J. MAC MCCLELLAN
were added to the strong winds to make
powerful turbulence—shear and mechani-
cal interference.
The people who design and certify airplanes use the term gust to define
turbulence. A gust is an increase in wind
impinging on the airplane, and a normal
category airplane certified under FAR Part
23—the rules covering airplanes that weigh
12,500 pounds or less for takeoff—must
survive a 50-foot-per-second gust without
exceeding its design limit g-load. That’s a
pretty big bump.
But the term gust can be misleading in
this context. In general we think of a wind
gust as being a substantial change in wind
velocity that lasts briefly. A gust that an
airplane experiences as turbulence can also
be a change in wind velocity, but it can also
be a big change in wind direction.
Strong wind blowing unimpeded does
not create turbulence because there is no
gust. But when the wind blows over
obstructions on the surface, the interference creates changes in both wind
direction and velocity—gusts. Obstructions
can cause strong wind to go up, and then
ultimately back down, as well as change
direction laterally, and, of course, create
large changes in velocity.
Wind can also create turbulence aloft
when it encounters air moving at a
Wind Shear
Turbulence
Wind Shear
different velocity or direction. On a windy
day the air is seldom moving as a uniform
mass. Often there are very abrupt changes
in wind direction and velocity with only
small changes in altitude. These are called
stratified winds and can occur at any time
but are pretty common during seasonal
changes such as spring when the temperature difference between various air
masses, and the temperature of the surface, can be great.
At the altitude where winds of different
strata merge the air becomes turbulent
very much as wind does when it is forced
over obstructions on the surface. An airplane flying in the zone where two wind
strata meet will experience turbulence—
sometimes even severe turbulence—and
also wind shear. When wind shears, it
changes direction, velocity, or both over a
short distance or small change in altitude.
What I experienced on that approach
was strong wind stratification and then
wind shear. The wind on the surface was
blowing about 20 knots with gusts above
30 knots, but at barely 1,200 feet above
the surface, the wind velocity was above
60 knots. When I descended through the
zone where the two wind fields merged,
there was powerful turbulence. Closer to
the runway the wind was also disturbed
by trees, hills, buildings, and other
