Why do planes use knots?

The knot predates aviation by centuries. Sailors measured speed by paying out a line knotted at regular intervals behind the ship and counting how many knots ran through their hands while a sand glass emptied. The unit that survived is one nautical mile per hour, and the nautical mile is the useful part: it was defined as one minute of arc of latitude, so a distance measured on a chart with dividers could be read directly off the latitude scale printed on the side. Since 1929 the international nautical mile has been fixed at exactly 1,852 m, roughly 1.15 statute miles, close enough to the original geometric definition for practical navigation.

Early aircraft used whatever their home country used: miles per hour in Britain and the United States, kilometres per hour in most of continental Europe. The shift to knots came with long-range navigation in the 1930s and 1940s, when aircrews borrowed marine charts, marine techniques and marine instructors. Dead reckoning, wind triangles and radio bearings are all easier when speed, distance and coordinates share the same base. Postwar airspace was organised around ICAO, and ICAO's Annex 5 lists the knot, the nautical mile and the foot as non-SI units that may be used alongside metric units for the foreseeable future. Air traffic control, charts, flight plans and instrument approach procedures were built on that convention, and changing it would mean touching every one of them at once.

Not everyone uses knots. Russia, China and several former Soviet states have historically flown in metres and kilometres per hour, which is why glass cockpits offer switchable units and why flights into those regions still require conversion tables and careful briefing. Weights, fuel and temperature are already metric in most of the world, and wind is given in knots even in countries that otherwise think in kilometres. The result is a hybrid that pilots learn to live with: altitude in feet, speed in knots, visibility in metres, pressure in hectopascals, and no plan to unify them soon.

A last point of nuance: the airspeed indicator shows indicated airspeed, which is what the wing feels, not the aircraft's speed over the ground. True airspeed corrects for altitude and temperature, and ground speed adds the wind. All three are quoted in knots, and the distinction is exactly why the unit matters: a pilot planning a 600 NM leg at 450 kn ground speed knows, without a calculator, that it will take one hour and twenty minutes.