Garmin G300 Pilot's Manual page 213

If the altimeter's barometric setting is adjusted while VNAV mode is active,
the Integrated Autopilot increases/decreases the descent rate by up to 500 fpm
to re-establish the aircraft on the descent path (without commanding a climb).
For large changes, it may take several minutes for the aircraft to reestablish on
the descent path. If the change is made while nearing the VNAV waypoint, the
aircraft may not reestablish on the descent path in time to level off at the VNAV
Target Altitude. In this case, the Integrated Autopilot will revert to Pitch Hold
mode upon passing the VNAV waypoint, and Selected Altitude Capture (ALTV)
mode will be automatically armed.
AUTOMATIC REVERSION TO PITCH HOLD MODE
Several situations can occur while VNAV Mode is active which cause the
Integrated Autopilot to revert to Pitch Hold Mode:
• Vertical deviation exceeds 200 feet during an overspeed
condition.
• Vertical deviation becomes invalid (the Deviation Indicator is
removed from the PFD).
Following a reversion to Pitch Hold Mode, VNAV Mode and the
appropriate altitude capture mode become armed to allow for possible
profile recapture.
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.
190-00921-00 Rev. D Garmin G300 Pilot's Guide
integrated autopilot
195