GNC 420(A) Pilot’s Guide and Reference
APPENDIX C
TROUBLESHOOTING
C-5
When does turn anticipation begin, and what
bank angle is expected?
The GNC 420 smooths adjacent leg transitions based
upon a nominal 15º bank angle (with the ability to roll up
to 25º) and provide three pilot cues for
turn anticipation:
1) A waypoint alert (‘NEXT DTK ###°’) flashes in the
lower right corner of the screen 10 seconds before
the turn point (Figure C-6).
Figure C-6 CDI at 1 nm Scale
2) A flashing turn advisory (‘TURN TO ###°’)
appears in the lower right corner of the screen
when the aircraft is to begin the turn. Set the HSI
to the next DTK value and begin the turn.
3) The To/From indicator on the HSI (or CDI)
flips momentarily to indicate that the aircraft
has crossed the midpoint of the turn. For
more information on waypoint alerts and turn
advisories, see Sections 6.2 and 6.3.
When does the CDI scale change, and what
does it change to?
The GNC 420 begins a smooth
CDI scale transition
from the 5.0 nm (enroute/oceanic mode) to the 1.0 nm
(terminal mode) scale 30 nm from the destination airport
(Figure C-7). The CDI scale further transitions to 0.3
nm (approach mode) at 2 nm prior to the FAF during an
active approach. To return the CDI to the 1 nm scale in a
missed approach situation, activate the missed approach
sequence by pressing the
OBS
Key, as described in Section
6.2, Flying the Missed Approach. The CDI scale is also
1.0 nm within 30 nm of the departure airport.
Figure C-7 CDI Scale Transition
0.3 nm
30 nm
2 nm
FAF
MAP
1 nm
1 nm
5 nm
1 minute
Approach
Enroute/Oceanic
Terminal
190-00140-20 Rev. K
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