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

Peter wrote:

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

Exactly. In the US, particularly in engineering, with units we are a bit like the Swiss with languages. We also have more Spanish speakers than Spain and no official US language, so flexibility in that respect also has its local benefits!

I will say I’m glad we in the US don’t mix meters for runway length with feet/min for climb rate, but I’m sure I’d get used to that too if it were necessary.

Last Edited by Silvaire at 08 May 21:15

Cobalt wrote:

Probably because the similar idea of having 10-hour days with 100 minutes of 100 seconds each wasn’t adopted either

Some gave it a try

I’m sure life would be a lot easier with decimal time, including 100 days in a year

Last Edited by Silvaire at 08 May 21:19

Don’t Russians measure altitude in metres?

Silvaire wrote:

I’m sure life would be a lot easier with decimal time, including 100 days in a year

The French revolutionary calendar had 12 months of 30 days each, with three 10-day weeks (décades), and 5-6 “bonus days” each year to get it to the required 365-and-a-bit.

Biggin Hill

Silvaire wrote:

Some gave it a try

In principle, a decimal clock should need only one scale for hours and minutes, with zero/ten at the top, which means zero hours and zero minutes. And the minute hand pointing at, say, 7, means 70 minutes.

But some watchmakers preferred five at the top (for noon, which is what people were used to, and they are a lot more likely to look at the watch at noon than at midnight) and hence you get awkward examples like in your photo which needs an awkward, separate minute scale offset by 180 degrees.

Biggin Hill

mh wrote:

… like medicine.

Good point. Medicine has lots of weird units, most notably mmHg for blood pressure or partial gas pressures (pCO2 etc.). Notably, East Germany was more advanced in that regard and mainly used SI units. My hospital has made an effort to at least convert blood sugar from mg/dl to mmol/l, which is weird to all the surrounding hospitals who still use mg/dl. A blood sugar level of 5 means you’re practically dead with mg/dl while it is normal for mmol/l (the conversion factor is 18 for those who are interested).
What I will probably not experience in my career as a doctor is a conversion of blood pressure from mmHg to Pa. A BP of say 120/80 mmHg being normal is so deeply ingrained in the entire population (wheter they have medical education or not) that it would require an enormous effort to change…for little benefit, as you never calculate anything with blood pressure anyways. I guess it is the same for most aviation applications and the corresponding imperial units.

Silvaire wrote:

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.

Very interesting Silvaire, I didn’t know any of that. I always thought the dominance of US units and standards in aviation had more to do with the victory in WWII, didn’t know the foundations were already laid in the interwar period. It is also strange that a norm-crazy nation such as Germany (DIN Norm) never got around to create and spread its own standards.

vic wrote:

I do all calculations for flight, if required at all , in metric units from the beginning, after PPL that is.

Really? Quite an effort, isn’t it? Personally I do convert the distances flown from nm to km after I finished flight planning, to get a better feeling of what kind of distances I am flying. Yet for all other applications I use nm, kts and ft as customary. Despite being a huge proponent of the metric system.

Low-hours pilot
EDVM Hildesheim, Germany

Well as said most of these things are arbitrary. As an Australian (metric country with km/h and metrers), I still think of length (runways and distances) in ft and nm. Visibility in metres.

I never convert to decimal in aviation and find the european use of metres for runways annoying.

My aircraft measures fuel in lbs which is a pain for Europe as you have to convert to and from litres but then this is also hard in the US as you have the same conversion to/from gallons.

Last Edited by JasonC at 08 May 22:37
EGTK Oxford

Perhaps this new-fangled “SI” system will turn out to be a passing fad.

I mean, the whole world measures shot gun barrel calibres in lead balls per pound, some Italian/Finnish Tikka rifles have Unified muzzle threads, and who but some Neo-Napoleonic Zealot could question the convenience (for body-piercings) of the American Wire Gauge’s ratio between successive sizes being the 39th root of 92?

