When does your GPS show a reverse course and your HSI show a front course?

I discovered a interesting situation using the Garmin 430 and the Avidyne equipment. If say you have your flight plan set to KCRQ, OCN, SLI, KSMO. After take off you do the double click to select the path from OCN to SLI (note that you will be Northbound on this leg). If before reaching SLI you turn around and go back to KCRQ without touching the GPS. The GPS does not realize that you are now Southbound so demonstrates reverse sensing. The Avidyne HSI shows your offset of course correctly.

So I took off, was out over the ocean, which showed the path to my right. The HSI also showed it to my right. I turned around and went back, still over the ocean, and the GPS still showed I was to the right but the HSI showed the path was to my left. I tried this on the GPS simulator and it works the same on the simulator. You would think since the GPS knows your heading that it would detect the correct offset of the path.

Any comments?

Unlike VOR navigation, GPS navigation is always “to” a point, and courses are directional; from point A to point B. There is no concept of “reverse sensing” in GPS navigation because there are no “from” courses (like VOR radials).

I always save my active flight plan in stored flight plan #1. In your instance, I could have and would have gone to that #1 flight plan and selected “Reverse and activate flight plan” from the menu. Then the waypoints would have been reversed and in the right order for your return, you could have activated the desired leg in the new direction.

How funny I did this flight both directions over the weekend. Likely doing it again this coming weekend. If you want a passenger or a ride let me know :slight_smile:

HI Gordon,

when I noticed this I was whale watching and hand flying so I was not using the avionics for directions or the AP. This is my default flight plan when I am flying locally to sight see. Wow, the whale watching this year has been terrific by the way. What I had noticed was that the HSI and the GPS did not agree. I thought it would be an interesting discussion, because until this slightly strange example I had never seen them disagree before.

Happy flying,