I use FSNAV as my flight planner. As part of it, when navigating between two points, whether a short leg (less than 20 miles); average leg (50 miles) or a long leg (greater than 50) FSNav identifies the magnetic course for that leg.So, since it's mag course, when navigating between two NAVAIDS (VORTAC) that mag course should become the reciprocal of the inbound radial or in other words, the OBS setting with a TO indication is the same as the magnetic course.Why is then, that when I set the mag course as my inbound radial and center the CDI on NAV1 that the moving map of the in-built FS GPS shows me off course and ATC keeps trying to give me an intercept heading? Or conversely, if I center the GPS, the CDI is showing me off-course? Should these two be the same???I've experimented with the variances between the two thinking it is magnetic variation but the delta between the two courses doesn't = the variation for the area. Thanks for any insight.

Just a guess here but I think the earth curvature plays a part in this. I don

I thought about that but it didn't sound right. If I were flying an airliner tran-continental or trans-oceanic, I wouldn't have thought about it twice. But I'm flying general aviation where my typical flights are 150 miles in length. On a 50 mile leg, VOR to VOR, the difference between the FS Nav magnetic course (centered CDI, TO indication) and the GPS course line is as much as five miles. When I check the variation at the two VOR's it's within one degree of being the same and often times is the same...I just can't figure out why my CDI indication and the GPS course line don't agree with each other.BobL

Can I ask what VORs you are using as I would like to try this out.

>Why is then, that when I set the mag course as my inbound>radial and center the CDI on NAV1 that the moving map of the>in-built FS GPS shows me off course and ATC keeps trying to>give me an intercept heading? Or conversely, if I center the>GPS, the CDI is showing me off-course? Should these two be>the same???>I notice the same problem when navigating within FS2004. I like to fly the old birds with the steam gauge panels and I did exactly what you described above but the GPS always shows me off of the airway! Is this just a long standing FS quirk?

A GPS track between two points will probably be Great Circle and I'll guess the formula FS uses is based on True rather than Magnetic bearings. How it then calculates the Mag heading I have no idea. It could be from start, mid or final point of each leg. I also wonder if it uses the MAGVAR table for this data or has a separate resource.For other magnetic calculations FS uses the MAGVAR table. Certainly in FS2002 this was very out of date - I think it was using 1997 values. Does anyone know if this was corrected/updated in FS2004? boneshttp://fsaviation.nethttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

>I notice the same problem when navigating within FS2004. I>like to fly the old birds with the steam gauge panels and I>did exactly what you described above but the GPS always shows>me off of the airway! Is this just a long standing FS quirk?> IN FSNav, if you press the + several times when flying, many times it will show you off the airway. The more you magnify the chart, the further it will show you are off. It may actually match the GPS, when it shows you are off. I've always assumed it was due to drift from cross winds, but never checked closer to try to figure out the cause. Bob