Aircraft safety is a major concern in aviation; in General Aviation, Military and Commercial Aviation, where passengers in airliners rely on the safe systems and qualified pilots. Each time there is an accident or a minor incident in aviation the NTSB looks to find out what the cause was. One of the most dangerous situations in aviation besides mid-collisions and near misses; is Windshear.
The occurs win the normal relative airflow change greatly in speed or direction as you either as change altitude or are simply flying forward and all of a sudden the wind is going a different way. Most experienced pilots have experienced this many times in their careers. In fact you may have noticed this phenomena yourself in a parking lot, it happens all the time. Have you ever observed or noticed flagpoles, which are quarter mile apart. One flag is blowing one direction and another is blowing a different direction. Yet they should be blowing the same way right? Indeed they should, but they are not always are they. Sometimes this is due to terrain and sometimes it is due to buildings in the way, while other times you are simply witnessing the exact problem pilots encounter when they try to fly through such situations.
Now this if you are up high and cruising at a safe speed all you feel is a little air turbulence, but when you are low and slow it is much more critical. This is because the air you are flying through which is holding you up all of a sudden is blowing a different direction and you cannot speed up fast enough in the new airflow to stay up there. Now you see the problem? The is one reason that airports pay very close attention to which way the wind is blowing as you always want to land and take off into the wind for safety.
Some Windshear experiences are well known and many airports are notorious for having them. Once this is known you can adjust your flying to make up for it by coming in a little faster or picking up a little more airspeed before taking off or climbing out to counteract the differences.
So at some airports this is predicable that you will have such wind currents at various times of the year, one serious airport in DFW, happens a lot there, quite serious, sometimes as much as 30-40 kts difference. If you have an emergency or something goes wrong due to these conditions or if you come in too hot (faster airspeed) to counteract this on a short runway somewhere and something else goes wrong, you will definitely need all the pilot skills you got.
Accidents are usually in aviation not just one thing, usually 3 or more things go wrong at once, a comedy of errors if you will, only there is nothing funny about crashing an aircraft. One or two problems may not even be serious, but it ought to wake you up when problems start and other situations start going South as it is a compounding issue. Then, well when some pilot has an accident, the news media and everyone says how could this happen? Wind shear. What they should be saying is how come it did not happen sooner? "Pilot skill" is the answer. The opposite of course being that of the potential eventualities which come to fruition through what we call "pilot error."
Greatest danger: Takeoff and landing
Windshear poses the greatest danger to aircraft during takeoff and landing, when the plane is close to the ground and has little time or room to maneuver. During landing, the pilot has already reduced engine power and may not have time to increase speed enough to escape the downdraft. During takeoff, an aircraft is near stall speed and thus is very vulnerable to windshear.