Wind shear is 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. Wind shear can occur at high or low altitude.
The following types of windshear exist:
Vertical windshear (vertical variations of the horizontal wind component, resulting in turbulence and affecting aircraft airspeed when climbing or descending through the shear layer); and,
Horizontal windshear (horizontal variations of the wind component (e.g., decreasing head wind or increasing tail wind, or a shift from a head wind to a tail wind), affecting the aircraft in level flight, climb or descent).
Windshear is associated usually with the following weather conditions:
– Jet streams;
– Mountain waves;
– Frontal surfaces;
– Thunderstorms and convective clouds; and,
Microbursts present two distinct threats to aviation safety:
– A downburst that results in strong downdrafts (reaching 40 knots vertical velocity); and,
– An outburst that results in strong horizontal windshear and wind-component reversal (with horizontal winds reaching 100 knots).
Doppler radar wind measurements indicate that the wind speed change a pilot might expect when flying through the average microburst at its point of peak intensity is about 45 knots. However, microburst wind speed differences of almost 100 knots have been measured.
Following precautionary actions are recommended if windshear is suspected:
FOR EDUCATIONAL PURPOSES ONLY
– Use maximum takeoff thrust
– Use the longest suitable runway
Approach and Landing:
– Use flaps 30 for landing
– Establish a stabilized approach no lower than 1000 feet.
– Use most suitable runway to avoids the areas of suspected windshear.
– Use ILS G/S, VNAV path or VASI/PAPI indications to detect flight patch
– Add an appropriate airspeed correction (Max. 20 knots)