at different speeds and/or directions at different levels, or all of the above.
Wind shear specifically refers to the area where two different "rivers" of air meet, usually one above the other, such that as an aircraft climbs or descends it encounters changes in the relative wind speed and/or direction, thus causing changes in airspeed, rate of climb, etc. It even will often cause turbulence.
Climbing or descending into air that has less headwind (or more tailwind) component will cause a loss of airspeed (the greater the differential, the more loss) and a consequent reduction in rate of climb or increase in rate of descent. If the aircraft is very slow (near stall) then even small changes can be hazardous, as the aircraft may stall. A larger aircraft is usually more at hazard than a smaller one, especially at low altitudes, because it has a lot more inertia, thus takes longer to accelerate back to the desired airspeed.
Going into more headwind/less tailwind reverses these effects, usually making it a less hazardous encounter.
Hope this helps.
GPWS2000 used to have a wind-sheer warning, which was great. Never seen it anywhere since.
I think it's too much for programmers.
Peter Sydney Australia
03-17-2002, 09:32 AM
Well, Larry covered the theory very well, as usual. As for experience, I encounter mild windshear quite often when coming in to land at my local airport. There are ~200 ft trees lining the north side of the airport and the prevailing winds here (VDF, Tampa, FL) are usually from the NE. If the winds are anything but calm, you can expect a nice little windshear and turbulence at around 100-300 ft AGL on final approach. The winds above the trees are stronger than those below the tree line and they are churning. Early on as a student, it would scare the heck out of me. However, I've learned to expect it and prepare for it. Occassionally, it will be especially strong and coupled with a canal several hundred feet off the end of the runway that on hot days will cause a little loss in lift (due to thermal difference between the water and the surrounding land), it can easily catch you off guard. I lost about 50-70 feet on one approach about a month ago. Doesn't sound like much, except I was only 200 feet AGL. That greatly increased the "pucker" factor on that approach. Almost sucked the entire seat inside me. :-lol After a go-around, I prepared for it on the second approach (increased speed and elevation) and made a good landing.
Windshear above temperature inversions can also be interesting. The standard rule of thumb states that if there is a temperature inversion with rather calm surface winds and winds in excess of 25 kts between 2000 and 4000 feet AGL, expect windshear and turbulence where the cool air and warm air mix. Well, on a cool Florida evening, I got to experience that first hand. However, my CFI knew it was there and instructed me accordingly. Because I was aware of it, I wasn't taken by surprise. It's nice when what you've read about and been taught in ground school is demonstrated in real life. Makes those concepts easier to remember, and makes you more aware of their possibilities in the future. In short, it makes you a better pilot.
Windshear can be very dangerous, or just annoying. It depends on the severity of the windshear, your current flight situation (landing, taking off), and your awareness of the surrounding conditions. A novice pilot is surprised when things go wrong. A pro is surprised when they don't. (I stole that from Rod Machado)
03-17-2002, 10:05 AM
Sometimes when you have wind shear conditions, it's also ripe for microbursts. A micro burst is a bubble of cold air that gets trapped above a warm air layer and then finds a spot to come down to the surface. What you end up with is a tornado sized column of air moving straight down at 50-150 mph with extreme vertical wind shear at the sides. That's what brought down that 737(?) at Dallas about 20 years ago, and I saw the results of one at Waukegan, Illinois about 6 years ago. It tossed around parked airplanes and took roofs off of homes for a radius of over 1 mile.
03-17-2002, 10:43 AM
What Silverblade describes above happens very often. Wind shear
detectors (wind measuring equipment) have been placed around
many airports in the U.S. and this has apparently cut down on
aircraft accidents caused by this weather occurence.
One I remember was back in the late sixties. If my memory is
correct, it was a Pan American B-727 or 737 departing runway
10 at KMSY. About a mile or so out of KMSY it was literally
thrown into the ground killing all on board and about a dozen
or so on the ground. It happened about 2 blocks from where a
dear friend of mine lived.
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