Compass turns revisited

I think you need to brush up on your Garmin GPS's. In the Cirrus, I
believe, the Garmins drive the Avidyne display.

Which is the PFD. The guidance in the latest revision of the PTS calls
for a non-precision approach to be performed without the PFD if the
aircraft is capable, and this one is.

One of the navigation pages
on the Garmin displays a CDI that scales properly to approach
sensitivity.

Sure - but it still only works for the GPS, not the VOR/LOC. You're
still limited to GPS approaches only.

That is much more accurate than following the magenta line for a
backup
approach.

This is not correct for three reasons.

First, the accuracy of the data is not affected by the presentation.
It is a function of satellite geometry and the underlying algorithms.
You're talking about precision, or resolution.

Second, you can zoom the map to a view of only a few hundred feet, and
if you so choose, you can configure the 430 to auto zoom as you get
closer. Your effective precision in this case is about 25 feet, which
is better than you can do with the CDI. This is better than the 430
can consistently do. In other words, you have accessible to you a
display with a level of precision not justified by the underlying
accuracy.

And finally, using the CDI only gives you information about your
position relative to the FAC. This is inherently wrongheaded. VOR/LOC
work that way because that's all the information you have, but the GPS
also has track information. The map display presents the same position
information as the CDI, with the same accuracy and any practically
usable precision, and it also presents the track information
graphically, such that the pilot can almost immediately see whether he
is converging with the FAC, diverging from it, or paralelling it -
without having to estimate this by monitoring the movement of the CDI
over time. This will allow the pilot to more consistently track the
course.

Thus, while I will grant you that there is indeeed a plan other than
watching the little airplane and its position and direction relative to
the courseline, it's a clearly inferior plan since it will yield
inferior results while requiring all the same equipment.

Michael