More Garmin GNC 355 (GPS 175, GNX 375) Questions

[Llewtrah381]

A few quick questions for those who have one of these units (or maybe a GTN, if the stuff is the same):

As noted before, I currently have a GNC 300XL which is working fine but was old-school when I bought it new in 2008. But I'm used to it and have no problems doing what I want to do, overall. Loading flight plans is a bit tedious but not bad, using the knobs and buttons. The good news is it's still pretty easy to do in turbulence.

For these units, I know the primary way to enter waypoints, etc. is via the touchscreen. Can it be done with the knobs? It's hard for me to discern that one way or the other in the manual. And overall, how have people found the units to be in turbulence? Feedback so far seems to be that it's a pain. Doing the plan on the ground is fine; my biggest concern is getting a re-route in the air and having to load it on a bumpy day.

Also, I see on page 3-66 of the Pilot's Guide that "Roll steering terminates when approach mode is selected on the autopilot. It becomes available once you initiate the missed approach." First, right now I have my 300XL connected to my Piper autopilot via a DAC GPSS, so the GPSS drives the autopilot in any phase of navigation, including down to the runway if I wanted to. Second, I'm also planning to connect the 355 to a GI 275, but now I'm not clear which one is actually providing the GPSS: the 355 or is that overridden by the 275? But more to the point, with this configuration (355 plus 275), will the autopilot be able to fly the approach or does it truly get disabled?

Frankly, even though the 300XL is getting long in the tooth and would have limited repair options, I'm wondering if the change is worth the considerable cost.

Appreciate the feedback.

 

[asicer]

For these units, I know the primary way to enter waypoints, etc. is via the touchscreen. Can it be done with the knobs?

GPS175 and GNX375: Yes.
GNC355: No (turning the knobs changes the COM frequency)

And overall, how have people found the units to be in turbulence?

Not too bad if you can use the knobs. Also, the GNX375 has an option for an external XDPR IDENT button. That could be useful in turbulence, too.

Also, I see on page 3-66 of the Pilot's Guide that "Roll steering terminates when approach mode is selected on the autopilot. It becomes available once you initiate the missed approach."

Huh, that's weird. I always thought it had the same availability as the GTN, G1000, etc.

Second, I'm also planning to connect the 355 to a GI 275, but now I'm not clear which one is actually providing the GPSS: the 355 or is that overridden by the 275?

The GPS provides the roll steering, the GI275 translates that into GPSS heading commands.

But more to the point, with this configuration (355 plus 275), will the autopilot be able to fly the approach or does it truly get disabled?

Pretty sure it can fly at the very least LNAV/LPV approaches.

 

[bbarrett]

The GI275 would let you get rid of the external GPSS emulator. I have a 650/275/century IIB setup, and the 275’s GPSS emulation is nice. Just make sure the installer adds an external switch to switch between heading and GPSS mode.

I’m surprised by the GPSS on final note, at least for a GPS approach. If you add a 275, the practical impact of that is likely small. Switch the AP to NAV mode (from HDG) at the FAF, switch the 275 from GPS to NAV1 source (skipping this step for a GPS approach), and you’re golden with a dial change and maybe a button press.

 

[Llewtrah381]

Appreciate the feedbacks. Very helpful.

I think I figured out the autopilot stuff: I was reading it as “…selected when on autopilot” when it actually says “…selected on the autopilot”. The Garmin 500 AP has an APR switch my Century doesn’t have.

 

[Randomskylane]

I just load all flight plans in ForeFlight on iPad and then it automatically goes to the 355. Just press messages and accept

 

[David Lloyd]

It is more difficult for me to make changes on my iPad in turbulence than pushing buttons on my GNX375. Every now and then, push the wrong button, it's just not a big deal.

When using GPSS from my 275, my Stec 60-2 must be in HDG mode. Once turned to the final approach course, switch from HDG to NAV for the coupled approach. At the missed, unsuspend and switch from NAV to HDG (275 still set to GPSS) and it flies the missed and hold. So many combinations of autopilots, navigators and displays require the pilot to understand how that avionics setup works.