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4.9.5. Statute-NM Interconversion. The miles scale of the computer is marked with a statute mile
index at 76 and a NM index at 66. The units are interconverted by setting the known distance under the
appropriate index and reading the desired unit under the other.
EXAMPLE: To convert 136 statute miles to NMs, set 136 on the minutes scale under the STAT index
on the miles scale. Under the NAUT index on the miles scale, read the number of NMs (118) on the
minutes scale (Figure 4.20).
Figure 4.20. Statute Mile, Nautical Mile, and Kilometer Interconversion.
4.9.6. Conversion of Nautical Miles (NM) or Statute Miles to Kilometers (km). A km index is
indicated on the miles scale of the computer at 122. When NM or statute miles are placed under their
appropriate index on the miles scale, kms may be read, on the minutes scale, under the km index.
EXAMPLE: To convert 118 NM to kms, place 118 on the minutes scale under the NAUT index on the
miles scale. Under the km index on the miles scale, read kms (218) on the minutes scale.
4.9.7. Multiplication and Division. To multiply two numbers, for example 12 X 2, the index (printed as
10 on the minutes scale) is placed opposite one of the numbers to be multiplied (12), and the product
(24) is read on the miles scale above the other number (2) on the minutes scale (Figure 4.21).
AFPAM11-216 1 MARCH 2001 127
4.9.7.1. To divide one number by another, for example 24/8, set the divisor (8) on the minutes scale
opposite the dividend (24) on the miles scale, and read the quotient (3) on the miles scale opposite the
index on the minutes scale (Figure 4.22).
Figure 4.21. Multiplying Two Numbers.
Figure 4.22. Dividing One Number by Another.
4.9.7.2. In the computations encountered in air navigation, as in the above examples, a mental estimate
will aid in placing the decimal point.
Section 4C— Effect of Wind on Aircraft
4.10. Basics. Any vehicle traveling on the ground, such as an automobile, moves in the direction in
which it is steered or headed and is affected very little by wind. However, an aircraft seldom travels in
exactly the direction in which it is headed because of the wind effect.
4.10.1. Any free object in the air moves downwind with the speed of the wind. This is just as true of an
aircraft as it is of a balloon. If an aircraft is flying in a 20-knot wind, the body of air in which it is flying
moves 20 NM in 1 hour. Therefore, the aircraft also moves 20 NM downwind in 1 hour. This movement
is in addition to the forward movement of the aircraft through the body of air.
128 AFPAM11-216 1 MARCH 2001
4.10.2. The path of an aircraft over the earth is determined by the two unrelated factors shown in Figure
4.23: (1) the motion of the aircraft through the airmass, and (2) the motion of the airmass across the
earth's surface. The motion of the aircraft through the airmass is directly forward in response to the pull
of the propellers or thrust of the jet engines, and its rate of movement through the airmass is TAS. This
motion takes place in the direction of true heading (TH). This motion of the airmass across the earth's
surface may be from any direction and at any speed. The measurement of its movement is called wind
and is expressed in direction and speed (wind vector [W/V]).
Figure 4.23. Two Factors Determine Path of Aircraft.
4.11. Drift Caused by Wind. The effect of wind on the aircraft is to cause it to follow a different path
over the ground than it does through the airmass. The path over the ground is its track. The terms true
course (TC) and track are often considered synonymous. TC represents the intended path of the aircraft
over the earth's surface. Track is the actual path that the aircraft has flown over the earth's surface. TC is
considered to be future, while track is considered to be past.
AFPAM11-216 1 MARCH 2001 129
4.11.1. The lateral displacement of the aircraft caused by the wind is called drift. Drift is the angle
between the TH and the track. As shown in Figure 4.24 the aircraft has drifted to the right; this is known
as right drift.
4.11.2. With a given wind, the drift will change on each heading. A change of heading will also affect
the distance flown over the earth's surface in a given time. This rate traveled relative to the earth's
surface is known as GS. Therefore, with a given wind, the GS varies on different headings.
4.11.3. Figure 4.25 shows the effect of a 270o/20 knots wind on the GS and track of an aircraft flying on
headings of 000o, 090o, 180o, and 270o. The aircraft flies on each heading from point X for 1 hour at a
constant TAS.
4.11.4. Note that on a TH of 000o, the wind causes right drift; whereas on a TH of 180o, the same wind
causes left drift. On the headings of 090o and 270o, there is no drift at all. Note further that on a heading
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