ILS Course Setting

Discussion in 'Cleared for the Approach' started by Michael A, Jun 10, 2022.

  1. Michael A

    Michael A Pre-Flight

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    Hi,

    I've noticed in some recent approach practicing that the GTN 750 Xi in my airplane loads course degrees on the ILS off by around 2 degrees to what is charted on the plate sometimes (sending it to a G5).

    My question is two-parted:

    1. Given how ILS signals work, does the course setting even theoretically matter at all? i.e if I had it off by 50 degrees it should still center the same way correct? Confusing for situational awareness and I wouldn't do it, but the question remains.

    2. If it does matter, what course setting do I trust? My current GPS database, or the approach plate? Gut reaction is the approach plate given its not an RNAV approach, but my instructor said he thought he read somewhere in the AIM to use the GPS version (which I cant seem to find anywhere).

    For either of these, I would greatly prefer references included in any answers.
     
  2. Clip4

    Clip4 Final Approach

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    It’s a digital system that is out of calibration.
     
  3. RussR

    RussR En-Route

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    Doesn't matter. Ever flown with a "normal" CDI (not an HSI)? you can spin that OBS around to any course you want and it makes no difference to the deviation displayed. Most people (I think) set it to the inbound course anyway as a reminder, but it's not necessary.

    On a G5, I'm not sure it will even let you manually change the course when it's on an ILS, but even if it does let you, it still makes no difference. The OBS knob is not part of the ILS system, as the ILS only has one "radial".

    It doesn't matter, and 2 degrees is certainly small enough to not have any effect on your flying of it anyway. But I will always use the one on the approach chart, it's the published value. You're flying the chart, not the GPS, and the chart is actually a regulation in itself, incorporated into 14 CFR Part 97 by reference.

    The whole reason for the difference is one of magnetic variation. All procedures are actually developed in True courses. The numbers are then adjusted for magnetic variation for the forms and charts. What varies is how the charts and the databases apply magnetic variation.

    The charts use a "magnetic variation of record" for every airport and facility. This is the "official" value of Mag Var, you might say. It's only updated when if differs from the actual mag var by more than a certain amount (different by type of facility or airport). This is the value used for the FAA forms and charts.

    The Garmin and Jepp databases, however, use the "actual" mag var given an internal model, when they convert from True.

    So you'll often see a couple degrees of difference in course.

    However, it makes no difference to the actual ground track, as the GPS is still using at its core the True course between fixes.
     
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  4. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    GPS uses a different set of coordinates for courses, so that 1-2 degrees is most likely that difference. 1-2 degrees isn’t going to make much difference flying the approach, since a localizer CDI deflection isn’t based on the course setting. And while the needle would act the same way if it was 50 degrees off, it would probably screw up your autopilot if you were flying it coupled.

    Again, 1-2 degrees isn’t going to matter, but technically, since the CDI is showing localizer deflection rather than GPS course deviation, it should be set to the course on the chart. Ask your instructor to find “somewhere in the AIM” for what he wants you to do.
     
  5. dmspilot

    dmspilot Final Approach

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  6. NealRomeoGolf

    NealRomeoGolf En-Route

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    It will let you.
     
  7. RussR

    RussR En-Route

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    I guess that makes sense since you don't have to load the procedure from the database, and would therefore have to set the course yourself. If you do load the procedure from the database, I assume it auto-sets the course, right? But you can then change it if you want to. It's been a year or so since I used a G5.
     
  8. NealRomeoGolf

    NealRomeoGolf En-Route

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    It auto sets but you can change it. I have one local to me that loads off by 1 degree and I always make it match the chart.
     
  9. John Collins

    John Collins En-Route

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    1) There isn't any bearing information provided by a localizer course, so what you set the OBS to on an HSI presentation is irrelevant to determining the deviation. It does however, aid with visualization because setting the OBS to the inbound course on an HSI or EHSI will provide a good visual reference for the inbound course. Read about the VOR system in the Instrument Flying Handbook and you will find details about using the OBS to set/select the course. Then read about the ILS and you won't find anything referring to an OBS, because it has no use in setting/selecting the CDI deviation on an ILS.

    2) What most pilots are unaware of is that VOR courses along airways and straight in approach courses to runways are determined by a magnetic variation determined a long time in the past. So for some VOR's, this can be back as far as in the 1960's or 70+ years ago. For airports and runways, this often goes back to the 1990's, still 30+ years ago. So at my airport, KUZA, the true heading of the ILS runway is 011 degrees, reference FAA NASR survey data. The magnetic variation for my airport was established in 1990 when the magnetic variation was 5W. All approaches to my airport use the 1990 variation values and derive a magnetic course of 016. See the ILS Rwy 2 at KUZA. But GPS uses the current magnetic variation, which now is 8W. So in the last 32 years, the magnetic variation changed by 3 degrees, so my GTN 750xi shows a magnetic heading of 019 when I am on course and tracking the localizer. So at my airport, unless you were flying the approach in the 1990's, the magnetic course has probably never matched the one charted on the ILS approach procedure. You are just now noticing because you installed a G5. What did not change is the true heading of the runway or the path that the localizer describes.

