Installation Instructions

190-01007-A3 GTN 6XX/7XX AML STC Installation Manual
Rev. 4 Page 2-23
the antenna’s susceptibility to RF radiation from components inside the aircraft.
2.4.5 Electrical Bonding
Electrical equipment chassis, shield/ground terminations, antennas, supporting brackets, and racks must
be electrically bonded to the aircraft’s main structure (metallic or tube/fabric aircraft) or instrument panel
(composite aircraft). Refer to SAE ARP 1870 Section 5 when surface preparation is required to achieve
electrical bond. The electrical bond must achieve direct current (DC) resistance less than or equal to:
• 2.5 milliohms to local structure in equipment mounting locations for metallic or tube and fabric
aircraft.
• 5.0 milliohms to the instrument panel for composite aircraft.
For some aircraft the instrument panel is attached with vibration mounts. For these aircraft, it must be
verified that the vibration-isolated instrument panel is grounded to the airframe metallic structure with a
bonding jumper the same or equivalent to the specification below. If no such jumper is installed, a
bonding strap meeting the following criteria must be installed to accomplish this:
• The cross sectional area of the strap must be greater than 0.016 square inches (approx 20800
circular mils). A 7/16 inch or wider tubular braid (QQB575R30T437, 24120 circular mils) or a
3/4 inch or wider flat braid (QQB575F36T781, 20,800 circular mils) must be used.
• The strap length should be as short as possible and must not exceed 6 inches. Detailed design of a
bonding strap meeting these requirements is shown in Section 3.6.8.
Compliance must be verified by inspection using a calibrated milliohm meter.
The antenna ground plane must be electrically bonded to the antenna baseplate. The electrical bond must
achieve direct current (DC) resistance less than or equal to 2.5 milliohms. Do not remove paint on outer
skin of aircraft under the footprint of the antenna baseplate unless necessary to meet bonding
requirements. The painted surface prevents corrosion and should be left intact if possible.
Brackets installed to main structure or instrument panel with four or more rivets can provide sufficient
electrical bond to allow equipment chassis or equipment rack to be bonded to the bracket. More rivets or
surface preparation may be needed for brackets that will carry large DC/AC or lightning currents.
The correct material finish is important when mating untreated or bare dissimilar metals. They should be
galvanically compatible. When corrosion protection is removed to make an electrical bond any exposed
area after the bond is completed should be protected again. Additional guidance can be found in
AC 43.13-1B and SAE ARP1870.
Typical electrical bonding preparation examples are shown in Figure 2-8, Figure 2-9, and Figure 2-10.
Aluminum surface preparation is detailed in Section 2.4.5.1.
2.4.5.1 Aluminum Surface Preparation
To prepare an aluminum surface for proper bonding, this general procedure should be followed. For a
more detailed procedure, reference SAE ARP1870 Sections 5.1 and 5.5.
1. Clean grounding location with solvent.
2. Remove non-conductive films or coatings from the grounding location.
3. Apply a chemical conversion coat such as Alodine 1200 to the bare metal.
4. Once the chemical conversion coat is dry, clean the area.
5. Install bonding aluminum tape or equipment at grounding location.
6. After the bond is complete, if any films or coatings were removed from the surface, reapply a
suitable film or coating to the surrounding area.