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FYI Garmin GNC 355 Version 3.21 released (April 2023)

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Forums Chief Pilot
Staff member
Oct 31, 1996
Type aircraft owned
Carbon Cub FX-3
Base airport
I decided to do a search to see if there were updates for my Garmin GNC 355 as I usually hear about it somewhere. It looks like in true Garmin form an update came out, was pulled, and now 3.21 is out with an April 2023 release date. I can't find the service bulletin which describes the release. For a moment I was praising Garmin about keeping the G3X manual up to date and now here we are showing poor form in not being able to find the technical document that describes a firmware update. So I'll give it a try, soon...FYI to anyone else with a GNC 355.

Installed today, no issues noted (yet).
There is a new feature on startup of the 355 post 3.21 install. Two new options to enter fuel on board and fuel flow appear on the right site of the screen on start up. In addition there is a new (I've never seen it before) Fuel Planning option under the utilities menu.

Given the fuel calculation function on the G3X I'm not sure I will be using this new function but without a G3X it would be helpful



I've suspected that there was a problem with my GNC 355 (wiring or configuration), and this may confirm it. Mine just shows dashes for the fuel flow. Like you wrote, it doesn't really matter since the info is on the G3X. I haven't yet updated to the 3.21.
I think the fuel calculator functionality is more for places that are retrofitting a COM unit with a GNC 355. In our case with have a GEA24 that gets sensor data and feeds this to the G3X which is how it gets fuel flow, etc. Now it is all on the BUS I believe but I don't think the GNC 355, that I know of, has any way of configuring or sensing a fuel flow sensor from a GEA24. I don't know avionics, this is just my suspicion.

The G3X has great tools for fuel management. Configuring the data bar at the top is what I do such as for going cross country I use Fuel Over Destination (FOD) for fuel calcs. My fuel flow is calibrated which you can do at each fill up as needed in the G3X on the fuel page by pressing Menu. It is very accurate for me which is nice. There are plenty of other helpful items you can put in the data bar as well. But for the GNC 355, I personally see no value in the new fuel calc functionality other than just a basic calculator.

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