.
speedbrakes retracted with decreasing altitude
. applicable target speed.
After the first “at” or “at or below” restriction, the path angle is level until intercepting the idle thrust descent path to the next altitude constrained waypoint.
Note: On airplanes with U10.2A and on, when passing top of descent following
high speed cruise operation (within approximately 6 knots of Vmo/Mmo,
cost index of 100 or higher), VNAV may revert to LVL CHG to prevent
overspeed. Reduce airspeed to the VNAV target descent speed prior to
reengaging VNAV.
Normally, the target speed is economy speed above the airspeed restriction altitude and 240 knots below that altitude, until deceleration for approach. VNAV will not permit descent below the airspeed restriction altitude until the airspeed is at or below the restricted value plus ten knots.
The start and end of the airport speed restriction deceleration segment is shown on the map as a green open circles with no labels.
Target speeds are changed by entries on the LEGS or DESCENT pages. Wind and thrust assumptions are changed on the DES FORECASTS page.
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737 Operations Manual
(U10) Deceleration points show on the map as green open circles with the label DECEL. Deceleration points show prior to:
.
airspeed constrained waypoints
.
holding patterns
.
approach flap extension. If more than one deceleration segment exists in the flight plan, only the next
deceleration point shows. Deceleration points can also show prior to cruise holding patterns or other speed reductions.
VNAV Path Descent
An E/D point must be defined in order to accomplish a path descent. It may be
defined manually or by the selection of an arrival procedure.
The FMC defaults to the path descent mode for planning purposes. If the
necessary information for a path descent is not available by the time the airplane
reaches the T/D point, the FMC will revert to the speed descent mode.
The path descent normally begins automatically at the calculated T/D point,
provided the MCP altitude is reset for the descent. If descent is not initiated by the
T/D, a path descent may not be available. At the T/D, the FMC commands idle
thrust and pitch to follow the descent path.
The descent complies with waypoint altitude restrictions by following the
calculated vertical path.
Note: A path descent uses the target speed for planning purposes only. There is no attempt to maintain the target speed.
A path descent will automatically revert to a speed descent, or VNAV will disengage, if all required parameters are not maintained during descent.
Note: When descending in VNAV PTH, the FCC will disengage VNAV and switch to LVL CHG if actual speed becomes equal to or slightly less than the minimum speed, denoted by the underspeed limiting symbol in the MCP IASD/Mach window. This can also happen in turbulence or gusty conditions when the minimum speed may momentarily increase due to G loading. See section 4.20, Minimum Speed Reversion.
The FMC uses a special program called “Energy Compensation” at certain times during an ACT PATH DES. This program goes into effect when the MCP has been temporarily set to an altitude above the planned descent path. The airspeed cursor will slowly move toward a slower airspeed while the “TARGET” speed on the FMC remains constant. The airspeed reduction improves the capability of recapturing the planned descent path. When the airplane is cleared to resume the descent, the airspeed will slowly build up to the FMC target speed as the airplane recaptures the planned descent path.
Copyright . The Boeing Company. See title page for details.
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Flight Management, Navigation -Flight Management System Operation 737 Operations Manual
The CDU message DRAG REQUIRED is displayed if an unexpected wind results in a significant increase in airspeed to maintain path. The CDU message DES PATH UNACHIEVEABLE is displayed if the FMC determines that the planned descent profile cannot be accomplished. VNAV disengages if a limit speed will be exceeded.
(U7 and on) A path descent must be initiated while within the allowable cross–track error for LNAV, however LNAV may be disengaged during descent while remaining in the path mode. To maintain a path descent under these conditions, the airplane must remain within a distance equal to the RNP from the LNAV course. If this distance is exceeded, VNAV will change to speed descent if no vertical angle is specified for the current leg. VNAV will disengage if there is a vertical angle specified.
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