Advanced Pilot Training: TM 1-205 Air Navigation - Section VI Effect of Wind

43. Definitions.-a. (1) Air speed.-True speed of an aircraft relative to the air. It is the true air speed unless otherwise stated. Air speed is obtained by correcting the calibrated air speed for density, using temperature and pressure altitude corrections.

(2) Ground speed.-Actual speed relative to the earth's surface.

(3) Drif t.-Angle between the heading and the track. It is named right or left according to the way the airplane is drifted.

(4) Drift correction.-Angle added to or subtracted from an aircraft's course (true) to obtain heading. In case of a right drift the angle is subtracted from the course to obtain the heading, and in case of a left drift it is added. A 5° drift correction to the right would be written as + 5°; the drift would be L.5°. A 5° drift correction to the left would be written -5°; the drift would be R. 5°.

(5) Course (C.).-Direction over the surface of the earth, expressed as an angle, with respect to true north, that an aircraft is intended to be flown. It is the course laid out on the chart or map and is always the true course unless otherwise designated.

(6) Magnetic course (M. C.) -Course (true) with variation applied.

(7) Compass course (C. C.)-Magnetic course with deviation applied.

(8) Course made good.-Resultant true direction the aircraft bears from the point of departure. (All courses are measured from north through east of 360°.)

(9) Track.-Actual path of an aircraft over the surface of the earth. Track is the path that has been flown. Course (true) is the path intended to be flown.

(10) Heading.-Angular direction of the longitudinal axis of the aircraft with respect to true north. It is the course with the drift correction applied. It is true heading unless otherwise designated.

(11) Magnetic heading.-Heading with variation applied.

(12) Compass heading.-Magnetic heading with deviation applied.

(13) Wind direction and force.-Wind is designated by the direction from which it blows. Force of the wind is expressed as the speed in miles per hour or knots.

(14) Velocity.-Rate of change of position in a given direction. it involves both speed and direction.

(15) Line.-A straight line may represent a velocity; direction is represented by the position of the line; speed by the length of the line.

b. No wind.- (1) General.-An airplane in flight, when there is no wind, will have the same air speed and ground speed. It will also have the same heading and course. This condition seldom prevails, so the effect of the wind must usually be taken into consideration. The change in rate of travel (air speed or ground speed) and the change in direction (course or heading) may be solved by the same vector diagram. There are two simple cases which do not need a diagram, and these are the rare occasions when the airplane course is exactly downwind or into the wind.

(2) Example- (a) An airplane, air speed 150 m. p. h., is flown directly into a 20 m. p. h. head wind. The ground speed then equals 130 m. p. h. and the heading is the same as the course.

(b) An airplane, air speed 150 m. p. h., is flown directly down wind in a 20 m. p. h. wind. The ground speed then equals 170 m. p. h. and the course and heading are the same.

44. Vector diagrams.-a. Velocities.-A velocity may be represented in a vector diagram by a straight line. The direction of the line is drawn in the same direction as the motion, and the length of the line represents the rate of motion. Two component velocities may be resolved into the resultant velocity.

b. Wind diagrams.-In the wind vector diagram the heading and air speed are drawn as one component and the wind direction and wind speed as the second component. The resultant then indicates the course and ground speed. (In the actual movement of the airplane over the earth, the resultant is the track.)

45. Usual problem, type 1-a. To determine heading and ground speed.- (1) The type of diagram described in this paragraph will hereafter be called type 1 in order to shorten the nomenclature. This type of diagram is constructed when it is desired to determine the heading and the ground speed, knowing the course, the air speed, and the wind velocity. A diagram of this type may be drawn directly on a map, using the map scale, or it may be constructed on an ordinary sheet of paper.

(2) In order to make the necessary allowance for the effect of wind and to find the compass heading from the compass course, the action of the wind upon an aircraft must be fully understood. A free balloon is carried with the wind and at the same speed as the wind, just as a cork is carried on the surface of a stream. If we substitute for the cork a toy motorboat which requires a minute to cross a small stream, and the stream is flowing at the rate of 10 feet per minute, even though the boat is headed directly across stream it will still feel the full effect of the current. During the minute of its crossing it will be swept 10 feet downstream and will reach the opposite bank at a point 10 feet below the point of departure. The solid line of figure 27 represents the path of the boat in crossing the stream. In exactly the same way an airplane in flight is subject to the full effect of the wind, even though the plane may be moving under its own power in an entirely different direction.

