Hold at VOR for 2v2

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.

"Roy Smith" wrote in message ...

You're at 9000 MSL, the elevation of the VORTAC isn't even
published, but you can guess it's about 1000 MSL,

Lousy guess, field elevation at BJC is 6012 or thereabouts.

"Roy Smith" wrote in message
...

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?


VOR site elevations are published in the Airport/Facility Directory.
Elevation at BJC VOR/DME is 5734 MSL.

I think that assumption was based on how most navaids work.

Oh, no doubt. One might be led to think that there is more protected
area on the other side (where you overshoot the navaid) than on the
side from which you're approaching. I vaguely recall seeing that was
true in the enoute segment in certain circumstances (don't hold me to
this), but was NOT true in the approach segment.

at what DME readout should you start your turn?

Exactly. And even if *I* could do it, can I expect a student to be as
proficient at it or understand the nuances as to when it should be
applied? An instrument pilot shouldn't have to be a TERPS expert to
fly safely.

I think it more prudent to hand a student a set of simple rules that
will always work, which is why I'm uncomfortable recommending leading
turns on non-GPS approaches. I think it's OK, but I can't *prove* it.

The GPS I fly with works all those numbers (OK, it doesn't know
the

With GPS, different story. I presume that following any lead provided
is both safe and legal.