"Icebound" wrote in message
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[...]
Never having flown a VOR course myself... I still doubt very much that any
two pilots (OR auto-pilots), flying reciprocal headings between two VORs,
would both be able to *simultaneously* hold a course to within 10 feet of
the centre-line for the whole course, considering the receiver errors and
that the VOR radial-signal *itself* probably varies more than that.

I could be wrong.

You are wrong.

For two pilots to *intentionally* stay exactly on course center on a VOR
airway would be challenging, granted. But the airway provides an
"attractor" for airplanes, and inasmuch as the airplanes average toward the
center of the airway, eventually a couple will come along flying the exact
same distance from the actual airway (whether that's 0.0 miles off-center or
3.9 miles off-center).

Like I said before, it's happened to me on several occasions (getting close
enough to other aircraft on an airway to require evasive action, that is).
That's with me handflying. Using an autopilot, VOR navigation can
theoretically be VERY good, especially close to the station (within 10-20
miles).

GPS increases the chances of collision, by reducing the average error. But
the issue did already exist with VOR navigation. Keep in mind that GPS
error is still going to be on the order 10 to 30 meters or so, just from the
position information standpoint, and then on top of that you still have the
problem of the airplane being kept exactly at the intended position (even
with an autopilot, there's going to be some slop, and not all pilots are
using autopilots in conjunction with their GPS navigation).

The total error even in the GPS case can be much larger than the wingspan of
typical GA aircraft, and so the same kinds of factors that protect against
collisions when using VOR navigation also protect against collisions when
using GPS navigation (though to a lesser degree).

Pete