[N19PEDL2]

Imperial units (specifically, feet) are dominant in aviation to measure flight altitude. Despite a recommendation of ICAO to use SI units since 1979, only China, Mongolia and a few former-USSR countries (not Russia) use metres to define flight levels. [0]

This leads to two interesting consequences for aircraft that perform international flights from/to China: [1]

- they need to be equipped with two altimetry systems;

- they need to perform small climbs or descents when entering or leaving Chinese airspace, as metric flight levels do not usually match Imperial ones (flight levels are defined by round numbers).

[0] https://en.wikipedia.org/wiki/Flight_level#Metric_flight_lev...

[1] https://skybrary.aero/articles/china-reduced-vertical-separa...

[azalemeth]

The main argument for imperial units in aviation I have seen relate to the fact that one nautical mile is almost exactly one minute of latitude on earth, and one knot is one nautical mile per hour, and therefore an aviator who may experience a total electrical failure and is left with his "six-pack" of unpowered, passive-only instruments could navigate with a wristwatch and paper chart relatively accurately.

I'm not saying I agree with it, but it is at least understandable!

[filmor]

The nautical mile is a sensible unit for anyone trying to navigate with charts (which, in the worst case, could happen to pilots). Feet, on the other hand, are nonsense.

[gaius_baltar]

> Imperial units (specifically, feet) are dominant in aviation to measure flight altitude. Despite a recommendation of ICAO to use SI units since 1979, only China, Mongolia and a few former-USSR countries (not Russia) use metres to define flight levels. [0]

Notice that before the end of WW2, continental European aviation was metric too. For this reason, the instrument panel from a Messerschmidt BF-109 is almost readable for a layperson: https://live.staticflickr.com/3746/14275081012_1f74be13b4_4k...