Even though fall is fast approaching, thunderstorms are still lurking out there over the horizon. They bring with them some of the nastiest weather a pilot and an aircraft can ever confront, including the sometimes-mysterious microburst. As AVweb's Linda Pendleton writes, we can't stay on the ground every time a thunderstorm pops up. But we can learn how to avoid their nastiest characteristics, including the microburst.
September 20, 2000
|About the Author ...
Linda D. Pendleton is Manager
of Computer Graphics and Animation for
She is also the author of a book, Flying Jets, and scriptwriter for
several of the training videotapes published by King Schools, including
"Navigation from A to Z," "METAR/TAF Made Easy," and "Handling Emergencies."
Linda is an ATP with Citation 500 and Learjet type ratings, and a CFI with
airplane, instrument and multiengine ratings. In her 10,000+ hours of flight
experience, she's flown US Mail, freight, corporate, charter, commuter, and
served as an FAA-designated examiner for the Citation 500.
cut my teeth as a pilot in the Midwest — the Chicago area to be exact — and
I learned lots about weather during those days. Among the things I learned is
that 97 percent of weather is safely navigable (by a qualified and current
pilot in a properly-equipped airplane, but that's another subject for another
article), and the important part is staying away from the other 3 percent
because it's definitely going to turn you into a statistic. The obvious safe
way to approach this subject is to never fly when there is a chance of
unpleasantness in the forecast, but as those of you in the Chicago area know,
had I done that I'd probably still have about 300 hours.
From March through October there were usually thunderstorms somewhere in
the Midwest and although most of those tended to be of the
isolated-to-scattered air mass type, they can still pack a punch. I've seen
the isolated magnificent specimen climb 60,000 feet into the sky and just dare
any puny airplane to come visit. One of the best things about thunderstorms,
though, is that they don't sneak up on you like other nastiness such as
airframe icing and fog can do. It's pretty hard to be stealthy when you're 10
miles tall and putting on a light show that can be seen for miles!
Where Do Microbursts Hang Out?
We've all been taught not to fly into cumulonimbus clouds and I can't
imagine any pilot among us who would do that — at least not willingly! True,
some are suckered in by embedded thunderstorms, but that's not common since
most try to remain VFR around the larger TRWs so we can see what we're flying
into. You can encounter plenty of unpleasantness outside of the main cloud of
a thunderstorm, however, and one of the most sinister of these is the
downburst or microburst. Some refer to this as wind shear and, when I taught
at FlightSafety, we used to refer to it as wind shear training, but wind shear
is a bit too general and a bit too benign a term for the
airplane-slapped-out-of-the-sky chain of events unleashed in a microburst.
What Are Microbursts, Anyway?
So, what is a microburst, anyhow, and what causes it?
microbursts are just parcels of cold air rocketing down out of a thunderstorm.
The late Dr. Theodore Fujita of the University of Chicago defined microbursts
as downbursts from a thunderstorm that are 2.2 nm (4 km) across or smaller and
are from five to seven minutes in duration. Dr. Fujita studied the 1975
Eastern Airlines (EAL) Flight 66 crash at JFK and determined that it was a
microburst that had downed that flight. The EAL 66 accident and an Allegheny
crash at Philadelphia in 1976 were the beginning of the recognition by the
aviation community that thunderstorms might pack a far more powerful punch
than was previously appreciated.
The FAA responded to this threat by developing the Low Level Wind Shear
Alert System (LLWAS) in the late 1970s. LLWAS consists of a network of
anemometers (wind sensors), which are strategically located on and around the
vicinity of an airport to measure the speed and direction of wind. The data
collected by the anemometers is transmitted to a master station (typically
located in the control tower) which detects wind shear or microburst
conditions. Once the system detects a wind shear or microburst, warnings are
presented to controllers who in turn relay the messages to pilots approaching
or departing the airport.
The Holes In The Net Are Too Big...
The LLWAS is still in use today, but microburst encounter events have shown
that, although we built a net to catch these little devils, we made the holes
too big! On a July day in 1988, there were several thunderstorms in the area
of the Denver Stapleton Airport. This is not an uncommon event in the summer
in Denver. Also, there were several United Airlines flights in the area. This
also is a rather common event in Denver. The airport was equipped with a LLWAS.
Also, the FAA was testing the new Doppler radar at an Air Force base southeast
of the airport.
Several flights operating in the area that day reported some pretty scary
microburst-related events. One pilot reported that he had initiated a
go-around and had takeoff power established for 55 seconds before the aircraft
began to climb. (That HAD to be the longest 55 seconds of his life.) Another
pilot reported a 65-knot airspeed change. Now during the whole time these
incidents were going on, the LLWAS reported a maximum of 15-knot gusts. The
sensor towers were far enough apart that the microbursts snuck in and were not
detected. The Doppler radar being tested, however, recorded the 65-knot speed
difference across the core of one microburst. Clearly, LLWAS is not a panacea
and, while it does work in the area immediately near and on the airport, the
system does little for planes and crews on approach or maneuvering in the
...But, What Causes A Microburst?
answer is: thunderstorms. You're already aware that there are severe up- and
downdrafts associated with cumulonimbus clouds. It's this air movement that
allows hail to form. The warmer water droplets from below are swept above the
freezing level by the updrafts. As the water freezes, it begins to fall again
until caught in another strong updraft. The size of hailstones reaching the
ground is a pretty direct indicator of the strength of the thunderstorm. It
takes some pretty strong updrafts to lift pieces of ice the size of tennis
balls back into the upper levels to let them accumulate another layer of ice.
