Aero-Tips!
A good pilot is always learning -- how many times have you heard
this old standard throughout your flying career? There is no truer
statement in all of flying (well, with the possible exception of
"there are no old, bold pilots.") It's part of what makes aviation
so exciting for all of us... just when you think you've seen it
all, along comes a scenario you've never imagined.
Aero-News has called upon the expertise of Thomas P. Turner,
master CFI and all-around-good-guy, to bring our readers -- and us
-- daily tips to improve our skills as aviators, and as
representatives of the flying community. Some of them, you may have
heard before... but for each of us, there will also be something we
might never have considered before, or something that didn't
"stick" the way it should have the first time we memorized it for
the practical test.
It is our unabashed goal that "Aero-Tips" will help our readers
become better, safer pilots -- as well as introducing our
ground-bound readers to the concepts and principles that keep those
strange aluminum-and-composite contraptions in the air... and allow
them to soar magnificently through it.
Look for our daily Aero-Tips segments, coming each day to you
through the Aero-News Network. Suggestions for future Aero-Tips are
always welcome, as are additions or discussion of each day's tips.
Remember... when it comes to being better pilots, we're all in this
together.
Aero-Tips 05.18.06
Microbursts are small scale, intense
downdrafts that, on reaching the surface, spread outward in all
directions from the downdraft center. Imagine turning a garden hose
on high and then pointing the spray directly down at a
sidewalk—what happens to the water is sort of like what
happens with descending air in a microburst. Microbursts cause
extremely hazardous vertical and horizontal wind shears at low
altitudes as high-speed air exists close to calmer skies.
(Above graphic from the Aeronautical Information
Manual)
Microbursts are not easily detectable by conventional weather
radar or wind shear alert systems, because they are small, of short
duration at any point on the ground, and they can occur over areas
without surface precipitation—remember that radar sees rain
or snow, not wind patterns.
Low- to mid-altitude cumulus clouds produce microbursts.
Microbursts commonly occur in the heavy rain portion of
thunderstorms, once strong downdrafts have developed. But they can
also descend from much weaker, benign-looking convective cells that
have little or no precipitation reaching the ground.
Microburst characteristics
- A microburst downdraft is typically less than a mile across as
it descends from the cloud base to about 1,000-3,000 feet above the
ground. Below that height, the downdraft changes to a horizontal
outflow that can extend to approximately 2 1/2 miles in
diameter.
- The downdrafts can be as strong as 6,000 feet per minute.
Horizontal winds near the surface can be as strong as 45 knots
resulting in a 90 knot change in wind speed across the microburst.
These strong horizontal winds occur within a few hundred feet of
the ground—where you'd have little room to recover from a
windshear event.
- Microbursts can be found almost anywhere that there is
convective activity. They may be embedded in heavy rain associated
with a thunderstorm or in light rain in benign appearing virga
(precipitation that evaporates before hitting the ground). When
there is little or no precipitation at the surface accompanying the
microburst, a ring of blowing dust may be the only visual clue of
its existence.
- An individual microburst will seldom last longer than 15
minutes from the time it strikes the ground. The horizontal winds
increase during the first five minutes, with the maximum intensity
winds lasting approximately 2-4 minutes. Sometimes microbursts
concentrate in a line and area activity may continue for as long as
an hour. Once microburst activity starts, multiple microbursts in
the same general area are not uncommon.
Aero-tip of the day: Visualize where
microbursts may appear beneath and downwind of large cumulus
clouds—and stay well clear of microbrusts.