[TowerTalk]Microbursts

[K4SB]

I thought a little more explanation concerning the weather
associated with
micro bursts might be interesting.

Incidentally, the airport at Denver was chosen as the first to
have detectors installed. Even with all the sophisticated
equipment used, this event still has a lot to go in the learning
curve.

Essentially, a micro burst, which is usually, but not always,
associated with
severe thunderstorm activity, consists of winds,which flow
opposite or almost so to each other, are confined to a very
narrow altitude band ( sorta like a channel which can be as
narrow as 100 feet, or may exist from the ground up to a few
hundred feet. The micro burst is formed by the interaction of
these 2 opposite or near so wind flows. The micro burst can by
itself have a wind speed in excess of 200 mph, although most are
less than that. A 100 mph micro burst is
not uncommon, and the one which brought down that Delta heavy jet
in Dallas a few years ago was thought to be less than 50 mph.

However, imagine this; an approach speed of about 160 mph, with
say a 20 or so
mph headwind. Effectively, your ground speed is then 140 mph as
you go toward the runway. Now, you get an equal but opposite flow
of wind which increased your
ground speed to 180 mph, and then fly into the micro burst, which
may have winds from any direction, and may be blowing not
horizontally, but vertically as well.

The effect is severe turbulence with an enormous downward
pressure exerted on the aircraft, with a severe loss in lift due
to decreased airflow over the wings.

Been a long time since I've had to deal with them, but it's part
of every Captain's recurrent training in a simulator twice a
year. The proceedure used is
an immediate increase to full power on all engines, while
rotating the nose of the aircraft upward, even to that point
where the stall indicators begin their
hideous warnings, to get a positive rate of climb. In reality,
even having done all proceedures properly, you may still hit the
ground violently, but not so much as if you had not initiated a
recovery. Of course, in the simulators, they don't program in
such that you cannot recover, but it is a very hairy experience,
even in a simulator. I once asked the check Captain giving me my
ride to program in maximum force. Now, we both agreed he would
give me a countdown to when the program executed, I was
forewarned, and primed, executed all the proceedures perfectly,
and crashed violently. The g forces on impact were such that
there would have been no survivors. ( and also brought forth an
angry complaint from the simulator operator )

Now, remember that these winds always occur close to the ground,
that they may
change direction in an instant, and could be from 100 to 200 mph
in force. Think about what that would do to a 100 foot tower with
a big array on it. Perhaps the
most vivid example would be 2 King Kongs, with hugh pipe
wrenches, both pulling and pushing on the top of your tower in
all directions.

I guarantee you this. That tower will come down, regardless of
it's make or guying.

OK, enough of the "there I was on my back at 20,000 feet
surrounded by 30 migs"
bit. I'm sure there have been vast improvements in the system
since I retired, and perhaps some of us might add to this. I hope
this has been interesting. 

73

Ed

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