Garmin GNC 420AW, Installation Manual

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400W Series Installation Manual Page 2-3
190-00356-02 Rev. G
3. ARINC 429 from GTX 33/330
4. RS-232 FADC
2.3.3 Multiple Heading Sources
The 400W Series can accept heading data from an ARINC 429 INS/IRU, ARINC 429 EFIS, GAD 42,
EHSI, GTX 33/330 transponder, ARINC 429 traffic advisory system, RS-232 FADC, and RS-232
WX-500 Stormscope®.
If multiple sources of heading data are supplied to the 400W Series unit, only valid data from the highest
priority source is used – preference is given to true heading (ARINC 429 label 314) if both true heading
(label 314) and magnetic heading (label 320) are received. If the highest priority source becomes
unavailable, data is taken from the next-highest priority source. The priorities of the heading sources are
as follows (from highest to lowest):
1. ARINC 429 INS/IRU
2. ARINC 429 EFIS
3. ARINC 429 from GAD 42
4. ARINC 429 from Sandel EHSI
5. ARINC 429 from GTX 33/330
6. ARINC 429 traffic advisory system
7. RS-232 FADC
8. RS-232 WX-500 Stormscope
2.4 Antenna Considerations
This section contains mounting location considerations for the antennas required for the 400W Series
units.
The GPS 400W requires a GPS/WAAS antenna, the GNC 420W/420AW requires a GPS/WAAS and
COM antenna, and the GNS 430W and GNS 430AW require four antennas: a GPS/WAAS antenna, a
COM antenna, a NAV antenna, and a Glideslope antenna. Structural substantiation for mounting of
antennas is beyond the scope of the 400W Series unit STC. For mounting the GPS/WAAS antenna, refer
to Garmin GA Antenna AML STC SA01695SE or other FAA approved data. For mounting the COM,
NAV, and Glideslope antennas, refer to the aircraft manufacturer’s data.
2.4.1 GPS Antenna Location
The GPS antenna is a key element in the overall system performance and integrity for a GPS/WAAS
navigation system. The mounting location, geometry, and surroundings of the antenna can affect the
system performance and/or availability. The following guidance provides information to aid the installer
in ensuring that the most optimum location is selected for the installation of the GPS antenna. The
installation guidelines presented here meet the intent of AC 20-138A section 16. The greater the variance
from these guidelines, the greater the chance of decreased availability. Approach procedures with vertical
guidance are the most sensitive to these effects. LNAV only approaches, terminal operations, and enroute
operations may also be affected. Because meeting all of these installations guidelines may not be possible
on all aircraft, these guidelines are listed in order of importance to achieve optimum performance. Items 3
below are of equal importance and their significance may depend on the aircraft installation. The installer
should use their best judgment to balance the installation guidelines.
1. Mount the antenna as close to level as possible with respect to the normal cruise flight
attitude of the aircraft. If the normal flight attitude is not known, substitute the waterline,
which is typically referenced as level while performing a weight and balance check.