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WAAS capable GPS approaches

Asked by: 8795 views Instrument Rating

If I have a WAAS GPS, will I always have a glide path needle that I can follow to the appropriate published minimums?  It seems like in the past, I didn't have a glide path unless there was an LPV or LNAV/VNAV minimum shown on the chart.  If there was only a LNAV minimum, I don't recall getting a glide path indication; however, I recently noticed getting a glide path needle on a LNAV approach which I used to descend to the MDA.

2 Answers

  1. Kent Shook on Oct 04, 2010


    It depends on what WAAS GPS you’re using and what software version it’s running, and the answer will lie within the manual for the GPS. Short answer: Not “always” but “usually.”

    On the Garmin 430W, there are several possible modes; the GPS will annunciate one of the following:

    1) LNAV: Fly to the LNAV minimums, no vertical guidance will be provided.
    2) LP: Fly to the LP minimums (this is a WAAS approach with no vertical component).
    3) LNAV+V: Fly to the LNAV minimums, but advisory vertical guidance will be provided.
    4) L/VNAV: Fly to the LNAV/VNAV minimums, advisory vertical guidance will be provided.
    5) LPV: Fly to the LPV minimums, GPS/WAAS glideslope will be provided.

    (You can find this info on Page 113 of the Garmin 430W Pilot’s Guide, Rev. G.)

    The advisory vertical guidance on LNAV+V or L/VNAV is a calculated glideslope between the points on the approach. It has not been surveyed by the FAA like an LPV or ILS glideslope, but it does lower your workload by avoiding the “Dive & Drive” nature of a normal non-precision approach, and should keep you above the minimum altitudes on the approach.

    For what it’s worth, in my experience, flying a non-WAAS (ie “RNAV(GPS)” without LPV minimums) approach, the 430W has always provided the advisory vertical glideslope in LNAV+V mode, it has never degraded to LNAV mode. I don’t think the older GPS overlay approaches (ie “NDB or GPS”) ever give you an advisory glideslope.

    Another good reference for further information is Max Trescott’s GPS and WAAS Instrument Flying Handbook.

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  2. John D. Collins on Jan 22, 2011

    A few points of clarification on the previous post.  The advisory glidepath only applies to an approach that has LNAV minimums as the highest level of service. It is normally provided on most overlay ( or GPS), GPS standalone, or RNAV (GPS) that have straight in minimums. The advisory glidepath is not on the approach chart, it is provided by the database provider/ manufacturer at their discretion. In my experience, 99+ % of the straight in approaches have the advisory glidepath. The advisory glidepath may be used as an aid to managing the descent path to the MDA, but the pilot is required to comply with any minimum altitude depicted on the approach chart.  The MDA may not be treated as a DA, the pilot is expected to level off at or above the MDA and not descend from the MDA until the required visual cues are available. In the case of a DA, the D(ecision) is made at the DA and momentum may carry the aircraft below the DA while the pilot initiates a missed approach. This kind of sink thru is not allowed on a MDA. Since the advisory glidepath is only provided as an aid, the pilot is free to follow it or ignore it and descend using the “dive and drive” method.
    With respect to a LNAV/VNAV (annunciated as L/VNAV), it has a DA and the vertical guidance is part of the charted approach. It has been surveyed for use with a glidepath and requires the pilot to fly the glidepath.
    All of the WAAS GPS glidepath indications are calculated by the GPS, regardless if the approach is a LPV, LNAV/VNAV, or LNAV+V.
    All downgrades from a vertically guided approach (LPV, LNAV/VNAV, LNAV+V) are to a LNAV without vertical guidance.

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