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5 Answers

LPV versus LNAV/VNAV minimums

Asked by: 3775 views Airspace

I am trying to find out why the LPV minimums are 2 feet higher than the LNAV/VNAV minimums on the RNAV Rwy 24 approach at KCGF. If you can find out you will be the only person that can, I have talked to numerous people that you would think could tell me but no one seems to know and that includes the FAA. I obviously have not talked to the right people. Thank's for your help!

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5 Answers

  1. John D. Collins on Oct 10, 2012

    Although this is the exception, it is the result of the two different methods of determining the DA. In particular, when there are obstacles close in to the runway, the geometry used to set the DA is different for an LPV and a LNAV/VNAV approach. The LPV evaluates obstacles based on a single sloped Obstacle Clearance Surface (OCS). The LNAV/VNAV evaluates obstacles based on two surfaces, one level surface starting near the threshold and extending back along the approach course typically 4000 +/- feet, then starting a slope upwards. Obstacles close to the threshold can get two results from the two evaluation methods. Also, an obstacle close in can be beyond the DA location of the LPV and therefore into the missed approach portion and according to the standards can force the DA up to compensate. Bottom line, it is rare, but it does happen, and is the result of two different geometry evaluations of obstacles.

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  2. Kris Kortokrax on Oct 10, 2012

    Where does your info come from?


    I found the following language:

    OCS SLOPE(S). In this document, slopes are expressed as run over rise; e.g., 34:1. The OCS is comprised of three longitudinal sections of differing slopes. For a 3° glidepath angle, the three sections are: Zero slope (elevation equal to ASBL), 27.03:1 slope, and 34:1 slope.

    It would be interesting to see the detailed design criteria.

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  3. John D. Collins on Oct 10, 2012

    The current TERPS for RNAV (GPS) approaches can be found in Order 8260.54A.

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  4. Kris Kortokrax on Oct 10, 2012

    And for WAAS approaches, in 8260.50. In both publications, the OCS for LPV approaches is shown as three segments (W OCS, X OCS and Y OCS), not a “single sloped OCS” as you stated earlier. This is the reason for my previous question.

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  5. John D. Collins on Oct 11, 2012


    I was referring to the W surface, which is the primary surface, not the secondary surfaces. I did not intend to provide a complete analysis of the TERPS, just an understandable comparison of the differences between how the DA is calculated for the LPV and LNAV/VNAV. Order 8260.54A is the most current TERPS on RNAV approaches and 8260.50 is no longer in the FAA library of orders on the RGL library web site.

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