Why doesn't the aviation industry use SI units?

AsheeshR
  • Why doesn't the aviation industry use SI units? AsheeshR

    This is a followup to What is the measurement system used in the aviation industry? and related to this question from History.

    I can understand the arguments as to why adoption of SI units would not make sense for the general population, but aviation is a specialised business. All professionals are highly trained, and would (should) be well versed in both systems anyways, so the transition would be much simpler from the point of view of human factors. The technology would probably be much harder to shift but again, with more and more displays and documentation going digital in cockpits, this again seems to be a not so huge problem. Maintenance and manufacturing is again fairly specialised and restricted to a smaller number of companies as compared to the general case.

    What are the historical factors that lead up to the adoption of imperial units in the industry? Why are they still being used when widely accepted scientific standards exist?

  • This is a historical development that dates back to that much of the early aviation equipment was sourced from the United States, and was consequently in imperial units. This in particular occurred after World War Two, and hence mixing them was a bad idea, and the imperial system stuck. Interestingly, the places where the US did not have a lot of influence- the former USSR and China for example- use metric.

    As for airspeed in Knots and distance in Nautical Miles, this comes from aviation's nautical heritage.

    All pilots can do the conversion, but it's rather that changing all the instruments from the imperial system to metric system that would cause a huge headache, among other things, and the cost would outweigh the benefits.

    • Instruments would have to be changed.
    • All reference material would have to be converted.
    • All maps/charts/etc. would have to be converted.
    • Pilots would have to become accustomed to the new units and to learn the changed values which they will probably have memorised, such as important airspeeds.

  • Non-SI is only used for altitude, distance and speed except in US and some other American countries.

    • Altitude is in feet because 1000 ft happens to be reasonable vertical separation and 1000 ft is easier to calculate with than the corresponding SI figure 300 m.
    • Distance is in nautical miles because it is related to the unit used for measuring latitude and longitude. 1 nautical mile corresponds to 1 degree latitude (and longitude on the equator), which makes it easier to calculate distances from navigational maps. If angles were converted to decimal, 1 km would be 1/100 gradian. Alas, angles and time were not converted.
    • Speed obviously based on the distance unit in use.

    Nevertheless if it was not for the prevalence of US-built planes after WWII, we would probably be using metric in Europe too as continental planes before WWII usually had instruments in metric.

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