"Richard Hertz" wrote in message
...
I have yet to try this myself, but two pilots I know suggested I try it.
The airways are defined by VOR radials. GPSs do not seem to navigate them
correctly. In fact, one of the pilots mentioned flew with an
"instructor"
who was relying on a GPS for lateral guidance on an IFR flight plan. The
pilot told the instructor to knock it off and use the vors as he was
seeing
CDI needle deflection. The instructor said not to worry.

After some time ATC informed them that they were off the airway (though
the
gps showed dead-on).


If the GPS is programmed properly it will follow airways just fine. Usually
in a case like this whoever put the flight plan into the GPS left out some
intermediate waypoint.

VOR's rarely agree with each other as closely as the GPS follows the
airways. If the GPS is properly set up it will rarely be more than a degree
off the airway, while the VORs can be as much as six degrees off. The GPS
sets up a single course for the entire length of the route segment, while
published airways may not have a single magnetic course for the route
segment due to change in magnetic variation. This is why the GPS is usually
a degree off what the published airway information is. If following the GPS
gets you so far off that ATC brings it to your attention, you probably
entered some bad data into the flight plan in the GPS.