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Pilot's Handbook of Aeronautical Knowledge

Effect of Wind

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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




Effect of Wind

The preceding discussion explained how to measure a true
course on the aeronautical chart and how to make corrections
for variation and deviation, but one important factor has not
been considered—wind. As discussed in the study of the
atmosphere, wind is a mass of air moving over the surface of
the Earth in a definite direction. When the wind is blowing
from the north at 25 knots, it simply means that air is moving
southward over the Earth's surface at the rate of 25 NM in
1 hour.

Under these conditions, any inert object free from contact
with the Earth is carried 25 NM southward in 1 hour. This
effect becomes apparent when such things as clouds, dust,
and toy balloons are observed being blown along by the wind.
Obviously, an aircraft flying within the moving mass of air
is similarly affected. Even though the aircraft does not .oat
freely with the wind, it moves through the air at the same time
the air is moving over the ground, thus is affected by wind.
Consequently, at the end of 1 hour of flight, the aircraft is in
a position which results from a combination of the following
two motions:
• Movement of the air mass in reference to the ground
• Forward movement of the aircraft through the air

Actually, these two motions are independent. It makes no
difference whether the mass of air through which the aircraft
is flying is moving or is stationary. A pilot flying in a 70-
knot gale would be totally unaware of any wind (except for
possible turbulence) unless the ground were observed. In
reference to the ground, however, the aircraft would appear
to fly faster with a tailwind or slower with a headwind, or to
drift right or left with a crosswind.

As shown in Figure 15-13, an aircraft flying eastward at an
airspeed of 120 knots in still air has a groundspeed (GS)
exactly the same—120 knots. If the mass of air is moving
eastward at 20 knots, the airspeed of the aircraft is not
affected, but the progress of the aircraft over the ground is
120 plus 20, or a GS of 140 knots. On the other hand, if the
mass of air is moving westward at 20 knots, the airspeed of
the aircraft remains the same, but GS becomes 120 minus
20, or 100 knots.

Assuming no correction is made for wind effect, if an aircraft
is heading eastward at 120 knots, and the air mass moving
southward at 20 knots, the aircraft at the end of 1 hour is
almost 120 miles east of its point of departure because of its
progress through the air. It is 20 miles south because of the
motion of the air. Under these circumstances, the airspeed
remains 120 knots, but the GS is determined by combining
the movement of the aircraft with that of the air mass. GS can
be measured as the distance from the point of departure to
the position of the aircraft at the end of 1 hour. The GS can
be computed by the time required to fly between two points a
known distance apart. It also can be determined before flight
by constructing a wind triangle, which is explained later in
this chapter. [Figure 15-14]

Motion of the air affects
Figure 15-13. Motion of the air affects the speed with which aircraft
move over the Earth's surface. Airspeed, the rate at which an
aircraft moves through the air, is not affected by air motion.

The direction in which the aircraft is pointing as it flies
is heading. Its actual path over the ground, which is a
combination of the motion of the aircraft and the motion of
the air, is its track. The angle between the heading and the
track is drift angle. If the aircraft heading coincides with the
true course and the wind is blowing from the left, the track
does not coincide with the true course. The wind causes the
aircraft to drift to the right, so the track falls to the right of
the desired course or true course. [Figure 15-15]