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Why are some LNAV/VNAV minimums higher than LNAV only?

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Instrument Rating

Hangar discussion has not yielded a definitive answer to this, so I thought I would ask here. Some RNAV (GPS) approaches with LNAV/VNAV minimums have a note that says "LNAV/VNAV: fly visual 119 degrees 2.2NM" or something similar. Why is it there? Aren't we always expected to fly visual to airport when going below minimums? Why is it on some plates and not others? We are specifially looking at the RNAV(GPS) RWY 12 at JEF (Jefferson City MO). While I'm already picking your brains, why is the LNAV/VNAV minimum higher than the LNAV only? Thanks in advance

3 Answers



  1. Paul on Aug 17, 2010

    To answer the first part of your question, East Hill Flying Club published a neat PDF on the “Fly Visual” Trap:

    http://ehfc.net/FlyVisualTrap.pdf

    It comes down to obstacles.

    “Why did the designers of the approach essentially choose to “slide” the entire approach
    away from the airport by the distance of the visual segment? In many cases, the
    underlying reason is that terrain in the missed approach area would necessitate
    unreasonably high minimums if the MAP were in its normal position. By displacing the
    MAP a few miles, the designers can build a missed approach segment that doesn’t have
    terrain problems (a situation well illustrated by the RNAV (GPS) X RWY 1 approach at
    Jackson Hole). Notice you are allowed to continue past the MAP/DA flying level with no
    runway in sight!!”

    The second part of your question has a similar answer. Those higher minimums are most likely due to obstacles in the missed approach area. A while back we answered a similar question about a situation where the DA on an ILS was higher than the MDA on the Localizer. Take a look at this post as well:

    http://www.askacfi.com/1432/ils-with-da-higher-than-mda.htm

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  2. John D. Collins on Jan 22, 2011

    I asked the same question over a year ago and contacted the FAA and told them there was no guidance in the AIM on the subject of “fly visual to airport”. They answered my question and then made updates to the AIM. See  5-4-5 Instrument Approach Procedure Charts, g. Visual Segment of a Published Instrument Approach Procedure
    The “fly visual to airport” means that you don’t have to meet the requirements of 91.175 in order to descend below the DA, but instead must meet the visibility requirements. This is because the DA is located further from the runway than the visibility minimum, so when the flight conditions are at minimum visibility, it would be impossible to see the runway.  It is a visual maneuver and you are not required to maintain the DA altitude, but in fact, altitude is at pilot discretion, so you can continue to descend.  You must assure your own separation from VFR traffic and from the terrain and must remain clear of clouds.
    Terrain clearance on a missed approach may require a “fly visual to airport” in order to meet the 200 ft/mile minimum climb gradient.  Terrain clearance is base on the climb commencing at the DA, but the location of the DA is too far from the runway to see it from the MAP.  This means that if the pilot decides to continue and land, and later is required to miss the approach, they have to have their own plan as to how they will avoid terrain. 
     

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  3. John D. Collins on Jan 22, 2011

    The second question was why the LNAV/VNAV DA is higher than the LNAV MDA.  Often it is due to the difference in the way obstacles close in to the runway affect the minimums.  In the case of a LNAV, it is simple, the minimums are 250 feet above the controlling obstacle. For a LNAV/VNAV it is more complicated as a sloped surface is used for the evaluation of obstacles.  If an obstacle penetrates this sloped surface, the height of the obstacle is projected back along the surface to a point that it no longer penetrates the surface, and the DA is located at this point.  The reason is that this forces the obstacle to be in the visual segment where the pilot can see it and avoid it.  In some cases, the LNAV can be considerably lower than the LNAV/VNAV, because in the case of a LNAV, a step down fix can be used after passing the obstacle, to further lower the MDA, whereas once the glideslope angle is determined for a LNAV/VNAV approach, the angle can’t change along the approach.
    As an aside, there is virtually no use for the LNAV/VNAV minimums on this approach for an aircraft that is equipped with a IFR WAAS GPS.  You will never see it annunciated on the GPS, instead you will only see LPV or if the integrity requirements aren’t met, it will downgrade to LNAV and there will be no vertical guidance. Nothing prevents the pilot from choosing to fly to the LNAV/VNAV minimums, but I can’t ever think of a good reason why one would want to do this.
    The LNAV/VNAV minimums are primarily there to confuse you for aircraft equipped with baro VNAV systems. In my opinion, anytime there is a LPV DA and a LNAV/VNAV DA, you can ignore the LNAV/VNAV if you have an IFR  WAAS GPS.
     

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