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must equal the difference between the TC and the TH which is set under the true index. As it stands, the
drift angle is 8o left, while the difference between TC and the indicated TH is 10o left.
4.14.6.2.7. Juggle the compass rose until the drift angle equals the difference between TC and TH. In
this example, the correct drift angle is 8o left. Now the wind triangle is set up correctly.
4.14.6.2.8. Read the TH (238o) under the true index.
4.14.6.2.9. Read the GS (179 knots) on the speed circle passing through the head of the wind vector.
4.15. Average Wind Affecting Aircraft. An average wind is an imaginary wind which would produce
the same wind effect during a given period as two or more actual winds which affect the aircraft during
that period. Sometimes an average wind can be applied once instead of applying each individual wind
separately.
AFPAM11-216 1 MARCH 2001 147
4.15.1. Authentically Averaging WDs. If the wind directions (WD) are fairly close together, a
satisfactory average wind can be determined by arithmetically averaging the WDs and windspeeds.
However, the greater the variation in WD, the less accurate the result will be. It is generally accepted
that winds should not be averaged arithmetically if the difference in directions exceeds 090o and/or the
speed of less than 15 knots. In this case, there are other methods which may be used to obtain a more
accurate average wind. A chart solution is shown in Figure 4.43.
4.15.2. Computer Solution. Winds can be averaged by vectoring them on the wind face of the DR
computer, using the square grid portion of the slide and the rotatable compass rose. Average the
following three winds by this method: 030o/l5 knots, 080o/20 knots and 150o/35 knots:
4.15.2.1. Place the slide in the computer so that the top line of the square grid portion is directly under
the grommet and the compass rose is oriented so that the direction of the first wind (030o) is under the
true index. The speed of the wind (15 knots) is drawn down from the grommet (A of Figure 4.44).
Figure 4.43. Solving for Average Wind Using Chart.
4.15.2.2. Turn the compass rose until the direction of the second wind (080o) is under the true index and
then reposition the slide so that the head of the first wind vector is resting on the top line of the square
grid section of the slide. Draw the speed of the second wind (20 knots) straight down (parallel to the
vertical grid lines) from the head of the first wind arrow (B of Figure 4.44).
4.15.2.3. Turn the compass rose so that the direction of the third wind (150o) is under the true index and
reposition the slide so that the head of the second wind vector is resting on the top line of the square grid
section of this slide. Draw the speed of the third wind (35 knots) straight down from the head of the
second wind arrow (C of Figure 4.44).
148 AFPAM11-216 1 MARCH 2001
Figure 4.44. Solving for Average Wind Using Computer.
AFPAM11-216 1 MARCH 2001 149
4.15.2.4. Turn the compass rose so the head of the third wind arrow is on centerline below the grommet.
Reposition the slide to place the grommet on the top line of the square grid section. The resultant or
average wind direction is read directly beneath the true index (108o). Measuring the length of the
resultant wind vector (46) on the square grid section and divide it by the number of winds used (3) to
determine the windspeed. This will give a WS of about 15½ knots. The average wind then is 108o/15 1/2
knots (D of Figure 4.44).
4.15.2.5. With a large number of winds to be averaged or high windspeeds, it may not possible to draw
all the wind vectors on the computer unless the windspeeds are cut by 1/2 or 1/3. If one windspeed is
cut, all windspeeds must be cut. In determining the resultant windspeed, the length of the total vector
must be multiplied by 2 or 3, depending on how the windspeed was cut, and then divided by the total
number of winds used. In cutting the speeds, the direction is not affected and the WD is read under the
true index.
4.15.2.6. Wind effect is proportional to time (Figure 4.45). To sum up two or more winds which have
affected the aircraft for different lengths of time, weigh them in proportion to the times. If one wind has
acted twice as long as another, its vector should be drawn in twice as shown. In determining the average
windspeed, this wind must be counted twice.
Figure 4.45. Weigh Winds in Proportion to Time.
4.16. Resolution of Rectangular Coordinates. Data for radar equipment is often given in terms of
rectangular coordinates; therefore, it is important that the navigator be familiar with the handling of
these coordinates. The DR computer provides a ready and easy method of interconversion.
4.16.1. Given: A wind of 340o/25 knots to be converted to rectangular coordinates (Figure 4.46).
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