Once the thunderstorm reaches the mature stage and the rain begins to fall,
the stage is set for a microburst. As the rain falls it is usually accompanied
by a downdraft. As this rain-laden air begins to fall, the rain begins to
evaporate. This evaporation cools the air further. The colder and denser air
now accelerates towards the ground. Upon ground contact, the air spreads out
in all directions. Characteristic patterns can sometimes be found in
vegetation struck by a microburst — the plants will be flattened out away
from the center of the ground strike as though a bomb had burst in the center.
So, What Makes These Things So Dangerous?
What makes microbursts so dangerous? Many things, not the least of which is
that they have the potential to swat airplanes out of the sky. Piston-powered
airplanes seem to have a slight advantage since they do not experience a
power-lag or spin-up time on the engines as do turbine-powered machines, but
they usually also have less power and less inertia to start with than does
Click for larger image.
One of the sneaky things about microbursts is that if you're a little short
on situational awareness, the apparent performance increase that can be the
first sign of a microburst in progress is sometimes ignored and the flight
presses on to further danger.
Take a look at the graphic to the right.
This is the classic microburst diagram we've all seen. Notice the airplane at
position A. It's just beginning to feel the effects of the outflow from the
microburst. This sudden increase in headwind will cause an apparent increase
in performance. The airspeed will increase, the rate of descent will decrease
and the almost overwhelming urge of a pilot at this point is to pull the
throttles back and push the nose over. Bad mistake. This is the time to get
out of Dodge. Microbursts are like everything else associated with
thunderstorms — there's no way to judge how bad they are from looking at them
and by the time you find out just how bad it is, it's too late. The proper
action at this point is to push up the power and abandon the approach.
If you press on, you will find yourself at position B in the
graphic. You will fly out of the headwind and into the tailwind. The
suddenly decreasing headwind and increasing tailwind will cause degradation in
performance equal in magnitude to the increase experienced at point A. You can
see that if the headwind portion of the microburst caused a 30-knot jump in
airspeed you're now going to lose 30 knots. If you pulled the throttles back
in response to the headwind increase, you'll be in real trouble now. This is
quickly becoming an unrecoverable situation.
When all the talk about wind shear started in the late 70s, I had a real
problem understanding how this happens. After all, we've all had it drilled
into our heads since that first hour of dual that airplanes only
"feel" wind on the ground and that once you're airborne the airplane
moves in concert with the air mass it's in. We've been taught to apply
windspeed to airspeed to arrive at a groundspeed figure. The thought that wind
could have an effect on airspeed was a hard one to swallow — and still is for
Oh, No, Not A Physics Lesson
The physics of wind shear can a bit daunting, but if you just get it into
your mind that in this instance the response of your airspeed and groundspeed
is going to be the reverse of what you would normally expect, I think you'll
see why these localized phenomena are so dangerous.
If you fly
into a microburst the first indication, as we discussed above, can be an
apparent increase in performance. To put it simply, the airplane tries to
maintain its same speed across the ground. Inertia keeps it from adjusting
quickly so the increasing headwind calls for an increase in airspeed to
maintain the ground speed. So far this doesn't sound too dangerous, and it
isn't — unless you press on. Then things get pretty scary pretty quickly.
If you fly through the headwind portion of the microburst you come to the
point (position C in the graphic) where the
headwind decreases and quickly becomes a tailwind. Now your airplane, not
knowing this is a wind shear, just tries to keep its speed across the ground
from changing. If you have responded to the headwind portion of the shear by
reducing power and pushing the nose down to maintain the glide path, you're
really going to be in hot water now. Your airplane indicating 80 knots is
going to try to maintain the 50 knots across the ground you had been
maintaining. Since the airplane cannot change the wind, it will change speed
— you will suddenly find yourself if a machine now indicating 50 knots and
sinking out of the sky.
Full power may or may not save you at this point, but it probably would
have if applied at the first indication of the microburst activity.
Once you are in the grips of a microburst, the maneuvers required for
escape may seem extreme, but they are your only hope. Apply full throttle
IMMEDIATELY and pitch up to a climb attitude — or an attitude that will
arrest your rate of descent. You may have to fly on the edge of a stall — in
a jet, you should pitch up to the point of stick-shaker activation — to
escape. It's best to think of this maneuver as an escape maneuver so you don't
confuse it with a normal go-around or missed approach. This is a far different
and more critical maneuver and you are trying to escape danger, not simply
abandon an approach. This is not the time to be "smooth and gradual"
in your application of power — firewall it! If you escape the microburst you
can worry about your engine later!
...The Key Is Avoidance
The key to successfully dealing with microbursts is awareness and
avoidance. Be aware of the weather conditions that can cause these powerful
phenomena and avoid them at all costs. Remember, an escape maneuver may or may
not work. Luckily, thunderstorms are not too stealthy and there are usually
ample warnings that they are in the area.
some DOs and DON'Ts that may help you deal with this weather:
DO get a thorough weather briefing including the possibility of
DO watch the temperature/dew point spread. Microburst activity is
more common and more severe with large temperature/dew point spreads. (The
falling rain has more opportunity to evaporate and cool the descending
DO maintain visual conditions any time thunderstorms are in the area.
DO perform an escape maneuver any time you think you may be dealing
with a microburst.
DON'T fly under ANY thunderstorm any time for any reason!
DON'T fly under the overhang of any thunderstorm.
DON'T fly into or near a rain shaft.
DON'T fly under virga — that's a built-in microburst.
DON'T depend on the fact that the flight that landed before you
didn't report any problems. Remember, microbursts have a short life span
and it may not have been there when he passed by. Also, they're relatively
small — his flight path may have been offset enough to miss it. Also, he
may just be too scared to talk!
Thunderstorms put out some of the most dangerous flying weather that can be
found, but luckily we are able to see and avoid them also!