Garmin G3X Pilot's Manual page 274

Integrated Autopilot
VNAV TARGET ALTITUDE CAPTURE MODE (ALTV)
VNAV Target Altitude Capture is analogous to Selected Altitude Capture
Mode and is armed automatically after the AP VNAV Softkey is pressed if the
VNAV Target Altitude is to be intercepted before the Selected Altitude. The
annunciation 'ALTV' indicates that the VNAV Target Altitude is to be captured.
Refer to Section 2.3 (Vertical Navigation) for more information on setting up
the VNAV target altitude.
As the aircraft nears the VNAV Target Altitude, the Integrated Autopilot
automatically transitions to VNAV Target Altitude Capture Mode with Altitude
Hold Mode armed. This automatic transition is indicated by the green 'ALTV'
annunciation flashing for up to 10 seconds and the appearance of the white
'ALT' annunciation.
At 50 feet from the VNAV Target Altitude, the Integrated Autopilot
automatically transitions from VNAV Target Altitude Capture to Altitude Hold
Mode and selects the VNAV target altitude as the new Integrated Autopilot
Altitude Reference. As Altitude Hold Mode becomes active, the white 'ALT'
annunciation moves to the active vertical mode field and flashes green for 10
seconds to indicate the automatic transition.
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Garmin G3X Pilot's Guide 190-01115-00 Rev. F