Garmin G500 Pilot's Manual page 29

A maximum of two GPS inputs are provided to the AHRS. If GPS information
from one of the inputs fails, the AHRS uses the remaining GPS input and an
alert message is issued to inform the pilot. If both GPS inputs fail, the AHRS
will continue to provide attitude and heading information to the PFD as long as
magnetometer and airspeed data are available and valid.
If the magnetometer input fails, the AHRS continues to output valid attitude
information and GPS Track information is used. However, the PFD heading
display is flagged as invalid with a red "X" and "TRK" is annunciated to the
right of the Track value in magenta. The Track value color changes from white
to magenta.
NOTE: In this case the magnetic standby compass and GPS ground track
can be used to keep the aircraft on the desired heading.
Map orientations change from HDG UP to TRACK UP and Map orientations
will change back to HDG up when heading is restored.
When heading fails, the heading bug is not removed and the GDU continues
driving the autopilot heading error output using track in place of heading.
Heading Failed
Figure 1-7 Track Mode shown as Active when Heading Info has failed
Failure of the air data input has no effect on the AHRS output while AHRS is
receiving valid GPS information. Invalid or unavailable airspeed data, in addition
to complete GPS failure, results in loss of all attitude and heading information.
190-00601-02 Rev. J Garmin G500/G600 Pilot's Guide
TOC Cover
Track Mode Active
SVT Is Turned Off
When Heading
Fails
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