Hi

I'm wondering which method SD uses to calculate a course and track between two points - the reason being that if I use the GPS on one of my frequent routes to just 'direct' to destination from origin, then this causes me to just clip a prohibited area. When I the same route plan in SD, it shows me as remaining clear. The GPS uses great circle, so either SD is using something else, or someone's data is incorrect. In any case, it would be good to know what SD is using, as it means the transfer of a planned route may not be flown the same when transferred to a non-SD device.

(Route is LFGB -> LFDP and the prohibited area is LFP2 Civaux, software is Windows PC V3.0.6.0)

SkyDemon uses a rhumb line (see this post http://www.skydemon.aero/forums/Topic7526.aspx)

Thanks - that was not the news I wanted, but at least clears it up. That seems to considerably reduce the value of SD for planning longer routes if they have substantial East-West legs

It is very easy to specify intermediate waypoints to eliminate the problem.

Sorry, I have to disagree with you there Tim!

It is easy to add some waypoints in SD along the rhumb line, but that then means creating them in the panel GPS, and entry of lat/longs must be one of the most error prone possibilities (DMS vs. DM.M vs. D.DD etc, E vs. W) to say nothing of tedious

On the other hand, there may be an easy way to add waypoints on the GC into SD but I haven't found it yet!

I clearly don't care about the difference because my aircraft has a range of about 300 nm. You would need a darn long route to come up with a considerable difference in distance, though.

-- Chris.

It's not the difference in the leg length, it's the difference in the route. It goes back to the 'plan the route, fly the plan' ethos. If it's difficult to fly the plan, it makes somewhat of a mockery of planning! If the plan says clear of all restricted/controlled airspace, it would be nice if that's what actually occurred in the flight.

Surely the easiest route to fly is the direct between two points?!

Similar to the distance, the difference in bearing between rhumb line and great circle course is small for shorter distances. It is worst for east-west courses at mid latitudes. What I don't understand, though, is the statement that "it is difficult to fly the plan". If I plan a route by rhumb lines, I can follow the pink line on the screen pretty easily since it is a constant course I am flying. A great circle route is generally a course with changing magnetic bearings as you follow the route.

I also don't understand what great circle and rhumb line have to do with staying clear of unwanted airspace?

Greetings, -- Chris.

Hi Chris

I could follow the pink line on SD, and that would reflect what was planned. However, SD is not connected to my autopilot (and a bunch of other aircraft equipment), whereas the panel mount is. However, the panel mount GPS is going to fly GC (I'm not aware of a TSO'd GPS that doesn't) and not rhumb line, so the plan and the route differ. As you state, for short legs, or nort-south legs this isn't much of a difference, but on longer east-west legs it is the difference in lateral (i.e north-south) position that makes a significant difference. In the example I started with, the rhumb line is several miles to the south of the GC, even towards the end of the leg, and the difference is between busting a prohibited zone and staying well clear.

Put another way, I plan A->B in SD, get a route that looks good (why add any more complexity if it doesn't need it?), but get in the aeroplane and put the same route in the GPS, and get something different. One way round this would be to create a few waypoints in SD on the rhumb line, but it is cumbersome (and extremely prone to errors) to put these into the aircraft GPS. Another way would be to put a bunch of waypoints into SD on the GC, but I haven't found an easy way to do this.

Does that explain the problem?

Ben

Yes, it does. The problem arises when you plan on SD and fly by GC. I guess the only workaround would be to plan the route on SD using segments short enough so that it doesn't matter. If you choose your waypoints cleverly, i.e., use airports or VORs, then you could enter them quickly in your onboard system.

Thanks for the explanation, -- Chris.