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Dead Reckoning (Part Two)

in Navigation

Please read Dead Reckoning (Part One) prior to reading this post.


Now, on a plain sheet of paper draw a vertical line representing north to south. (The various steps are shown in Figure 15-21.)

Figure 15-21. Steps in drawing the wind triangle.

Figure 15-21. Steps in drawing the wind triangle.

Step 1

Place the protractor with the base resting on the vertical line and the curved edge facing east. At the center point of the base, make a dot labeled “E” (point of departure), and at the curved edge, make a dot at 90° (indicating the direction of the true course) and another at 45° (indicating wind direction).

Step 2

With the ruler, draw the true course line from E, extending it somewhat beyond the dot by 90°, and labeling it “TC 090°.”

Step 3

Next, align the ruler with E and the dot at 45°, and draw the wind arrow from E, not toward 045°, but downwind in the direction the wind is blowing, making it 40 units long, to correspond with the wind velocity of 40 knots. Identify this line as the wind line by placing the letter “W” at the end to show the wind direction.

Step 4

Finally, measure 120 units on the ruler to represent the airspeed, making a dot on the ruler at this point. The units used may be of any convenient scale or value (such as ¼ inch = 10 knots), but once selected, the same scale must be used for each of the linear movements involved. Then place the ruler so that the end is on the arrowhead (W) and the 120-knot dot intercepts the true course line. Draw the line and label it “AS 120.” The point “P” placed at the intersection represents the position of the aircraft at the end of 1 hour. The diagram is now complete.

The distance flown in 1 hour (GS) is measured as the numbers of units on the true course line (88 NMPH, or 88 knots). The true heading necessary to offset drift is indicated by the direction of the airspeed line, which can be determined in one of two ways:

  • By placing the straight side of the protractor along the north-south line, with its center point at the intersection of the airspeed line and north-south line, read the true heading directly in degrees (076°). [Figure 15-22]

Figure 15-22. Finding true heading by the wind correction angle.

Figure 15-22. Finding true heading by the wind correction angle.

  • By placing the straight side of the protractor along the true course line, with its center at P, read the angle between the true course and the airspeed line. This is the WCA, which must be applied to the true course to obtain the true heading. If the wind blows from the right of true course, the angle is added; if from the left, it is subtracted. In the example given, the WCA is 14° and the wind is from the left; therefore, subtract 14° from true course of 090°, making the true heading 076°. [Figure 15-23]

Figure 15-23. Finding true heading by direct measurement.

Figure 15-23. Finding true heading by direct measurement.

After obtaining the true heading, apply the correction for magnetic variation to obtain magnetic heading, and the correction for compass deviation to obtain a compass heading. The compass heading can be used to fly to the destination by dead reckoning.


To determine the time and fuel required for the flight, first find the distance to destination by measuring the length of the course line drawn on the aeronautical chart (using the appropriate scale at the bottom of the chart). If the distance measures 220 NM, divide by the GS of 88 knots, which gives 2.5 hours, or 2:30, as the time required. If fuel consumption is 8 gallons an hour, 8 x 2.5 or about 20 gallons is used. Briefly summarized, the steps in obtaining flight information are as follows:

  • TC—direction of the line connecting two desired points, drawn on the chart and measured clockwise in degrees from true north on the mid-meridian.
  • WCA—determined from the wind triangle. (Added to TC if the wind is from the right; subtracted if wind is from the left).
  • TH—direction measured in degrees clockwise from true north, in which the nose of the plane should point to make good the desired course.
  • Variation—obtained from the isogonic line on the chart (added to TH if west; subtracted if east).
  • MH—an intermediate step in the conversion (obtained by applying variation to true heading).
  • Deviation—obtained from the deviation card on the aircraft (added to MH or subtracted from, as indicated).
  • Compass heading—reading on the compass (found by applying deviation to MH) which is followed to make good the desired course.
  • Total distance—obtained by measuring the length of the TC line on the chart (using the scale at the bottom of the chart).
  • GS—obtained by measuring the length of the TC line on the wind triangle (using the scale employed for drawing the diagram).
  • Estimated time en route (ETE)—total distance divided by GS.
  • Fuel rate—predetermined gallons per hour used at cruising speed.

NOTE: Additional fuel for adequate reserve should be added as a safety measure.

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