Let’s cover the difference between Variable Wind and a Sudden Wind Shift.
Variable Wind: wind that changes its direction frequently.
Sudden Wind Shift: this is a line or narrow zone where there is an abrupt change in wind direction
The approach shown in the video into Seldovia, Alaska has both variable wind as reported on the ASOS and sudden wind shifts. Think of variable wind as the wind in one particular place swinging around (i.e. when you see the windsock swinging around). A sudden wind shift also occurs as the wind is changing speed and direction quite abruptly in a short distance as we travel down the runway.
Windshear
Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal wind shear.
Microburst
- Position #1: Increasing performance, increase in airspeed and lift
- Position #2: Performance increase dissipates and airspeed begins decaying along with downdraft sink
- Position #3: Performance decrease, large loss of airspeed combined with sinking
- Position #4: Downdraft abates and aircraft is left with far less aispeed than when it originally entered the microburst. Airspeed is likely to be dangerously low at this point if the aircraft has not already stalled.
The Scenario
Imagine you are approaching to land on runway 36 with a nice steady breeze of 9 knots from 090 degrees. There are some hangars upwind of the runway with the hangar rows perpendicular to the runway. As you approach with your proper x-wind correction and begin to flare just a few feet above the runway you start getting pushed off centerline downwind. As you correct with aileron and rudder to maintain centerline you notice your correction seems to large and you drift back upwind of the centerline. What you are experiencing here is variable wind speeds as the wind accelerates in velocity, being squeezed between the hangar rows. Recognizing this may occur due to the wind direction and terrain features on the ground prior to entering into the flare can help you plan for this and accomplish a more stable approach and landing.