Garmin G3X Pilot's Manual page 382

AFCS
2) Use the NOSE UP/DN Wheel or FMS Joystick to adjust the commanded
airspeed while maintaining the same power, or reduce power to allow
descent in Indicated Airspeed Mode while the autopilot maintains the
current airspeed.
3) As the aircraft nears the Selected Altitude, the flight director transitions to
Selected Altitude Capture Mode, indicated by the green 'ALTS' annunciation
flashing for up to 10 seconds.
The green 'ALT' annunciation flashes for up to 10 seconds upon reaching
50 feet from the Selected Altitude; the autopilot transitions to Altitude Hold
Mode and levels the aircraft.
1
ALT Mode
VNAV descent to VNAV Target Altitude:
1) Select VNAV flight control:
a) GMC 305: Press the VNV Key to arm VNV Mode. The white
annunciation 'VNAV' appears.
Or: G3X: Press the AP VNAV Softkey to arm Vertical Path Tracking Mode.
The white annunciation 'VNAV' appears.
360
Cruise Altitude of 12,000 MSL
2
Selected Altitude of 10,000 MSL
IAS Descent
Garmin G3X Pilot's Guide
3
ALT Mode
190-01115-00 Rev. K