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precaution is unnecessary. The scale on all parts of a Lambert conformal chart is essentially constant.
Therefore, it is not absolutely necessary to pick off minutes of latitude near any particular parallel except
in the most precise work.
4.6.2. Plotting and Measuring Courses. Any straight line plotted on a Lambert conformal chart is
approximately an arc of a great circle. On long distance flights, this feature is advantageous since the
great circle course line can be plotted as easily as a rhumb line on a Mercator chart.
4.6.2.1. However, for shorter distances where the difference between the great circle and rhumb line is
negligible, the rhumb line is more desirable because a constant heading can be held. For such distances,
the approximate direction of the rhumb line course can be found by measuring the great circle course at
midmeridian as shown in Figure 4.9. In this case, the track is not quite the same as that indicated by the
course line drawn on the chart, since the actual track (a rhumb line) appears as a curve convex to the
equator on a Lambert conformal chart, while the course line (approximately a great circle) appears as a
straight line. Near midmeridian, the two have approximately the same direction (except for very long
distances) along an oblique course line as indicated in Figure 4.10.
AFPAM11-216 1 MARCH 2001 119
Figure 4.9. Use Midmeridian to Measure Course on a Lambert Conformal.
Figure 4.10. At Midmeridian, R/L and G/Circle Have Approximately the Same Direction.
4.6.2.2. For long distances involving great circle courses, it is not possible to change heading
continually, as is necessary when following a great circle exactly, and it is customary to divide the great
circle into a series of legs, each covering about 5o of longitude. The direction of the rhumb line
connecting the ends of each leg is found at its midmeridian.
4.6.3. Measuring Distance. As previously stated, the scale on a Lambert conformal chart is practically
constant, making it possible to use any part of a meridian graduated in minutes of latitude to measure
NM.
4.6.4. Plotting on a Gnomonic Chart. Gnomonic charts are used mainly for planning great circle
routes. Since any straight line on a gnomonic chart is an arc of a great circle, a straight line drawn from
the point of departure to destination will give a great circle route. Once obtained, this great circle route is
transferred to a Mercator chart by breaking the route into segments as shown in Figure 4.11.
4.6.5. Plotting Hints. The following suggestions should prove helpful in developing good plotting
procedures:
120 AFPAM11-216 1 MARCH 2001
Figure 4.11. Transferring Great Circle Route From Gnomonic to Mercator Chart.
4.6.5.1. Measure all directions and distances carefully. Double-check all measurements, computations,
and positions.
4.6.5.2. Avoid plotting unnecessary lines. If a line serves no purpose, erase it.
4.6.5.3. Keep plotting equipment in good working order. If the plotter is broken, replace it. Keep sharp
points on dividers. Use a sharp pencil and an eraser that will not smudge.
4.6.5.4. Draw light lines at first, as they may have to be erased. When the line has been checked and
proven to be correct, then darken it if desired.
4.6.5.5. Label lines and points immediately after they are drawn. Use standard labels and symbols.
Letter the labels legibly. Be neat and exact.
Section 4B— DR Computer
4.7. Basics. Almost any type of navigation requires the solution of simple arithmetical problems
involving time, speed, distance, fuel consumption, and so forth. In addition, the effect of the wind on the
aircraft must be known; therefore, the wind must be computed. To solve such problems quickly and with
reasonable accuracy, various types of computers have been devised. The computer described in this
pamphlet is simply a combination of two devices: (1) a circular slide rule for the solution of arithmetical
problems (Figure 4.12), and (2) a specially designed instrument for the graphical solution of the wind
problem (Figure 4.13).
4.7.1. The slide rule is a standard device for the mechanical solution of various arithmetical problems.
Slide rules operate on the basis of logarithms. Slide rules are either straight or circular; the one on the
DR computer is circular.
AFPAM11-216 1 MARCH 2001 121
4.7.2. The slide rule face of the computer consists of two flat metallic disks, one of which can be rotated
around a common center. These disks are graduated near their edges with adjacent logarithmic scales to
form a circular slide rule approximately equivalent to a straight, 12-inch slide rule. Because the outer
scale usually represents a number of miles and the inner scale represents a number of minutes, they are
　
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