Chart and KLN 94 don't agree

Discussion in 'Cleared for the Approach' started by TBalch, Oct 31, 2017.

  1. TBalch

    TBalch Filing Flight Plan

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    I'm new to this forum, so please refer me to the previous post if this topic has already been covered. I fly lots of Angel Flight trips from California's Central Valley to Palo Alto, and with the weather changing, I'd really like to get this resolved. Yesterday I flew the GPS RWY 31 approach in VMC, with the approach loaded in both my KLN 94 and ForeFlight, and I noted these discrepancies: On the leg from DOCAL to PUDBY, the chart says 329 degrees, the KLN said 330, and ForeFlight said 335. On the leg from PUDBY to RWY31, the chart says 322, the KLN said 323, and ForeFlight said 327. I think part of the difference is that the magnetic variation has not been updated for a while, so the chart is a little off. I'd appreciate an explanation of the differences and how to actually fly the approach.
     
  2. Clark1961

    Clark1961 Touchdown! Greaser!

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    Welcome to POA.

    The question which comes to mind is to ask if both databases are current? Obviously they should agree.
     
  3. denverpilot

    denverpilot Taxi to Parking

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    The difference between the chart and the KLN may very well be differences in mag variation between the two sources.

    As I recall, Foreflight just calculates great circle routes between any two waypoints. I wouldn't use any heading their flight planning tool spits out for anything related to IFR or using a real IFR nav source. It'll be close-but-no-cigar if you're going for one degree of error or less.

    I don't have time to look up the reference in their manual right now, but you can. :)
     
  4. dmspilot

    dmspilot En-Route

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    I wouldn't trust Foreflight for anything.
     
  5. denverpilot

    denverpilot Taxi to Parking

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    I'd trust them to be a nice VFR tool, and EFB, like they're supposed to be. :)
     
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  6. BigBadLou

    BigBadLou En-Route

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    Welcome to PoA!
    A) I'd fly the approach by the IFR approved GPS backed up with a paper plate. I would not trust a tablet and an aviation app for navigation, they are not certified for navigation.
    B) What do you mean by "ForeFlight said XXX"? How did it say it? Did you add the WPs to the routing or something?

    Palo Alto has a magnetic variation of 13.7 degrees (currently, per the airport diagram). So it doesn't seem like the 5-degree difference btw FF and GPS could account for that. Does FF correct route headings based on expected/forecast winds?
    I bet if you ask them, they would answer. The FF team is pretty responsive. And I am pretty curious about the answer too.
     
  7. dmspilot

    dmspilot En-Route

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    I saw the same thing with the KLN-89B. The GPS knows where you are and where you're supposed to be, that's all that matters. The 1° difference between the chart and the GPS display seems inconsequential. Foreflight is the odd man out, and should obviously be disregarded. There is only one way to fly the approach.
     
  8. TBalch

    TBalch Filing Flight Plan

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    Thanks for the responses. They prompted a second look and I found that the information ForeFlight provided (which I use for situational awareness only) was heading, not desired track. So the difference is only a degree between the chart and the KLN, which I'd bet is a magnetic variation issue, and as suggested above, is not worth losing any sleep over.
     
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  9. denverpilot

    denverpilot Taxi to Parking

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    Foreflight does a fine job of displaying plates. I wouldn't bother messing with "dead tree technology" for any "concerns" about the EFB part of it.

    It's the flight planner that's a tad goofy, and even then, it'll get someone where they're going, VFR, just fine.
     
  10. Ted DuPuis

    Ted DuPuis Administrator Management Council Member

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    Following the KLN94 - it's the IFR-approved GPS and is what you're supposed to be following.
     
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  11. BillTIZ

    BillTIZ Final Approach

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    Charts on ForeFlight are situational awareness. Follow the installed approved IFR GPS unit. Are you sure the database in the KLN is current?

    And for the post about Great Circle, headings don’t make much difference for great circle until the distance between 2 points is about 300nm. And then only for east west routes.
     
  12. denverpilot

    denverpilot Taxi to Parking

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    Yeah. I didn’t bother looking at his waypoints, I just knew they do that and have seen the error on long routes across the US east/west when playing with it.
     
  13. luvflyin

    luvflyin En-Route

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    By "ForeFlight said," do you mean what it shows as HDG in NavLog? Or the Track you are actually flying on the 'Instrument Panel?' What it shows as HDG in NavLog takes into account forecast winds. I'm sitting here on the couch and DOCAL to PUDBY shows 332 at 4000 and 337 at 12000. If you mean the Track you are flying, that of course is affected by what the real time winds are. Course Next on Instrument panel gives the same 329 that the Chart says.
    The 329/330 difference between the Chart and the KLN I'm sure is no different than Garmin430 would show. I don't remember the details but words like variation, declination and World Geo mapping reference or something like that get thrown around explaining it.
     
  14. aterpster

    aterpster Pattern Altitude

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    They're TF legs; i.e., a great circle track between waypoints. You fly what it takes keep the CDI centered. TF = track to fix.
     
  15. iamtheari

    iamtheari Cleared for Takeoff

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    With up-to-date databases in my Garmin 430W and the up-to-date charts on ForeFlight, I almost always see 1 degree of difference between them. It is 100% of the time for the two GPS approaches at my home airport. The chart says the final approach course is 306 and the 430W says DTK is 307. (126 and 127, respectively, for the other approach.) This triggers my OCD a little bit but I decided I will just accept it until my flying is good enough to track a course to less than 1 degree of error.
     
  16. BillTIZ

    BillTIZ Final Approach

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    The approach plate is written to 306 dependent on the charting and mag var at the time of publication and rounding the tenths of degrees up or down. The gps navigator has coordinates for points along the course, will compute the course, apply the mag var in the database which may be more current than the old chart data, round accordingly and provide the DTK info. If you can fly to 1degree tolerance on an HSI or DG marked in 5degree increments, you do have OCD issues.
     
  17. SbestCFII

    SbestCFII Line Up and Wait

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    It's not unusual to find a degree of so difference between a charted track and what the GPS wants you to do. I've noted that on both the KLN-94 I had in my Cherokee and my 430W in the Warrior I have now. Like I tell my students, the EFBs tablets running Foreflight, Garmin Pilot, WingX, etc. are nothing more than substitutes for a knee-board, with some useful bells and whistles (mainly Nexrad and traffic), but they are NOT flight instruments.
     
  18. aterpster

    aterpster Pattern Altitude

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    I've seen as much as a 3 degree difference. So long as the pilot understands this doesn't matter on TF legs, no big deal. The procedure designer uses one mag var assumption and the avionics uses a different assumption.
     
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  19. John Collins

    John Collins Pattern Altitude

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    ForeFlight will correct for the forecast winds and heading is depicted, not the track in the Nav Log. If you are following the CDI attached to the KLN, you fly to keep the needle in the center and whatever heading keeps Track = Desired Track to keep from drifting off center. The precise number is irrelevant, anyone should be able to fly with a MH = DTK +/- 10 degrees if needed to hold the track.