Garmin G3X Pilot's Manual page 383

b) G3X: From the PFD or PFD Page, press the ALT Softkey and turn the
FMS Joystick to set the Selected Altitude to the flight plan's VNAV
Target Altitude of 10,000 feet.
Or: G3X: From the PFD or PFD Page, press the ALT/PTCH Softkey and turn
the FMS Joystick to set the Selected Altitude below the flight plan's
VNAV Target Altitude of 10,000 feet.
If the Selected Altitude is not at least 75 ft below the VNAV Target
Altitude, the flight director captures the Selected Altitude rather than
the VNV Target Altitude once Vertical Path Tracking Mode becomes
active (ALTS is armed rather than ALTV). If no altitude select value has
been entered in the altitude select window, VNAV will flash white one
minute prior to descent and the VNAV profile will not be captured.
2) When the top of descent (TOD) is reached, the flight director transitions to
VNAV Mode and begins the descent to the VNAV Target Altitude. Intention
to capture the VNAV Target Altitude is indicated by the white 'ALTV'
annunciation.
3) As the aircraft nears the VNV Target Altitude, the flight director transitions
to VNV Target Altitude Capture Mode, indicated by the green 'ALTV'
annunciation flashing for up to 10 seconds.
The green 'ALT' annunciation flashes for up to 10 seconds upon reaching 50
feet from the VNV Target Altitude; the autopilot transitions to Altitude Hold
Mode and levels the aircraft at the vertical waypoint.
190-01115-00 Rev. K Garmin G3X Pilot's Guide
AFCS
361