Air Navigation: Lesson One

The in-flight information screen and I had a difference of opinion.

Stick with this one, there’s a point to it.

Distances like miles and kilometres are created by imagining that the circumference of the Earth is a true circle. Cars, trains and people travel along this so-called ‘Great Circle’, so it’s easy to determine distance.

Aeroplanes, however, fly at a height above the great circle (my trans-Atlantic flight was at 36,000 feet). So travelling 10km at that height wouldn’t get you as far as travelling 10km at ground level. So we need something else: nautical miles (nm). This is calculated by looking at the angle between the two pints you are flying between. One nautical mile is one minute of arc (one sixtieth of one degree). Thus the Earth’s circumference is 21,600 nm. Speed is measured in nautical miles per hours, or knots (kts).

So would someone explain to me why, during yesterday’s flight, all the distances were given in kilometres and miles, and the speeds in kh/m and mph? Are these people trying to undermine 65+ years of Air Cadet teaching??