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Why are the minima on a full ILS generally lower than the minima on a LPV approach?

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I am a mostly retired CFI who does try to keep up on the literature and tries to keep my skills current. A friend, who is also kind of a CFI-I student asked a question the other day that I acnnot answer, "Why are the minima on a full ILS generally lower than the minima on a GPS --LPV approach?" I suspect that it has something to do with the lack of outer/middle markers as well as approach light systems on the GPS approach, or with the lack of continuous monitoring of the approach components by ATC, or with less clearly defined obstruction clelarance because of the differences in the localizer/glideslope needles. I will be interested in your answer.

3 Answers

  1. Gary Moore on Aug 11, 2010

    Well – interesting question – although it might be flawed 🙂 With admittedly only a breif searching through approaches – I find LPV minimums as low as some ILS…so on any given approach it could be something unique to that particular approach. Quality of navaids, reliability of WAAS data – ancillary approach aids – such as lighting and or monitoring equipment (as you suggested).

    The best info on LPV i could find was here -> http://bit.ly/cPT3x8

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  2. Wesley Beard on Nov 09, 2010

    The minimums have to do with approach lighting aids, the type of runway (precision, non-precision) etc.  Also, when they created the tolerances for the WAAS approaches they are more lenient than an ILS at the same distance from the airfield.  This undoubtedly adds fee to the minimums.  There are some LPV approaches that are only feet above the ILS when the terrain is extremely flat.
    Another GPS term “LAAS”  (Local Area Augmentation System) is designed to the same tolerances of a CAT III approach.  In these approaches you could land with 0 visibility on the GPS.  That is pretty cool technology.

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  3. John D. Collins on Jan 22, 2011

    Originally the WAAS signal integrity only supported DH’s of 250 feet, but over time the experience and software improvements in the system, allowed DH’s as low as 200 feet. Now there are well over 400 LPV approaches with a DH of 200 feet. In order to qualify for the lowest minimums the airport must meet certain criteria, such as runway length, a parallel taxiway, approved approach lighting system, and precision markings.  You can still get a LPV with out meeting these requirements and can still have a DH of 200 feet without precision markings and an approved lighting system, but you don’t get the extra 1/4 mile credit and the minimum visibility will be 3/4 of a mile, not 1/2 a mile.  See KLKR RNAV RWY 6 approach for an example.
    For LPV approaches with a DH 250 feet or higher, the vertical integrity required to be met is 50 meters.  For LPV with a DH under 250 feet, the vertical integrity requirement is tighter at 35 meters. 
    The survey requirements are the same for an ILS and a LPV. Unlike ILS systems at untowered airports whose ILS is not monitored, all, repeat all, RNAV (GPS) approaches are monitored by the WAAS organization, remember that is what all those ground stations are for.  As a result of being monitored, the vast majority of RNAV (GPS) approaches can be used for the purpose of filing as an alternate if you have a TSO C145a/146a GPS and you plan on the NPA minimums of a LNAV.

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