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Why doesn't aviation use standard (metric) units?

Silvaire wrote:


Re airspeed calibration, continental Europeans would prefer km/hr and once had it, Americans and some others prefer mph and once had it… Both are now stuck with knots which make no intuitive sense to anybody and have no utility in the age of GPS. Typical aviation practice

This topic popped up in the Morane thread recently and I’d like to expand on it.

Aviation seems to be stuck with units that nobody uses in daily life the world over, such as nm or kts, or very few people use outside of the cockpit, such as ft.

The SI units such as m and it’s derivatives are so widely used nowadays that they have been formally adopted by all countries including the anglophone ones that used to have Imperial units. Except the US and Liberia all countries have committed to metrication. With aviation being very international, why don’t we use the units of measurement everyone else uses?

Obviously history plays a major role but this doesn’t seem like a convincing argument to retain nonstandard units.

Last Edited by MedEwok at 08 May 19:02
Novice pilot
EDVM Hildesheim

… like medicine.

Glider pilots in Germany fly metric though. For engineering, the US system is silly, for the pilot it’s irrelevant. Just use the numbers in the poh.

mh
Inside the sky.
EDXE, EDXF, Germany

The UK hasn’t converted.

The maritime world uses NM as does aviation as it is sensible as derived from latitude.

It would be far too hard to change the unit of altitude and not necessary – ft work just fine.

EGTK Oxford

The boating world use NM, kts and feet, not just aviation. ft is gradually being replaced by m though, but NM and kts are going strong. NM and kts are physical quantities tied to the planet earth. A much better system than meter. meter is no better than ft. They are both just arbitrary measures of length.

old habits die hard

For myself, i think in knots, and altitude in feet feels natural. Any (aircraft) speed in km/h or altitude in meters is meaningless to me, i have to start doing mental calculations (but it also took me also 10 years to get used to the EURO as currency)

That being said, most ultralights I see have speed in km/h (but altitude still in feet), so it appears to be changing.

I think also many french aircraft have airspeed in km/h, a french aircraft I often fly has a (second) altimeter in meter (It also had a pressure gauge in “Pieze”, but that’s another topic i guess).

EBTN, EBST, Belgium

MedEwok wrote:

Obviously history plays a major role but this doesn’t seem like a convincing argument to retain nonstandard units.

Its not history, it’s money invested. In the 1920s and 1930s there was a big effort to standardize aircraft production in the US. Aviation has been living off it so to speak ever since. One of the most obvious things is standardized aircraft screws, nuts, rivets, high pressure hose, electrical connectors, splices and terminations, rod end bearings and all sorts of other things in the AN (Army Navy, at the time there was no US Air Force) and NAS systems. As a result I can walk across the airport and buy new aircraft spec fasteners and fittings called out by the majority of GA aircraft parts catalogs, regardless of where and when that aircraft was made. That system has never been properly replicated by others, or in other units, because it was the result of a big effort at a time when it was considered important enough to make the (government) investment.

LeSving wrote:

They are both just arbitrary measures of length.

The basis of any unit of length (or anything else) is arbitrary to 99.9999% of the people using it. Decimalization is relevant, and easy for humans with 10 digits, but has nothing to do with units.

Last Edited by Silvaire at 08 May 20:56
Question is, how relevant are Nm and kts for practical flying today ? Does it matter at all in flight planning and in the air when everybody uses electronic aids for that ? I do not mean the use in training to get the PPL ! From seven billions of human beings only max. 400 millions remain in the imperial system, USA, Liberia and Myanmar. I do all calculations for flight, if required at all , in metric units from the beginning, after PPL that is. Sure , one altimeter is in feet, for radio calls. International pressures seem to have forced Russia lately to convert to feet in aviation – missed the chance to join with China to convince the rest to return to metric units, as was whole of Europe till after WW2 , UK excepted.

Just recently I asked a Diamond mechanic about metric aviation fasteners and if Diamond uses imperial bolts etc. . He could not point me to any outlet for “aviation” bolts in metric, instead we have simply tensile strength stamped on any bolt head you require, and no, Diamond (like Socata , Piaggio?) is metric all out, so what. Special wing bolts with close tolerance shafts will be available as spares anyway, any other bolt you get at your hardware store. Simplification in the imperial system was long overdue and pressing in the pre war times and remained a complete mess till late sixties, less so with metric gear. As time goes, even in USA modern manufacture slowly turns metric struggling to keep effective with the rest of the planet.
Vic

Last Edited by vic at 08 May 21:11
vic
EDME

The UK engineering scene converted to SI units many years ago. I was doing them exclusively at college, 1973-75. They are great for engineering calculations because, for most cases, there are no conversion factors. However the advantage is overcooked when you consider the USA got men to the moon and back, on ft-lbs, inches, feet, etc. It is fine if you are a proper engineer and have a good feel for the underlying mechanics/physics and then it is just some conversion factors.

However as Jason and Silvaire say there is little need to go SI in aviation. One uses relatively few units there; just speed (kt), altitude (ft), volume (litres in Europe mostly) and maybe a few others. I convert between litres and USG when calculating the fuel totaliser error after each fillup (3.78). Distance in metres is the odd one out but we don’t have to do calculations or conversions; it’s a matter of checking if e.g. the required vis is better than what the approach plate says, and it could equally specify the vis in fathoms…

Mechanics need to understand ft-lbs and in-lbs which some find sufficiently taxing to get it wrong but I doubt that Nm would make it much easier for them when it comes to setting the torque wrench which has two scales anyway.

I just think that when you have any human activity which has enough really big fish to fry, nobody bothers frying the smaller fish

There is also the AMU (aviation monetary unit) which equals $1000 / €1000 / £1000 and it facilitates productive discussions of aircraft ownership generally and avionics upgrades specifically

Administrator
Shoreham EGKA, United Kingdom

I am sure prominent members of this forum will vouch for the rip-off prices charged for aviation hardware that is metrically dimensioned.
I have to say that I am 100% happy flying aircraft with ANF fittings (even though it is made in Europe) and flying in Knots, feet and doing my fuel calculations based on gallons or pounds! Can you imagine what the cost of changing all the instruments to metric would be?

I think I shall now go to my local and have a pint of beer.

LeSving wrote:

The boating world use NM, kts and feet, not just aviation. ft is gradually being replaced by m though, but NM and kts are going strong. NM and kts are physical quantities tied to the planet earth. A much better system than meter. meter is no better than ft. They are both just arbitrary measures of length.

A nautical mile is 1/21,600 of the earth’s circumference, or one minute (1/60th) of a degree (1/360th of a full circle)

A kilometre is 1/40,000 of the earth’s circumference, or a “centigrade”, 1/100th of a grade (1/400th of a full circle)

The “grade” and the decimal subdivisions are the original metric equivalents of the degree and minute. So the metric system is as internally consistent as the customary nautical system, but that lesser known part of it was never adopted. Probably because the similar idea of having 10-hour days with 100 minutes of 100 seconds each wasn’t adopted either, and having a system for angles not evenly divisible by 24 hours is just awkward for navigation (16 2/3 grades per hour instead of 15 degrees per hour isn’t great)

Last Edited by Cobalt at 08 May 21:02
Biggin Hill
139 Posts
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