Greg Esres wrote:

Can anyone suggest any reasons why you shouldn't use GPS or DME to
lead your turns when approaching a fix, such as an IAF, on a non-GPS
approach? (Meaning that you don't wait for complete reversal of the
nav instrument.)

The AIM has a couple of requirements for a "complete reversal" of the
TO/FROM indicator, but they aren't instructions about how to fly an
instrument approach. Regardless, those injunctions may be outdated in
a GPS world.

"5-3-5. AIRWAY OR ROUTE COURSE CHANGES" states you can lead a turn.
Whether you can descend to the altitude for the next segment during the
turn has been addressed at FAA/Industry meetings but the feds won't
resolve the issue.

With GPS, the correctly flown fly-by waypoint will never cross over the
fix, rather the arc of the turn will depart the inbound course at the same
distance as it intercepts the outbound course. The consesus at the
aforementioned meeting is that descent to the altitude of the outbound
segment should be able to commence at the bisector of the turn, provided
groun speed is limited to a value to assure that the aircraft remains in
protected airspace.