Why doesn't the aviation industry use SI units? 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?
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. Also the procedures for instrument flying were first developed in the USA using feet.
Distance is in nautical miles because it is related to the unit used for measuring latitude and longitude. 1 nautical mile corresponds to 1 minute latitude (and longitude on the equator), which makes it easier to calculate distances from navigational maps using the grid lines as scale (large area navigation maps need to preserve angles, so they can't have constant scale). If angles were converted to decimal, 1 km would be 1/100 gradian. Alas, angles and time were never converted to decimal.
Speed obviously based on the distance unit in use.


Nevertheless if it was not for the prevalence of US-built planes after WWII and more advanced state of aviation in the USA at that time, we would probably be using metric in Europe too as continental planes before WWII usually had instruments in metric.