GTN750 LNAV+V Question

Discussion in 'Cleared for the Approach' started by Walboy, Jan 3, 2017.

  1. Walboy

    Walboy Line Up and Wait

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    I had a nice opportunity to practice approaches in IMC several days ago here in S AZ. I practiced the KCGZ GPS RWY 5 approach multiple times and each time my GTN indicated LNAV+V minimums. However, on one of the approaches the CDI's (GI-106A) glide slope needle was flagged. There were no messages indicating any sort of degraded performance. Unit seems to work flawlessly otherwise.

    What would cause this? I can't find any explanation in the GTN manual.
     

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  2. gsengle

    gsengle Cleared for Takeoff

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    Only works on my 530 if your database is current.


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  3. tsts4

    tsts4 Line Up and Wait

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    Sounds like to me there was a momentary glitch in the comms between the 750 and the 106 caused by either something internal to the devices or a loose connection in the wiring harness and nothing to do with the approach,database, or GPS reception.
     
  4. gsengle

    gsengle Cleared for Takeoff

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    Is your database current? From what I understand if your database isn't current Garmin locks out the GS.


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  5. tsts4

    tsts4 Line Up and Wait

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    The OP said he shot this approach multiple times with no problems and the GS flagged only once--doesn't sound like a database currency issue to me.
     
    Last edited: Jan 3, 2017
  6. MAKG1

    MAKG1 Touchdown! Greaser!

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    Usually when the GP flags on a Garmin, it means SBAS (WAAS) is not available.

    Note it' s not a GS. That's for ILS's. It's a glidepath. GTN650 annunciations don't distinguish, but G1000 does.
     
  7. Walboy

    Walboy Line Up and Wait

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    Database was current. The CDI glidepath needle was flagged, but the lateral navigation needle was not. I guess I need to read up on what indications I can expect to see if WAAS is not available. I just noted that the LNAV+V was lit up indicating to me that lateral navigation was available.
     
    Last edited: Jan 3, 2017
  8. blueskyMD

    blueskyMD Pre-takeoff checklist

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    I have aspen PFD and G 530 WAAS. I shoot LNAV regularly. GS always works - database current or not. Unusual for WAAS not to work when it was working during the previous approaches
     
  9. Checkout_my_Six

    Checkout_my_Six Final Approach

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    Not sure if this was your issue....but for me (using the 530W) if you didn't sequence properly to the segment on the approach you won't get the glide slope.

    I saw this as I was transitioning from a hold to the approach. Instead of deleting the hold I went direct to one of the fixes. I got the localizer but no vertical guidance till I was past the first fix.
     
  10. Walboy

    Walboy Line Up and Wait

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    Everything sequenced properly on all the approaches. ATC cleared me direct to the IAF, I crossed the IAF, I flew the procedure turn, noted LNAV+V once I turned inboud. I'm not sure when the flag first popped up. I first noticed it when I crossed the FAF, turned inbound and cut power.

    I tried calling Garmin, but after being on hold for 29 minutes, I gave up.
     
  11. James331

    James331 Touchdown! Greaser!

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    ....quit picking all those nits


    And as someone following the complementary v nav, or a ILS/LPV how does that change HOW I follow it???

    I'll follow a ILS glide path down to 200', just the same as I would follow a complementary v nav down to 500', or whatever.
     
  12. Checkout_my_Six

    Checkout_my_Six Final Approach

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    btw.....my 530W nav data is two years old and still provides gps glide slope guidance. :D
     
  13. MAKG1

    MAKG1 Touchdown! Greaser!

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    Always? They are different at Livermore. Better uncouple your autopilot by 1700 if you're using the GS. LPV can go down to 600 MSL.
     
  14. James331

    James331 Touchdown! Greaser!

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    So do you not keep the needles centered on one vs the other or something?
     
  15. Z06_Mir

    Z06_Mir Cleared for Takeoff

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    I'm pretty sure that my GTN750 will show the glideslope even if the database isn't current. I've been doing a lot of practice approaches lately without a current database and definitely have gotten vertical guidance for LNAV/VNAV and LPV where available. I have noticed that if you turn (or get turned, which you shouldn't in the real world) inside the FAF it won't sequence and you have to go missed and come back around to hit the FAF. Probably a good thing to have in reality flying, but doing practice approaches sometimes you end up turning before the FAF.
     
  16. MAKG1

    MAKG1 Touchdown! Greaser!

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    As an instrument pilot, you should know about false GS. There is no such thing as false GP. So, yes, you do fly them differently.
     
  17. James331

    James331 Touchdown! Greaser!

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    Yeah, there are some different failure modes

    But when it comes to actually flying it, shy of trying to dive bomb onto the vertical, it's the same thing, needles centered till you hit the magic numbers.
     
