WX BRIEF

Better knowledge of downbursts can save you from dangerous flying experiences

Strong storm-related winds can pummel unsuspecting aircraft.


Photograph of a wet microburst accompanied by heavy rainfall. The yellow arrows indicate the air flow within the downburst.

All of these elements contributed to what NTSB described as "an encounter with and failure to escape from a microburst-induced windshear that was produced by a rapidly developing thunderstorm located at the approach end of Rwy 18R."

This was not the first—or last—time that windshear from a thunderstorm downburst had brought down an aircraft. Each year, dozens of pilots tempt fate by flying through or beneath a thunderstorm.

Some escape unscathed, but others wind up in the accident files.

Following the 1975 crash of a Eastern Air Lines Boeing 727-200 on ap­proach to JFK (John F Kennedy, New York NY) after it encountered a microburst, the low level windshear alert system (LLWAS) was developed and installed at a number of major airports. Urban encroachment gradually reduced the effectiveness of these systems.

In 1993, airports began installing terminal Doppler weather radars (TDWRs). Today TDWRs are more often used at large airports to identify and report on low-level windshear.
The LLWAS at CLT in 1994 did not help US Air Flight 1016. Pilots must be on alert whenever flying near a thunderstorm, and assume that beneath the convective cell will be an outflow that can turn violent without warning.

Storm force

An intense wet microburst descends from a thunderstorm near Bixby OK on Jul 25, 2012. Chilled dense air falls toward Earth and may reach speeds of over 100 kts before spreading out at the ground. Although affecting areas less than 3 miles in diameter and lasting only a few minutes, microbursts have contributed to several fatal aircraft accidents.

The forces generated by even a small thunderstorm far exceed the forces that allow a pilot to maintain positive control of an aircraft. They even exceed the amount of force required to cause structural failure of the aircraft itself. At their core, however, thunderstorms are simply windstorms.

A thunderstorm forms when warm and humid air at the surface is uncorked to rise rapidly through the colder, drier atmosphere above. As the warm, humid air rises, it is able to spread out and cool. This cooling lets the moisture condense into cloud droplets and eventually into rain droplets. The release of energy when moisture condenses mitigates the cooling somewhat, but it also makes the air more dense.

With the storm's relatively warm air being pumped up continually from below, the air in the upper parts of the storm becomes colder and more dense than the now heated and moisturized air around it.

This denser air descends as a downdraft. In most storms that pop up on warm summer afternoons, the appearance of a downdraft marks the beginning of the end of the storm, as the downdraft falls through the cloud, disorganizing the updrafts and evaporating moisture as it warms through compression. If there is wind aloft strong enough to tilt the storm, the downdraft may be able to coexist with the updraft and maintain the storm cell for much longer.

Downdrafts can be quite intense. In strong storms, they can attain speeds of over 100 kts. But, as they descend, they may pass through a high-moisture region and evaporate a great deal of the moisture. The process of evaporation requires energy drawn from the dry air.

Extraction of energy cools the air further, increasing its descent rate until it may exit the cloud base at speeds over 150 kts. These relatively short-lived bursts of water-cooled air are known as downbursts.

When a downburst exits the cloud, it may have a couple of thousand feet or less before it reaches the ground, so its strength does not dissipate much. The downburst wind that strikes the surface spreads out in all directions, sometimes with enough force to topple trees or damage structures.

It is this radial damage pattern that meteorologists use to determine whether it was produced by a downburst rather than of a tornado (which would create a more chaotic but linear damage pattern).

Micro and macrobursts

Downbursts are classified into 2 types, depending on the area they affect. A downburst impacting an area more than 2.5 miles (4 km) in diameter is a macroburst, while a downburst affecting a smaller diameter area is a microburst.

Downbursts are also further divided into dry and wet varieties. In regions of lower surface humidity, much storm rainfall evaporates before it reaches the ground. The downburst aids in that evaporation, producing virga. As a result, a dry downburst will reach the ground in dry air and may only be identifiable by a radial dust cloud kicked up beneath the storm. Wet downbursts, on the other hand, occur within rain shafts that reach the ground.

The acceleration of the downdraft in a wet downburst is thought to be due as much to evaporative cooling as to the drag produced by the falling precipitation. Wet downbursts can be more difficult to identify, though their gust front boundaries are often accompanied by a rotor or shelf cloud—and, unlike a dry downburst, they may be seen on radar. Also, the bases of clouds that produce wet downbursts tend to be far lower than those that produce dry downbursts.

As the downburst outflow spreads out from its impact point, it meets resistance from the surface air and curls upward into a ring-shaped vortex. This means that an aircraft encountering the vortex will have to contend with not only horizontal windshear, but vertical shear too.

Flying a downburst

Most recurrent pilot training emphasizes downburst identification and recovery. Pilots are trained to notice that if, while conducting a stabilized approach, their airspeed suddenly increases and the aircraft balloons above the glidepath, they have likely encountered a microburst or windshear.

Pilots are taught to fight the urge to reduce power for a return to the flightpath, and instead apply full power to avoid the imminent loss of altitude. If entering a downburst head on, this is indeed a good identification and recovery scenario.

In such conditions, the aircraft would fly first into the oncoming outflow vortex. The leading edge of the vortex would push the aircraft upward. Immediately afterwards, the strong headwind of the outflow would cause an increase in airspeed.

For a properly trimmed aircraft this would mean a sudden increase in altitude. A normal response to such a situation would be to reduce power, or pitch down, to return to the glidepath. But, almost immediately after the aircraft flies through the headwind region, it encounters the downburst core.


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