continues an idle thrust descent using VNAV SPD.
Upon reaching the next altitude restriction, VNAV commands level flight using
VNAV PTH. The thrust mode changes to FMC SPD.

Descent and Approach
After EPG, VNAV continues the idle thrust descent using VNAV SPD.
Prior to the approach, VNAV decelerates to approach speed. The FMC prompts
manual flap extension.

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Flight Management, Navigation -Flight Management System Operation 737 Flight Crew Operations Manual

Vertical Angle
A vertical angle can be assigned to a waypoint from the navigation database. This vertical angle defines a VNAV path between the waypoint and the waypoint preceeding it. This feature can be available in approaches, approach transitions, and STARs. For example, the vertical angle for the glidepath of an ILS approach would typically be 3 degrees. This angle is displayed on the ACT RTE LEGS page above the speed/altitude line for the associated waypoint. Vertical angles may be expected in any approach ending at RWXXX or MAXXX. The E/D will be RWXXX or MAXXX, and the E/D altitude will be either threshold crossing height (TCH – typically 50 feet above the touchdown zone elevation) or the altitude specified at MAXXX.
If a path (VNAV PTH) descent is active when a vertical angle leg becomes active, the path mode will remain active, but VNAV will follow the vertical angle rather than the idle thrust descent path.
If the vertical angle leg becomes active during a speed (VNAV SPD) descent, the VNAV mode will change to VNAV PTH automatically, and there will be no SPEED prompt on the descent page.
Early Descent
A descent in VNAV started before the top of descent point is an early descent. If
a path descent is planned, VNAV commands a 1000 fpm descent until the idle
descent path is intercepted. If a speed descent is planned, VNAV commands an
idle thrust descent.

To start an early descent, use DES NOW prompt on the DES page.

YD651 - YD655
An early descent can also be started by pushing the altitude intervention switch.

DES NOW (VNAV PTH)
With a VNAV path descent planned, VNAV starts an early descent at 1000 fpm and captures the idle descent path. VNAV uses FMC SPD for the autothrottle mode and VNAV PTH for the pitch mode.
Copyright . The Boeing Company. See title page for details.
11.31.36  March 28, 2005

Flight Management, Navigation -Flight Management System Operation
737 Flight Crew Operations Manual

DES NOW (VNAV SPD)
With a VNAV speed descent planned, VNAV starts an idle thrust early descent. VNAV does not attempt to capture the VNAV descent path. VNAV uses VNAV SPD for the pitch mode and the autothrottle commands IDLE, followed by ARM.
Approach
The FMC transitions to “on approach” when the airplane is within:
.  
2 NM of the first approach waypoint (including approach transitions such
as arcs and procedure turns), or

.
2000 feet of airport elevation, whichever occurs first.

When the FMC is “on approach”, the following features are available:
.
UNABLE RNP alerting levels are higher

.  
when preparing for a missed approach and the MCP altitude is set at least
300 feet above the current airplane altitude, VNAV will continue to
command a descent

.  
if the airplane is more than 200 feet below the vertical path, VNAV
commands zero vertical speed until intercepting the path.

Note:  Display of a specified path angle is not limited to approaches. A path angle may be defined for a leg in a STAR and displays on the RTE LEGS page for the procedure.
The FMC transitions out of “on approach” under the following conditions:
.
selecting TO/GA

.
the airplane lands

.
the waypoint cycles to the first waypoint of the missed approach

.
executing a direct-to waypoint in the missed approach. The following situations are generally encountered during approach operations,

but are not determined by “on approach” logic: YD651 - YD655
.
If speed intervention is engaged:

.  
during a path descent with flaps up on an idle leg, VNAV switches to VNAV SPD

.
with flaps down, VNAV remains in VNAV PTH
YD652 - YD655

.  
when a point to point (geometric path) leg is active, VNAV remains in VNAV PTH

.  
while a vertical angle leg (GP x.xx on RTE LEGS page) is active, VNAV remains in VNAV PTH

.  
if a vertical angle leg (GP x.xx on RTE LEGS page) becomes active,
VNAV switches to VNAV PTH without pilot action

.  
　
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