A vertical path suitable for use of VNAV is one that approximates 3° and crosses the runway threshold at approximately 50 feet. To obtain such a VNAV path, maximum use of the navigation database is recommended. For approaches where an RNP is specified, or approaches where a DA(H) is used, the waypoints in the navigation database from the FAF onward may not be modified except to add a cold temperature correction, when appropriate, to the waypoint altitude constraints. With respect to the construction of a suitable final approach path, there are two types of approaches in the navigation database:
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approaches with a glide path (GP) angle displayed on the final approach
segment of the LEGS page. The final approach segment is completely
compatible with VNAV and complies with final approach step down
altitudes (minimum altitude constraints).
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approaches where no GP angle is published and where the approach end
of the runway is defined by a runway waypoint (RWxx) or a missed
approach point fix (MXxx or a named waypoint) exists. Normally these
waypoints display an approximate 50 foot threshold crossing altitude
constraint and may be used “as is” for VNAV. If the RWxx waypoint
altitude constraint does not coincide with approximately 50 feet, this
waypoint may be modified with a threshold crossing altitude of
approximately 50 feet.
Note: Threshold crossing altitude normally require entry of a four-digit
number. Example: enter 80 feet as 0080. VNAV may be used for approaches modified in this way; however, the approach should be flown by constant reference to raw data (VOR, NDB, DME, etc.) and compliance with each minimum altitude constraint is required. Use of a DA(H) is not appropriate when the final approach is manually constructed in this manner.
Note: ILS approaches are in the process of being coded with the appropriate threshold crossing height. When coded in this way, approaches may be used as an overlay for a LOC or NDB approach, for example, and the need for this waypoint altitude constraint modification by the crew is eliminated.
VNAV should not be used for approaches containing no waypoint (RWxx or MXxx) coincident with the approach end of the runway since the path does not provide a normal glide path. An example of such an approach is a VOR approach with a MAP beyond the runway threshold. These approaches are gradually being re-coded to permit VNAV path operation.
Note: Approaches designated for circling only are not VNAV compatible.
With the autopilot engaged, the EICAS alert message AUTOPILOT and VNAV mode fail indications are available to alert the flight crew of potential problems with the flight path.
Copyright . The Boeing Company. See title page for details.
October 31, 2003 FCT 757 (TM) 5.33
When appropriate, crews should make cold temperature altitude corrections by adding a correction from an approved table to the waypoint altitude constraints. VNAV PTH operation and procedure tuning function normally; however, the airplane follows the higher of the glide path angle associated with the approach (if available) or the geometric path defined by the waypoint altitude constraints.
When on final approach, VNAV should be used with speed intervention active to reduce workload. Adding speed constraints to the final approach waypoints is not recommended because of the extra workload, no safety benefit, and reduced ability to make last minute approach changes.
To prevent unnecessary leveloffs while descending in VNAV prior to the final approach, reset the MCP altitude selector to the next lower constraint prior to altitude capture when the altitude restriction is assured.
Use of Altitude Intervention (As installed) during Approaches using VNAV
Altitude intervention is appropriate during approaches only if the AFDS enters VNAV ALT mode above the approach path and descent must be continued. Entering VNAV ALT mode can occur if passing a waypoint on the approach and the crew has failed to reset the MCP altitude to a lower altitude. If this occurs, set the MCP altitude to the next lower altitude constraint or the DA(H) or MDA(H), as appropriate, and select altitude intervention. When VNAV altitude intervention is selected, VNAV path deviation indications on the map display disappear momentarily while the path is recalculated, but should reappear.
If altitude intervention is selected when on-approach logic is active, typically after the airplane has sequenced the first approach waypoint, level flight is commanded until reaching the VNAV path, then the airplane captures the VNAV path.
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