More info on GNS 430/530 WAAS upgrade

Ron Natalie wrote:

Matt Barrow wrote:


I'd like to dump my KLN94 and go to a GPS480 with an AviDyne MX-500. One
problem is the GPS480 does not support curved flight paths (GAMA) that a GPS
430 does.

It will fly them (at least it does holds, procedure turns, and DME
arcs). Maybe it doesn't reflect htem properly on the GAMA graphcis
to the AVIDYNE (I have an MX20)

ARINC 424 specifications provide for two different type of curved legs AF (arc to
fix) and RF (radius to fix). AF legs are for DME ARCs. RF legs are for the new,
advanced instrument approach procedures, which thus far are limited to
RNP-qualified aircraft and flight crews.

Garmin claims their panel mounts will be able to do RF legs, but I remain
skeptical. I can see it working but only in a robust installation with an
electronic HSI (where the course setting auto-slews to keep up with the RF leg's
constant course change) and with a good steering computer for either a flight
director or nav mode of the autopilot.

With an AF leg you have 4 miles of protected airspace (plus a 2 mile secondary)
each side of the ARC, so some plus-or-minus 0.5 mile zig-zagging is
insignificant. Not so with RF legs where not only is the radius of turn typically
much tighter than a DME ARC, the protected airspace may only be 0.6 of a mile each
side of centerline. And, an RF leg of one radius and direction can be immediately
followed by a different RF leg of a different radius and/or direction. This is a
much more complex flight path to manage than a DME ARC or a holding pattern
(Garmin's holding patterns are not real-world patterns in any case; rather just
symbols for holding course and turn direction.)