Garmin G1000 NXi Pilot's Manual page 402

AUTOMATIC FLIGHT CONTROL SYSTEM
When a descent leg is captured (i.e., vertical deviation becomes valid), Vertical Path Tracking becomes
active and tracks the descent profile (next figure). An altitude capture mode ('ALTS' or 'ALTV') is armed as
appropriate.
GPS is Selected
Navigation
Source
If the altimeter barometric setting is adjusted while Vertical Path Tracking is active, the flight director
increases/decreases the descent rate by up to 500 fpm to re-establish the aircraft on the descent path
(without commanding a climb). Adjusting the altimeter barometric setting creates discontinuities in VNV
vertical deviation, moving the descent path. For large adjustments, it may take several minutes for the
aircraft to re-establish on the descent path. If the change is made while nearing a waypoint with a VNV
Target Altitude, the aircraft may not re-establish on the descent path in time to meet the vertical constraint.
For approaches providing vertical guidance, the computed enroute/terminal baro-VNAV solution remains
active until the FAF waypoint becomes the active leg. Once the FAF becomes the active leg, VNAV continues
to remain active if the aircraft is in a VPTH-coupled descent. If VNAV is not in a VPTH-coupled descent,
the enroute/terminal baro-VNAV solution is removed to make way for the approach descent path to be
displayed on the VDI.
When the active leg is prior to the FAF waypoint, the computed enroute/terminal baro-VNAV solution
remains active (i.e., does not apply the VPTH-coupled restriction) even if the TOD is located on the FAF-
1 to FAF leg. The system operates in this way to avoid situations in which the aircraft was previously
intercepting the descent path after the FAF-1 waypoint (i.e., on the FAF leg), but is now intercepting it
before the FAF-1 waypoint due to changes in the aircraft altitude. In that situation, baro-VNAV would have
previously not been active since the TOD would have been on the FAF leg, but then became active once the
system detected the aircraft altitude intercepted the descent path prior to the FAF-1 waypoint.
Rather than allowing a situation in which VNAV was previously inactive but became active without pilot
modification, the system removes the VNAV solution once the FAF becomes the active leg (assuming the
system is not VPTH coupled). It is at that point the approach descent path would become active on the VDI.
382
Vertical Path
Tracking Active
Command Bars Indicate Descent to
Terminal
Maintain Required Vertical Speed
Phase of
Flight
Figure 7-16 Vertical Path Tracking Mode
Garmin G1000 NXi Pilot's Guide for the Cessna NAV III
VNV Target Altitude
Capture Armed
VNV Target
Altitude
Required
Vertical
Speed Bug
Vertical Deviation
Indicator (VDI)
190-02177-02 Rev. A