Garmin G3X Touch Pilot's Manual page 350

AFCS
If the localizer deviation is less than half-scale when NAV is touched
or when the NAV Key is pressed, the autopilot will activate LOC Mode
and track the localizer, otherwise LOC Mode is armed and the pilot is
responsible for intercepting the localizer. In some cases it may be necessary
to use HDG Mode with LOC Mode armed to follow a vector to the localizer.
BACKCOURSE APPROACH
NOTE: When making a backcourse approach, set the Selected Course to the
localizer front course.
NOTE: 'BC' will not be displayed on the G3X Touch HSI.
Backcourse Mode captures and tracks a localizer signal in the backcourse direction.
Backcourse is indicated by 'BC' in the autopilot status box.
If GS and LOC Mode are armed, the flight director can only capture the localizer
front course, indicated by 'LOC' in the autopilot status box. If only LOC Mode is armed
(not GS Mode), the flight director will capture either the front course or the backcourse
based on the angle at which the aircraft intercepts the localizer.
Selecting BC Mode:
1) EXTERNAL NAVIGATOR: Tune and activate the desired LOC frequency.
2) G3X Touch: Ensure that LOC1 is the selected navigation source. The HSI
should automatically slew the needle to the correct course.
3) G3X Touch: From the PFD touch the autopilot then touch NAV.
Or:
Press the NAV Key on the GMC control unit.
If the aircraft heading is within 75˚ of the localizer backcourse then BC
Mode will be armed. If the aircraft heading is within 105˚ of the localizer
front course, LOC Mode will be armed.
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Garmin G3X Touch ™ Pilot's Guide for Certified Aircraft
190-02472-00 Rev. D