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Flight Management, Navigation -
Flight Management System
Operation
11.31.24 Boeing Proprietary
If flight plan modifications or unknown winds occur when below the first speed
constraint, VNAV continues to maintain the path and allows the speed to vary up
to the following limits:
• with greater than 15 knots below the target speed, and autothrottles not
active, the CDU help window message THRUST REQUIRED displays.
The airspeed may decrease to minimum maneuvering speed.
Subsequently, VNAV commands the airplane to fly below the path to stop
the deceleration. If VNAV can no longer maintain the airplane within 150
feet of the path without further deceleration, speed reversion occurs, the
pitch mode annunciation changes from VNAV PTH to VNAV SPD,
VNAV resets the target speed to 10 knots less than the target speed (not
less than minimum maneuvering speed), and the CDU help window
message THRUST REQUIRED displays again.With autothrottles active,
this condition should not occur as throttles are in SPD and target speed is
maintained during the descent
• with greater than 10 knots above target speed, the CDU help window
message DRAG REQUIRED displays. The airplane may accelerate up to
15 knots above target speed to maintain the path. The maximum speed
excursion allowed is 5 knots above the transition speed after the airplane
is below transition altitude for the destination airport or 5 knots below the
flaps placard speed if flaps are extended. If further correction is required,
VNAV may allow the airplane to rise up to 150 feet above the path to stop
the acceleration. If VNAV can no longer maintain the airplane within 150
feet of the path without further acceleration, speed reversion occurs, the
pitch mode annunciation changes from VNAV PTH to VNAV SPD,
VNAV resets the target speed to 250 knots, and the CDU help message
DRAG REQUIRED displays again.
Early Descent
When a descent is started before the T/D, VNAV commands a descent at a reduced
descent rate until the idle descent path is intercepted.
Start an early descent by selecting the DES NOW prompt on the DES page or by
pushing the MCP altitude selector. In an early descent, the autothrottle mode
annunciation is initially THR, followed by HOLD, allowing the pilot to adjust the
rate of descent. The pitch mode is VNAV SPD.
787 Flight Crew Operations Manual
Flight Management, Navigation -
Flight Management System
Operation
Boeing Proprietary 11.31.25
Use the DES NOW prompt on the VNAV DES page. VNAV starts an early descent
and captures the idle descent path.
Use the MCP altitude selector to start an early descent. Within 50 nm of the top of
descent point, VNAV starts an early descent and captures the idle descent path.
Use the MCP altitude selector to start a cruise descent. If the distance from the top
of descent is more than 50 nm, VNAV begins a cruise descent to the new cruise
altitude. VNAV may not capture the idle descent path since the target airspeed is
economy cruise and the descent path is based on idle thrust and economy descent
airspeed. In the example, VNAV levels at the new cruise altitude.
Approach
The FMC transitions to "on approach" under the following conditions:
• the airplane is in the descent phase and flaps are out of UP
• a VFR approach has been created and incorporated in the active flight
plan and,
• the airplane has sequenced the FAXXX or
• the airplane is enroute to a direct-to or intercept-to the RWYYY
waypoint and the airplane is within 25 nm of the runway threshold
1 DES NOW
2 Within 50 nm of Top of Descent Point
3More than 50 nm from Top of Descent Point
T/D
1
T/D
50 NM
2
50 NM
OLD CRZ ALT T/D
NEW CRZ ALT T/D
3
787 Flight Crew Operations Manual
Flight Management, Navigation -
Flight Management System
Operation
11.31.26 Boeing Proprietary
• a published instrument approach has been selected and incorporated in
the active flight plan and,
• the airplane has sequenced the first waypoint on the published
approach or
• the airplane is enroute to a direct-to or intercept-to waypoint (DIRECT
displays at 1L on RTE page) and the airplane is within 12 nm of the
runway threshold
The approach condition may be delayed if the flight crew manually inserts,
bypasses, or deletes an approach waypoint on the LEGS page
The FMC transitions out of "on approach" under the following conditions:
• the pilot selects TO/GA
• the airplane lands
• the airplane flies beyond the last waypoint in the approach (missed
　
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