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Pilot's Handbook of Aeronautical Knowledge
Weather Theory
Wind and Currents

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Effects of a microburst wind.

Low-Level Wind Shear
Wind shear is a sudden, drastic change in wind speed and/or
direction over a very small area. Wind shear can subject an
aircraft to violent updrafts and downdrafts, as well as abrupt
changes to the horizontal movement of the aircraft. While
wind shear can occur at any altitude, low-level wind shear is
especially hazardous due to the proximity of an aircraft to the
ground. Directional wind changes of 180° and speed changes
of 50 knots or more are associated with low-level wind shear.
Low-level wind shear is commonly associated with passing
frontal systems, thunderstorms, and temperature inversions
with strong upper level winds (greater than 25 knots).

Wind shear is dangerous to an aircraft for several reasons. The
rapid changes in wind direction and velocity change the wind's
relation to the aircraft disrupting the normal flight attitude and
performance of the aircraft. During a wind shear situation,
the effects can be subtle or very dramatic depending on wind
speed and direction of change. For example, a tailwind that
quickly changes to a headwind causes an increase in airspeed
and performance. Conversely, when a headwind changes
to a tailwind, the airspeed rapidly decreases and there is a
corresponding decrease in performance. In either case, a
pilot must be prepared to react immediately to the changes
to maintain control of the aircraft.

In general, the most severe type of low-level wind shear
is associated with convective precipitation or rain from
thunderstorms. One critical type of shear associated with
convective precipitation is known as a microburst. A typical
microburst occurs in a space of less than one mile horizontally
and within 1,000 feet vertically. The lifespan of a microburst
is about 15 minutes during which it can produce downdrafts
of up to 6,000 feet per minute (fpm). It can also produce a
hazardous wind direction change of 45 degrees or more, in
a matter of seconds.

When encountered close to the ground, these excessive
downdrafts and rapid changes in wind direction can
produce a situation in which it is difficult to control the
aircraft. [Figure 11-17] During an inadvertent takeoff into
a microburst, the plane first experiences a performance-increasing
headwind (1), followed by performance-decreasing
downdrafts (2). Then, the wind rapidly shears to a tailwind
(3), and can result in terrain impact or flight dangerously
close to the ground (4).

Microbursts are often difficult to detect because they occur
in relatively confined areas. In an effort to warn pilots of
low-level wind shear, alert systems have been installed at
several airports around the country. A series of anemometers,
placed around the airport, form a net to detect changes in
wind speeds. When wind speeds differ by more than 15 knots,
a warning for wind shear is given to pilots. This system is
known as the low-level wind shear alert system (LLWAS).

It is important to remember that wind shear can affect any flight and any pilot at any altitude. While wind shear may be
reported, it often remains undetected and is a silent danger
to aviation. Always be alert to the possibility of wind shear,
especially when flying in and around thunderstorms and
frontal systems.