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.
11.31.28 D6-27370-866-EGP
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.
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.
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.
September 15, 2016
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