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uk_figs

Well Known Member
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Learning the operation of my new 375 GPS navigator and trying to understand how GPSS mode works in a autopilot. The question is if I select a "direct to" on the GPS navigator (autopilot off) and then get vectored off the track and some time later select the autopilot GPSS mode would you expect the autopilot to fly direct to the waypoint from the current position or turn to intercept the original origin to waypoint track.
Figs
 
Generally, GPSS really only adds turn anticipation. Meaning that it will start a turn before the waypoint, to most effectively change course, without overshooting the waypoint. How early it starts the turn is based upon ground speed and degrees of direction change. Need to be a tad cautious, as some waypoints are considered fly over waypoints and you are expected to fly over the waypoint before executing a course change. I suspect many forget this years after their IFR test.

If you fly off course and re-engage the AP, it will make a track to intercept the exact course it was last told to execute. If you want to go direct from the present, off course point to the next waypoint, just press DIR ENT ENT, assuming it works the same as the old 430 that I use. Never have used the touch stuff. If ATC puts you on a vector back to the original course after the deviation, some AP's will allow heading mode and have nav on standby to acquire. If not, go to heading mode and as the plane indicator reaches the magenta line, switch to nav mode for final intercept. I don't believe that I have ever been given a vector back to course; Always "resume direct" or given multiple vectors until told to resume direct.

This my understanding, but sure someone will correct me if I missed something.

Larry
 
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In an IFR environment one does not want the autopilot arbitrarily changing the course to a navigation waypoint on AP engagement . In my experience with a 430W and GPSS capable autopilot, reengagement causes the AP to steer to an intercept course to rejoin the original selected course.
 
It will turn and try to intercept the original track. To go "direct" to the destination or waypoint from your new off-track position, just press the "direct-to" button again and reselect your target and it will establish a new line from your current position to the target which you can fly with GPSS.
 
It will turn and try to intercept the original track. To go "direct" to the destination or waypoint from your new off-track position, just press the "direct-to" button again and reselect your target and it will establish a new line from your current position to the target which you can fly with GPSS.

HOW it intercepts the original course is autopilot dependent/configurable however. On mine, you can choose an intercept angle to the original course, and if this is very shallow and the next WPT is far enough away, it would work it's way back over very slowly, vs a steep intercept angle is going to fly you over to the original course rapidly and then turn onto it.

All that said, rarely (ever?) has ATC wanted me to get back on the line I was on, so D-> <enter><enter> is usually the right answer.

Also, I had to upgrade my autopilot to the GPSS/GPSV option, utilizing the ARINC data to be able to do coupled approaches, and it appears that with having GPSS, not only does it do turn anticipation, it flys the procedure turn and holds, whereas it didn't before.
 
Also, I had to upgrade my autopilot to the GPSS/GPSV option, utilizing the ARINC data to be able to do coupled approaches, and it appears that with having GPSS, not only does it do turn anticipation, it flys the procedure turn and holds, whereas it didn't before.

That will depend on the navigator being used - but with both a G430W and IFD440 yes it will work as you describe.
 
Also, I had to upgrade my autopilot to the GPSS/GPSV option, utilizing the ARINC data to be able to do coupled approaches, and it appears that with having GPSS, not only does it do turn anticipation, it flys the procedure turn and holds, whereas it didn't before.

Thats the way the GNX375 works with the Trutrak gemini setup I have, the only thing I had to figure out was that you needed to be in altitude hold mode when arriving at the selected transition fix and then the system flies the entire approach. I am working on my instrument rating (a bit late I know:) ) so I am learning about the system operation hence the initial post to check if the problem was a system issue or operator issue.
Figs
 
Just for information I checked out the operation of the Gemini yesterday and as noted in the comments above if you go off track after entering a "direct to" (ATC vectors etc.) when you re-engage the GPSS mode the AP turns to get back to and intercept the original "direct to" track. The turn is quite abrupt which made me think I had some kind of problem but turns out was just operator lack of understanding of how the system works.

Learn new stuff every day:)
Thanks for the inputs.
Figs
 
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