The statute mile is a land-based unit of exactly 5280 feet — Page 28, Lesson 39

I want to walk you through the different distance measurements used in aviation, starting with the statute mile. The statute mile is a land-based unit of exactly 5280 feet. While it's widely used on the ground, it's hardly ever used in aviation nowadays. Older airspeed indicators used to be calibrated in miles per hour, or mph, and some American light aircraft still use them, but that's rare now. You will also still see visibility given in statute miles in the USA. Other than that, the only time a modern pilot needs to know about mph is to inform passengers in UK or US aircraft how fast the aircraft is travelling in the same units they use in their cars. Now, the really important one for us is the nautical mile, or NM. The nautical mile is the most important large measure of distance used in aviation because it can be related directly to the angular measurements of the latitude and longitude graticule of the Earth. The ICAO definition — that's the International Civil Aviation Organization — says the nautical mile is a measure of distance of 1852 metres. There is also the Standard Nautical Mile, defined as a length of 6080 feet. You might notice that 1852 metres converted to feet at a factor of 3.28 does not give exactly 6080 feet. For all EASA ATPL practical use, either 1852 metres or 6080 feet will give close enough answers to any problems you will have to solve. Because nautical miles are based on Great Circle arcs, the definition of the nautical mile is similar to the definitions of latitude and longitude. Remember these key relationships: one minute of latitude equals one nautical mile. So one degree of latitude equals 60 minutes, which equals 60 nautical miles. However, one minute of longitude equals one nautical mile at the equator only. Since there are 90 degrees of latitude change between the Equator and the Poles, the distance between the Equator and the Poles is 90 times 60, which is 5400 nautical miles. The circumference of the Earth at the Equator is 360 degrees times 60 minutes, which gives 21,600 nautical miles. Now let's talk about variations in the length of a nautical mile. The full definition of the length of a nautical mile is that it is the length of arc of a Great Circle which subtends an angle of one minute at the centre of curvature of the Earth's surface. It might seem that the longer the Earth's radius, the longer the nautical mile, and that would be true if we used geocentric latitude. However, our system of latitude and longitude is based on geodetic latitude, not geocentric latitude. Because the Earth is flattened at the Poles, the radius of curvature is increased, and a greater arc is required to subtend an angle of one minute at the centre of curvature. Hence, a nautical mile is longer at the Poles than at the Equator.