Why do airplanes use nautical miles instead of kilometres?

It is more practical for pilots and even for Sea Captains to calculate the distances if we use the nautical miles. I will give you an example. Suppose, you want to fly from:

Longitude (25 degrees West to 45 degrees West) and you are on a Latitde of 60 degrees North. What will be the distance that will be covered?

So, we have the formula;

Departure = Change in Longitude x 60 x Cos lat
                  = (45-25) x 60 x cos 60
                  = 600 nautical miles.

Why we put Cos lat is because, as we move from equator either to South or the North, the earth converges, decreasing the distance. This gives a simple cosine relation. At equator: Cos 1 (one) and at the poles Cos 0 (zero). If you are exactly on the equator, you can just multiply the change in latitude by 60 and get the answer.

One degree of latitude is 60 nautical miles. So the distance between North pole and equator is;
 
* 90 (the amount of degrees between them) x 60 =  5400 nm (which is like 10000 km)

Hope this helps!

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