WX BRIEF

Knowing how to handle adverse blows on approach and landing

Strong winds and unexpected gusts are leading factors in weather-related ground accidents.


Thunderstorms are a common cause of severe and gusty surface winds. Pilots should always avoid flying at low altitudes near or beneath a storm cell, and exercise extreme caution when operating at an airport with storms in the vicinity.

The problem is that although you may think you've avoided the nasty forces within the cell, the reality is that they are quite capable of leaving the clouds. A storm cell is not limited to the cloud it produces.

Rather, it extends from the surface, where it pulls in warm and humid surface air and spits out its cold downdraft, to well above the cloud top where hail may be tossed an ad­ditional several thousand feet into the air.

As we fly at low altitude beneath or near a storm cloud, we place ourselves in the path of any current of cold dense air that is being ejected from the storm base. Downdrafts in mature or decaying cells can easily travel at over 10,000 ft per minute.

Since wind power is calculated using the cube of the windspeed, a 100-kt downdraft a kilometer in diameter contains about the same kinetic energy as 4 Airbus A380s at cruise speed—more than enough to knock your aircraft out of the sky.

What's more, when that downdraft hits the ground, it has nowhere to go but radiate outward along the surface. Professional pilots are trained to respond to the aircraft's cues that it has entered a downburst environment, but the reality is that it may happen in a matter of seconds and at too low an altitude and too slow a speed to recover in time.

In a direct penetration, the aircraft would first encounter an abrupt headwind, followed by a loss of altitude due to the downdraft core, and finally a loss of lift as a result of the tailwind. The usual problem is that in correcting for the initial headwind, the aircraft is slowed to a point where it stalls as soon as the tailwind hits.

More often, though, landing or departing aircraft are hit on the side by outflow coming from an adjacent storm cell. While unlikely to cause much of a change in airspeed, the leading edge of a downburst gust front can contain a severe rotor that may cause an immediate loss of control.

At low altitude there may be little time to recover. The most appropriate course of action when faced with mature storm cells within a mile or two of your runway is to either hold until they've moved on, or divert to an alternate. Pilots who do attempt an approach in such conditions often opt for a higher and faster approach to create a bit of a cushion should they en­counter adverse winds.

Finally, there is one thing that pilots should remember when dealing with winds near the surface. The winds reported in the Metar or on ATIS have generally been observed by an anemometer located on a 10-m (33-ft) tower.

Winds normally decrease in speed closer to the ground, which means that pilots should expect winds above 33 ft to be stronger while winds in the ground effect region are likely to be a few knots weaker.

Karsten Shein is a climatologist with the National Climatic Data Center in
Asheville NC. He formerly served as an assistant professor at Shippensburg University. Shein holds a commercial license with instrument rating.

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