GNC 420(A) Pilot’s Guide and Reference
APPENDIX C
TROUBLESHOOTING
C-3
NORMAL (no ‘OBS’
annunciator)
OBS
Automatic sequencing of
waypoints
Manual sequencing- ‘holds’
on selected waypoint
Change in HSI does not
affect CDI deflection
Manually select course to
next waypoint from HSI
Always navigates ‘TO’ the
active waypoint
Indicates ‘TO’ or ‘FROM’
waypoint
Must be in this mode for
final approach course
Cannot be set for final
approach course or
published holding patterns
When OBS mode is active, the GNC 420 allows the
pilot to select the desired course to/from a waypoint using
the HSI (much like a VOR) and display a to/from flag for
the active-to waypoint. If an external course input is not
available, select the OBS course on-screen, via a ‘Select
OBS Course pop-up window. With OBS mode cancelled,
the CDI always displays a ‘TO’ indication for the next
waypoint once the aircraft has crossed the active waypoint
(provided the active waypoint is not the last waypoint).
Refer to Section 6.2 for an example using the
OBS
Key.
One application for the
OBS
Key is holding patterns.
The
OBS
Key is used to suspend waypoint sequencing
and select the desired course along the waypoint side of
the hold. For many approach operations, setting and
resetting of
waypoint sequencing is automatic. Holding
patterns that are part of an approach automatically
disable waypoint sequencing, then re-enable waypoint
sequencing after one time around the holding pattern.
A ‘SUSP’ annunciation appears directly above the
OBS
Key (Figure C-3) to indicate that automatic
waypoint
sequencing is temporarily suspended and course selection
is not available. If more than one trip around the holding
pattern is desired, press the
OBS
Key to again suspend
waypoint sequencing. An example of this operation is
an approach which begins with a holding pattern at the
initial approach fix (IAF). See Section 6.2, Flying an
Approach with a Hold for more information on the ‘SUSP’
annunciation and approaches with holding patterns.
Figure C-3 ‘SUSP’ Annunciation
190-00140-20 Rev. K
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