Glideslope intercept on an LNAV+V approach

Discussion in 'Cleared for the Approach' started by RussR, Aug 29, 2017.

  1. luvflyin

    luvflyin En-Route

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    LOL. I was following along with foreflight a few weeks ago on a Southwest flight. Never occurred to me to turn the volume down. The person sitting next to me seemed a little worried.
     
  2. aterpster

    aterpster Pattern Altitude

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    Use the earphones. ;)
     
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  3. SbestCFII

    SbestCFII Line Up and Wait

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    The LNAV+V gives a "best-fitting line" along the charted non-precision step-down altitudes on the procedure. I teach being (always) at the correct GS intercept altitude and the use the +V for descent guidance while cross-checking the altimeter. The +V guidance is a function of the WAAS-correct GPS satellite geometry independent of temp. As long as you're at or perhaps slightly above the +V glideslope, you'll be fine to follow that to the MDA. As far as descent rate, just use what's reasonable to hit those targets while staying as close as possible to the aircraft's normal approach speed.
     
  4. Clip4

    Clip4 En-Route

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    Remember "High to low look out below", altimeter accuracy is effected by non standard temperature.
     
  5. Clark1961

    Clark1961 Touchdown! Greaser!

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    High to low look out below refers to the error which results from initially setting the altimeter in an area of high pressure then flying to an area of low pressure without updating the setting.

    Yes, low temperatures cause an altimeter to read higher than standard temperature but it has nothing to do with high temperatures.
     
  6. aterpster

    aterpster Pattern Altitude

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    Another saying, "Low or cold, look out below."
     
  7. Clip4

    Clip4 En-Route

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    [​IMG]
     
    Last edited: Sep 2, 2017
  8. Clark1961

    Clark1961 Touchdown! Greaser!

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    Oooh looky looky looky clippy found a cartoon on the internet that is worded very poorly. Temperature effect on the altimeter is not related to flying from warm air to cold air. Does clippy think that approaches with low temperature notes only have a problem if someone flies in from a warm area?
     
  9. luvflyin

    luvflyin En-Route

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    Yeah, that's true. There is also "low to high, clear blue sky." You get more space from the rocks this way but it can cause problems with space from aluminum if the other piece of aluminum is locked on to something like a glideslope, which doesn't care about pressure or temperature while you are underneath it using an altimeter for your altitude.
     
  10. Clark1961

    Clark1961 Touchdown! Greaser!

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    The nice thing about having a glide slope around is that it means we have weather data broadcast pretty much continuously.
     
  11. Ryan F.

    Ryan F. Pre-takeoff checklist

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    Interesting thread. I'm just getting back into instrument instruction after a several-year layoff. Indeed, these +Vs add a new wrinkle which could confuse the heck out of an instrument student. Things have already become far more complicated since my heyday as an instructor... hell, I flew NDB holds and approaches on my CFII checkride, and all of those NDBs in my old home area have been decommissioned now for years. It's a different era for sure. But, all for the better.

    It's a little tempting to write the +Vs off as marketing propaganda. They're not real procedures, you certainly can't descend below a published minimum altitude to follow an advisory "glideslope" down, etc. But they're pretty nifty in a pinch, too. I've experimented with the LNAV+V to my home airport, which doesn't have a LPV option due primarily to terrain. I flew the +V profile down to well below minimums on a clear VFR day and although it was a little "tight" with regards to trees and hills, it was still plenty good enough to get me on the ground with ILS-like accuracy. Of course that's a double-edged sword; the approach isn't surveyed in that manner and all it takes is a new cell tower or growing trees, over time, to turn what seems like a nice 'out' into a potential disaster.

