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2 Answers

Why are the LPV minimums lower than the ILS minimums?

Asked by: 3264 views Instrument Rating

I noticed that at KEEN, the RNAV 2 approach has an LPV DA of 843, and the ILS 2 has a DA of 860. I can’t figure why this is.

2 Answers



  1. Bryan on Jul 27, 2022

    AIM 1-1-18 b.2 explains how LPV decision altitudes are determined: LPV minima takes advantage of the high accuracy guidance and increased integrity provided by WAAS. This WAAS generated angular guidance allows the use of the same TERPS approach criteria used for ILS approaches. LPV minima may have a decision altitude as low as 200 feet height above touchdown with visibility minimums as low as 1/2 mile, when the terrain and airport infrastructure support the lowest minima. LPV minima is published on the RNAV (GPS) approach charts (see paragraph 5-4-5, Instrument Approach Procedure Charts).

    AIM 1-1-9 d.2 explains how ILS decision heights are determined: The glide slope transmitter is located between 750 feet and 1,250 feet from the approach end of the runway (down the runway) and offset 250 to 650 feet from the runway centerline. It transmits a glide path beam 1.4 degrees wide (vertically). The signal provides descent information for navigation down to the lowest authorized decision height (DH) specified in the approved ILS approach procedure. The glidepath may not be suitable for navigation below the lowest authorized DH and any reference to glidepath indications below that height must be supplemented by visual reference to the runway environment. Glidepaths with no published DH are usable to runway threshold.

    So in the case of KEEN, that particular ILS deployment provides descent information down to 372 feet above the ground while the LPV system with WAAS accuracy gets down to 355. It just so happens that the LPV is lower at this airport. ILS DH is usually lower than LPV DA, but not always and the better GPS gets, the more we will see LPV DA’s lower than ILS DH’s.

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  2. Russ Roslewski on Jul 28, 2022

    The final segment evaluation for an LPV is the same as that for an ILS since the two were standardized several years ago. This means that any obstacle in the final approach segment will affect each procedure equally, and result in the same DA. So the difference is not in final.

    However, the missed approach evaluations are very much different between LPV and ILS, and this is where difference can sometimes affect the DA – since if an obstacle in the missed approach is too tall, one of the possible solutions (often the only solution) is to raise the DA so that the aircraft starts out at a higher altitude on the missed to begin with.

    In general terms, the evaluated area for an ILS missed approach is typically wider than on an LPV missed approach, simply because the ILS missed is going to use some form of navigation like VOR or even NDB, whereas the LPV missed will use GPS – and GPS is certainly more accurate. So the ILS missed approach area, being wider, will pick up more obstacles.

    When you have an ILS DA higher than the corresponding LPV, and the finals are otherwise identical (same FAF and angle), the missed approach is the typical reason.

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