.
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.
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YD651 - YD655 During descent, VNAV ALT engages if the airplane levels at an MCP altitude not in the FMC.
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.
YA613 - YD651, YK101 - YK104 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 achievable. 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 during the idle portion of the descent.
YA613 - YD651, YK101 - YK104 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 IAS/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.
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The CDU message DRAG REQUIRED is displayed if an unexpected tailwind 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.
YA721, YA722 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.
YA801 - YA803 (SB changes YA613 - YA687, YC153 - YC413) 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 twice 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 and LNAV is not engaged.
YA809, YD651 - YK104 (SB changes YA613 - YA687, YC153 - YC413) 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. VNAV will remain in path regardless of cross–track.
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