LNAV lower than LNAV/VNAV?

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1. Best Answer
   
   

   
   

   John D. Collins on Apr 18, 2011

   Andreas,

    

   This is not that uncommon.  Note that there are LPV minimums and the DA is 363 feet lower than the LNAV.  There is no “known bad WAAS reception” that affects the approach, it all has to do with obstacles and approach geometry.

   A major difference between the LPV and the LNAV/VNAV is the width of the area that is evaluated for obstacles, with the LPV tapering to about +/- 700 feet at the threshold.  The LNAV and the LNAV/VNAV have a wider primary area that is evaluated for obstacles and is fixed at .6 NM (+/- 3646 feet) either side of the centerline.  Because the LPV has an angular primary protected area that gets smaller and smaller as the threshold is approached, obstacles that affect the LNAV and LNAV/VNAV may be outside of the primary protected area for the LPV. Note that the area evaluated for obstacles for the LPV is the same as the area evaluated for the ILS.

    

   Since the LNAV/VNAV is evaluated for obstacles by a sloping Obstacle Clearance Surface (OCS) that is offset from the threshold (usually a couple of hundred feet), it may encounter obstacles that penetrate the OCS.  In this case, the DA is moved further from the runway and because it is on a slope, and forcing a higher DA.  The amount it is moved is based on projecting the obstacle height that penetrates the OCS further from the threshold until the height of the obstacle equals the height of the OCS.  This keeps the obstacle in the visual segment of the approach so the pilot may see and avoid it.

    

   Note that there is a step down waypoint that only applies to the LNAV MDA.  This is likely to be where the obstacle is located that causes the LNAV/VNAV to be raised is located.  But it must be further to one side of the centerline than the protected area for the LPV, as the LPV does not seem to be affected by this obstacle.  The LNAV/VNAV can’t change the GS angle to a different one when it clears the obstacle, so it forces the obstacle to be placed in the visual segment as discussed above.  On the other hand, once the obstacle is cleared on the LNAV approach at the step down waypoint, the pilot may descend to the lower MDA.

    

   All this said, if you are using a WAAS GPS, there is rarely a good reason why you would ever fly to the LNAV/VNAV minimums.  The LPV is lower and so is the LNAV.  The WAAS GPS will annunciate either LPV or if there is a downgrade, it might annunciate LNAV, but you will never see it annunciate LNAV/VNAV.  I tell my students that for all practical purposes, you can ignore the LNAV/VNAV minima line anytime there is also a LPV minima.  The pilot is authorized to fly the LNAV/VNAV procedure, but there rarely is a good reason to do so, unless there isn’t a LPV on the same procedure.  The LNAV/VNAV was primarily intended for aircraft that have a Baro-VNAV capability, which is very uncommon in GA aircraft excepting some turbine or jets.

    

   If you want to learn more about how RNAV (GPS) approaches are constructed, search on the document “Order 8260.54A United States Standard for Area Navigation (RNAV)”

    

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