(3) For example, if a plane were headed due east from A at an air speed of 100 m. p. h., it would reach a point B 100 miles away in just I hour if there were no wind. If a wind were blowing from the north the plane would actually arrive in 1 hour at a point to the south of point B. In order to be able to make good the desired course to the east, the airplane must be headed into the wind by an amount which will counteract the tendency of the wind to cause the airplane to be drifted south of the 90° course. This change of heading (the heading will be less than 90°) will also cause a change in the ground speed. The example in b below shows how the heading and ground speed are determined by constructing a diagram.

b. Type I diagram.- (1) First example.-To find heading and ground speed.

(a) Given: Desired course 90°.

Air speed 100 m. p. h.

Wind velocity 20 m. p. h. from 315°

Required: Heading.

Ground speed.

(b) The first step is the construction of a north-south line. A mental calculation or rough sketch indicates that this line may be placed close to the left side of the paper, as all of the diagram will be on the right side of it. So it is drawn close to the left border of the paper and then a starting point A is chosen.

(c) From A the course line 900 is drawn as AX. (See fig. 28.)

(d) From A the wind line AB is drawn down wind from 3150.

(e) The distance AB is made equal to 20 units of the scale it is desired to use.

(f) From B the point C is located so that by construction the distance BC equals the air speed for 1 hour or 120 m. p.h.

(g) The triangle ADC is constructed in this example so that ABCD is a parallelogram.

(h) The required heading will be the direction indicated by the line AD (same as shown by line BC.)

(i) The required ground speed will be shown by the number of units in the length of the line AC.

(j) It usually is not necessary to complete the parallelogram, as the required heading and ground speed may be determined from the triangle ABC. Note that the angle NAD, which is the heading angle, is the same as the angle NEC (CB is continued through B to E). The heading may therefore be determined by finding the direction of BC and so the side AD is not actually needed.

(2) Second example.-To find heading, ground speed, and time.

(a) Given: Course 243°.

Air speed 140 m. p. h.

Wind 20 m. p. h. from 278°.

Distance A to M 248 miles.

Required: Ground speed out.

True heading out.

Time for flight.

(b) Determine the position of the north-south line and point A. (See fig. 29.)

(c) Draw AX 243°.

NOTE.-This line need be only long enough to help complete a 1-hour diagram. In this type of problem it does not have to extend all the way to the point M.

(d) Draw AB, wind velocity 20 m. p. h. from 278°.

(e) Draw BC, air speed 140 m. p. h.

(f) Measure AC, ground speed.

(g) Measure angle D, heading angle.

(h) Divide distance 248 by ground speed 123.

(i) Change resulting hours and fractions to hours and minutes.

U) The required answers are given in (f), (g), and (i) above.

46. Rules and helps. - a. The principal source of accuracy in diagrammatical solutions is neatness. Lines should be drawn with a hard lead pencil sharpened to a fine point. The paper should be placed on a hard surface and in sufficient light.

b. To avoid mistakes, all lines should be labeled as soon as they are drawn. This will insure easier checking of the diagram after completion.

c. Read the problem through two or three times so that there is no doubt as to what is required and the methods to use. It is helpful to make a rough approximate sketch before starting the detailed accurate diagram.

d. Place the diagram correctly in the available space. If the diagram will all be done on one side of the N-S line, do not draw this line down the center of the paper.

e. Use 1 hour units unless there is a reason for using more or less than this amount.

f. There are sometimes several ways to work some types of diagrammatical solutions. The ones shown in this manual are best for all types of such problems.

g. Wind always blows the aircraft from the heading to the course.

h. Course and ground speed are always on the same line.

i. Heading and air speed are always on the same line.

j. In all diagrams illustrated in this section of the manual the wind is always plotted downward.

k. Only true directions are used in diagrams. If the data give a compass course, change it to a true course, using arithmetic, before drawing the diagram. If the problems give a compass heading, change it to a true heading, arithmetically, before starting to draw.

47. Compass heading. -a. Course and heading. - (1) It has already been shown that the compass course is the direction by compass in which a plane should be headed in order to reach its destination in still air or with the wind parallel to the course. It was also defined as the true course plus or minus variation and deviation but with no allowance for wind. To avoid any confusion at this point remember that-

(a) Compass course is the true course plus or minus variation and deviation but without allowance for wind effect.

(b) Compass heading is the true course plus or minus variation and deviation and including allowance for wind. It is the direction by compass in which the plane is pointed.

(2) Another method of illustrating the differences between course and heading is to enumerate them as follows:

1. True course.                                        1. True course.

2.                                                            2. Drift correction.

3.                                                            3. True heading.

