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GPSS question

 
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Njacko
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PostPosted: Tue Jun 12, 2018 1:09 am    Post subject: GPSS question Reply with quote

Question for Maule avionics and autopilot gurus...

It’s taken a while, but I’ve just got around to adding an ST-901 GPSS module so that my GTN 650 can drive my S-TEC 60-2 autopilot. GPSS transforms the accuracy with which the AP follows a procedure or route and once it has captured a straight leg it follows it like a laser, but there’s still a couple of hundred yards overshoot on a 90 degree corner:



It looks as if the GTN 650 is anticipating the turn, as it is supposed to do, but just initiating it a few seconds too late.

Is this to be expected? I wonder whether it has to do with the “roll” servo operating on the rudder, or perhaps the S-TEC limiting roll to 90% (?) of a standard rate turn?

Of course, it doesn’t matter in practice, but it just looks a bit untidy to anyone watching a radar screen Rolling Eyes
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VA Maule
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PostPosted: Tue Jun 12, 2018 5:36 pm    Post subject: Autopilot Reply with quote

My s- tec30 coupled up to a Garmin 530W does about like what you show with the screen shot. As the course angle gets more acute the overshoot gets further off course but comes back like yours. But it sure does lighten the workload at times and thus far (horizontal guidance only) is rock solid to center line on GPS approaches right down to the flair ( should I fail / forget to disconnect A/P Wink ). Granted that's a straight line & holding patterns are somewhat elliptical rather than the race track but ATC hasn't grumbled at me yet.
With angles of less about 60* course track deviation isn't noticeable.
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Kirk
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PostPosted: Tue Jun 12, 2018 11:58 pm    Post subject: Reply with quote

Not many autopilot systems handle more than a 45 degree intercept without some overshoot. Really, the majority have to be watched with more than 30 degrees.

Looking at your track line, I’d say that is pretty good performance.
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captnkirk
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PostPosted: Wed Jun 13, 2018 12:39 pm    Post subject: Reply with quote

Overshoots are normal the question is does it fall within required nav accuracy. A RNP of 10 miles is what is needed typically for general aviation. Certain approaches require 3 and the airlines can do some down to .3 but that takes special aircraft and crew training and authority . Most IFR approaches will not give you that sharp of a turn on normally it will require some type of course reversal procedure in IFR conditions. Also some autopilots fly over a fix while others will flyby to smooth the intercept.
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VA Maule
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PostPosted: Wed Jun 13, 2018 3:09 pm    Post subject: Reply with quote

To expand on Captinkirks comments as I recall from instrument training way back then when a 300XL was state of the art I remember that some way points were designated as fly over ( which would have to cause an overshoot if corse alinememt changes) and some/ most except from the final approach fix inbound the fly by is accepted and expected as I was taught that the " MAGIC " in the box knows witch ones are witch . If I recall correctly maybe it's time for an instrument profincy check? Embarassed
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skymarc
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PostPosted: Wed Jun 13, 2018 3:15 pm    Post subject: Reply with quote

GPSS gets deactivated when on approach mode.

I leave it on nav/GPSS mode with gpss activated until the last straight leg before the FAF THEN I switch to approach mode. This will give you more accuracy in the turns.
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Njacko
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PostPosted: Fri Jun 15, 2018 11:35 am    Post subject: Reply with quote

Thanks folks.
I’m due to fly a 400 mile round trip on Sunday, hopefully VFR, but with a veteran Wellington and Liberator pilot to keep me company, so I guess the GPSS AP will get some use while we natter about aeroplanes and whatnot...
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maules.com
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PostPosted: Fri Jun 15, 2018 3:03 pm    Post subject: Reply with quote

Sounds like you'll have good company!
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