12/7/2023 0 Comments Vnav lnav![]() Some countries even mandate the use of continuous descent final approaches (CDFAs) on non-precision approaches. 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. A stabilized approach can be maintained from the FAF to the landing when a constant-rate descent is used. A constant-rate descent has many safety advantages over non-precision approaches that require multiple level-offs at stepdown fixes or manually calculating rates of descent. Like flying any other IAP, the pilot must see and avoid any obstacles in the visual segment during transition to landing. This rate of descent can be flown with the Vertical Velocity Indicator (VVI) in order to use the VDA as an aid to flying a stabilized descent. Pilots may use the published angle and estimated/actual groundspeed to find a target rate of descent from the rate of descent table published in the back of the U.S. The presence of a VDA does not guarantee obstacle protection in the visual segment and does not change any of the requirements for flying a nonprecision approach. Pilots must still respect any published stepdown fixes and the published MDA unless the visual cues stated 14 CFR §91.175 are present, and they can visually acquire andavoid both lit and unlit obstacles once below the MDA. There is no implicit additional obstacle protection below the MDA. However, pilots should be aware that the published angle is for information only − it is strictly advisory in nature. Stabilized descent is a key factor in the reduction of controlled flight into terrain (CFIT) incidents. It provides a means for the pilot to establish a stabilized descent from the FAF or step-down fix to the MDA. The VDA provides the pilot with advisory information not previously available on nonprecision approaches. 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). VNAV information appears on selected conventional nonprecision, GPS, and RNAV approaches (see “ Types of Approaches” post). The pilots, aircraft, and operator must be approved to use advisory VNAV inside the FAF on an instrument approach. 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 stepdown fix altitudes, using the primary barometric altimeter. Modern RNAV avionics can display an electronic vertical path that provides a constant-rate descent to minimums. Traditionally, the only way to get vertical path information during an approach was to use a ground-based precision NAVAID. One of the advantages of some GPS and multi-sensor FMS RNAV avionics is the advisory VNAV capability.
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