Par 602 Page 109
(a) Spray hinges and latches at pivot points with Water-Displacing Corrosion Preventive Compound, MIL-PRF-81309, Type II.
NOTE: The purpose of Water-Displacing Corrosion Preventive Compound is to displace any entrapped water between hinge and latch components. The subsequent application of a lubricating oil may then fill the void areas and provide lubrication. Without the Water-Displacing Corrosion Preventive Compound, the lubricating oil will not displace the entrapped water because it is less dense than the water.
(b) Apply Lubricating Oil, General Purpose Preservation, conforming to MIL-PRF-32033, to all moving parts.
(7) Preserve unpainted metallic hardware, hinges, and latches not requiring lubrication per Chapter 4, paragraph 407.
l. Shelves, Bulkheads, and Crevices. Inspect shelves, bulkheads, crevices, and corners for signs of dust, lint, debris, and corrosion. Figures 6-5 and 6-6 show lint and debris accumulation on a shelf. Pay particular attention to cracked, chipped, peeling, or deteriorating paint and sealant. Treat in accordance with subparagraph 602b.
m. Moisture Traps and Cavity Areas. Inspect for moisture traps and cavities at the rear of equipment shelves. Figure 6-3 shows evidence of a moisture entrapment area on the avionics shelf. Treat in accordance with subparagraph 602b.
(1) Moisture traps and cavities may be filled with Sealing Compound, conforming to MIL-PRF-81733, when authorized.
(2) Apply sealant and finish in accordance with Chapter 5, paragraphs 504 and 505.
n. Electrical Bonding and Grounding Straps. Bonding and grounding straps used on aircraft and avionics equipment may exhibit galvanic corrosion when not protected. Figure 3-8 and Figure 6-7 show examples of a corroded bonding strap and attach hardware. In most cases the material used for bonding and grounding straps is different than the mating surfaces of the aircraft and avionics equipment. Treat these areas in accordance with Chapter 7, paragraph 702.
Par 602 Page 110 FIGURE 6-5. LINT ACCUMULATION ON A SHELF
Par 602 Page 111 FIGURE 6-6. LINT AND DEBRIS ACCUMULATION ON A SHELF
603.
REPAIR OF AVIONICS SYSTEMS, EQUIPMENT, AND COMPONENTS.
a. Antenna Systems. Antenna systems are normally exposed to fairly severe environments. Without adequate corrosion protection, these systems can fail via shorts, open circuits, loss of dielectric strength, signal attenuation, poor bonding, or electromagnetic interference (EMI). Structural damage to the aircraft can also result. Antennas mounted on the fuselage require openings in the aircraft structural skin to route the various cables to the antenna.
The area around the antenna mounting is susceptible to moisture intrusion from rain, runway deicing fluids/materials, condensation, aircraft wash, and internal fluids (i.e., fuel, oil, lavatory and galley products, etc.). Antennas mounted on the lower fuselage are particularly susceptible to corrosion. The inspection and treatment process is outlined in the following paragraphs.
(1) Visual Corrosion Inspection. A visual examination of installed antennas and mounting areas can reveal evidence of a corrosion attack. Cracks, splits, and peeling of the exterior paint finish and sealant are a good indicator of possible corrosion damage. Evidence of corrosion deposits at the antenna mounting areas is the most obvious indication that an attack has taken place. Examine for a grayish-white to white powdery deposit (aluminum oxide). Figure 6-8 shows a corroded antenna mounting area and electrical plug.
(2) Antenna Mounting Area Preparation Procedures. When corrosion is visually evident or highly suspected, corrective action, including further inspection by disassembly, is
Par 603 Page 112 necessary to determine the full extent of any damage and prevent further deterioration. Corrosion treatment includes stripping exterior finishes in the affected area, removing corrosion products, cleaning, and applying surface treatment for avionics bonding. The recommended procedures that may be used on the antenna base and mating aircraft structure for corrosion removal, cleaning, and mounting preparation are as follows:
(a) Remove dirt, grime, oil, and grease from antenna mating surfaces using a cleaning cloth or cheesecloth dampened with an approved solvent. A clean surface will allow proper evaluation of the extent of corrosion damage. Figure 6-10 shows a corroded antenna mounting area on a removed lower fuselage skin.
