Garmin GNC 355 and CDI Scale setting - all the time

flyzone

Pre-takeoff checklist
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Oct 10, 2014
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Flyzone
I am curious to know if others set the CDI scale on their Garmin GPS equipment to the 0.3nm setting and leave it there. I used to do this on my old (non-IFR)250XL and it drove my old Century autopilot much more accurately. I know it can autoscale up to a 2nm deflection but I don't fly that sloppy but I'm wondering about the downsides of my using this setting all the time in an IFR environment.
 
I don’t know what equipment you fly with, but a .3 for intercepting an arc, radial, or LOC would not be good. The box default is best.
 
The default is AUTO which in the approach environment downshifts to 0.3 anyway where intercepting arcs, radials and LOC are mostly done. So that's not right. Essentially a permanent 0.3 setting would apply to the enroute and terminal environments as well which are 2nm and 1nm scales by default respectively. I can see why this might apply to a VOR which can wander but GPS's do not as far as I know.
 
Going back to the old days with a IIMorrow Loran, then a slide in replacement GX55, I always used the smallest scale. With the present GNX375, the 2 mile scale made me feel like wasn't able to fly accurately and the autopilot seemed to agree. Set to the 1 mile, still feels a little sloppy but the autopilot tracks pretty good in the nav mode. Autopilot works great at .3 once the nav is captured but getting to that point was prone to overshoots.
 
If you don't have roll steering with your autopilot, then tracking may be improved with the greater CDI sensitivity of 0.3. Even with this setting, on an RNAV(GPS) approach, the CDI FSD will switch to angular +/- 2 degrees for the WAAS GPS, making the final approach leg more sensitive than +/- 0.3.
 
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