737 Flight Crew Training  Manual
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:
.  
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).

.  
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. ILS approaches coded with the appropriate threshold crossing height may be used as an overlay for other approaches such as LOC or NDB.
VNAV should be used only for approaches that have one of the following features:
.  
a published GP angle on the LEGS page for the final approach segment

.  
an RWxx waypoint coincident with the approach end of the runway

.  
a missed approach waypoint before the approach end of the runway, (e.g.,
MXxx).

These features permit construction of a normal glide path. VOR approaches with the missed approach point on the LEGS page beyond the runway threshold and circling only approaches do not have these features.
Copyright . The Boeing Company. See title page for details.
5.40  FCT 737 (TM) October 31, 2006

737 Flight Crew Training  Manual
When appropriate, crews should make cold temperature altitude corrections by applying a correction from an approved table to the waypoint altitude constraints. The FMC obtains the GP angle displayed on the LEGS page from the navigation database. This GP angle is based on the standard atmosphere and is used by the FMC to calculate the VNAV path which is flown using a barometric reference. When OAT is lower than standard, true altitudes are lower than indicated altitudes. Therefore, if cold temperature altitude corrections are not made, the effective GP angle is lower than the value displayed on the LEGS page. When cold temperature altitude corrections are made, 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.
Note: Temperature corrections redefine the glide path only if the FMC has the
geometric path option installed. Reference the applicable FCOM for
optional equipment installation.
When on final approach, VNAV may be used with speed intervention active (if installed) to reduce workload. Adding speed constraints to the final approach waypoints is not recommended because of the extra workload, lack of safety benefit, and reduced ability to make last minute approach changes.
To prevent unnecessary level offs while descending in VNAV before the final approach, reset the MCP altitude selector to the next lower constraint before altitude capture, when compliance with 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.
　
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