•  Movement of the air mass in reference to the ground  •  Forward movement of the aircraft through the air mass  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.  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 16-13,  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  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 16-14]  The direction in which the aircraft is pointing as it flies is  called heading. Its actual path over the ground, which is a  combination of the motion of the aircraft and the motion of  the air, is called track. The angle between the heading and  the track is called drift angle. If the aircraft heading coincides  with the TC and the wind is blowing from the left, the track  does not coincide with the TC. The wind causes the aircraft  to drift to the right, so the track falls to the right of the desired  course or TC.  [Figure 16-15]  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. In other words, 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 TC.  [Figure 16-17]  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 16-16]  Figure 16-13.  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.  16-9  090°  Groundspeed 120 knots  WINDS ARE CALM  090°  Groundspeed 140 knots  WINDS 270° AT 20 KNOTS  090°  Groundspeed 100 knots  WINDS 090° AT 20 KNOTS 