Dead reckoning is navigation solely by means of computations based on time, airspeed, distance, and direction. The products derived from these variables, when adjusted by windspeed and velocity, are heading and groundspeed. The predicted heading will guide the airplane along the intended path and the groundspeed will establish the time to arrive at each checkpoint and the destination. The word “dead” in dead reckoning is actually derived from “ded,” or deduced reckoning. Except for flights over water, dead reckoning is usually used with pilotage for cross-country flying. The heading and groundspeed as calculated is constantly monitored and corrected by pilotage as observed from checkpoints.
The Wind Triangle or Vector Analysis
If there is no wind, the airplane’s ground track will be the same as the heading and the groundspeed will be the same as the true airspeed. Only on rare occasions does this condition exist. A wind triangle, the pilot’s version of vector analysis, is the backbone of dead reckoning.
The wind triangle is a graphic explanation of the effect of wind
upon flight. Groundspeed, heading, and time for any flight can be determined
by using the wind triangle. It can be applied to the simplest kind of cross-country
flight as well as the most complicated instrument flight. The experienced
pilot becomes so familiar with the fundamental principles that estimates
can be made which are adequate for visual flight without actually drawing
|The beginning student, however, needs to develop skill in constructing these diagrams as an aid to the complete understanding of wind effect. Either consciously or unconsciously, every good pilot thinks of the flight in terms of wind triangle. If a flight is to be made on a course to the east, with a wind blowing from northeast, the airplane must be headed somewhat to the north of east to counteract drift. This can be represented by a diagram as shown in figure 8-17. Each line represents direction and speed. The long dotted line shows the direction the plane is heading, and its length represents the airspeed for 1 hour. The short dotted line at the right shows the wind direction, and its length represents the wind velocity for 1 hour. The solid line shows the direction of the track, or the path of the airplane as measured over the Earth, and its length represents the distance traveled in 1 hour, or the groundspeed.||
|In actual practice, the triangle illustrated in figure 8-17 is not
drawn; instead, construct a similar triangle as shown by the black lines
in figure 8-18, which is explained in the following example.
Suppose a flight is to be flown from E to P. Draw a line on the aeronautical chart connecting these two points, measure its direction with a protractor, or plotter, in reference to a meridian. This is the true course which in this example is assumed to be 090° (east). From the National Weather Service, it is learned that the wind at the altitude of the intended flight is 40 knots from the northeast (045°). Since the National Weather Service reports the windspeed in knots, if the true airspeed of the airplane is 120 knots, there is no need to convert speeds from knots to MPH or vice versa.
|Figure 8-18.—The wind triangle as is drawn in navigation practice. Blue lines show the triangle as drawn in figure 8-17.|
|Now on a plain sheet of paper draw a vertical line representing north
and south. (The various steps are shown in figure 8-19.) 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).
With the ruler, draw the true course line from E, extending it
somewhat beyond the dot by 90°, and labeling it “TC 090°.”
|Figure 8-19.—Steps in drawing the wind triangle.|
|The distance flown in 1 hour (groundspeed) is measured
as the numbers of units on the true course line (88 nautical miles per
hour 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°).
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 groundspeed 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 will be used. Briefly summarized, the steps in obtaining flight information are as follows:
• TRUE COURSE—Direction of the line connecting two desired points, drawn
on the chart and measured clockwise in degrees from true north on the mid-meridian.
• WIND CORRECTION ANGLE—Determined from the wind triangle. (Added to TC if the wind is from the right; subtract if wind is from the left.)
• TRUE HEADING—The 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; subtract if east.)
• MAGNETIC HEADING—An intermediate step in the conversion. (Obtained by applying variation to true heading.)
• DEVIATION—Obtained from the deviation card on the airplane. (Added to MH or subtracted from, as indicated.)
• COMPASS HEADING—The reading on the compass (found by applying deviation to MH) which will be 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).
• GROUNDSPEED—Obtained by measuring the length of the TC line on the wind triangle (using the scale employed for drawing the diagram).
• TIME FOR FLIGHT— Total distance divided by groundspeed.
• FUEL RATE—Predetermined gallons per hour used at cruising speed.
NOTE: Additional fuel for adequate reserve should be added as a safety measure.