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15-9
Figure 15-13. Motion of the air affects the speed with which aircraft move over the Earth’s surface. Airspeed, the rate at which an aircraft moves through the air, is not affected by air motion.
090°
Groundspeed 120 knotsWINDS ARE CALM090°Groundspeed 140 knotsWINDS 270° AT 20 KNOTS090°Groundspeed 100 knotsWINDS 090° AT 20 KNOTS
Effect of Wind
The preceding discussion explained how to measure a true course on the aeronautical chart and how to make corrections for variation and deviation, but one important factor has not been considered—wind. As discussed in the study of the atmosphere, wind is a mass of air moving over the surface of the Earth in a definite direction. When the wind is blowing from the north at 25 knots, it simply means that air is moving southward over the Earth’s surface at the rate of 25 NM in 1 hour.
Under these conditions, any inert object free from contact with the Earth is carried 25 NM southward in 1 hour. This effect becomes apparent when such things as clouds, dust, and toy balloons are observed being blown along by the wind. Obviously, an aircraft flying within the moving mass of air is similarly affected. Even though the aircraft does not float freely with the wind, it moves through the air at the same time the air is moving over the ground, thus is affected by wind. Consequently, at the end of 1 hour of flight, the aircraft is in a position which results from a combination of the following two motions:
• Movement of the air mass in reference to the ground
• Forward movement of the aircraft through the air mass
Actually, these two motions are independent. It makes no difference whether the mass of air through which the aircraft is flying is moving or is stationary. A pilot flying in a 70- knot gale would be totally unaware of any wind (except for possible turbulence) unless the ground were observed. In reference to the ground, however, the aircraft would appear to fly faster with a tailwind or slower with a headwind, or to drift right or left with a crosswind.
As shown in Figure 15-13, an aircraft flying eastward at an airspeed of 120 knots in still air has a groundspeed (GS) exactly the same—120 knots. If the mass of air is moving eastward at 20 knots, the airspeed of the aircraft is not affected, but the progress of the aircraft over the ground is 120 plus 20, or a GS of 140 knots. On the other hand, if the mass of air is moving westward at 20 knots, the airspeed of the aircraft remains the same, but GS becomes 120 minus 20, or 100 knots.
Assuming no correction is made for wind effect, if an aircraft is heading eastward at 120 knots, and the air mass moving southward at 20 knots, the aircraft at the end of 1 hour is almost 120 miles east of its point of departure because of its progress through the air. It is 20 miles south because of the motion of the air. Under these circumstances, the airspeed remains 120 knots, but the GS is determined by combining the movement of the aircraft with that of the air mass. GS can be measured as the distance from the point of departure to the position of the aircraft at the end of 1 hour. The GS can be computed by the time required to fly between two points a known distance apart. It also can be determined before flight by constructing a wind triangle, which is explained later in this chapter. [Figure 15-14]
The direction in which the aircraft is pointing as it flies is heading. Its actual path over the ground, which is a combination of the motion of the aircraft and the motion of the air, is its track. The angle between the heading and the track is drift angle. If the aircraft heading coincides with the true course and the wind is blowing from the left, the track does not coincide with the true course. The wind causes the aircraft to drift to the right, so the track falls to the right of the desired course or true course. [Figure 15-15]
15-10
Figure 15-15. Effects of wind drift on maintaining desired course.
Heading
WindTrackDrift angleDesired course
Figure 15-16. Relationship between true, magnetic, and compass headings for a particular instance.
Heading
TNMNCNTH-088°MH-078°CH-074°VAR 10° EDEV 4°
Figure 15-14. Aircraft flightpath resulting from its airspeed and direction, and the wind speed and direction.
Airspeed eff
ect (1 hour)20 knotsDistance covered over ground (1
hour)
The following method is used by many pilots to determine compass heading: after the TC is measured, and wind correction applied resulting in a TH, the sequence TH ± variation (V) = magnetic heading (MH) ± deviation (D) = compass heading (CH) is followed to arrive at compass heading. [Figure 15-16]
By determining the amount of drift, the pilot can counteract the effect of the wind and make the track of the aircraft coincide with the desired course. If the mass of air is moving across the course from the left, the aircraft drifts to the right, and a correction must be made by heading the aircraft sufficiently to the left to offset this drift. To state in another way, if the wind is from the left, the correction is made by pointing the aircraft to the left a certain number of degrees, therefore correcting for wind drift. This is the wind correction angle (WCA) and is expressed in terms of degrees right or left of the true course. [Figure 15-17]
　
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