AOPA Pilot Magazine - September 2022 - 88

RUDDER & WRENCH
FLYING SMART /
Listening to your aircraft
Planning-and a light touch-can mitigate
the effects of wind shear
BY CATHERINE CAVAGNARO
takeoff and landing distances, I plan for
calm winds or even a tailwind component
to figure a worst-case scenario. That way,
any headwind component will improve
performance, but I won't rely on it. Cutting
it close is not in my playbook.
Since stopping distance is a function
of the square of the ground speed, it's
important to be miserly with approach
speeds. Interestingly, the FAA advisory
circular on wind shear (AC 00-54), reports
that " an additional 20 knots at touchdown
can increase stopping distance by as much
as 25 percent and, in some cases may exceed
brake energy limits. " But if an airplane that
normally touches down at 60 knots does so
at 80 knots instead, the attendant increase
in stopping distance would be more than
75 percent ((80/60)^2 = 1.78) beyond the
usual value. The advisory circular must be
referencing airplanes that normally touch
down at 170 knots or more for landing
distance to increase stopping distance by
only 25 percent!
The second problem involves motion
MUCH OF MY flight from Tennessee to
New Hampshire earlier this summer
involved testing the integrity of my seat
belt and shoulder harness as my Beechcraft
Bonanza Niky bounced up and down in
unstable air. And the Nashua (ASH) ATIS
dispelled any notion that there would be
relief as I neared my destination. As the
tower controller cleared me to land on
Runway 14, he issued a wind shear advisory
and warned me that the last aircraft noted
an airspeed loss of 10 knots on short final.
I acknowledged the information and
resolved to add a few knots to my approach
airspeed lest my flight become fodder for
another accident report.
Wind shear is a large change in
wind speed or direction over a small
distance in space and can occur in any
direction. The typical culprits involve
temperature inversions, approaching
fronts, and thunderstorms, which can
be responsible for microbursts, the worst
type of wind shear.
88
AOPA PILOT / September 2022
I confess that when I began my
aviation studies, I found the FAA guidance
on wind shear confusing because some of
the effects on performance are stated with
little or no explanation. My knowledge
sufficed to pass my private pilot written
and practical exams, but when I dove in
to shore up my deficiencies, I realized
that there are two competing problems
with wind shear which involve a different
frame of reference for the aircraft motion.
Considering these two competing factors
separately helped me understand wind
shear, and operate safely to and from
airports with gusty wind conditions.
The first problem with wind shear is
that the changing direction and strength
of wind can skew anticipated takeoff and
landing distances. For these operations, the
frame of reference is motion with respect
to the ground, as this is what allows us to
clear obstacles at the end of the runway on
takeoff as well as touch down and stop in a
confined area. Therefore, when calculating
with respect to the air mass in which
we fly and is arguably more critical than
the first. When air is smooth, it's not an
issue beyond ensuring that we fly with a
reasonable angle of attack (AOA). But in
gusty conditions, winds change abruptly,
which can mean large changes in airspeed
and AOA, and cause a sudden loss of lift.
If the airplane is operating at an already
high AOA, as is the case during takeoff
and landing, that can mean stalling near
the ground. To mitigate such risk, adding
to the approach speed makes sense.
Most pilots follow a rule that adds half
the gust factor to the approach speed to
avoid an inadvertent stall without setting
up a long landing or even runway overrun.
For example, if the winds are 18 knots,
gusting 30, adding (30-18)/2 = 6 knots to
the approach speed will mean a lower AOA
and a greater margin over the stall. I tried
without success to trace the origin of this
rule, but it's clearly a happy medium that
addresses the two competing problems. Of
course, there is no guarantee of success, but
it seems to have done a good job for pilots
over the years.
The situation is especially tricky when
the destination runway is short, and the
winds are gusty. John Denker in his online
ALEX WILLIAMSON
AOPA Pilot Magazine - September 2022

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