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What is Great Circle & Rhumb Line ?

Air Navigation - Lesson 2

Introduction:

As the long-range and ultra-long-range non-stop commercial flights slowly turn into reality, the accurate characterization and optimization of flight trajectories become even more essential.

The Earth is not a perfect sphere and due to rotational and gravitational effects the shape is more of an oblate spheroid

Due to the economy of flight, we are particularly interested in the shortest distances between two arbitrary points on Earth.

In a flight path from New York to Madrid, if I asked you which line is shorter, you’d say the straight one, right?

However, a straight line in a 2-dimensional map is not the same as a straight line on a 3-dimensional globe. This is why flight paths take an arc route between an origin and a destination.

Now here’s how the same flight paths look like on a sphere.

 

We can navigate in at least three different manners depending on what kind of flight are we going to perform or plan:

Great-circle navigation or orthodromic navigation is the practice of navigating an aircraft along a great circle. Such routes yield the shortest distance between two points on the globe.

A straight line drawn on a gnomonic chart would be a great circle track. When this is transferred to a Mercator chart, it becomes a curve.

• In navigation, a Rhumb line or loxodrome navigation is an arc crossing all meridians of longitude at the same angle, that is, a path with the constant bearing as measured relative to true or magnetic north.

On a Mercator projection map, any rhumb line is a straight line; a rhumb line can be drawn on such a map between any two points on Earth without going off the edge of the map.

Finally, there is one special route which takes advantage of upper winds currents flow called jetstreamsNorth Atlantic Tracks or called scientifically brachistochrone routes.

These heavily traveled routes are used by aircraft traveling between North America and Europe, flying between the altitudes of 29,000 and 41,000 feet inclusive in the The North Atlantic High Level Airspace (NAT HLA).

They are aligned in such a way as to minimize any headwinds and maximize tail winds impact on the aircraft. This results in much more efficiency by reducing fuel burn and flight time.

To make such efficiencies possible, the routes are created twice daily to take account of the shifting of the winds aloft and the principal traffic flow, eastward in North America evening and westward twelve hours later.

But we will cope this argument better in the next lesson.

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