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  1. Username Protected
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       #1  

    Odd HDG - NAV behavior

    I keep meaning to bring this topic up as it has happened a few times to me and I still am unclear what conditions are causing it. On departure given a heading with the expectation to join a DP route out. Shortly after calling departure AND while still in the turn in HDG mode they give the instruction to go direct to a fix. I go to my FPL select the point and hit Direct To and move to NAV mode. NAV mode will then nearly stop the turn even though it might only be another 30 degrees in the current direction of the turn and then it likely will re-start the turn.

    I've also had it a few times want to turn the opposite direction and each time it catches me off guard and I immediately disconnect AP and fly it.

    I have yet to figure out what I am doing wrong and feels like my solution should just be to continue in HDG mode and turn it to the fix then go Direct To.

    This something anyone else has seen or experienced and can share what I am doing wrong?
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    #2  
    My P300/G3000 does exactly the same thing. I do not know why it does it.
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    #3  
    Quote Originally Posted by (Username Protected) View Post
    I have yet to figure out what I am doing wrong and feels like my solution should just be to continue in HDG mode and turn it to the fix then go Direct To.
    While mine always restarts the turn, it does tend to turn a lot slower in NAV mode than HDG mode. So I tend to do Direct To, roll the HDG bug to roughly to the required heading, then when the plane gets close go to NAV.
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    #4  
    After reading this thread I started paying attention. On our way out to Denver on Thursday I was on an assigned departure heading ~180 degrees from the DTK. I was then cleared on course and spun the heading knob about 90 degrees to get the turn started. Per normal I hit the DIRECT button and then switched to NAV. The jet responded as you've noted above; it stopped the turn.

    I think this is normal. When you hit Direct the CDI is automatically in-line (no course deviation) so the airplane is going to level the wings. Once the CDI starts off track the AP will resume trying to hold course and restart the turn. Remember NAV doesn't look at the HDG bug, only the CDI.

    The obvious procedure should be to do as above but stay in HDG mode until within ~10 degrees of the DTK, hit DIRECT again (unless you are trying to intercept a specific radial/SID course) and then engage NAV mode.

    I have also noted that the jet turns faster in HDG mode; that is probably a function of maximum bank in HDG vs NAV.
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    #5  
    (Username Protected):
    Good explanation - thanks for outlining the system logic on this.
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    #6  
    Quote Originally Posted by (Username Protected) View Post
    After reading this thread I started paying attention. On our way out to Denver on Thursday I was on an assigned departure heading ~180 degrees from the DTK. I was then cleared on course and spun the heading knob about 90 degrees to get the turn started. Per normal I hit the DIRECT button and then switched to NAV. The jet responded as you've noted above; it stopped the turn.

    I think this is normal. When you hit Direct the CDI is automatically in-line (no course deviation) so the airplane is going to level the wings. Once the CDI starts off track the AP will resume trying to hold course and restart the turn. Remember NAV doesn't look at the HDG bug, only the CDI.

    The obvious procedure should be to do as above but stay in HDG mode until within ~10 degrees of the DTK, hit DIRECT again (unless you are trying to intercept a specific radial/SID course) and then engage NAV mode.

    I have also noted that the jet turns faster in HDG mode; that is probably a function of maximum bank in HDG vs NAV.
    Yes, I think this is basically right -- when you hit direct-to, it plots a turn-towards course that assumes you are currently flying straight at the current heading, so it bakes in time/angle to start the turn... but if you are already turning then the initial course (which starts at the current heading) would have the AP level out before it starts to turn.

    As for turning the opposite direction... the main reason I would see this is if the direct-to from the current heading thought it was faster to turn the opposite direction - again, it doesn't take into account that you are banked and the heading is quickly changing... it just looks at the current heading and determines a turn/course to the point, which may be the opposite direction if you are doing close to a 180 reversal.

    I usually just ignore it.. (as long as it isn't reversing course) - it will start to level the wings but quickly re-join the turn... if you are really picky then as others have mentioned, wait to hit direct-to until you are within a few degrees of the expected course.
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    #7  
    That's Garminology at its best. Unfortunately. As as soon as you execute the Direct to, the Garmin plots a "smart" turn based on your current heading/speed/etc. So it rolls out your current turn and starts fresh... Not like a commercial grade FMS that does a smooth intercept. The way to keep it smooth for pax is to use HDG mode to get you pointed in the right direction and then do another Direct to to fine-tune things before you go to NAV mode...

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