![]() ![]() The WAAS GPS is able to be used to fly the LNAV/VNAV even though there are minor differences in the path. One other difference between a WAAS calculated GS is that it is a straight line in space, much like an ILS generated GS. Since it is not affected by temperature, the WAAS calculated GS remains relatively fixed in space. The WAAS 2D position is more accurate and the vertical is calculated using the same satellites. For this reason, the advisory GS should only be used down to the MDA.Ī WAAS GPS calculates the GS solely using the GPS constellation and WAAS satellites. The difference is that a LNAV procedure has not been evaluated by TERPS for use of a GS, whereas the LNAV/VNAV has. An advisory glidepath is calculated using the same process for a LNAV procedure. ![]() The barometric calculation also means that the GS path follows the curvature of the earth. 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 pressure and altimeter setting are inputs to the Baro-VNAV equipment which calculates a glideslope based on altitude verses position. With this type of equipment, the 2D position of the aircraft is determined by a RNAV system, typically a non WAAS GPS. 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. When I fly a LNAV or LNAV/VNAV approach with my GNS530W WAAS GPS, it uses the +/- 2 degree FSD. My point is that the CDI FSD varies based on the equipment capabilities. A WAAS GPS can also meet the requirement, but the FSD will be the lower of +/. A Baro VNAV system using the standard GPS service (not WAAS) can meet this requirement. The equipment that is approved to fly the lateral course can be more but not less precise than the. To fly a RNAV approach with LNAV or LNAV/VNAV minimums requires lateral guidance with a Full Scale Deflection (FSD) no greater than. LNAV is an approach specification, as is LNAV/VNAV, LP, and LPV. The minimum LNAV/VNAV DA permitted is 250 AGL and for the LPV is 200 AGL. 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. For an LPV, obstacles must not penetrate a sloping surface that continues down close to the threshold. However, if obstacles are not an issue, the LPV specification normally allows for a lower DA. 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. The DA on the LNAV/VNAV is generally higher than the DA on a LPV, but not always. 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 +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. You will never see it charted on an approach chart. The LNAV+V advisory glideslope is not a part of the LNAV procedure. A Baro-VNAV approach certified and equipped aircraft may fly a LNAV/VNAV procedure, so can most aircraft equipped with a WAAS GPS unit. Baro-VNAV is a technology, LNAV/VNAV is an approach specification and is an APV (Approach with Vertical). ![]() The use of Baro-VNAV as an approach type in the question is not very precise. 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. For a non vertically guided procedure, this is at a fix or a specified location along the final approach course. 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.įor a vertically guided procedure, this is at the point on the GS where the aircraft reaches the DA. ![]()
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