This topic often produces a lot of confusion in the General Navigation module, so here is an article to clear up any misconceptions and provide some simple but effective tips for solving these types of questions!

Background: Great Circles and Rhumb Lines

The shortest distance between two points on the curved surface of the Earth is called a Great Circle. Flying this shorter route actually involves constantly altering the aircraft’s track! If you were to fly from point A to point B using a constant track with respect to True North, you would actually be flying a longer distance! This constant track is called a Rhumb Line. The image below shows how these appear in 3D.

Here is how a great circle track and a rhumb line track appear on a Mercator map (in 2D), for a flight from Madrid to New York. The Great Circle seems like the further of the two tracks, but this is only because it is being represented on a 2D map.

Drawing Diagrams representing Convergency and Conversion angles on a Mercator map:

Most questions involving Convergence and Conversion angle in Gen Nav are made infinitely more simple by learning how to draw the two diagrams below. To help you draw them, just remember to draw the Great Circles as being concave to the equator. Easy way to remember this: “Mr Mercator lives in a cave on the equator.”

Northern Hemisphere:

In the diagram above, point A and point B are in the Northern Hemisphere – the Great Circle track is concave to the equator which would be at the bottom of the diagram. You can see visually from he diagram that the Great Circle track at B is greater than the Great Circle track at A. The rhumb line track is constant.

Southern Hemisphere

In this diagram, point A and point B are in the Southern Hemisphere – the Great Circle track is concave to the equator, which would be at the top of the diagram. Note that in this case, the Great Circle track at A is greater than the Great Circle track at B. The Rhumb Line track is once again constant.