  18. RotorDude

    RotorDude Pattern Altitude

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    Maybe it's something to do with air data hookup (which I don't have), but my GTN never indicates minimums, on any approach. What am I missing?
     
  19. RussR

    RussR Line Up and Wait

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    Although I agree with you that the terminology of glidepath or glideslope is virtually irrelevant, the last bit of your statement isn't quite true.

    An LNAV+V glidepath is not quite the same as any of the others: LPV, LNAV/VNAV, or ILS.

    LNAV+V plots an advisory glidepath that has not been TERPS-evaluated by the FAA, but has been entered into the database by the Jeppesen or Garmin or whoever is supplying the database. As such, you are still flying to an MDA, not a DA.

    On an ILS, LPV or LNAV/VNAV, you fly down to the DA, make your decision, and in the process of executing the missed approach, may temporarily dip down below that DA. That is fine and expected.

    On an LNAV+V, you fly down to the MDA, and need to level off at that altitude without dipping below it. You can stay there, level, and drive it in to the MAP, but whether you do that or start your climb right away, there is no protection below the MDA. This may require you to begin leveling off, and therefore going slightly above glidepath, before you get to that MDA.

    So you do and should fly them a bit differently.
     
  20. James331

    James331 Touchdown! Greaser!

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    So you're flying the complementary vnav down to a number, like I said, be it DA or MDA, you keep the needles centered and fly till you hit said number, land or go around.
     
  21. Walboy

    Walboy Line Up and Wait

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    What I was referring to is the green annunciator at the lower left side of the unit as it sequences from terminal to approach mode. Some RNAV approaches have different minima like the RNAV approach shown below. I don't have air data hookup. I don't have an autopilot either.

    GTN750.JPG
     

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  22. RotorDude

    RotorDude Pattern Altitude

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    No problem. I somehow imagined you were getting minimum descent information (like the big boys), which would have been very cool. :)
     
  23. Walboy

    Walboy Line Up and Wait

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    Nah...just a lowly Cherokee...but hey...it's paid for ;)
     
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  24. MAKG1

    MAKG1 Touchdown! Greaser!

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    That's how you fly an MDA. For a decision height, you need to level off a bit earlier, or you'll bust through it. ACS standard for a DH is -0, not -100 like other minima. The rule of thumb I use in a 182 is 50 feet high to begin the level-out. It will be more on a faster airplane. You also continue level at the DH until you reach the MAP (or decide you can't land). An MDA goes missed immediately.
     
  25. azure

    azure Final Approach

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    It might just be a matter of wording, or for that matter, my own reading skills... but it sounds to me as if you have MDA and DA reversed as far as your first sentence is concerned. So, just for clarity (paraphrased from the P/CG):

    DA is the altitude at which you must make the decision to continue or go missed. Your momentum will carry you a bit lower than the charted altitude, and that is expected.

    MDA is Minimum Descent Altitude and is a hard minimum: you aren't authorized to continue below the MDA. If flying an advisory GS, you have to level off a bit early so as not to bust it.

    Actually, rereading the rest of your post, it sounds as if you have the two reversed consistently, not just in your first sentence. But I'm not familiar with the ACS so I'm not sure about that. But I'd be very surprised if the ACS criteria were in conflict with the standard definitions from the P/CG.
     
  26. RussR

    RussR Line Up and Wait

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    I agree. @MAKG1, you have it backwards. DA is used on the vertically-guided approaches, MDA on the non-vertically guided. DA has the expected height loss below it, MDA is the hard altitude.
     
  27. MAKG1

    MAKG1 Touchdown! Greaser!

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    Mea culpa. Yes, it's backwards.

    Precision (including LPV) = descend to altitude, then decide to go missed, possibly passing the minimum while executing the missed.

    Nonprecision = don't go below the minimum at all; level off if necessary to the MAP.
     
  28. mfisher

    mfisher Pre-Flight

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    I had a similar problem recently.

    The 430 stopped giving LPV approaches and would default to LNAV. This continued happening for a couple months. Later, I noticed on one cross country that every time transmitting on 132.025 caused the GPS to completely lose reception and go into "Ded Reckoning Mode". A minute after transmitting, it would gain reception again. This was IFR at night. The 496 was not affected. After getting a frequency change, I didn't see the problem again. I was able to shoot the ILS and land at my destination. But I was now willing to fly IMC again until it was fixed. The avionics shop diagnosed interfere between the comm antenna and the WAAS antenna and after $700 and a new antenna (bent instead of straight) both problems (unable to pick up LPV and losing reception when transmitting on a range of frequencies) disappeared.
     
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