The statute mile, abbreviated stat.m, is 5280 feet — Page 28, Lesson 39

Let’s start with the statute mile, because it’s the one you’ll barely use. The statute mile, abbreviated stat.m, is 5280 feet. It’s the mile you know from roads and cars. In aviation, it’s almost gone. Older airspeed indicators were calibrated in mph, and a few American light aircraft still are, but that’s rare now. The one place you’ll still meet it professionally is visibility, which is still given in statute miles in the USA. Otherwise, the only time a modern pilot needs mph is to tell passengers in UK or US aircraft how fast they’re going in the same units their cars use. So, statute mile: 5280 feet, ground use, almost no aviation use. Now the nautical mile, the NM. This is the most important large distance measure in aviation, and the reason is that it ties directly to the angular measurements of the latitude and longitude graticule — the grid of lines on the Earth. The ICAO definition is that the nautical mile is a measure of distance of 1852 metres. There’s also the Standard Nautical Mile, defined as 6080 feet. Now, you might notice that 1852 metres converted to feet at a factor of 3.28 does not give exactly 6080 feet. That’s fine — for all EASA ATPL practical use, either 1852 metres or 6080 feet will give close enough answers to any problem you have to solve. Here’s the key relationship. Because nautical miles are based on Great Circle arcs, the definition is similar to the definitions of latitude and longitude. Remember this: one minute of latitude equals one nautical mile. One degree of latitude equals 60 minutes, which equals 60 NM. But — and this is the critical contrast — one minute of longitude equals one NM at the equator only. Away from the equator, longitude minutes shrink, so that rule doesn’t hold. Let’s build on that. There are 90 degrees of latitude change between the Equator and the Poles, so the distance between them is 90 times 60, which is 5400 NM. The circumference of the Earth at the Equator is 360 degrees times 60, which is 21,600 NM. Now, variations in the length of a nautical mile. The full definition is that the nautical mile 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. Let me unpack that. Imagine a Great Circle arc on the Earth’s surface. The angle at the centre of curvature — the point you’re measuring from — is one minute. The length of arc that corresponds to that one-minute angle is the nautical mile. 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. But our system of latitude and longitude is based on geodetic, 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 longer nautical mile. So the nautical mile isn’t a fixed length everywhere; it varies with the Earth’s shape, and that variation comes from the difference between geodetic and geocentric latitude.