Greg Esres wrote:
To me the question is how to give a student a criteria for leading
turns on an instrument approach. I can't tell from TERPS whether this
was ever intended; it does suggest that the framers assumed that
navaids would be flown over, rather than by.

I think that assumption was based on how most navaids work. You don't
really know you're over a VOR until you get a full TO/FROM flip. Even
with DME, slant range makes it pretty difficult to calculate leads for
turns. You're at 9000 MSL, the elevation of the VORTAC isn't even
published, but you can guess it's about 1000 MSL, your true airspeed is
170 kts, you've got a 30 kt left-quartering tailwind, and you need to
make a 70 degree turn to the right; quick, at what DME readout should
you start your turn?

The GPS I fly with works all those numbers (OK, it doesn't know the
wind, but it does know my groundspeed, and uses horizontal distance
instead of slant), gives me an audible "boing" and a 10-second countdown
to "turn now", and if I zoom the map in far enough, even draws me a nice
magenta curved line on the screen to follow.