I've always thought that when flying on autopilot and using the GPS as the Nav source, that the autopilot got directional information from the GPS. What I found was that when flying the C172 or C182, if you set the gryo drift to on and fly along awhile, if you hit the D key to realign the DG, the plane changes heading. This seems to show that the autopilot is getting heading information from the DG and not the autopilot. Is there another explanation for this?

>This seems to show that the autopilot>is getting heading information from the DG and not the>autopilot. >I hope you realize that the sentence above makes no sense whatsoever. Autopilot is not getting the heading information from autopilot (itself) - AP is getting the heading information precisely from the DG. Information from the DG about the current heading coupled with the desired course information is what drives AP.Michael J.

Yup, Michael's right about the autopilot getting its info from the DG and other instuments, although he's a little harsh in the way he put it. In our C340 I correct the HSI every ten-fifteen minutes so that the GPS can continue to track true.So if you're going to use drift, make sure that you keep up with it. If it gets to be a problem, just hit the "D" key and it will align for you. Happy flying

I like the sentence in your signature!"The best part about simulated flying is your hands don't smell like Avgas and you don't have to clean the bugs off the windshield!"Let's put it the other way:"The best part about real flying is your hands don't smell like Cofee, and you don't have to clean the bugs off the software!"haha

Yeah, that's a good one! I didn't think about the "bugs" double meaning. The other great thing is that you don't have people buggin you to let them on to the internet when you're flying.:+

Thanks for the info, and yes, I did misword it. I assumed that the autopilot got all it's nav info from the GPS. I apparently assumed incorrectly.

In fact GPS could theoretically give you the heading information - by knowing the current position and your position a couple of seconds later the heading could be derived. But most likely an AP designer would tell you that such indirect information that comes with some 'delay' is not precise enough to drive AP's servos in an unambiguous fashion.Michael J.http://www.reality-xp.com/community/nr/rsc/rxp-higher.jpg

>In fact GPS could theoretically give you the heading>information - by knowing the current position and your>position a couple of seconds later the heading could be>derived. Not at all. That would give you track information, which is signifigantly different from heading information in a crosswind situation. Dan

Dan, you are of course absolutely correct. I completely forgot about it.Michael J.