    I think as instructors we teach these GPS approaches but stick with the concept of approaches which contain glideslopes and DAs, vs. ones which do not. The +V is a unique outlier which we just have to remember is something dreamed up by avionics manufacturers, not any regulatory body, and carries no weight, so to speak. It is true, though, that CDFAs are the way of the future, certainly accepted by industry as superior in all meaningful ways to "dive 'n drive" with all factor being equal, so this is another step towards eliminating those step down fixes which require pilots to be concerned with power and possibly configuration changes close to the ground. That's a good thing. We just have to clear up confusion regarding when and why you can descend with our students.
     
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  12. MauleSkinner

    MauleSkinner En-Route

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    ...although it's part of a certificates piece of equipment, and is therefore very fair game for an oral/checkride.
     
  13. midlifeflyer

    midlifeflyer Final Approach

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    Very fair game, particularly since not understanding what it is and its limitations is arguably a "killer item," and not just a checkride failure is one follows it below a step-down before reaching the step-down.
     
  14. aterpster

    aterpster Pattern Altitude

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    +V is not something dreamed up my avionics vendors. See:

    https://www.faa.gov/documentlibrary/media/advisory_circular/ac_120-108.pdf

    for a discussion of the vertical descent angle (VDA).

    There were lots of discussions leading up to the VDA concept. It also led to more emphasis on the visual segment, whether it is 34:1 clear or, if not, whether it is 20:1 clear. If I were teaching this stuff these days I would place as much emphasis on the visual segment as the advisory nature of the VDA (+V). The VDA should clear final step-down fixes (SDF) where they exist, although the SDF is a barometric/regulatory floor.

    The real hazard is below MDA, particularly at unfamiliar airports. Some pilots use the path vector (green donut) on Garmin avionics, to approximate a VDA on IAPs without +V. Done skillfully, this can be an aid on an IAP with straight-in minimums. But, an approach that is aligned with the runway, with circle to land only minimums, it can be a dangerous trap below MDA. A case in point is KRIR.
     
  15. luvflyin

    luvflyin En-Route

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    Jepp and the Gov sure have different ideas on what obstructions to display on that approach
     
  16. aterpster

    aterpster Pattern Altitude

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    Indeed they do. In my view, the procedures designers should have mandated charting of the 1,369 high point. It's 1,342' terrain with a fancy home atop the high point. The high point is 1,200 feet to the left of the final approach course centerline. Descending on the nominal path of 3.05 degrees, an airplane would be descending through 1,280' msl abeam the home. Granted, the IAP is NA at night, but what about 1,200 and 1 in moderate rain? Attached is a topo of the final approach course. I shift the final from red to blue where an 1860 MDA intercepts a 3.05 degree path to the runway. Check the coordinates of the house in Google Earth.

    RIR.jpg
     
  17. Ryan F.

    Ryan F. Pre-takeoff checklist

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    Yes, of course you are correct about that. The actual implementation (including the option to even include it in the first place, and any naming convention) was left up to the vendor. I suppose calling it marketing propaganda is a bit unkind to CDFA in general. Wasn't my intention. It's a nice tool, but my point has been simply to emphasize that it's nothing more than an advisory glidepath - there's no regulatory strength behind it other than the FAA gently encouraging industry to build vertical profiles into their approach databases.

    There are quite a few approaches in which the advisory glideslope's primary benefit is below the final step down fix, particularly LNAV approaches with no VNAV or LPV variants available. It's a bit eye-opening to think about people following that down below minimums. Definitely needs to be included in the curriculum.
     
  18. olasek

    olasek Pattern Altitude

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    And I think this is the key!

    EDIT: it would mean that during very hot weather you may be exactly on the glide path yet your altimeter may be showing you are too low at step-down fixes. I am thinking aloud - you are still OK (legally) to be exactly on the path.
     
    Last edited: Sep 23, 2017
  19. aterpster

    aterpster Pattern Altitude

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    +V won't be available until the PFAF is the active waypoint (assuming VTF is not used). Prior to that you have to use Baro VNAV (if equipped). The Baro VNAV path rises and falls with temperature changes, unlike the WAAS +V path.