Definition
A unit of distance equal to 5,280 feet (approximately 1,609 meters), used in aviation primarily to express surface visibility in weather reports and forecasts. One statute mile is shorter than one nautical mile (1 NM ≈ 1.15 SM).
Plain English
The regular kind of mile used on roads and in everyday life. In aviation, it shows up mainly when talking about how far you can see — visibility is reported in statute miles, not nautical miles.
Context Anchor
Seen in airport weather reports, instrument approach minimums, and weather requirements for flight operations.
Derivation
Statute' comes from Latin statutum, meaning 'a law or decree.' The statute mile was the mile fixed by English law in 1593 — hence 'the legal mile.' Aviation kept it for visibility because weather observers on the ground have always measured distance in statute miles, while pilots use nautical miles for navigation over the curved Earth.
Why Pilots Care
Weather and approach minimums are published in statute miles, so a pilot must know the exact distance to decide whether legal requirements are met.
Grounding Statement
If a weather report says visibility is 2 SM, it means you can expect about two ordinary land miles of horizontal visibility.
Intuition Check
Do not read SM as nautical miles. In weather reports and approach minimums, SM means statute miles: ordinary land miles of 5,280 feet each.
Example Sentence 1
The METAR reported visibility as 2 SM in light rain, just above the approach minimum.
Example Sentence 2
Part 135 daytime VFR in Class G airspace requires 1 SM of visibility.