(b) Remove existing fillet sealant from the mating surface of the antenna and aircraft structure/skin with a nonmetallic scraper (see Figure 6-9).
(c) Remove paint as required from the area surrounding the antenna mounting with an epoxy paint remover as described in Chapter 5, paragraph 502.
(d) Thoroughly clean the stripped area with a water-moistened cleaning cloth or cheesecloth.
(e) Remove corrosion deposits to the limits of the OEM’s maintenance instructions with 320-grit abrasive cloth, conforming to A-A-1048, or an abrasive nylon mat, conforming to A-A-58054, Type I. Ensure complete corrosion removal from the antenna base and mating aircraft surface.
(f) Wipe off corrosion removal products using a cleaning cloth or cheesecloth dampened with Isopropyl Alcohol, TT-I-735. Allow the surface to air dry.
(g) If bare metal was exposed on the antenna base or aircraft structure/skin, treat the cleaned surface with an avionics grade Chemical Conversion Coating conforming to MIL-DTL-81706, Class 3, in accordance with Chapter 5, paragraph 503.
Par 603 Page 113 FIGURE 6-7. CORRODED BONDING STRAP AND ATTACH HARDWARE
Par 603 Page 114 FIGURE 6-8. CORRODED ANTENNA MOUNTING AREA AND ELECTRICAL PLUG
Par 603 Page 115 FIGURE 6-9. NONMETALLIC SEALANT REMOVAL TOOLS NO TE: 1. All dimensions are in inches unless otherwise specified.
2. Material: Micarta suggested.
Par 603 Page 116 FIGURE 6-10. CORRODED ANTENNA MOUNTING AREA (
3) Rigid Antenna Mounting (without gasket). The mounting bases of rigid antennas vary in shape and size. The following installation procedures are typical and may be used for mast-type antennas (blade, spike, whip base, or long wire mast base) not requiring a gasket.
Figures 6-11 and 6-12 show typical blade mounted antennas. The recommended procedures for the application of corrosion prevention measures and attachment of the antenna to the aircraft structure/skin are as follows:
(a) Clean and remove any corrosion from the antenna base and aircraft structure/skin as described in subparagraph 603a(2).
(b) Remove corrosion from screw fastener countersinks and fastener bore areas on the antenna base in order to provide good electrical conductivity from the base to the screw fasteners. Remove corrosion deposits to the limits of the OEM’s maintenance instructions with 320-grit abrasive cloth, conforming to A-A-1048, or an abrasive nylon mat, conforming to A-A-58054, Type I.
Par 603 Page 117
(c) Clean the base of the antenna and mating aircraft structure/skin with a clean cloth or cheesecloth dampened with Isopropyl Alcohol, TT-I-735. Allow both surfaces to air dry.
(d) Apply avionics grade Chemical Conversion Coating, conforming to MIL-DTL-81706, Class 3, to bare areas on the antenna base, fastener countersinks, and mating aircraft structure/skin in accordance with Chapter 5, paragraph 503.
(e) Install antenna per the OEM’s maintenance instructions or as follows:
1. Apply an even coating of Corrosion Preventive Compound, conforming to MIL-PRF-16173, Grade 4, to the antenna base and mating aircraft structure/skin. Avoid applying the material in the antenna base fastener countersink areas. Wipe any Corrosion Preventive Compound from the fastener countersink areas using an approved solvent or Isopropyl Alcohol, TT-I-735, as described in subparagraph 603a(3)(c). Position the antenna and install, set, and torque the attach fasteners per the OEM’s maintenance instructions.
NOTE: Mask as required an area just outside of the antenna base on the aircraft structure/ skin using pressure-sensitive tape.
