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Figure 4.30. Mark Each Vector of Wind Triangle.
4.14.1.2. Remember that wind direction (WD) and windspeed (WS) compose the wind vector. True
airspeed (TAS) and true heading (TH) form the air vector and GS and track compose the ground vector.
4.14.1.3. The ground vector is the resultant of the other two; hence, the air vector and the wind vector
are always drawn head to tail. An easy way to remember this is that the wind always blows the aircraft
from TH to track.
4.14.1.4. Consider just what the wind triangle shows. In Figure 4.31, the aircraft departs from point A on
the TH of 360o at a TAS of 150 knots. In 1 hour, if there is no wind, it reaches point B at a distance of
150 NM.
4.14.1.5. In actuality, the wind is blowing from 270o at 30 knots. At the end of 1 hour, the aircraft is at
point C 30 NM downwind. Therefore, the length BC represents the speed of the wind drawn to the same
scale as the TAS. The length of BC represents the wind and is the wind vector.
4.14.1.6. Line AC shows the distance and direction the aircraft travels over the ground in 1 hour. The
length of AC represents the GS drawn to the same scale as the TAS and windspeed. Thus, the line AC,
which is the resultant of AB and BC, represents the motion of the aircraft over the ground and is the
ground vector.
4.14.1.7. Measuring the length of AC determines that the GS is 153 knots. Measuring the drift angle,
BAC, and applying it to the TH of 360o, results in the track of 011o.
4.14.1.8. If two vectors in a wind triangle are known, the third one can be found by drawing a diagram
and measuring the parts. Actually, the wind triangle includes six quantities; three speeds and three
directions. Problems involving these six quantities make up a large part of DR navigation. If four of
these quantities are known, the other two can be found. This is called solving the wind triangle and is an
important part of navigation.
AFPAM11-216 1 MARCH 2001 135
Figure 4.31. Wind Triangle.
4.14.1.9. The wind triangle may be solved by trigonometric tables; however, this is unnecessary since
the accuracy of this method far exceeds the accuracy of the data available and the results needed. In
flight, the wind triangle is solved graphically, either on the chart or on the vector or wind face of the
computer.
4.14.1.10. The two graphic solutions of the wind triangle (chart solution and computer solution) perhaps
appear dissimilar at first glance. However, they work on exactly the same principles. Plotting the wind
triangle on paper has been discussed; now, the same triangle is plotted on the wind face of the computer.
4.14.2. Wind Triangles on DR Computer. The wind face of the computer has three parts: (1) a frame,
(2) a transparent circular plate which rotates in the frame, and (3) a slide or card which can be moved up
and down in the frame under the circular plate. This portion of the computer is illustrated in Figure 4.32.
4.14.2.1. The frame has a reference mark called the TRUE INDEX. A drift scale is graduated 45o to the
left and 45o to the right of the true index; to the left this is marked DRIFT LEFT, and to the right,
DRIFT RIGHT.
136 AFPAM11-216 1 MARCH 2001
Figure 4.32. Wind Face of DR Computer.
AFPAM11-216 1 MARCH 2001 137
4.14.2.2. The circular plate has around its edge a compass rose graduated in units of 1 degree. The
position of the plate may be read on the compass rose opposite the true index. Except for the edge, the
circular plate is transparent so that the slide can be seen through it. Pencil marks can be made on the
transparent surface. The centerline is cut at intervals of two units by arcs of concentric circles called
speed circles; these are numbered at intervals of 10 units.
4.14.2.3. On each side of the centerline are track lines, which radiate from a point of origin off the slide
as shown in Figure 4.33. Thus, the 14o track line on each side of the centerline makes an angle of 14o
with the centerline at the origin. And the point where the 14o track line intersects the speed circle
marked 160 is 160 units from the origin.
Figure 4.33. Speed Circles and Track Lines.
138 AFPAM11-216 1 MARCH 2001
4.14.2.4. In solving a wind triangle on the computer, plot part of the triangle on the transparent surface
of the circular plate. For the other parts of the triangle, use the lines that are already drawn on the slide.
Actually, there isn't room for the whole triangle on the computer, for the origin of the centerline is one
vertex of the triangle. When learning to use the wind face of the computer, it may help to draw in as
much as possible of each triangle.
4.14.2.5. The centerline from its origin to the grommet always represents the air vector. If the TAS is
150k, move the slide so that 150 is under the grommet; then the length of the vector from the origin to
　
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