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.....
.....so I loaded up the plane and moved to Middle-EEEE..........OIL..that is......
by Brute » Wed Nov 17, 2004 6:43 pm 
