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I figured this was probably a no brainer, since Europe uses the metric system, I assume the airspace does... But then I remembered that aviation likes international standards, and I know the US uses Imperial so, does Europe as well?

And, yes or no, why do they take that choice?

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European Civil Aviation Authorities use the imperial system to depict altitudes and airspace restrictions (feet), speeds (knots) and distances (nautical miles).

Some VFR aircraft have speed indicators in km/h, gliders can have altitude and variometer in meters and meters/second and apart from a special ICAO VFR chart being available in metric in Germany, the AIP itself and all other charts always use imperial units.

Related question: What is the measurement system used in the aviation industry?

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Europe uses imperial for distance (nm), speed (knots) and altitude (feet). Russia and China are the only major countries that use meters for altitude - and it causes all sorts of confusion.

Some aircraft (more in europe) use kilometers and statute miles/hour on the ASI - but the same is true in the US.

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  • $\begingroup$ The nautical mile is now (since 1954) defined in terms of meters... $\endgroup$
    – copper.hat
    Feb 10, 2015 at 3:26
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    $\begingroup$ @copper.hat As are all US customary units of measure that I'm aware of; the yard was first defined in meters in 1893, and was revised in 1959. Other units of measure are similarly defined in terms of SI units, or in terms of units defined in terms of SI units. $\endgroup$
    – cpast
    Feb 10, 2015 at 6:04
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    $\begingroup$ @cpast: I generally have a preference for metric (consequence of a childhood spend doing multi-base arithmetic with acres, roods, furlongs, pounds, schillings & pence), but prefer the nautical mile because of the relationship with the Earth's circumference. $\endgroup$
    – copper.hat
    Feb 10, 2015 at 6:18
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    $\begingroup$ Russia has already switched to feet-based flight levels, though they still use metres below transition altitude. $\endgroup$
    – Jan Hudec
    Feb 10, 2015 at 10:39
  • $\begingroup$ There are aircraft that use nautical miles for distance and statute miles for speed? $\endgroup$
    – Gabe
    Feb 10, 2015 at 19:31
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As others have mentioned, the basic units to define the airspace are feet and nautical miles.

However, some other values are also used when navigating the airspace, and a few are from the SI system. For instance:

  • pressure settings: hPa (aka millibars, mbar)
  • runway lengths: meters
  • visibility: meters
  • temperature: centigrades

Regulations (VFR):

  • minimum horizontal distance from obstacles: meters

To see the SI units for pressure, visibility and temperature in a real-life setting, just have a look at a TAF/METAR for any European airport.

The mixture of units can seem strange, but there is also a method to the madness. You know that whenever you hear a measurement in feet, it is about a vertical distance, and anything expressed in meters (or kilometers) will be horizontal. A small contribution to the clarity of communications.

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  • $\begingroup$ I've most commonly seen runway lengths in feet, even on airport diagrams. It's true that metric value is often given as well, but doesn't seem to be the primary thing. $\endgroup$
    – yo'
    Feb 10, 2015 at 16:28
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    $\begingroup$ @yo' Just checked the official information for Heathrow, EGLL, Charles de Gaulle, LFPG and Schiphol, EHAM. That's three of Europe's largest airports, and they all uses meters for RWY length (and width). Are you sure you didn't see a US "translation" (Jeppesen or similar)? $\endgroup$
    – Monolo
    Feb 10, 2015 at 16:47
  • $\begingroup$ Ah yeah I used Jeppesen, right. $\endgroup$
    – yo'
    Feb 10, 2015 at 16:56
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    $\begingroup$ These are the critical differences. European ATC will variously give altimeter settings in hPa but pilots can request inHg, afaik (if they don't have dual-legend altimeters, for example) $\endgroup$
    – J...
    Feb 11, 2015 at 13:46

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