Although A_A is right about the general benefit of the society, he is, like me, from a country that has been quietly adopting SI unit for a long time now. But different nations have it differently. Obviously the French pioneered the SI system which is conquering the world for its simplicity and consistency, while the British always found the decimal system to be far to boring to fully adopt, having invented the unquestionably more colourful Imperial System (IS?).
The pound (lb) is 14 stones, while there is 16 ounces to the pound. One ounce is 16 drachms, while for very light items the useful “grain” is exactly 1/7000 pound. At the other end of the scale, a hundredweight is 8 stones, or 112 pounds. For aerodynamic studies from British literature it is useful to know the slug, which is the weight that a one pound force (1 lbf) will accelerate exactly 1 ft/s². It is slightly more than 32 pounds. Air density is often given in slugs per cubic feet. And then we all know the quarts of oil we pour into the Lycomings, i.e. 1/4 of a gallon – oh, is that an imperial or the smaller U.S. gallon? The quart is, of course, two pints.
The British length units are too many to mention, but I have included a strategic map below.
SI units of pressure are super easy to visualize once you realize a standard atmosphere is 101,325 Pascals, the SI unit. It all makes sense then 
Preference and theory notwithstanding, the reality is that units of measure make very little difference in the efficiency at which the world operates, and choice is good. I use whatever is convenient – for some things I use whatever version of metric units work easiest (for example lbm versus lbf is confusing to me relative to Kg and N) but for structural engineering I’ve always found it easier to use US units…For example this situation in strength of materials is particularly easy to remember:
Steel: Density 0.28 lb/cu in, Stiffness (E) = 28 million psi
Aluminum: Density 0.10 lb/cu in, E = 10 million psi
Titanium: Density 0.16 lb/cu in, E = 16 million psi
That’s useful to me… and I am absolutely unconcerned with who if anybody cares, one way or the other. In relation to the thread I’m mostly interested in buying as much Avgas as I can eat (and no Diesel whatsoever) for $4.24 per US gallon, a few minutes pay.
Silvaire wrote:
Preference and theory notwithstanding, the reality is that units of measure make very little difference in the efficiency at which the world operates, and choice is good.
If everyone use the same units, yes. Otherwise things like this can happen.
I think the benefits of choice generally outweigh the issues with choice. That said there are examples where I agree it is just silly to mix systems of units – I do appreciate that in the US we don’t use different units for runway length and altitude.
Ok, but why do you do it for runway length, visibility and distance?
My issue with runway length and altitude and vertical speed (and also terrain height) is that I do takeoff/landing and climb/descent performance estimates using these parameters in combination, all in feet and minutes. Typically this involves mental trigonometry and for me to do that, the units need to be the same. The one parameter that needs conversion is knots to feet/min, and one knot is very close to 100 ft/min. Easy.
I might separately do en route distance/speed/time calculations in miles and mph (or nautical equivalents) using distance and speed (and maybe visibility?) in combination, with the outputs in tenths of an hour. Fuel burn is in gallons per hour and capacity is in gallons. No need to convert anything to feet or minutes.
Silvaire wrote:
SI units of pressure are super easy to visualize once you realize a standard atmosphere is 101,325 Pascals
We usually use bar. 1 bar = 100 kPa = 1 atm (approximately) in engineering and millibar in meteorology. 1 mbar = 1 hPa, hence isobar. SI is the standard, and used in standards, but there are lots of old and “other” units in use for everyday use. bar is not SI by the way 
Numerically, bar is “better” than Pa, easier to relate to, and of course super easy to convert to Pa if “needed”.
Sea and air is NM, kts and feet and so on for various other stuff. A funny thing is that US measurements are defined in terms of SI units. They have no definition on their own, like the SI system has. Hence it really is just an awkward way of using the SI system. It is no different than the use of bar, but bar has a super easy conversion.
I think it makes sense (particularly when not working on paper) to use units which work out relatively easily in the head. feet and inches have the advantage of being a fairly intuitive size and many common factors for division.
The SI system if I recall for the metre was defined as a ten millionth the distance from north pole to equator. Although their measurement was out, so we’ve just had to redefine it as an arbitrary length based on the vacuum speed of light.
I’ve yet to hear someone suggest we switch from degrees to radians for flying, despite them being more sensible for other mathematics.
I think the whole thread should be moved to Pilot Jokes
Karl_Acht wrote:
I think the whole thread should be moved to Pilot Jokes
Exactly I really need explanation on how inch of feet or any unit is intuitive. All measures are men invented and whatever you learn to use in particular activity simply works. When I started to fly (gliders, 36 years ago), we used meters, m/s, km/h, QFE and everything worked perfectly. Some towing aircrafts were US made with feet fpm, mph and some were Yugoslav made with meters, m/s and km/h but we knew how to easily convert and calculate. In diving I use meters and bars and no way I can use feet and psi although I use them in flying.
