If I do a relatively long East/West leg (yesterday was Biggin to Exeter, for example) if find that the line on SD is significantly displaced from that on the plumbed in GPSs (530/430/Pilot IIIc) to the extent that there is probably over a mile between them at the mid point. Thus, one can have a significant fly right on one and fly left on the other.

To decide whether SD is using a rhumb line or great circle route I plottet a single leg from 48N005W to 48N22E in SD. The course apears as a straight line, coinciding with the 48° latitude. This is a rhumb line. Therefore SD calculates rhumb lines rather than great circle courses.

If you want to track the centerline of an airway I suggest you pick intermediate waypoints.

I don't see this as a particular problem because it only ever results in significant deviations when you plot very long legs, running predomininantly in an easterly or westerly direction. Over a course from Brest (LFRB) to Nyireghaza (LHNY) the GC course is 1043 nm long as compared to the rhumb line course which comes out at 1047 nm. I can live with that. Just don't use it to cross the Atlantic.

-- Chris.

It's only a problem in that it doesn't follow the standard, which is GC. Any GPS you use in tandem will use GC, meaning that the courses deviate.

SkyDemon uses rhumb lines for planning and navigation. The track and heading shown in the PLOG is a rhumb line, because there isn't a decent way of representing a GC in the PLOG as by its nature, flying a GC requires periodic updating of the heading you're flying.

A long time ago we actually used to have a tool which helped you plot a series of rhumb lines tracking along a GC, but nobody used it so it eventually got removed when the parent feature was being updated.

Tim, I quite understand your reasoning. But you could show the course to steer at the current location. That course would update as you fly along, naturally. But at any point in time, when you view the PLOG, you'd see the appropriate course at this time.

-- Chris.

We could change the DI/HSI instrument to work with great circles instead of rhumb lines, in theory. But what is shown on the map would still be rhumb lines, and the PLOG would also be giving rhumb line headings to steer. This sounds like a potentially confusing state of affairs to me.

I can't really think of anything that would help you. What exactly is it that you're after here? We could have a tool that offered to turn a rhumb line into a great circle, putting points every 20 miles or something, I suppose.

I am divided myself on this. I think that, on balance, SD should work the same as all modern GPSs and paint a GC. The track on the plog should be the initial track to follow the GC, and live plog used in flight should present the instantaneous heading.