A Pilot's Accident Review by John Lowery

Author:John Lowery
Language: eng
Format: epub
Publisher: Aviation Supplies and Academics, Inc.
Published: 2015-10-27T00:00:00+00:00

Figure 6-14. Vortex ring. (From “The Microburst as a Vortex Ring,” Caracena, NOAA 1982)

The insidious danger of the microburst is that its effects can be encountered in clear air, well in front of or behind a thunderstorm. In fact, a microburst can be dry, with its effects being shown only by the telltale ring of dust (the vortex ring) it kicks up on the ground. Because it most often develops from a small, rapidly building cell lacking significant moisture it is unlikely to be detected by radar.

A microburst is typically about two miles wide, and produces a stream of cold air that falls rapidly to the ground and then spreads out and curls back up. It resembles water spouting from a garden hose pointed straight down. Its horizontal winds increase in intensity during the first five minutes with the maximum velocity lasting approximately two to three minutes. An airplane flying through it will encounter a stiff headwind from the outward flow, then a severe downdraft, and finally a tailwind. Sometimes microbursts concentrate in a line, and their activity may continue for up to an hour. Once they start, multiple microbursts in the same general area are common and should be expected.

Over the years, microbursts have caused numerous airline and general aviation crashes. One spectacular accident occurred some years ago before the phenomenon was well recognized. The flight crew of an L-1011 landing at Dallas-Fort Worth International Airport continued their approach into the rain shaft of a cumulonimbus cloud. According to the NTSB, at low altitude they encountered severe wind shear from a microburst produced by rapidly developing thunderstorm. The airplane crashed about 6,300 feet north of the approach end of runway 17L, hitting a car on a highway north of the runway and killing the driver. Finally, it struck two water tanks on the airport and broke apart. Of the 163 people aboard, only 26 passengers and three flight attendants survived. The potential for catastrophe in a light GA aircraft is obvious.

Thunderstorms represent a large turbulent, system of churning, up-and-downdrafts. The AIM warns of severe turbulence up to 20 to 30 miles from a severe thunderstorm, and 10 miles in less-severe storms: “No flight path through an area of strong or very strong radar echoes separated by 20–30 miles or less may be considered free of severe turbulence” (see AIM 7-1-28(c)).

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