If we are talking convenience for dead reckoning nav (DR NAV), for distance/speed unit, I would keep NM for curved earth maps and change time units in multiple of 6 depending on the pilot “time clock tick capability”: For most GA & CAT ops, one would go for unit of Speed = NM/6min which is 1/10 of your TAS reading (you fly at 200KTS, you do 20nm in 6min, just divide by 10 easy, which is the usual size of my nav legs?)
Works the same way with km and km/h. Doesn’t work with furlongs/fortnight, though.
In the end it’s completely irrelevant. Just don’t mix too much. Conversions are always easy, as it will remain a simple multiplication. No matter what factor is used.
mh wrote:
Works the same way with km and km/h. Doesn’t work with furlongs/fortnight, though.
Yes, works on kmh and mph as you estimate stuff in the car as well…I was referring if you have a map with NM, then 1h is not that convenient for flying, so let’s debate time units but definitely 14 days will not work
alioth wrote:
wind speeds are all reported in knots, and 1 min latitude is roughly 1 nm (and all charts have those tic marks running vertically up lines of longitude) and NOTAMs will define things like special use airspace in NM from a navaid. My plane’s in MPH but I still flight plan in knots
Obviously you have to take the data in whatever form it comes. My point was that if nautical miles had never been invented it would make no difference to me, and in fact I’d prefer that they had never been invented because my life would be better without them. Today, nautical miles are just another one of those archaic things like Q-codes etc etc that could be thrown away.
I’m sure somebody, somewhere gets some utility out of being able to equate distance and degrees latitude without arithmetic, it’s not me and I don’t think it’s anybody flying light aircraft in the age of GPS etc.
Alternately if we’re in love with knowing every movement & speed on the earths surface in relation to its equivalent degrees of latitude, we could get rid of statute miles, meters etc and use nautical miles exclusively in all areas of life 
For the anecdote, there was a purist movement in physics that considered metric SI system (kg-m-s) to be “more scientific” in the 18xx’s compared to units of measurements that relate to human body/life (e.g. yards, foot, day…), this took the UK 60 years to implement, the funny part is that the scientific guy who pioneered the “merits of metric system for science” never had faith in relativity/quantum nor in heavy-than-air aviation in 1900 (airplanes cant fly and we know everything), a good lesson on how much you could be wrong, given what happened to aviation/physics in the next 5 years
For physics, things have evolved a lot since the 19 century (theoretical physic units), so the debate on “pure/absolute units” is well settled today unless someone throw the whole [gravity/relativity/quantum] lot in the bin with a new fresh stuff
For aviation, this leave us with personal taste/ego and convenience arguments, as mh said, all of it is irrelevant as long as you have a conversion table and you can do multiplications
Ibra wrote:
the debate on “pure/absolute units” is well settled
I suggest we use Plack units. They are as pure as it gets. “Report distance 460000000000000000000000000000000000000 final.”
That will be very convenient: you will be very lucky to get a “correct clearance read back”? even way more lucky to get a correct “position & speed report” ?
Those magnitudes would make the TK exams Q&A very interesting, for now I am happy to use NM & hours for familiarity and convenience, but I think something has to be done with RT frequencies in mHz and Sqwak codes in base 8…
Finding something convenient is not easy neither, off topic on number base, lot of math/computer purists have claimed it is easy to do basic calculations outside base 10, especially divisions in base 8, 12 or 16….but I will find it very difficult to teach my children how to count first on those bases before doing any basic operation (obviously their hands have 5, 10 fingers), so only base 2 seems ok for both counting & operations but those numbers take a lot of space
What is an “aviation unit” anyway ? 
There is no such thing IMO.
The odd thing with aviation, compared with cars and boats is that we indeed have made a mix of different units. The way I look at things:
The way it “should” be for all things is:
Gliders have it almost right, except they use km/h instead of m/s for who knows what reason. They do use m/s for VSI though. In meteorology all winds are in m/s. m/s to knots is an easy conversion with a factor of 2 (1.94 to be more exact).
As with others I think it’s irrelevant what we use today. An EFIS can be set up to use anything in any case, and switch between them. Out of curiosity the “mile” or mil is from latin. The original Roman definition was 1000 passus. One passus was two walking steps taken by the roman army. 1 mile (mil) was originally approximately 1482 meters.
In Norway and Sweden one mile (mil) is 10 km. That was decided in 1889. Before 1889 it was 11295m and 10689m respectively. We use this for distances above approximately 10 km and we also use it for fuel consumption (liters per mil). A car today typically do 0.5-1 liters per mil.
We also have “sea mile”. Today (from 1875) one sea-mile is 4 nautical miles, or 1/15 degree. It’s seldom used today.
In it’s simplest form the units for speed don’t matter.
There is always a number, or a couple of numbers that are needed to fly the aeroplane.
Say 75 for the climb and approach without flap, then 60 over the fence with full flap
Who cares what the unit is, so long as the number in your head matches the number on the ASI
Once I’m in the cruise I go as fast as I can whatever that is, so long as I respect the marks on the ASI
LeSving wrote:
Gliders have it almost right, except they use km/h instead of m/s for who knows what reason. They do use m/s for VSI though. In meteorology all winds are in m/s. m/s to knots is an easy conversion with a factor of 2 (1.94 to be more exact).
Not everywhere, but I have seen the few who appears to have done some FAI competitions, they tend to have standardized avionics in SI & km/h, it was always hard to sell them in the UK unless you find someone who cares about numbers in relative terms…
Neil wrote:
In it’s simplest form the units for speed don’t matter.
As far as “flying for fun” go, you can all do by “constant angle aspect” (for height & distance) and “attitude & power” (for speed & performance) and following ground/map features, it is only where you are around “borderline or precision flying” where using wrong units can spoil your day (e.g. long nav, imc flying, fuel planing, atc environment…)
A minor bugbear of mine is that people talk about “thousands of km” instead of megametres, and “millions of km” instead of gigameters :-) Although by the time you’re getting to gigametres, you may as well start using light seconds…
