The 2 Basic Types of Aircraft Navigation Routes

19/04/2023

There are two basic route types in air navigation, the “great circle” and the “rhumb line” – what’s the difference between the two? First of all, we should point out that the naked eye can deceive.

According to common sense as well as what we all learned in school, the shortest distance between two points is a straight line. But…that’s true on a flat, two-dimensional surface. When it comes to going from point A to point B by plane, straight lines go out the window, since now we’re dealing with the spherical surface which is the Earth. And that’s where these two rout types come in: is where you can choose two types of route: orthodromic and loxodromic.

The Great Circle (Orthodromic) Route

The word orthodromic comes from the Greek orthos, meaning “straight”, and dromos, “race” or “road” – hence this is the route that’s the straightest or shortest. Essentially it’s the arc of a great circle that joins two points on the Earth an imaginary line that splitting the planet into two identical halves. Vertically, each of the meridians would be a great circle, for example; horizontally, only the equator would be.

So if a plane departs from point A we need to imaginarily connect it to point B through that circular route. On a map, which is in two dimensions, the great circle route will be represented by a curved line, and precisely because it is two-dimensional, you might be surprised to see that to fly for example from Buenos Aires to Beijing that line has to pass over northern Europe – but you can see it’s true that it’s the fastest and most direct if you run your finger over a globe between these two points.

The Rhumb-Line (Loxodromic) Route

Loxos from the Greek means oblique, so if an orthodromic path is a segment of a great circle, loxodromic describes a segment of a spiral, and while it isn’t necessarily the shortest, it may sometimes still turn out to be the better option. A 16th-century cartographic discovery revealed that following the earth’s surface in a fixed direction (the one marked by the compass) wouldn’t be a great circle but rather a spiral that would close on itself to infinity. In other words, when pilots undertake a loxodromic route, they’re following a fragment of a spiral on the terrestrial sphere that would gradually approach one of the poles without ever reaching its centre, with a smaller and smaller radius.

On a two-dimensional map this route is represented by a straight line, since an aircraft does travel in a straight line without turning or changing course. But again, the view deceives us: although on the two-dimensional plane we see that it’s much shorter, the truth is that in fact it’s longer.

So Why Is Rhumb Line Route Sometimes Better than Great Circle?

It has to do with the angle at which the aircraft cuts across each meridian. In the case of the great circle route, the trajectory a plane must take intersects each meridian at a different angle. This implies that you have to make constant course changes, since the navigation routes are designed to trace straight segments and not curves. Whereas in the case of the rhumb, the plane flies in a straight line. Yes, it flies over a sphere, but it doesn’t spin as it moves.

The great circle route does offer the advantage of saving time and fuel, whereas the rhumb line’s is that it’s easier to follow, since the route is straightforward and unchanging. Therefore it depends on what type of flight is planned. The usual thing is to use the rhumb line on short flights – since the difference in time and energy expenditure is minimal – and the great circle on longer hauls, since here the priority is to use less fuel.

As we noted above, an airplane cannot follow a curve, but it can draw small straight segments that amount to the same thing. On Flightradar24.com for example, it’s easy to discern the path of an airplane between two points at a great distance: it’s not curved, but rather composed of short straight lines which when joined together look very much like a curve. Similarly, short-haul flights are often represented by fairly straight lines.

Of course, these types of route are the ones that an aircraft chooses in an ideal situation. But there are many other factors that affect the final route of an airplane, from the already predefined.

The ‘Single European Sky’ Seeks to Improve Air Traffic

This is an iniative (SES for short) which has been promoted since 1999 to unite the currently fragmented air traffic management system at the institutional, operational, technological, and control and supervision levels. On the one hand, SES is about preparing European airspace for the future and minimising the environmental impact of civil (and, where possible, military) aviation. The division of airspace would no longer have national borders to be managed by functional blocks, which would make traffic more efficient. It also seeks the modernisation of the system required by the growing traffic. The steps which have been taken in this direction over the years have always been with an eye toward making the European air-navigation system efficient, safe and respectful of the planet, and from which the rest of the world would also benefit.

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