Well… perhaps the very same NNZ by whose diktat a tin of Tate & Lyle’s “golden syrup” (essential ingredient of Yorkshire Parkin) is marked as containing 907 g instead of 2 lb…

Last Edited by Jacko at 08 May 23:53
Glenswinton, SW Scotland, United Kingdom
MedEwok,

our aircraft is fully metric, Russian of course, but it would be the same if it was Italian, French, German, Czech, Polish etc. , had these aircraft not been partly imperialised in recent times, so no longer original. The only imperial instrument in the Yak is the feet altimeter, next to the original meter type. That is just for radio calls because sadly aviation in Europe was changed to obsolete units in the aftermath of last war. Again, what is so puzzling about keeping all metric apart from altitude callouts ? Nobody cares if you do your maths metric when the speedo is metric, the fuel gauge in liters, temps in Celsius, and so on. Come on, most of you living in Europe will be perfectly familiar with kph, meters, liters, kilograms, when entering your car. So where is the problem with keeping same in your aircraft ?? Troubles getting used to this – why on earth, unbelievable to me ! Your tablets are adjusted to metrics in a second, when doing flightplanning, altitude is kept at feet, reason clear. So why would one change instruments one way or the other, when nobody cares, except for altitude that is used in radio calls . But speed etc. is nothing I´d tell on the radio or anybody will ask for.
As to imperial sizes, wire gages, small screws, thread pitches – I lived and worked in NZ all of 1982, own a few british classics – the system is ridiculously unefficient to work with. You need all sorts of files with all dimensions to finally identify what you got on the bench or in the drawing sheet. Example: The higher the small screw number the smaller the diameter of the thread. Wire diameters – no way to simply measure them, you have to look in the books to get the gage number for ordering.Traditional spanners are stamped with the thread dia. not the size across flats ! So figure if you got a thread different to the standard and look for a fitting spanner ! No wonder everybody uses monkey wrenches (adjustable open end spanners) . I could go on a while with funny relics of olden times that make any simple job a real waste of time while working out what sizes you actually want. You see sizes in fractions like 9/16 " but your micrometer is in thou, so out with the calculator again – unbelievable .
I guess UK will drive on the right side of the road before USA turns fully metric – or maybe China will take them over, they own most of it anyway already.

Vic
vic
EDME

Jacko wrote:

Perhaps this new-fangled “SI” system will turn out to be a passing fad.

I don’t have strong feelings one way or the other about whether an atmosphere pressure should equal 101300 of the preferred units or 14.7. Either way my wife knows very well that a (German) ‘pound’ is equal to 500 g, and also that its entirely correct for car wheel diameters to be measured in inches, tire pressure in bar and etc. She does get frustrated by ‘cups’ as a unit of measure and wonders why Americans don’t have or need gram scales (or any of kind of scales) in the kitchen

MedEwok wrote:

I didn’t know any of that. I always thought the dominance of US units and standards in aviation had more to do with the victory in WWII, didn’t know the foundations were already laid in the interwar period. It is also strange that a norm-crazy nation such as Germany (DIN Norm) never got around to create and spread its own standards

I don’t know if interwar Germany established any such aviation standards, they would have been lost anyway. IIRC we once discussed whether the Russians had ever done anything similar, but I don’t remember a clear conclusion. Regardless, the US auto industry and a large contingent of Government people were involved in the US and Henry Ford’s factories surely made a whole lot of planes with US Government orchestrated standardization. Companies such as Cessna took that ball and ran with it postwar.

vic wrote:

I lived and worked in NZ all of 1982, own a few British classics – the system is ridiculously unefficient to work with. You need all sorts of files with all dimensions to finally identify what you got on the bench or in the drawing sheet. Example: The higher the small screw number the smaller the diameter of the thread. Wire diameters – no way to simply measure them, you have to look in the books to get the gage number for ordering.Traditional spanners are stamped with the thread dia. not the size across flats ! So figure if you got a thread different to the standard and look for a fitting spanner

None of the old UK-style Whitworth, BA, Cycle Thread etc stuff was ever used on US aircraft, and isn’t in use on anything else today – unless you happen to own an antique SU carb, or three. That said, if you happen to own a Tiger Moth or Chipmunk, you’ll also have the pleasure of working on something without US standardized AN fasteners etc. This was a major factor in my decision not to buy a Portuguese built Chipmunk a few years ago.

The old Spanish built and East Block stuff is easier. You can’t buy any replacement aviation-grade hardware at reasonable cost, but at least the tools you use on your motorcycles will fit what you already have.

vic wrote:

But speed etc is nothing I´d tell on the radio or anybody will ask for.

Interesting!

Last Edited by Silvaire at 09 May 01:25
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