Wind shear is defined as a
change in wind speed and/or direction over a small area.
Wind shear can be vertical or horizontal and can occur at any altitude.
While wind shear can cause uncomfortable turbulence and aircraft controllability
problems at any altitude, it is of most concern at lower altitudes, simply because
of the risk that an altitude loss could result in the airplane impacting terrain.
Wind shear at low altitudes is referred to, logically, as low level wind shear,
Hazardous wind shear is common with passing frontal
systems and thunderstorms. It may also exist as an unseen hazard, producing
clear air turbulence. In low level temperature inversions, pilots can
expect to encounter wind shear when the wind speed at 2,000 to 4,000 feet above
the surface is at least 25 knots.
Pilots must be aware of the effects wind shear can have on airplane performance.
A few scenarios involving wind shear are described below to aid in this
Tailwind Becomes A Headwind
With a tailwind, the airplane is flying with the wind, like a boat moving with the
water's current. If that
tailwind shears to a headwind, the airplane all of
a sudden finds itself blasted in the face with wind. The airspeed increases.
With this additional airspeed, the wings produce more lift and the flight controls
become more effective. The pilot can see the airspeed jump up on the airspeed
indicator and feel the airplane's performance increase.
Headwind Becomes A Tailwind
In this case, the airplane is flying into the wind. When the headwind shears
to a tailwind, the airplane experiences a loss of airspeed. This loss of airspeed
can be seen on the airspeed indicator, and the loss of performance can also be seen
and felt by the pilot.
Microbursts are incredibly hazardous. Microbursts are an intense downdraft
of air. When an airplane flies through a microburst, this downward moving
air tries to push the airplane into the ground. Mircobursts have overcome even large
jet aircraft, and can easily force a piston powered general aviation
aircraft into terrain.
A microburst is typically less than
one mile wide and occurs within 1000 feet vertically,
lasting about 15 minutes before it dissipates. A typical general aviation
airplane is capable of climbing at a rate of 700 to 1500 feet per minute, while
a microburst can produce downdrafts of
up to 6000 feet per minute. Even as
the pilot is climbing through the air at the airplane's maximum rate of climb, that
air is descending at a significantly greater speed, so the airplane descends
It gets worse than that, however. When the air from the microburst strikes
the ground, it spreads out
in all directions. As a result, microbursts often
produce wind speed changes of 45 knots or more.
As an airplane flies into a microburst, it will initially encounter the outflow
as an increasing headwind. Next, the headwind will fall off as the airplane
enters the downdraft. As the airplane progresses, the downdraft becomes a tailwind.
So, airplane performance increases initially, then the microburst produces decreasing
airplane performance from that
Microbursts are hard to detect and usually remain in a somewhat
LLWS Alerting Systems
In an attempt to warn pilots when LLWS exists at a particular airport, LLWS alerting
systems have been installed at many airports. These systems consist of wind measuring devices
placed at different locations on the airfield. These wind measurements are
compared in order to detect the presence of wind shear. These
systems are not common at smaller airports, and are unable to detect LLWS in the
area surrounding the airport.