The Nautical Mile, as a unit of measurement, was first defined as an international standard in Monaco, at the First International Extraordinary Hydrographic Conference in 1929. Prior to that, there was no international standard for measuring distances either while traveling in water or, or more importantly, over water.
The United States did not adopt the Nautical Mile as its standard measurement in 1929, but did jump on board in 1954 and now recognizes this international standard. A nautical mile, logically based on the circumference of the earth, is equal to one minute of latitude, and is slightly longer than a statute mile (which is used for measurements of distance on land.)
Nautical Miles Versus Statute Miles
In the world of aviation, the standard way to measure distance is the nautical mile. One exception is when stating Visual Flight Rules (VFR). According to the Aircraft Owners and Pilots Association, when a pilot refers to VFR, they mean:
"Three statute miles' visibility and far enough away from the clouds that they don't hassle you, you don't interfere with aircraft flying under instrument rules that are coming out of the clouds, and you can see where you're going and spot other airplanes."
In this case, pilots use the statute mile (or the familiar land mile, which is 5,280 feet and it's based on paces) as opposed to the nautical mile. These VFR requirements, which all pilots know, refers to the basic VFR weather minimum (14 CFR 91.155) and is specific to various types of airspace and altitudes.
The VFR weather minimum boils down to the rationale of needing greater visibility and more distance from clouds when flying above 10,000MSL (otherwise know as "mean sea level," which measures aviation altitude.) This minimum exists because pilots need more time to see and avoid other aircraft while traveling in and out of clouds.
Cloud Clearance is Another Exception
Another exception to the iron-clad universality of the nautical mile's use in aviation is measurement regarding cloud clearance. Cloud clearance measurements employ statute miles rather than nautical miles. A precise cloud clearance requirement is necessary because it's hard to know if the visual out the hangar door window is accurate with variable meteorological conditions.
Below are the definitions of both nautical miles and statute miles, along with a few easy conversions:
Statute Mile (SM):
- 1 SM = 1,609 meters
- 1 SM = 5,280 feet
- 1 SM = .869 NM
Nautical Mile (NM):
One nautical mile, as defined by NOAA, is: “A unit of distance used in marine navigation and marine forecasts. It is equal to 1.15 statute miles or 1,852 meters. It is also the length of 1 minute of latitude.”
- 1 NM = 1,852 meters
- 1NM = 6,076 feet
- 1NM = 1.151 statute miles
While flying, distance is generally measured in nautical miles and visibility is usually stated or forecasted in statute miles.
When exploring nautical miles, it's worth noting the difference with often-used terms such MPH and knots.
- MPH: In statute miles, speed measurements are given in miles per hour, the same as in vehicles.
- Knot: The standard speed measurement in aviation is the knot. One knot is equal to one nautical mile per hour. Airspeed indicators on aircraft are calibrated in knots.