    If you identify your airport that you were doing the ILS at, I can confirm what the GPS should have been reading.
     
  10. Palmpilot

    Palmpilot Touchdown! Greaser! PoA Supporter

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    One thing I noticed when acting as a safety pilot in a plane that had a mechanical HSI: When the pilot under the hood neglected to set the CDI to the localizer course, it happened to be off sufficiently so that it made it look like he was flying toward the localizer when he was actually flying away from it. (I played dumb and let him struggle with it for a while, because I figured that way he would be more likely to remember it in the future.)
     
  11. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    It makes no difference on the needle deflection on an HSI either, all turning the course arrow does is get the CDI lined up with the runway heading.
     
  12. Palmpilot

    Palmpilot Touchdown! Greaser! PoA Supporter

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    That's correct. The problem was that the CDI was not oriented straight up and down, which made it look like he was headed toward the final approach course when he was actually headed away from it.
     
  13. dtuuri

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    You would be a good instructor. ;)
    Not correct. You're talking about a LOC. For correct "sensing" (feel free to use your own term) it needs to reference the inbound course. Unless your navigator fixes it for you, that is. I'm not sure about all these modern gizmos.
     
  14. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    What I said is precisely correct and YES I'm talking about the LOC. The needle displaces the the same way no matter how the course arrow is set. However if you're flying the front course, you do want the thing pointed in the direction you're going.
     
  15. asicer

    asicer Final Approach

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    I think you mean that the HSI D-bar displaces the same way relative to the course pointer while everyone else is speaking relative to the pilot's eye, right? (i.e. if the pilot sees the course pointer straight down and the D-bar displaced left, then if the pilot twists the course pointer to be straight up they'll see the D-bar displaced right)

    Note: I generally use "D-bar" for HSI and "needle" for CDI, even though CDI's can be rectilinear rather than pendulum.
     
  16. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    “Reverse sensing” is more properly called “confusing interpretation, mostly because I set it up wrong.” ;)
     
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  17. IK04

    IK04 En-Route PoA Supporter

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    I only use the term "needle" with an RMI type indicator, an ADF or any type of display that rotates in the center of a round dial. Otherwise is use the term "course deviation indicator (CDI)."
     
  18. aterpster

    aterpster En-Route

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    All the old CDIs (circa 1960) were pendulum. BTW, thanks for the way to express them.
     
  19. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    And a lot of them had a blue side representing the shaded side of the localizer feathered arrow, and I’m recalling a yellow side for the unshaded side. Made a lot more sense then.
     
  20. asicer

    asicer Final Approach

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    I've seen some EHSI have both what I call a "D-bar" and a CDI. Although apparently some are able to detect a back course and apply the reverse sensing.
    [​IMG]
     
  21. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    They generally “detect a back course” if the HSI course needle is more than 90 degrees off the nose. It still indicates “left” or “right” exactly the same way.
     
  22. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    There's no need to "reverse sense" on an HSI (digital or otherwise). The sensing isn't really backward on a reverse corse to begin with, it's just that you're holding the conventional CDI upside down. If you set the course arrow to the front course on the HSI the indicator is rotated around with the heading card into the right orientation. As stated, the course arrow does nothing with regard to needle displacement on a LOC, it just moves the CDI around on the heading card so it looks right.

    I suspect the BC here is for the benefit of the autopilot and has squat to do with the presentation to the pilot. Autopilots just get L/R signals from the HSI and hence can get confused if they are tracking inbound on the BC (or even outbound on the front course).
     
  23. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    It probably has more to do with communication between the automation and the pilot. Older autopilots had a BC button that would tell it to fly the opposite direction (left or right) from the CDI without an HSI. The BC annunciation tells the pilot that the automation is going to fly toward the needle, even though the HSI is pointed “backwards”. It will show up not only when you’re flying a back course, but also when you’re downwind for a front course.
     
  24. asicer

    asicer Final Approach

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    Look carefully. There's both a D-Bar and a CDI needle in that picture. The course pointer is pointed down and D-Bar is deflected right. The CDI needle below the "BC" indicator is also deflected right. If it weren't for the BC detection, the lower CDI needle would be deflected left.

    There's also a VDI along the right edge of the screen but it's X'd out.
     
  25. MauleSkinner

    MauleSkinner Touchdown! Greaser!

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    I see all of that, and it makes perfect sense based on what I said. How are you thinking “BC detection” detects a back course that makes what I said untrue?
     
  26. asicer

    asicer Final Approach

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    Now that I think about what you said, I think we might actually be in violent agreement. :)
     
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