4. Variation.                                             4. Variation.

5. Magnetic course.                                 5. Magnetic heading.

6. Deviation.                                            6. Deviation.

7. Compass course.                                 7. Compass heading.

In the first group of seven lines there has been no correction for drift, while in the second group the drift correction has been made.

Each starts with true course and each ends with a compass direction. If the drift correction is applied to line 5 of the first group it will then become line 5 of the second group. Line 7 may be changed in the same manner.

b. Application- (1) In actual use the true heading is changed to the compass heading before it is used by a pilot. To do this, he applies variation and deviation to the true heading which then becomes the compass heading.

Example: If the problem illustrated in paragraph 45b (2) had included, in the data given, variation 11°  east and deviation 2° west, it would be possible to complete it for a required compass heading. The true heading was determined to be 247 1/2 °or 248°. Taking the latter figure, the next steps would be

True heading 248°.

Variation 11° east (subtract). Magnetic heading 237°.

Deviation 2° west (add).

Compass heading 239° (answer).

(2) In order to make good a course of 243°, with the wind and air speed as shown in paragraph 45b (2), and variation and deviation as shown above, the pilot would head the airplane so that the compass reading would be 239°. The resulting compass heading in this case would not be many degrees differentt from the original course. Often in actual practice this difference will be quite large.

48. Reverse problem, type 2. -a. To find course and ground speed.-(1) In the preceding discussion the usual type of problem has been considered, namely, determining from the chart and from the wind data, when planning a flight and before taking off, the distance between points, the compass heading to be followed, and the ground speed.

(2) The second case is concerned with plotting on the chart while in flight, from the observed compass heading and ground speed, the track being made good and the position of the plane along the track at any time. It may seem that this should never be necessary if the course is properly determined before beginning the flight; however, wide departures from the charted route are altogether possible, intentionally or otherwise. In this event it may happen that after leaving a certain position the only data which can be obtained are compass heading, approximate ground speed, and elapsed time.

(3) Essentially, this problem is the reverse of the first, In type 1 we start with the true course measured on the chart and apply the drift correction, variation, and deviation in order to obtain the compass heading. In type 2, starting with the compass heading observed in flight, all these factors are included and must be taken away in order to obtain the true course to be plotted on the chart. Obviously then all the rules of type I must be reversed. Whatever would have been added then must be subtracted now, and vice versa.

(4) (a) This process of taking away or changing may be called "rectifying," and to do so requires three steps:

1. Rectify the compass heading for deviation to obtain the magnetic heading (magnetic direction in which the plane is pointed).

2. Rectify the magnetic heading for variation to obtain the true heading (true direction in which the plane is pointed).

3. Rectify the true heading for wind to obtain the true course (track) being made good over the ground.

(b) The first two steps are done by arithmetic in order to obtain the true heading. The last step may be done graphically or by mechanical means, by use of tables, by using a computer, or even by mathematical means.

b. Type 2 diagram.-(1) First example.

(a) Given:      Compass heading 820.

Air speed 100 m. p. h.

Wind 20 m. p. h. from 315°.

Variation 10° east.

Deviation 2° west.

Required:       Course (or track) being made good.

        Ground speed.

(b) By arithmetic find the true heading. This may be done by reversing the rule of "East is least, west is best," or it may be done by filling in the known values as follows:

(c) Although this may seem longer than the method of reversing a previously learned rule, it has the advantage of allowing a rapid check of the arithmetic by using, from top to bottom of column B, the same rule used before.

(d) The diagram is now drawn as follows (fig. 30) : From point A on the N-S line draw AB equal to the true heading 90° in direction and to the air speed in length. Then from point B draw BC equal to the wind velocity. Connect A and C, and AC will represent the course (or track) and also the ground speed.

(e) Note that in this diagram, type 2, the wind is drawn from a point 1 hour's air speed away from the starting point. Also that the rule of the wind blowing the airplane from heading to track applies as it should.

(2) Second example.

(a) Given:      Compass heading 242°.

Air speed 100 m. p. h. Wind 18 m. p. h. from 200°.

Variation 11° west.

Deviation 2° west.

Required:       Course (or track).

        Ground speed.

(b) Change compass heading to true heading.

(c) Draw AB (fig. 31) equal to heading and air speed.

(d) Draw BC equal to the wind velocity.

(e) Draw AC, which equals the course (track) and the ground speed.

49. Solving for Wind.-Another type of wind diagram may be used if the heading and air speed and also the course and ground speed are known.

Example (fig. 32) : Let AB equal the true heading and air speed.

Let AC equal the course and ground speed. Then BC will represent the wind velocity, and the wind would be blowing in the direction f rom B to C with speed equal to the length BC.