Garmin G3X Pilot's Manual page 346

AFCS
Activating Vertical Navigation Mode:
➊
When a flight plan is active, VNAV data is valid, and the VNV Key (GMC 305)
or AP VNAV Softkey is pressed, VNAV mode is armed in preparation for descent
path capture. 'VNAV' is annunciated in white in the G3X Autopilot Status Box. If
applicable, the appropriate altitude capture mode is armed for capture of the VNAV
Target Altitude (ALTV) or the Selected Altitude (ALTS), whichever is greater.
➋
When a descent leg is captured (i.e., vertical deviation becomes valid), VNAV Mode
is activated and tracks the descent profile. An altitude capture mode (ALTS or ALTV)
is armed as appropriate.
➌
When approaching the VNAV Target Altitude (or Selected Altitude) the system
automatically transitions to ALTS or ALTV Mode with ALT Mode armed.
324
Vertical Navigation
Mode Active
Vertical Navigation Mode
Garmin G3X Pilot's Guide
Vertical Navigation
Target Altitude Capture
Mode Armed
Selected
Altitude
Vertical
Deviation
Indicator
Required
Vertical
Speed Bug
190-01115-00 Rev. K