GPS approach step-down fixes

"Peter" wrote in message ...

The other question I would ask here is whether an FAA approved IFR GPS
is *capable* of continuously displaying the distance to the MAP,
during the flight from the FAF to the MAP, if there are *any*
waypoints between the two.

I don't think it is. I don't think it's possible to have

FAF X Y Z MAP

and somehow force the GPS to display the distance *to the MAP* while
one is flying past X,Y,Z.

The GPS will always display the distance to the next waypoint in the
database.

In other words, I don't think it's possible to get the GPS to produce
a DME-like distance readout.

One way to achieve a distance readout to some waypoint which is past
the current one would be to have a sort of "invisible" attribute on a
waypoint (in the GPS database) causing the GPS to ignore it for the
distance calculation.

Yes, it is possible for at least some GPSs to do what you doubt.
I don't have a TSO-C129 GPS to compare with,
but my TSO-C146 unit will display distance to MAP, ignoring stepdowns,
but possibly only when it can compute an uninterrupted glide path to MAP.
In that case, even though named stepdown fixes exist, they aren't used.
That's the behavior I've seen in the U.S.; I don't know the rules behind it.

Of course, in the UK, with EGNOS not yet available,
TSO-C146 units could not compute that glide path,
so I shouldn't try to guess what they'd do without augmentation.
But I'd expect them to revert to TSO-C129-like behavior.
I think Julian may experiment with a Garmin 430/530 simulator,
and he'll surely tell us if he turns up behavior different from the GNS480.