VERTICAL NAVIGATION

One of the advantages of some GPS and multi-sensor FMS RNAV avionics is the advisory VNAV capability. Traditionally, the only way to get vertical path information during an approach was to use a ground-based precision NAVAID. Modern RNAV avionics can display an electronic vertical path that provides a constant-rate descent to minimums. Since these systems are advisory and not primary guidance, the pilot must continuously ensure the aircraft remains at or above any published altitude constraint, including step-down fix altitudes, using the primary barometric altimeter. The pilots, airplane, and operator must be approved to use advisory VNAV inside the FAF on an instrument approach.

VNAV information appears on selected conventional nonprecision, GPS, and RNAV approaches (see Types of Approaches later in this chapter). It normally consists of two fixes (the FAF and the landing runway threshold), a FAF crossing altitude, a vertical descent angle (VDA), and may provide a visual descent point (VDP). [Figure 5-15] The published VDA is for information only, advisory in nature, and provides no additional obstacle protection below the MDA. Operators can be approved to add a height loss value to the MDA, and use this derived decision altitude (DDA) to ensure staying above the MDA. Operators authorized to use a VNAV DA in lieu of the MDA must commence a missed approach immediately upon reaching the VNAV DA if the required visual references to continue the approach have not been established.

A constant-rate descent has many safety advantages over nonprecision approaches that require multiple level-offs at stepdown fixes or manually calculating rates of descent. A stabilized approach can be maintained from the FAF to the landing when a constantrate descent is used. Additionally, the use of an electronic vertical path produced by onboard avionics can serve to reduce CFIT, and minimize the effects of visual illusions on approach and landing.

WIDE AREA AUGMENTATION SYSTEM

In addition to the benefits that VNAV information provides for conventional nonprecision approaches, VNAV has a significant effect on approaches that are designed specifically for RNAV systems. Using an FMS or GPS that can provide both lateral navigation (LNAV) and VNAV, some RNAV approaches allow descents to lower MDAs or DAs than when using LNAV alone. The introduction of the Wide Area Augmentation System (WAAS), which became operational on July 10, 2003, provides even lower minimums for RNAV approaches that use GPS by providing electronic vertical guidance and increased accuracy.

The Wide Area Augmentation System, as its name implies, augments the basic GPS satellite constellation with additional ground stations and enhanced

Figure 5-15.VNAV Information.

position integrity information transmitted from geostationary satellites. This capability of augmentation enhances both the accuracy and integrity of basic GPS, and may support electronic vertical guidance approach minimums as low as 200 feet HAT and 1/2 SM visibility. In order to achieve the lowest minimums, the requirements of an entire electronic vertical guidance system, including satellite availability; clear obstruction surfaces; AC 150/5300-13, Airport Design; and electronic vertical guidance runway and airport requirements, must be satisfied. The minimums are shown as DAs since electronically computed glidepath guidance is provided to the pilot. The electronically computed guidance eliminates errors that can be introduced when using barometric altimetry.

RNAV (GPS) approach charts presently can have up to four lines of approach minimums: LPV, LNAV/VNAV, LNAV, and Circling. Figure 5-16 shows how these minimums might be presented on an approach chart, with the exception of GLS.

• GLS — The acronym GLS stands for The Global Navigation Satellite System [GNSS] Landing System (GLS). GLS is a satellite based navigation system that provides course and glidepath information meeting the precision standards of ICAO Annex 10. Procedures based on the local area augmentation system (LAAS) will be charted separately under the GLS title as these systems are implemented. NOTE: On RNAV approach charts the GLS minima line has been used as a placeholder only. As WAAS procedures are developed, LPV lines of minima will replace the “GLS DA-NA” lines of minima.
• LPV — APV minimums that take advantage of WAAS to provide electronic lateral and vertical guidance capability. The term “LPV” (localizer performance with vertical guidance) is used for approaches constructed with WAAS criteria where the value for the vertical alarm limit is more than 12 meters and less than 50 meters. WAAS avionics equipment approved for LPV approaches is required for this type of approach. The lateral guidance is equivalent to localizer accuracy, and the protected area is considerably smaller than the protected area for the present LNAV and LNAV/VNAV lateral protection. Aircraft can fly this minima line with a statement in the Aircraft Flight Manual that the installed equipment supports LPV approaches. Notice the WAAS information shown in the top left corner of the pilot briefing information on the chart depicted. Below the term WAAS is the WAAS channel number (CH 50102), and the WAAS approach identifier (W17A), indicating Runway 17R in this case, and then a letter to designate the first in a series of procedures to that runway.
• LNAV/VNAV — APV minimums used by aircraft with RNAV equipment that provides both

Figure 5-16. RNAV (GPS) Electronic Vertical Guidance Approach Minima.

lateral and vertical information in the approach environment, including WAAS avionics approved for LNAV/VNAV approaches, certified barometric- VNAV (Baro-VNAV) systems with an IFR approach approved GPS, or certified Baro-VNAV systems with an IFR approach approved WAAS system (See RNAV APPROACH AUTHORIZATION section for temperature limits on Baro- VNAV). Many RNAV systems that have RNP 0.3 or less approach capability are specifically approved in the Aircraft Flight Manual. Airplanes that are commonly approved in these types of operations include Boeing 737NG, 767, and 777, as well as the Airbus A300 series. Landing minimums are shown as DAs because the approaches are flown using an electronic glidepath. Other RNAV systems require special approval. In some cases, the visibility minimums for LNAV/VNAV might be greater than those for LNAV only. This situation occurs because DA on the LNAV/VNAV vertical descent path is farther away from the runway threshold than the LNAV MDA missed approach point.

• LNAV — minimums provided for RNAV systems that do not produce any VNAV information. IFR approach approved GPS, WAAS, or RNP 0.3 systems are required. Because vertical guidance is not provided, the procedure minimum altitude is published as an MDA. These minimums are used in the same manner as conventional nonprecision approach minimums. Other RNAV systems require special approval.
• Circling — minimums that may be used with any type of approach approved RNAV equipment when publication of straight-in approach minimums is not possible.