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Barometric Vertical Navigation (VNAV) Instrument Procedures


Barometric Vertical Navigation (BARO VNAV) is a navigation system which presents computed vertical guidance to the pilot referenced to a specific Vertical Path Angle (VPA). The onboard avionics computer resolves vertical guidance data based on barometric altitude and is either computed as a geometric path between two waypoints or a angle from a single waypoint.

Decision Altitude (DA) is a specified altitude in mean sea level in a approach with vertical guidance at which a missed approach must be initiated if the required visual reference to continue the approach has not been established.

Decision Altitude (Height) (DA(H)) The height is used for advisory reference only and does not necessarily reflect actual height above underlying terrain.

Final Approach Segment The final approach segment for a VNAV approach begins at the final approach fix (FAF) and ends at the runway threshold. A waypoint is established at the FAF and the runway threshold.

Height Above Threshold (HATh) For VNAV the HATh is defined as the height above the runway threshold elevation. Also know as TCH of threshold crossing height.

Obstacle Clearance Surface (OCS) An imaginary surface in the primary final approach obstacle clearance area being used for obstacle evaluation.

Vertical Path Angle (VPA) Used in connection with barometric VNAV operations to specify the vertical angle required on a segment.

Waypoint A predetermined geographical position used for route definition specified in latitude/longitude. For VNAV the terms waypoint and fix are the same.

Fix Displacement Tolerance The rectangular area formed around and centered on the plotted position of a waypoint. This describes the region within which the aircraft could be placed when attempting to over fly the waypoint considering all error components. Its dimensions are plus-and minute the along-track (ATRK) and cross-track (XTRK) fix displacement tolerance values found in the applicable lateral RNAV criteria. For VNAV the ATRK fix displacement error is 0.3 NM.

 General Criteria  

The lateral shape of a VNAV instrument procedure is identical to the LNAV(GPS).

The vertical profile provides for an obstacle free area between the runway and the start of the Obstacle Clearance Surface (OCS)

The VNAV Secondary Area Elevation begins at the edge of the Primary Area and at the elevation of the OCS. and proceeds outward at a 7:1 slope. For a VPA (GS angle) of 3 degrees the OCS slope is 23.4:1.

The distance from the runway threshold where the OCS surface starts (Section 1) is defined by the formula D = (250-HATh)/.052407779

When you draw the VNAV surface (Menu Draw - IAP - RNAV - VNAV) be sure to extend it 0.3 NM past the FAF waypoint.

After you display the procedure and save to the database open the database, select the procedure using the scroll keys you will need to regenerate (Re-gen Command Button) the procedure and study (Study Command Button) the location. The results box will open and display the maximum allowable height.

From the Results Form you can select Visual Segment to automatically build the visual segment and calculate the results if site is inside the visual.