|CHAPTER 3. Navigation
LNAV (lateral navigation), like a conventional localizer, provides lateral approach course guidance. LNAV minimums permit descent to a prescribed minimum descent altitude (MDA). The LNAV procedure shown on the chart in Figure 3-48 offers an MDA of 1,620 feet.
LNAV/VNAV (lateral navigation/vertical navigation) equipment is similar to ILS in that it provides both lateral and vertical approach course guidance. Since precise vertical position information is beyond the current capabilities of the global positioning system, approaches with LNAV/ VNAV minimums make use of certified barometric VNAV (baro-VNAV) systems for vertical guidance and/or the wide area augmentation system (WAAS) to improve GPS accuracy for this purpose. (Note: WAAS makes use of a collection of ground stations that are used to detect and correct inaccuracies in the position information derived from the global positioning system. Using WAAS, the accuracy of vertical position information is increased to within 3 meters.) To make use of WAAS, however, the aircraft must be equipped with an IFR approved GPS receiver with WAAS signal reception that integrates WAAS error correction signals into its position determining processing. The WAAS enabled GPS receiver shown in Figure 3-49 allows the pilot to load an RNAV approach and receive guidance along the lateral and vertical profile shown on the approach chart in Figure 3-48.
It is very important to know what kind of equipment is installed in an aircraft, and what it is approved to do. It is also important to understand that the VNAV function of non-WAAS-capable or non-WAAS-equipped IFR approved GPS receivers does not make the aircraft capable of flying approaches to LNAV/VNAV minimums.
LPV can be thought of as “localizer performance with vertical guidance.” Procedures with LPV minimums use GPS information to generate lateral guidance, and IFR-approved GPS/WAAS receivers to generate vertical guidance similar to an ILS glideslope. Several manufacturers now offer FMS/ GPS RNAV units capable of flying approaches to LPV minimums.
GPS or RNAV (GPS) Approach Waypoints
Figure 3-50 shows a GPS approach loaded into an FMS/GPS RNAV. As previously noted, approaches must be selected from a specific approach menu in the FMS. The software then loads all of the waypoints associated with that procedure from the database into the flight route. It is not possible for you to enter or delete, separately or individually, waypoints associated with the approach procedure.
Once loaded, a GPS or RNAV (GPS) approach is shown in the FMS display as a collection of waypoints with a title that identifies the approach. Four waypoints in every approach procedure have special designations: (1) initial approach waypoint; (2) final approach waypoint; (3) missed approach waypoint; and (4) missed holding waypoint.
Flying a GPS or RNAV (GPS) Approach
Most FMS require the pilot to choose whether to simply load, or load and activate, instrument approach procedures. When ATC tells you to expect a certain approach, select that approach from the menu and load it into the flight plan. Loading an approach adds its component waypoints to the end of the flight plan, but does not make them active. Once ATC clears you for the approach (or, alternatively, begins providing radar vectors to intercept the final approach course), you must remember to activate the approach to receive course guidance and auto-sequencing. You must be careful not to activate the approach until cleared to fly it, however, since activating the approach will cause the FMS to immediately give course guidance to the initial approach fix or closest fix outside the final fix, depending on the unit’s programming. In the case of a vectors-to-final approach, activating the vector-to-final causes the FMS to draw a course line along the final approach course.
Once you have loaded and activated the GPS or RNAV (GPS) approach procedure, flying it is similar to flying between any other waypoints in a programmed flight route. However, you must be prepared for two important changes during the approach.
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