Neil wrote:
When ATC tell me “speed 180 to 4DME” they mean 180kts, and the DME is reading in NM
Yes, but if you avoid flights where you have to talk to ATC (as I believe Silvaire does) then the point is moot…
Silvaire wrote:
I don’t generally fly directly north-south
The point is that you can just take whatever non “vertical” distance and bring it against one of the latitude lines and get the distance you want. You don’t really need a ruler even.
I still don’t really see a “value” in having common speeds with cars or boats. I could say that >90% of the vehicles have flow have SI units, but I don’t find it any issue fly in ft and nm. The only time I have to do conversions is when having to talk in non-aviation context (tell our passengers what our speed is etc). I would be annoyed to have to convert statute miles to nautical miles in an aviation context (e.g. "G-XXXX maintain 120 kts minimum on approach), just like I am having to convert USG to Litres (a similar issue brought down an airliner: https://en.wikipedia.org/wiki/Gimli_Glider).
I don’t really mind using different units (I have trouble thinking of heights in anything other than ft these days), as long as within the same activity, things are consistent.
Silvaire wrote:
My flight planning is done in statute mph if the plane is in statute miles. The scale of the chart is whatever scale is on the ruler I might use for that planning (or the marks on the string on the wall), my ruler has both nautical and statute and I don’t generally fly directly north-south. I set GPS (Foreflight) to match the aircraft’s ASI calibration, i.e. in statute miles when the ASI is in statute miles.
Airborne_Again wrote:
So you’ve created your own personal air navigation infrastructure. (You’ve hinted at that in many previous posts, too.) Very well, but most people would find it most convenient to use the existing one. And then they will want to use knots and NM
Actually time/angle units (base 60min/12h/24h, base 180/360) are not easy to mix with decimal base 10 system when calculating angular/distance speeds: the decimal system is the least convenient for integer fractions while time/angle units work seamlessly for integer fractions (12h or 24h/day has many divisors, 60min/hour has a lot of divisors and 360 just don’t count….)
The only easy way to mix distances with angles/time for day to day VFR navigation are “1/60 rules” and “6min rules”, so if someone has a “golden distance unit for navigation” it will be just as crap as the decimal base 10 system for integer fractions…
For legacy reasons, the NM/KTS were an attempt to link distance to “angle = time” using earth surface (this get rid of integer factions on metric system with decimal base 10, but if you don’t feel the need for this or you have a modern computer then you can use whatever you want 
)
Ibra wrote:
For legacy reasons, the NM/KTS were an attempt to link distance to “angle = time” using earth surface (this get rid of integer factions on metric system with decimal base 10, but if you don’t feel the need for this or you have a modern computer then you can use whatever you want
Exactly. That is a great way to explain it.
I have to smile at the idea of ”creating your own personal air navigation infrastructure”. What I’ve described is a few elements of the (very most) basic methods of flying from A to B, as practiced by most people I fly with, most of the time. We have good weather in general. As a result, accessible Class E airspace and runways are the primary elements in making a plane useful… and not much else is needed from people chatting on the radio or otherwise scurrying around on the ground.
Getting into and out of home base requires ATC and in particular a few USAF people to keep GPS running are much appreciated. Foreflight is also great infrastructure. As is the most basic element, trucks and their drivers to fill lonely self serve fuel tanks, and their credit card readers.
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?)
This will tell you how much “degrees you span on earth surface for a reasonable tick size of pilot/atc time”, I think it will work well as most of aviation tasks that are done on curved earth map as these can be chunked in 6 min intervals, I personally don’t think in seconds or hours while flying…
This is why old sailing maritime navigation was done in NM/day (same spirit but no ATC, you adjust course when you have the chance to see the “Polar Star” or “Noon Sun” on the next day and really nothing new or insightful in between)
The exceptions, if you fly an F16 you might want to use V = NM/6 seconds, if you fly a motor-boat you may want to use V = NM/60min, if you fly a balloon you may want to use V = NM/6h, all these will make you DR on a map easier
If one does not fly DR and does short distances (this is what most of us do for fun flying), then who cares what one uses for NAV (on a 100km task the earth is flat and the universal/atomic clock from GPS will fit the bill ),
Personally, I don’t see much of earth curvature on UK CAA 1/5 mill map and I never felt much of it while flying with my PPL, I still prefer to use the “6min rule” and NM tough 
Noe wrote:
I don’t really mind using different units (I have trouble thinking of heights in anything other than ft these days), as long as within the same activity, things are consistent.
That’s really the issue with the 1970s introduction of knots into FAA certified light aircraft, as minor an issue as it is. I happen to own one of each now, but there are few aircraft that I’d want to buy that have an ASI calibrated in knots, most of them are in statute miles per hour and that’s not going to change. I can do mental arithmetic on a 15% difference, no big deal, but I will be doing it for the rest of my flying lifetime.
I guess if it were a big enough concern, one could install a dual calibrated ASI to match both TC/POH and ATC, but they are messy looking and hard to read..
Airborne_Again wrote:
MedEwok wrote: Incidentally Russia, which also uses mmol/l does also use meters and km/h in aviation…According to their AIP they do use feet/flight levels as the primary unit for altitudes with metres as a secondary unit.
They used to use meters and km/h. Now that their airspace is a way of extracting money out of third-party airlines flying Europe/Asia routes, they adapted to their customer base.
Silvaire wrote:
It’s just that nautical miles and knots add no value to light aircraft operations
I find they do – 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.
My plane is all instrumented in US gallons but the wing stickers are in litres and of course fuel (avgas) is always purchased in litres However I always fill right up so the units of fuel sold don’t matter; I just set the fuel totaliser to “86” each time.
One has just got to live with it.
alioth wrote:
I find they do – 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.
I don’t think that much people use Map/Gps coordinates to measure distance, visualize track, localize notams…so most of us did forget how useful they are?
You will be lucky to find a modern GPS or GeoApp today that still has an easy interface to input/output raw coordinates (instead of WPT names), the last one I tried along these lines was a Trimble 2000T panel GPS (has a big red VFR only disclaimer, looks old but not obsolete)
You can blame this on geeks working on GoogleMap and PokemonGo: they had their own taste of how earth should be visualized/mapped, but their stuff is not compatible with VFR/IFR topics
