Gdc 74A; Grs 77 - Garmin G500 Pilot's Manual

1.1.3

GDC 74A

The GDC 74A Air Data Computer (ADC) compiles information from the
pitot/static system and an Outside Air Temperature (OAT) sensor. The GDC 74A
provides pressure altitude, airspeed, vertical speed, and OAT information to the
G500 system. The GDC 74A communicates with the GDU 620 and GRS 77
using an ARINC 429 digital interface.
1.1.4

GRS 77

The GRS 77 is an Attitude and Heading Reference System (AHRS) unit that
provides aircraft attitude information to the G500 display. The unit contains
advanced tilt sensors, accelerometers, and rate sensors. In addition, the GRS 77
interfaces with both the GDC 74A Air Data Computer and the GMU 44
magnetometer. The GRS 77 also utilizes GPS data forwarded from the GDU
620. Actual attitude and heading information is sent to the GDU 620 using an
ARINC 429 digital interface.
The IGRF (International Geomagnetic Reference Field) model is contained in
the GRS 77 and is only updated once every five years. The IGRF model is part
of the Navigation Database. At system power-up, the IGRF models in the GRS
77 and in the Navigation Database are compared, and if the IGRF model in the
GRS 77 is out of date, the user is prompted to update the IGRF model in the
GRS 77. The prompt will appear after the G500 splash screen is acknowledged
on the MFD.
1-4
Figure 1-4 GDC 74A Air Data Computer
Figure 1-5 GRS 77 AHRS
Garmin G500 Pilot's Guide 190-01102-02 Rev. E