737 Flight Crew Operations Manual
Flight Management, Navigation -
Flight Management System
Operation
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
11.31.34 D6-27370-866-EGP
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.
(SB Changes YK981 - YQ456)
An early descent can also be started by pushing the altitude intervention switch.
YW273, YW274
(SB Changes YK981 - YQ456)
A CRZ DES will occur upon lowering the MCP ALT to a lower altitude, but at or
above any descent constraint altitude and pressing ALT INTV if the airplane is
further than 50 nm from the top of descent at the current cruise altitude. If within
50 nm of the top of descent, the Early Descent mode will be invoked. In the
previous Operational Flight Programs (OFPs), this action resulted in always going
into the Early Descent mode of operation regardless of distance from the top of
descent.
• This allows the crew to initiate a cruise descent using the altitude
intervention feature on the Mode Control Panel when the airplane is not
in close proximity to the Top of Descent. A distance of 50 NM is used
as a determinant for this operation.
• Altitude Intervention may be used to initiate early descent when the
airplane is 50 nm or less to Top of Descent.
• If Altitude Intervention is used to initiate descent when 50 NM or less to
T/D with VNAV engaged and the MCP ALT below current altitude,
Early Descent vertical speed commands of -1000 fpm will be generated
by the FMC for Autopilot V/S tracking until path intercept or MCP ALT
level off occurs.
September 15, 2016
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