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Why if the GPS directs me to KDM VOR and the DTK is 152 , why when I tune the KDM VOR on the 152 radial is it not corresponding with the GPS , I've been 2nm off GPS course but nailing the VOR...why???

hint........what's the difference between magnetic and true?

Do you have NAV one set to GPS mode or still on VOR?  True vs. Mag course shouldn't be the problem.  Try doing the same thing with other VORs.

Do you have NAV one set to GPS mode or still on VOR?  True vs. Mag course shouldn't be the problem.  Try doing the same thing with other VORs.

Are VOR based airways straight lines or great circle courses?  What does a GPS plot?

Is this the possible solution ?

The VOR airways are depicted on a mercator chart, where meridians and parallels of latitude appears as parallel lines. The drawn victor airway is thus a

So is the simple answer: The world is round

Okay, I'll stop answering questions with questions now....

So is the simple answer: The world is round

......and charts are flat........ but it's not that simple unfortunately.

why when I tune the KDM VOR on the 152 radial is it not corresponding with the GPS , I've been 2nm off GPS course but nailing the VOR...why

He didn't ask why his Nav wasn't indicating properly, he asked why it didn't agree with the GPS......He was using both of them to go to the same place and they were telling him to go to two different places.....wu'll sort of....

First, all airway bearings(radials)are computed using great circle (geodesic) calculations in true rather than magnetic. The math to accomplish this is the same for Jeppessen, the FAA, the military, and NACO. This means that airways that run North or South will usually be reciprocals of each other. Find an airway that runs East/West , on the other hand, and you will notice that the outbound course indicated on opposing ends(VOR outbound radial) is not a reciprocal. The longer the airway and the further South or North the airway is located , the greater the difference in reciprocals.

Second, station declination (magnetic variation) may vary at opposite ends of the airway in question. If you look at a VFR chart, I believe(it's been awhile since I've flown VFR!) you'll find a dashed, curved lined which is green(?), runs North/South and has a number indicating the number of degrees of magnetic variation.  

Finally, a GPS is certified under what is known as ARINC 424 specs. The airways conform to these same specs. I can hear the gears turning ".....then they should be indicating the same thing after all!"
Most current avionics systems do not carry the published radial in their airborne databases. Instead, they obtain the latitude and longitude for the VOR's from their own database, for each end of the airway and, in turn, perform their own calculations, onboard, to obtain the radials.  Practically all of the current systems on the market today use the same formulas that result in the same true bearings but after that many variations occur.
From the true bearing , some manufacturers apply the station declination from the database. Because of space limitations, many avionics systems don't carry the station declination onboard, but instead, have a couple of different means of determining the magnetic variation-not, station declination-in  the area of each VOR. One avionics system actually carries a magnetic earth model in it's database, with the idea that the avionics will only be good for a 20year life cycle, at best!  ::) What this means, is that when it is first installed, it will be acceptably accurate. By the time it is 10 years old, it will be "dead on" and by the time it is a full 20.....well.....it'll be as good as ......new?!
Other systems apply the variation that is correct at the time of flight, but that can be as many as four degrees in error, since VOR's are not realigned at the local magnetic variation until there is a significant difference between station declination and the actual magnetic variation at the VOR site. This is because the local magnetic variation continues to change over the surface of the Earth.
The good news is that both devices will get you to the same destination. The only problem is that the bearings may not agree precisely and on an East/West course, at the midpoint of the airway, there may be as much as several miles of latteral disagreement.....

[quote]Is this the possible solution ?

The VOR airways are depicted on a mercator chart, where meridians and parallels of latitude appears as parallel lines. The drawn victor airway is thus a

........so I was driving home last night, patting myself on the back for being such a knowledgable and helpful guy, when it hit me.......that probably isn't correct(entirely).

The first two points are definitely a factor in this problem. The last point that I raised(GPS programming anomalies) should not be.  The entire first post is accurate and does apply to real airplanes BUT the GPS is not a self contained computer in FS world, it's just a program. It's brought up here frequently: "the FS program assumes an angular planet as opposed to a spherical one". A VOR gives linear guidance, while a GPS plots a great circle course.  The end affect is that the discrepancy resembles what actually occurs in the real airplane.
so.........

Is this the possible solution ?

The VOR airways are depicted on a mercator chart, where meridians and parallels of latitude appears as parallel lines. The drawn victor airway is thus a  straight "rhumb line".  This at least gives me the impression that VOR radials are rhumb lines.
HOWEVER, the GPS course line is based on gnomonic projection, ie the great circle track will appear as a straight line. As you know the great circle track is curved on a mercator chart  

...........is probably not so far off after all!  :)

Sweet Jesus, I should buy a ticket in the state Lottery  ;D

Sweet Jesus, I should buy a ticket in the state Lottery  ;D

They have that over there? I thought that was an exclusively  American vice.