Garmin GFC 600 Pilot's Manual page 54

AFCS
When the AFCS is interfaced to a Directional Gyro (as opposed to an HSI), Navigation
mode cannot be armed. The pilot must align the aircraft to be on course with the CDI
displaying less than half scale of deviation and then press the NAV key to force capture
in Navigation mode. The GMC 605 will display a message 'SET HDG=CRS' while any
Navigation mode is active.
When the AFCS is interfaced to a DG, set the Selected Heading on
NOTE:
the DG to the same value as the Selected Course when a Navigation Mode
is active.
Some navigators that combine both GPS and VHF Nav functions are capable of
automatically switching the CDI source from GPS to VOR/LOC when programmed to
transition from a GPS flight plan to a Localizer approach. The AFCS does not support
automatic transition from a Navigation mode to an Approach mode and will revert to
ROL mode if it senses that the navigation source has changed.
CAUTION:
capability should be configured with automatic switching disabled.
If Navigation mode is active and any of the following occur, the AFCS reverts to Roll
Hold mode (wings rolled level):
• Different VHF Nav frequency tuned while in VOR or LOC Navigation mode (note
that in some installations Navigation mode may remain engaged if there is still
valid deviation after the frequency changes from VOR to VOR, or from LOC to LOC)
• Navigation source switched (e.g. GPS to VOR/LOC, or VOR to LOC)
Pressing the CWS button and hand-flying the aircraft does not change the selected
course while in Navigation mode. Upon release of the CWS button the autopilot will
guide the aircraft back to the selected course (or GPS flight plan).
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Navigators or PFDs with automatic CDI source switching
Garmin GFC 600 Pilot's Guide 190-01488-00 Rev. B