If we go to the thesaurus and look up “course”, “course” will certainly appear among the words with the same meaning. However, in aviation there is an important difference. Aircraft courses and headings are not the same thing, or at least they almost never are. Wind and magnetic fields play a major role here. Let’s take a closer look.
Aircraft Courses and Directions
First of all, it is interesting to review the meaning of direction. According to the RAE, it is the “path or course that a body follows in its movement”. In the case of an aircraft, it is essential to have a reference point.
When we think about the course of an airplane (or course: at this point they are still the same), we will need a compass to mark those references. Compasses mark the magnetic north of the Earth and the needle moves on a circumference divided in 360 degrees. Thus, all the values we are going to see will be expressed in angles.
True North and Magnetic North
The compass is a magnet… and so is the Earth. The measuring device points to the North Pole. However, magnetic north (that is, the one marked by the compass) and true north are not exactly the same point.
The meridians, which are the ones that mark the time zones, converge at the same point to the north, which is where the planet’s axis of rotation is located. Well, that is the true (or geographic) north.
Magnetic north is the point to which compasses point. It is close to true north, but not exactly in the same place. In fact, since it began to be measured in 1831, it has been observed that it is moving at a rate of 15 kilometers per year. It is important to be aware of this variation, which must be corrected during flights.
We can define the course as the planned route between point A and point B. This is not the dictionary definition, but it is a very precise one in aeronautical terms.
And here appears the first angle that interests us, which is the true course: it is the angle between true north and the course of the aircraft. The true course is measured in degrees and in the same direction as clockwise. Thus, with a simple protractor we can measure the true course on a map, since all meridians point to true north.
As we have already seen, true north and magnetic north do not coincide. That is to say, between both there are some degrees of difference, and to that angle that both form we know it as magnetic variation. Well, the magnetic course is the angle between the magnetic north and the course of the plane.
How do pilots interpret all these measurements?
Depending on where the aircraft is, the angle between course and true north will be greater or less than the angle between course and magnetic north. If the variation is to the east (i.e., magnetic north is to the east of true north), we must subtract the magnetic variation from the true course to obtain the magnetic course. And if it is to the west, we must add it.
Heading and the effect of the wind
So far we have seen the importance of differentiating the two northerlies in order to be able to orient an aircraft correctly on its course. Now, let’s see how the wind affects, because this will determine the heading.
If the airplane experiences the action of a crosswind, it will displace the aircraft from its trajectory. That is, it will take it off its intended path. To counteract this trajectory drift, what the pilot does is to point the nose of the aircraft slightly into the wind. In other words, the aircraft varies the course to follow the desired course (provided that the wind modifies the latter).
Thus, we have a definition for the heading: it is the direction in which the nose of the aircraft points.
The angle between true north and the heading of the aircraft.
In this case, the angle is determined by magnetic north and the aircraft heading.
The drift correction angle
In addition to courses and headings, some other concepts are worth knowing. We have already seen that the force of side winds affects the trajectory. This displacement is called drift and the aircraft modifies its heading so that the course remains the same. That is, there is an imaginary line that goes from the origin to the destination, which would be the course, and another that starts from the same point, but deviates slightly from the course. The angle formed between these two lines is known as the drift correction angle.
This is another value that affects aircraft courses and headings. The compass measures magnetic north, but other magnetic fields may interfere and give an erroneous result. The engine and radio equipment are two of the main interferences, which are corrected by systems built into the aircraft.
Photo | aapsky