Why doesn't the aviation industry use SI units?.

I believe only a few countries use meters and other SI units in air transportation, including India and China (please correct if wrong).

Using kts and NM in navy -- built from the length of the 1' arc on a meridian -- simplified nautical mental arithmetic, and likely also helped at the beginning of aviation.

Current ICAO recommendations

In Annex 5 - Units of Measurement to be Used in Air and Ground Operations, ICAO allows 3 non-SI units (knot, foot, nautical mile) to be used...

as alternative units because of their widespread use and to avoid potential safety problems which could result from the lack of international coordination concerning the termination of their use.

... but overall states in the same document that only SI units should be used, the same units for all members. Nevertheless the termination dates for alternative units have not been specified:

It is intended that the use of the non-SI alternative units [...] will eventually be discontinued in accordance with individual unit termination dates established by the Council. Termination dates, when established, will be given in Chapter 4.

Case of vertical measurements / altitude

Vertical measurement in meters may be the more complex due to the existence of flight levels in feet, with the full system of vertical separation based on FL.

This topic was discussed in Using feet vs meters for altitude?. The current question is not a duplicate of the latter.

Other non-SI units

Other non-SI measurements seem more easy to remove:

  • Horizontal distances in feet and nautical miles may be expressed in meters, as already done for runway lengths. It is not as important as it was to mentally translate a latitude offset in NM.
  • Atmospheric pressure expressed in Hg column height may be expressed in Pascal. Many pressure instruments can be already calibrated both in inHg and hPa.
  • Speeds expressed in knots (sometime in statute miles per hour) may be expressed in km/h.

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Wouldn't it be useful (and safer) in the long term if we could remove most of the imperial units from new aircraft instrumentation, procedures, ATC, documentation and maps (at least for anything other than altitudes)

Just asking what is the current view from the air transport community itself.

  • $\begingroup$ @falstro: Yep, didn't see it before posting. However the discussion may go further. FL in feet may make sense, but what about NM, kts, Hg inches, etc? $\endgroup$
    – mins
    Feb 10, 2015 at 19:39
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    $\begingroup$ What benefit is there to using SI? Aviation is self-contained to a significant degree, so I don't think there's a significant cost of unit conversion anywhere in the system. Any advantage would have to be pretty significant to overcome the safety issues of switching systems. $\endgroup$ Feb 10, 2015 at 19:40
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    $\begingroup$ Things like the 60:1 rule would then become the 60:1.852 rule (or the 15,000:463 rule if you made them whole numbers, equally bad). So to find my lead radial on a 15nm arc, at 250kts it would then be, 27.78x/1.852=60 = 15x=60 = 4, and then 4.63/1.852 = 2.5, so then 4*2.5= 10. 10 rads. Using NM I can do the calculation in my head in about 10 seconds. Yeah... no thanks. As you said, using NM makes aviation calculations easy and there is no reason to change it. $\endgroup$ Feb 11, 2015 at 0:44
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    $\begingroup$ aviation.stackexchange.com/questions/2572/… $\endgroup$
    – Federico
    Feb 11, 2015 at 5:32
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    $\begingroup$ In light of that question, this one is definitely a dupe. $\endgroup$
    – reirab
    Feb 11, 2015 at 5:34

1 Answer 1


For me metric or imperial is not the question, from my perspective the key is standardization. Having 2 different unit systems has only got drawbacks:

  • Customers: many airline's customers are living in a metric system, and they do not know what's is a mile or a feet. Is a matter of making flights more comfortable.
  • Cost reduction: units transformation is finally a cost. Engineer at manufacturers and airlines need to spend some time using a calculator to understand what is meant by other unit system. Maybe is only 30 minutes per month, but if we are talking about 2000 people, we are reaching a cost that is not adding value of 1000h per month. Finally this cost is somehow appearing in our airplane tickets.
  • Reduce human factor: we all are humans and exposed to errors. The less opportunity we have to make an error the best. Already there had been an issue with satellites with unit conversion. Reducing this difficulty is finally improving the safety of the air transport.
  • Increase competence in the sector: suppliers from, let's say, metric system have an slight competitive disadvantage when competing for a contract with imperial units. The more competition in airline services, airplane manufacturers suppliers, handling services the lower the price of our tickets.

Having 2 standards is a cost that is not producing any value and is actually a sector issue, moreover, a society issue. From my point of view the industry should try to move to a single standard. Metric or Imperial, I don't mind.

Next discussion will be which side of the road we should drive...

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    $\begingroup$ "The wonderful thing about standards is that there are so many of them to choose from!" $\endgroup$
    – voretaq7
    Feb 10, 2015 at 21:41
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    $\begingroup$ When you say "airline customers" do you mean the airlines who are customers of the aircraft manufacturers or the passengers who are customers of the airlines? If it's the former, I would strongly disagree with your assertion that many of them don't know what a foot or a nautical mile is. I would say all of them, outside of perhaps a few domestic Russian and Chinese are very familiar with both. If you're talking about the passengers, who cares? The passengers don't communicate with ATC (hopefully not, at least.) In-flight entertainment systems can and do display both systems of units. $\endgroup$
    – reirab
    Feb 11, 2015 at 5:27
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    $\begingroup$ I was referring to the passengers, professionals shall be familiar with them. Passengers are intereacting with units with communications from pilots, flight entertainment or websites. I remember the website of an airline which I connected to, where everything was in imperial, so I was not able to calculate the distance directly. $\endgroup$ Feb 11, 2015 at 6:34
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    $\begingroup$ Using inHg to calibrate a barometer / an altimeter looks broken science, and adds to profane and professional confusion. Many European countries have required years ago that all weather reports use Pa and not cm/in Hg or mbar for atmospheric pressure. This was not a big challenge. That avoided headaches to understand if we were talking about Hg mass or Hg weight or which section of the pipe we were using :-) $\endgroup$
    – mins
    Feb 11, 2015 at 7:06

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