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3 Answers

Confused about LNAV/VNAV

Asked by: 9083 views Instrument Rating, Student Pilot

I'm going through a few practice instrument tests and came across a question asking: "The missed approach for the BARO-VNAV and LNAV is initiated at the" with a correct answer of: B -- missed approach point. I'm confused, as "BARO-VNAV and LNAV" reads to me as "LNAV/VNAV" which is listed on an approach plate as having a DA. The materials I've found about it are leaving me somewhat lost as I believe there is also a LNAV+V function, which I understand to be just an advisory glideslope (paired with the LNAV) for a smooth transition down to the MDA. Is this what this question is referring to? Is the DA on the LNAV/VNAV portion higher than the minimums for the LPV because of the use of WAAS and the fact that it's using a Baro-VNAV which is not as accurate? Or have I missed something? If someone could explain it simply to lay the foundation for me to read further that would be great!

3 Answers

  1. John D. Collins on Jul 08, 2013

    The Pilot Controller Glossary defines:

    MISSED APPROACH POINT− A point prescribed in each instrument approach procedure at which a missed approach procedure shall be executed if the required visual reference does not exist.

    For a vertically guided procedure, this is at the point on the GS where the aircraft reaches the DA. For a non vertically guided procedure, this is at a fix or a specified location along the final approach course. How the missed approach point is determined is different for different types of approaches, but by definition, each approach procedure has a missed approach point.

    The use of Baro-VNAV as an approach type in the question is not very precise. Baro-VNAV is a technology, LNAV/VNAV is an approach specification and is an APV (Approach with Vertical). A Baro-VNAV approach certified and equipped aircraft may fly a LNAV/VNAV procedure, so can most aircraft equipped with a WAAS GPS unit.

    The LNAV+V advisory glideslope is not a part of the LNAV procedure. You will never see it charted on an approach chart. The +V is a feature provided solely by the manufacturer of the vertical guidance equipment to aid the pilot in establishing a constant angle descent from the FAF to the MDA. The +V should not be used below the MDA as there is no assurance of obstacle or terrain clearance from the MDA to the runway.

    The DA on the LNAV/VNAV is generally higher than the DA on a LPV, but not always. The specifications for the two types of APV procedures are different and sometimes produce backwards results, particularly when there are obstacles close in to the runway or in the missed approach segment. However, if obstacles are not an issue, the LPV specification normally allows for a lower DA. For an LPV, obstacles must not penetrate a sloping surface that continues down close to the threshold. For a LNAV/VNAV, it has two surfaces that are evaluated for obstacle penetration, a sloping one to a point approximately 4000 feet from the threshold and from that point a level surface that is 250 feet above the highest obstacle to the threshold. The minimum LNAV/VNAV DA permitted is 250 AGL and for the LPV is 200 AGL.

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  2. Fly92020 on Jul 17, 2013

    John’s answer is all accurate, but here’s a more succinct answer to your question, “why is the DA on the LNAV/VNAV portion higher than the minimums for the LPV…”

    LNAV uses lateral deviation (1,824′ @ full deflection), LPV uses angular deviation (like a localizer; 131′ @ full deflection); as you get closer on an LPV approach the tolerance is tighter, allowing for lower descent toward the rocks. LPV will pretty much (not always) get you as low as an ILS; generally 200′ AGL.

    Couple other caveats: you must have a note in the AFM that airplane is approved for LPV approaches (WAAS alone is not enough); and LNAV+V is just a calculated glidepath; think of it as a pseudo-glideslope… it’s not really using a signal for vertical descent like LNAV/VNAV does.

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  3. John D. Collins on Jul 17, 2013

    LNAV is an approach specification, as is LNAV/VNAV, LP, and LPV. To fly a RNAV approach with LNAV or LNAV/VNAV minimums requires lateral guidance with a Full Scale Deflection (FSD) no greater than .3 Nm or a little over 1800 feet. The equipment that is approved to fly the lateral course can be more but not less precise than the .3 NM FSD. A Baro VNAV system using the standard GPS service (not WAAS) can meet this requirement. A WAAS GPS can also meet the requirement, but the FSD will be the lower of +/- .3 NM or +/- 2 degrees. My point is that the CDI FSD varies based on the equipment capabilities. When I fly a LNAV or LNAV/VNAV approach with my GNS530W WAAS GPS, it uses the +/- 2 degree FSD.

    LNAV/VNAV specification predates the LPV specification and was designed around Baro-VNAV equipment used by mostly by turbojet and turboprop aircraft with an FMS. With this type of equipment, the 2D position of the aircraft is determined by a RNAV system, typically a non WAAS GPS. The barometric pressure and altimeter setting are inputs to the Baro-VNAV equipment which calculates a glideslope based on altitude verses position. There isn’t a real GS, it is calculated. Since it is based on the sensed barometric pressure which is converted to a MSL altitude, the GS is affected by temperature and moves upward on hot days and downward on cold days. The barometric calculation also means that the GS path follows the curvature of the earth. An advisory glidepath is calculated using the same process for a LNAV procedure. The difference is that a LNAV procedure has not been evaluated by TERPS for use of a GS, whereas the LNAV/VNAV has. For this reason, the advisory GS should only be used down to the MDA.

    A WAAS GPS calculates the GS solely using the GPS constellation and WAAS satellites. The WAAS 2D position is more accurate and the vertical is calculated using the same satellites. Since it is not affected by temperature, the WAAS calculated GS remains relatively fixed in space. One other difference between a WAAS calculated GS is that it is a straight line in space, much like an ILS generated GS.

    The WAAS GPS is able to be used to fly the LNAV/VNAV even though there are minor differences in the path. Using a WAAS GPS to fly the LNAV/VNAV path is more accurate and is unaffected by temperature or remote altimeter sources.

    A significant reason for the generally higher DH for a LNAV/VNAV verses the LPV has more to do with the procedure has to work with the Baro-VNAV equipment which doesn’t have the vertical or lateral accuracy and integrity of the WAAS GPS. In other words, it must handle the lowest common denominator of equipment. The original version of the LPV specification also had a 250 foot minimum DH. The service was improved over time and a LPV200 specification was developed. The original LPV specification requires a vertical integrity of 50 meters and still does anytime the DH is 250 feet or higher. However, if the DH is below 250 feet, the vertical integrity is lowered to 35 meters.

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