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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?

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asheeshr
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4 Answers4

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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.

Jan Hudec
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  • 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. – reirab Feb 11 '15 at 05:39
  • @reirab: Radians are different thing. Still does not explain why gradians did not replace degrees. – Jan Hudec Feb 11 '15 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. – reirab Feb 11 '15 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 :-). – copper.hat Feb 11 '15 at 18:00
  • @copper.hat: It's rather that in the area where it would help, navigation, backward compatibility was more important than warts of the system as it would mean redrawing all the navigational maps and that was a lot of work back then. Plus for all the warts of the sexagesimal system it has great advantage of having many integral factors. – Jan Hudec Feb 11 '15 at 19:36
  • With grads we would have to change to the one-in-sixty-four rule (which has a nice binary ring to it). – copper.hat Feb 12 '15 at 00:44
  • The nautical mile and derived units are the only unit system based on the earth being spherical. As such, they are more accurate both in units calculations and human factors. If you ever have to plot manually on a map with a divider, the ease of the nautical mile becomes readily apparent. – TechZen Jun 02 '15 at 18:47
  • @TechZen: The fact Earth is spherical has absolutely nothing to do with this. The coordinate system used for specifying position does, as the text already explains. Both nautical miles and metres are derived from such systems (so nautical mile is not the only one), alas different ones and the one metres are derived from never caught up. As the answer already explains as well. – Jan Hudec Jun 03 '15 at 07:02
  • @JanHudec: Degrees > Gradians because of numerics. (Summary: 360 is "better" than 400 from a numerical standpoint, especially if you're going to break it up into parts -- which you're doing all the time with degs / grads.) Similar holds true for time -- nice "decimal" numbers might look better, but the current (Sumerian) 60 * 60 * 24 is better for subdivisioning. – DevSolar Jun 03 '15 at 09:39
  • @DevSolar: Yes, that is probably the main reason why it stuck for civil time (and physicists don't care, because they can simply use seconds and ignore the larger units). But for angles you don't really need division to equal parts much, so I suspect not wanting to redo maps is more likely reason there. – Jan Hudec Jun 03 '15 at 09:46
  • @DevSolar: Sometimes I am indeed wondering why we are not using base-12 instead of base-10. Because while using fingers for base-10 is more obvious than using them for base-12, they can be used for base-12 too (4 fingers times 3 parts, the other hand works as index) and the additional divisors are often useful. – Jan Hudec Jun 03 '15 at 09:51
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    @JanHudec: We don't usually need the subdivision-features that base-12 would make (marginally) easier, so the benefit of switching to base-12 (compared to the royal PITA it would be in everything from language to engineering to computing) is negligible. We already had to cope with the effects of base-2, and the not-quite-SI Ki, Mi and Gi bastard prefixes. ;-) – DevSolar Jun 03 '15 at 09:59
  • @JanHudec - I disagree, The fact that the earth was sphere did play a role that is why it is one nautical mile was defined as one arc minute along any meridian, a measure meaningless on any surface other than a sphere. Your definition is later, hacked on version. The nautical mile, like many pre-metric system, evolved out of the nature and scale of the phenomena observed as well the angle being the only form of accurate repeatable measurement. It worked fantastically back when most calculation where done by various divisions and ratios instead of numerically. – TechZen Jun 06 '15 at 01:57
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    @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. – Jan Hudec Jun 06 '15 at 20:00
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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.

fooot
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Skip Miller
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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.
Thunderstrike
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    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. – Jan Hudec Mar 22 '14 at 22:22
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    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. – David Richerby Nov 23 '14 at 09:45
  • And air traffic controllers would have to become accustomed as well. The transition would surely result in more than a few improper headings. – usernumber Nov 23 '14 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. – reirab Feb 11 '15 at 06:00
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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.

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