# 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?

• possible duplicate of Using feet vs meters for altitude? Mar 22, 2014 at 17:42
• @flyingfisch This is a more general question about the units system being used for altitude, fuel, length, ground speed, air speed, pressure, and also by on-ground maintenance in terms of tools and measurements. Mar 22, 2014 at 17:52
• ah ok, understood. ;-) Mar 22, 2014 at 17:54
• possible duplicate of What is the measurement system used in the aviation industry? Mar 22, 2014 at 22:05
• I've added answer to the linked question that attempts to answer this one as well. Hm, I'll probably copy it here. Mar 22, 2014 at 22:07

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.

• Well, approximately 1 gradian, at least. The reason that angles were never generally converted to gradians is probably because radians are much, much more useful for the vast majority of purposes. Feb 11, 2015 at 5:39
• @reirab: Radians are different thing. Still does not explain why gradians did not replace degrees. Feb 11, 2015 at 10:56
• Yeah, they are different for sure, but with engineering and science using radians for most purposes anyway, there probably still wasn't nearly as much incentive for them to push the switch than with most other units of measure. Feb 11, 2015 at 15:13
• One practical issue with radians is that they are irrational. A degree is approximately the daily change in angle to the sun, I would guess that it is deeply buried in our psyche :-). Feb 11, 2015 at 18:00
• @TechZen: The fact that the Earth was a sphere did play exactly the same role in why meter was defined the way it was, as a 1/100th of a gradian along any meridian. The units are both geocentric. The distinguishing feature is which matches the coordinate system of maps we use. And the only reason we use that particular coordinate system is historical; any coordinate system would make just as much sense, but long ago we arbitrarily chose one and it stuck. Jun 6, 2015 at 20:00

You state that "the transition should be much simpler in terms of human factors."

Ideally, yes. In practice, no. When mistakes happen, they may well be lethal. The Gimli Glider incident was partly caused by a confusion between which system was used to measure fuel quantity. From Wikipedia:

The subsequent investigation revealed a combination of company failures and a chain of human errors that defeated built-in safeguards. Fuel loading was miscalculated due to a misunderstanding of the recently adopted metric system which replaced the imperial system.

The fact that the Gimli Glider incident ended with no loss of life doesn't negate my point: Human factors such as fatigue will find a way to defeat the best plans of the Safety Engineers. One of the human factors of which you speak is "what you learned first, you learned best" — and this learning is likely to predominate in an emergency situation.

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.
• As it currently stands, pilots have to regularly switch the units during flight. Russia, CIS and China are not exactly negligible part of the world. Mar 22, 2014 at 22:22
• Note that the US does not use imperial units (or, as Americans often call them, English units): it uses US customary units. The US fluid ounce is a few percent different from the imperial fluid ounce but a US pint is only 16 ounces compared to 20 in imperial; similarly for multiples such as quarts and gallons. Also, a US hundredweight is 100 pounds, whereas an imperial hundredweight is 112, with a concommitant difference in the definition of a ton. Nov 23, 2014 at 9:45
• And air traffic controllers would have to become accustomed as well. The transition would surely result in more than a few improper headings. Nov 23, 2014 at 16:05
• I would say that the conversion issue isn't the main issue at all. It's a big issue, but I think the larger issue is that feet, nautical miles, and, consequently, knots are objectively more useful/convenient units for aviation, for the reasons Jan Hudec listed. Feb 11, 2015 at 6:00

The first part of the answer is that it is too difficult to change. It would involve lots of rewriting rules and manuals and retraining.

The second part is that it does not really matter. As a pilot you (or I) do not really care how height or speed or whatever is measured. In the procedures we follow it says height 2000 and speed 200 (or whatever). And the units are the same as the instruments are calibrated in. If you the units where different, the instruments and the procedures would show different values. As long as they are in the same units it would not matter.