System Operation
3-86
GNS-XLS Flight Management System
Rev. 3
Sep/96
One minute prior to arriving at #TOD, the system issues the VNAV
WPT ALERT message and the discrete waypoint light will flash for
10 seconds, then go steady.
NOTE: Changing CRUISE ALT on the CDU to a lower altitude
should only be done after the aircraft has departed cruise altitude, or
#TOD at the current cruise altitude will be lost.
Pre-Selected Altitude Intercept Point (#PRESL)
When the system has an input from an Altitude Pre-selector and the
aircraft is flying toward this altitude, a profile waypoint (#PRESL)
appears on the VNAV Page. #PRESL, however, never becomes the
Vertical To Waypoint. When the Pre-selector input is valid, ETE and
RANGE to #PRESL can be found on the VNAV DATA Page.
NOTE: With certain types of Pre-Selectors installed (analog), it may
be necessary to manually enter a CRUISE ALT when the PRESEL
ALT is set higher than the cruise altitude. The system does not read
analog Pre-selector output until the aircraft’s baro is within approxi-
mately 1000ft of the pre-selected value.
Descent Reference Waypoints
Descent Reference Waypoints have a fixed altitude crossing, i.e.
Cross-At type altitude constraint. To create a Descent Reference
Waypoint, the pilot can program a Flight Path Angle or a Cross-At
altitude constraint.
If a programmed FPA violates a prior vertical constraint, the system
reassigns the Descent Reference Waypoint, using the Default FPA
from the VNAV DATAPage, as illustrated in Figure 3-141.
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