Landing Take-off
Figure I - 4: Effects of longitudinal wind shears on flight path
Figure I - 5: Effect of crosswind shears on flight path
H.2. VERTICAL WIND SHEARS
H.2.1. EFFECT ON ANGLE OF ATTACK
In a level flight, the airflow hits the wing horizontally. When the aircraft flies in a downdraft or updraft, the resultant airflow (i.e. nominal airflow + downdraft/updraft) hits the wing with an angle to the horizontal. This angle depends of the airspeed and the velocity of the downdraft or updraft. The pitch attitude remains unchanged.
The initial effect of a downdraft is then a transient reduction of the Angle Of Attack (AOA) that leads to a transient lift reduction. On the contrary, the initial effect of an updraft is a transient increase of the AOA that leads to a transient lift increase.
As a consequence, the initial effect of a downdraft on the flight path is the same as the one with a decreasing headwind or increasing tailwind (see Figure I - 4). And the initial effect of an updraft is the same as the one with an increasing headwind or decreasing tailwind.
When the aircraft passes the vertical wind shear, the aircraft naturally returns to equilibrium thanks to its longitudinal stability. Without any pilot actions, pitch oscillations may occur with a period of approximately 5 seconds.
H.2.2. DOWNBURST EFFECTS
A downburst is a powerful downdraft produced by a thunderstorm. As it approaches the ground, the downburst splits in all directions.
Figure I - 7: Downburst effect Figure I - 8: Downburst side view
Figure I - 7 and Figure I - 8 illustrate the effects of downburst on the flight path assuming that the downburst is centered on the glide path. Effects of vertical wind shears on AOA and of longitudinal wind shears are combined.
When the downburst is centered on the glide path, the effects of the downburst are sequenced in three steps:
1.
The aircraft encounters an increasing headwind. The aircraft flies above the glide path (See Figure I - 4).
2.
The aircraft comes into the center of the downburst and encounters a vertical wind shear. In a downburst, the AOA and then the lift are reduced. The aircraft passes below the glide path (See Figure I - 4).
3.
The aircraft encounters an increasing tailwind. The lift increases; the aircraft may regain or overshoot the glide path according to the magnitude of the tailwind (See Figure I - 4).
A downburst centered on the glide path is the worst wind shear case when approaching the runway. Indeed, the aircraft encounters wind shears in opposite directions along the flight path plus a downdraft.
When the downburst is not centered on the glide path, the aircraft encounters less critical but non-negligible effects: airspeed, drift, and descent rate vary. See situations 1 and 2 of Figure I - 9.
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