View Full Version : Should I ask...?
07-30-2002, 04:32 AM
What is wind shear? How does it affect aircraft?
07-30-2002, 05:04 AM
I'm no expert, but I believe it occurs when you have 2 wind directions at different altitudes which are moving in opposition to each other. This can create a problem when the aircraft passes through the junction of these layers as it reduces lift sometimes drastically, causing the aircraft to "drop", possibly several hundred or even thousand feet. It has caused some crashes to due airframe damage when the aircraft hits the second layer.
07-30-2002, 05:20 AM
I dont know how it is caused but isnt it when wind is pushing down on an aircraft???
07-30-2002, 07:13 AM
I think that you prefer to fly the big iron and it's usually a bit less serious with them -unless you are flying at critical situations such as landing or T/O where you are near a stall speed. In FS2002 I'v seen a situation where I was climbing through a few wind layers in a light plane and the wind essentially changed directions from 35 kts out of 12:00 to 35 kts or so out of 6:00. This causes an effective immediate change in airspeed (not ground speed) of about a negative 70 kts. This causes a plane that cruises at about 100kts to then be flying at 30kts. oops... I should say falling at 30 kts.
The wind speed differential can be that dramatic occasionally in real life , or more so but I think that it happens always in FS2002 as there's no gradual change in wind speed in the default weather DL. It becomes more noticeable when the wind takes a sudden 180 deg. turn or near mountainious areas.
One can also get problems in real life from wake turbulence from crossing behind the larger jets if not careful. This is similar to wind shear.
07-30-2002, 07:49 AM
In convective clouds, you have updrafts and downdrafts. In the
developing stages of a convective shower/thunderstorm, mostly
updrafts are present. During the mature stage, you have both.
During the dissipating or decaying stage, mainly downdrafts.
Downdrafts are extremely dangerous to aircraft mainly during
landings and takeoffs. I'll try and do a little for instance
here. Lets say you are departing on runway 9. There is a
thunderstorm a mile or two east of runway 9. Downdrafts coming
out of the base of the thunderstorm are causing the wind to be
westerly. When you get more directly under the storm, winds are
more downward, and as you progress more to the east, winds then
become easterly. This downward and then easterly wind takes away
the lift from your aircraft. Hope my explanation helps.
07-30-2002, 07:53 AM
Forgot to mention in my above statement.
Wind shear is basically winds that change direction over a short
distance, which is what you have in the above scenario. You also
have wind shear in other instances but thunderstorms probably
the most pronounced.
07-30-2002, 08:17 AM
If you have ever seen a fast moving stream that runs into another stream at an angle and seen the water churn, that is the liquid version of wind shear. Its the boundry where two different layers of air are comming together and mixing. Not only can wind direction or velocity cause it, but abrupt temperature differences can create shear.
Sometimes if you look up at the clouds and they look like someone plowed them into a series of regular closely spaced furrows you are looking the results of wind shear.
If you could see the air that you fly in, most people would be too frightened to fly.
07-30-2002, 08:41 AM
LAST EDITED ON Jul-30-02 AT 08:42AM (EDT)[p]Here is my view of it. Very rarely is it a problem on takeoff. Generally planes don't takeoff into thunderstorms. Yes I know you may have, but generally they don't. Know landing is different. Planes almost always down here in florida dodge thunderstorms when landing. What happens is when landing lets say the plane is on final for runway 27 (since most storms is florida come from the west). As the plane descends a downdraft from behind it pushes the plane down and takes away most of the lifting pressures from the wing.
As a side note this summer I was flying from Nashville, Tenn. to Bangor, Maine. The pilot tells us about 60 miles out from bangor that the airport is socked in with thunderstorms and the only way in is either fly 250 miles west and around the airport and come in from the north or climb to fl390 and descend over the airport. So since we didn;t have enough fuel for option one we descend over the airport. And let me tell you when you fly anywhere near thunderstorms the downdrafts play havoc on the planes. So to make a long story short. He went up to 360 hit the spoilers and basically put us in nose dive. I was really impressed with that young comair pilot.
07-30-2002, 08:54 AM
Windshear has nothing directly to do with thunderstorms, although it is often associated with it by virtue of the weather systems that thunderstorm activity bring with them.
There are four common sources of low level wind shear: Frontal activity, thunderstorms, temperature inversions, and surface obstructions.
`Wind shear` is simply a generic description for a change in wind speed and/or direction over a short distance. It can occur either horizontally or vertically and is most often associated with strong temperature inversions or density gradients.
In its extreme forms windshear will cause even the bggest, safest airliner to crash, or climb & lose altitude at a fantastic rate. It is a major source of airliner fatalities in the last few years, and is possibly one of the factors in otherwise `inexplicable` accidents and crashes.
07-30-2002, 08:54 AM
Here is a good link to explanation of wind shear and it's effect on airplanes.
07-30-2002, 09:30 AM
LAST EDITED ON Jul-30-02 AT 09:34AM (EDT)[p]Wind shear is caused by two adjacent layers of air moving at considerably different speeds and, often, in different directions.
In normal (mild) conditions, the wind speed and direction change gradually with altitude. But occasionally (and commonly around some storms) there is a rather abrupt change of speed and/or direction within a very few feet of altitude change. This is wind shear.
If the aircraft climbs or descends into a suddenly increasing headwind (or decreasing tailwind), its airspeed increases considerably (by almost the amount of the change in wind) almost instantly, and the aircraft has an abrupt increase in its rate of climb (or decrease in rate of descent). As inertia is gradually overcome, the airspeed and rate of climb decrease to approximately what they were before the shear was encountered. This effect is usually not particularly dangerous, though it can sure throw off the pilot's planning if it is on short final, and a go-around may be needed if the change results in not being able to reasonably make the runway.
But if the aircraft climbs or descends into a suddenly decreasing headwind (or increasing tailwind), the exact opposite happens; the airspeed suddenly decreases, and the rate of climb decreases (or rate of descent increases) drastically. If this happens on short final, or even just at low altitudes, especially at approach speeds, it can be very dangerous, because it may take quite a bit of altitude loss to regain the airspeed and stabilize the rate of descent. This is especially bad in larger aircraft at low speeds because they have so much inertia that it takes a while to accelerate. A lighter aircraft recovers more quickly, since it has less inertia, though it can still be a problem in many situations.
Up and down drafts CAN have similar effects, but they are usually short lived, since you quickly fly past the effect (or encounter the opposite effect), where with wind shear the aircraft must recover without benefit of leaving the new batch of air.
Microbursts are only down -- a rapidly descending column of air, usually from a thunderstorm (often a dry one), and it covers a much larger area (and usually is more severe) than an ordinary downdraft, and it usually goes all the way to the ground. It is as usually more dangerous than even a strong wind shear, since it covers a broad area and the air itself, which supports the aircraft, is descending rapidly, unlike the wind shear which is only a speed change. And once this column of air hits the ground, it has to go somewhere, so it rushes out in all directions, usually generating low-level wind shear all the way around it for several miles. This is very often the cause of the wind shear that you hear about in the news.
Due to inertia, larger aircraft don't get tossed around as much by wind shear or up/down drafts, but they take longer to recover from the effects. Light aircraft get tossed around quite a bit, but recover quickly.
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