System Maintenance Manual

G1000/GFC700 System Maintenance Manual – C90A/C90GT/C90GTi King Air Page 2-5
190-00682-01 Revision G
2.1.8 GDC 74B Digital Air Data Computer (2)
The Garmin GDC 74B computers compile information from the pitot/static system and various outside air
temperature (OAT) and awareness sensors and provide digital air data computations to the G1000 system.
The GDC 74B communicates with the GIA 63W, GDU 1040A, and GRS 77 using ARINC 429 digital
interface. The unit is mounted behind the instrument panel. Power is provided to both GDC 74B units by
the triple-fed bus. Additionally, a secondary power source is provided to the No. 1 computer by the center
bus. The GDC 74B connects to existing pitot/static ports via existing tubing in C90A and C90GT airframes.
For C90GTi airframes new pitot/static tubing and hardware is purchased from Hawker Beechcraft Company.
Reference drawing Main Instrument Panel Installation KingAir C90A/C90GT, p/n 005-00375-28 for details.
2.1.9 OAT Probe (2)
The Garmin GTP 59 OAT Probes provide the GDC 74B with air temperature data. The OAT probes are
mounted to the bottom of the King Air fuselage at F.S. 113.5. The probes receive power directly from the
GDC 74B airdata computers.
2.1.10 GRS 77 Attitude & Heading Reference System (2)
The Garmin GRS 77 AHRS units provide attitude and heading information to the G1000 system. The units,
mounted nose equipment bay, contain advanced tilt sensors, accelerometers, and rate sensors. The unit
interfaces with the GDC 74B and GMU44 Magnetometer and utilizes GPS signals from the GIA 63Ws.
Actual attitude and heading information is sent using ARINC 429 digital interface to both GDU 1040As and
GIA 63Ws. Power is provided to both GRS 77 units by the triple-fed bus. Additionally, a secondary power
source is provided to the No. 1 GRS 77 by the center bus. The GRS 77 interfaces with and provides power
to the GMU 44 Magnetometer. The GRS 77 supplies attitude and heading information directly to the PFDs,
MFD, and GIAs.