Garuda Flight GA-200 & Windshear
What Factors Affect Flight Navigation and Landing in a Crash?
When the pilots of Garuda Flight GA-200 claimed they were affected by windshear, the pilot community stood up and listened.
Windshear or microbursts are a severe problem for all airliners that fly the world’s airways. It is most problematic on departure or approach. A sudden shift in wind direction or speed (or a violent downburst of air in the case of a microburst) causes the aircraft to increase or loose the ability to fly. Pilots are trained in recognition and recovery of windshear events.
The pilot, upon recognition of the situation will, if taking off below the decision speed, stop the aircraft in what is called a RTO (rejected take off.) If it is after the decision speed (V1) the pilot will fly continue to fly the aircraft by adding maximum power and flying a windshear profile. This profile is either computer generated (in new aircraft) or pilot trained in older aircraft. On approach the pilot will abort the landing attempt and climb back up to a safe altitude under all conditions – the go around.
As stated earlier, windshear has an effect on the landing profile that the pilot does not desire. The shear causes a rapid loss of airspeed that is necessary for the aircraft to continue to fly. The result is the aircraft crashes short of the runway. Examples of this are well known around the world – Delta Airlines Flight 191, Eastern Airlines Flight 66, USAir Flight 1016, Continental Airlines Flight 426 and Avianca Airlines Flight 52*.
However, in only one recent case has windshear been cited as the reason for an overshoot of the runway during a landing attempt. In all the cases above and several others not cited, whindshear caused the aircraft to crash short (before reaching) the runway. Only in the case of American Airlines Flight 1420 did the aircraft depart the end of the runway. While windshear was a contributing factor it did not cause the accident. When looking at the arguments of Garuda Flight 200’s pilots, none of the windshear factors were present.
Windshear has a multitude of profiles that are well known throughout the piloting industry. Depending on where the shear is, with reference to the flight path of the aircraft, determines what type, intensity and effect, the shear will have on the aircraft. Based upon this the pilot takes the appropriate actions that come with training and experience. The most destructive (to aircraft) is what pilots call “the dead man’s profile” or “the dead man’s rise.” This is the profile where the shear’s location is between the aircraft and the runway. The initial affect of this type of shear is to add a forward vector of wind speed to the aircraft so as the aircraft sees an increase in its performance. It requires less power for the aircraft to maintain a proper descent gradient and the natural tendency of the pilot is to reduce thrust.
This is the “dead man’s rise” for the aircraft tends to gain altitude even with the power reduced. This rise is immediately followed by a massive downburst of wind physically pushing the aircraft toward the ground (in the case of a microburst) or a rapid change in the direction of the wind in the case of a windshear. The change in direction, which happens in both cases of windshear or microbursts, has the effect of decreasing performance of the aircraft. Since the pilot has already decreased the power, the decrease in performance, caused by the shear, is compounded by this power reduction. There is insufficient time for the aircraft’s inertia to change and the aircraft will crash.
Pilots are trained to recognize the environment where windshear can occur. Scientific data has determined that windshear occurs in all parts of the world; however, the most destructive shears are associated with but a few climatologically known situations. None of these situations were present on March 7th, 2007 in Yogyakarta and one must seriously question the claims made by the pilots of Flight GA-200.
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