Timothy wrote:
Surely from a time and fuel point of view, you want to be told you are flying four miles, not ten?
Generally yes, but this particular example illustrates a problem with this approach. Let me label the points.
Having entered a flight plan A-B-C, I would expect the distance A-B to be the actual distance and the distance B-C to be shortened. I would expect this even more when actually flying from A towards B. But in that case the distance B-C has to be given initially as –1 mile and switch to 2 miles (4 divided by 2) when the navigator selects the next flight plan leg halfway through the turn.
And even if this is the behaviour that makes the most sense, it would be confusing to see a negative distance initially… Is this is what actually happens? (I guess I have to try it on the simulator.)
Timothy wrote:
Surely from a time and fuel point of view, you want to be told you are flying four miles, not ten?
While we are at atomic precision, are you sure fuel consumption numbers are the same when flying arcs vs straight lines?
(I bet you the maths of that involve Navier-Stikes, Frenet–Serret formulas and few Christoffel symbols 
)
Surely this computation of curved tracks is a can of worms. Is that really what is happening?
I would expect to see 10nm 100% of the time, in any flight planning app or any GPS.
Peter wrote:
Is that really what is happening?
I actually don’t know, but it’s my best guess from the evidence I see.
I’ve set up a flight plan in Garmin’s GTN 650 simulator similar to the figure above.
The distance A-B is 1.9 NM and the distance B-C is 1.5 NM. The course change is ≈320°.
Before arriving at A, the flight plan shows a distance A-B of 1.1 NM and B-C of 0.7 NM. After passing A the distance to go to B changes to 1.9 and continues to show the direct distance to B until halfway to the turn when the flight plan is sequenced to the leg B-C.
Unfortunately the distance B-C remains at 0.7 NM after passing the turn — i.e. there is no compensation for showing the actual distance to B rather than the distance to go until sequencing the flight plan. So until the flight plan is sequenced, the total distance shown is higher than the actual distance to go.
Does any certified aviation GPS do this?
I could test it, but I believe my KLN94 would show
10nm at A
7.5nm halfway between A and B
5nm at B
2.5nm halfway between B and C
and since you will not be flying over B it will switch the legs when you are abeam B. It would be quite interesting to get a movie of that. But I am 100% sure it will show 10nm in the flight plan, because it has no practical way of calculating the length of the curve.
alioth wrote:
The bizarre thing about Skydemon isn’t really that it uses a rhumb line, but it uses a rhumb line for planning, but when you actually fly the route it uses great circle instead!
No it doesn’t!
No it doesn’t!
Are you able to supply detail?
boscomantico wrote:
The matter is closed.
So 3.2 won over 3.14 ?
But I am 100% sure it will show 10nm in the flight plan, because it has no practical way of calculating the length of the curve.
Yes, the GTN does that.
