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3 Answers

LNAV/VNAV and LPV Approaches

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General Aviation

I understand that approaches with LNAV/VNAV minimums may be flown by two types of systems, one is a TSO C145A/146A WAAS GPS and the other is a Baro VNAV system.

Could somebody kindly answer the following questions:

  • Why a pilot of a WAAS GPS equipped airplane would choose to fly an LNAV/VNAV approach instead of an LPV approach? Are there perhaps airports that have LNAV/VNAV approaches, but don’t have LPV approaches? If that is the case, what is the reason for not having LPV approaches, considering that ground equipment is not necessary for both types of approaches?
  • Are all the ‘non WAAS’ Garmin 430 ‘Baro’ equipped?
  • I would imagine that even in a small airplane the altimeter that provides altitude information to the ‘Baro’ GPS is a separate altimeter from the altimeter in the panel of the airplane and has to be adjusted by the pilot as the panel altimeter. And the corrections required for very cold temperature are inputted from a table in a similar way. Could you please confirm?

Thanks

3 Answers



  1. Mark Kolber on Oct 23, 2016

    To answer your first question, why someone with a properly certified GPS would choose LNAV/VNAV mins over LPV, take a look at the RNAV 21 into Raleigh Executive,
    http://155.178.201.160/d-tpp/1611/09472R21.PDF

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  2. Charles22 on Oct 23, 2016

    Thanks, Mark. I can see what you mean. The minimums for the LNAV-VNAV approach are lower than the minimums for the LPV approach. I can’t figure out the reason for that. I have read on this website about an LNAV approach with minimums lower than the minimums for the corresponding LNAV-VNAV and about a localizer approach with minimums lower than the minimums for the corresponding ILS approach. In those cases the reasons for the unusual situations were obstacles in the final approach areas. The profiles of the approaches were completely different (LNAV versus LNAV-VNAV, localizer versus ILS) and the lateral navigation only approaches allowed to deal with the obstacles with lower minimums.
    But at Sanford the LNAV-VNAV approach and the LPV approach should have very similar profiles, even if not identical, because the LNAV-VNAV approach can be flown with the ‘baro’ equipment instead of a WAAS GPS. Could you or some other instructor shed some light on this?
    Thanks

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  3. John D Collins on Oct 23, 2016

    The LNAV/VNAV procedure and LPV procedures are base on different criteria to evaluate obstacle clearance. The LPV is a straight line in space whereas the LNAV/VNAV is a curved path (following the curvature of the earth) and designed for avionics that have a barometric altimeter input that measures MSL height along the curvature of the earth. The WAAS usage of the LNAV/VNAV is based on certain limitations in the vertical path GS indications that the WAAS system generates, allowing them to be close enough to the path for the LNAV/VNAV.

    Regardless, anytime you have two ways of evaluating a GS, each with its own TERPS obstacle clearance requirements, there are bound to be cases such as the Raleigh one. The LPV TERPS evaluates a sloped surface that is a straight line in space, all the way to the runway. The LNAV/VNAV has a sloped segment and a level segment that are evaluated along the final approach course. When the sloped obstacle surface reaches a certain distance from the runway, it levels off for a level segment for the balance of the distance. That makes obstacles that are very close into the runway can penetrate the LPV obstacle surface, but not the LNAV/VNAV level surface. When an obstacle penetrates the LPV surface, the height of the obstacle is projected back along and up the obstacle surface until it reaches the height of the obstacle on the surface. The DA is placed above this point. This assures that the obstacle will be in the LPV visual segment to be avoided and that if a missed approach is required, the obstacle will be avoided on the miss which allows for an additional sink value below the DA.

    Bottom line, two TERPS specifications, two answers.

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