2. (Alternate method 1.) Mix and apply an even coating of Polysulfide Sealing Compound, MIL-PRF-81733 or AMS-3276, to the antenna base and mating aircraft structure/skin in accordance with Chapter 5, subparagraph 505d. Within the working time of the sealant, position the antenna and install, set, and torque the attach fasteners per the OEM’s maintenance instructions. Ensure sealant squeeze-out from all sides of the antenna base.
3. (Alternate method 2.) Position the antenna and install, set, and torque the attach fasteners per the OEM’s maintenance instructions.
4. Clean the outside edge of the antenna using an approved solvent or Isopropyl Alcohol, TT-I-735, as described in subparagraph 603a(3)(c).
5. Conduct an electrical resistance test in accordance with subparagraph 603a(12).
FIGURE 6-11. BLADE ANTENNA INSTALLATION FIGURE 6-12. BLADE ANTENNA INSTALLATION Par 603 Page 118
Par 603 Page 119
6. (For all installation methods.) Apply a fillet seal of Polysulfide Sealing Compound, MIL-PRF-81733 or AMS-3276, to the antenna base and mating aircraft structure/skin, and coat the fastener heads using a spatula or sealant gun in accordance with Chapter 5, subparagraph 505d to form a watertight seal.
7. Remove masking tape within the working time of the sealant.
8. Allow the sealant to fully cure before any flight operations.
(4) Rigid Antenna Mounting (with gasket). These procedures are applicable to blade or spike antennas that require a conductive gasket between the antenna base and aircraft skin/structure. The following procedures are recommended for the application of sealant, corrosion prevention, and mounting the antenna base.
(a) Clean and remove corrosion from the antenna mounting and screw countersink areas and apply chemical conversion coating in accordance with subparagraphs 603a(3)(a) through (e).
(b) With the gasket in place on the antenna base and the coaxial connector mated, apply a coating of Corrosion Preventive Compound, conforming to MIL-PRF-16173, Grade 4, using a brush. Apply compound to the skin around the edge of the coaxial cable hole and to the coaxial connector. Ensure the Corrosion Preventive Compound will not interfere (insulate) with the conductive gasket and the antenna when mated to the fuselage skin.
(c) Clean any Corrosion Preventive Compound from the fastener countersink areas using a clean cloth or cheesecloth dampened with Isopropyl Alcohol, TT-I-735.
(d) Position the antenna base. Ensure the countersink area is clean under the fastener heads. Set and torque the fasteners.
(e) Conduct an electrical resistance test in accordance with subparagraph 603a(12).
(f) Clean and seal the antenna base to the fuselage skin/structure and fastener heads in accordance with subparagraphs 603a(3)(e)6, 7, and 8.
(5) Flush or Dome Antenna Mounting. These installation procedures are applicable to flush or dome covered (radome) antennas. These antennas are usually installed on aircraft as part of the primary structure. The radiating elements of the antenna and fiberglass cover are normally individual units. Figure 6-13 shows a flush mounted antenna radome. The recommended procedures for applying corrosion preventives to these antennas are:
(a) Clean and remove corrosion from the antenna and fuselage skin/structure as described in subparagraph 603a(2).
Par 603 Page 120
(b) Install the antenna in accordance with the OEM’s installation instructions. Prior to attaching the cover or dome (radome), spray a coat of (avionics grade) Water-Displacing Corrosion Preventive Compound, conforming to MIL-PRF-81309, Type III, on the internal areas of the coaxial and antenna connectors. Shake out excess. Mate the coaxial connector with the antenna. Spray a coat of Water-Displacing Corrosion Preventive Compound, conforming to MIL-DTL-85054, on the exterior of the antenna, coaxial connectors, and all other exposed metallic hardware. Mount the antenna and secure fasteners.
FIGURE 6-13. FLUSH MOUNTED ANTENNA RADOME (
c) Position the cover or dome (radome) and install, set, and torque the attach fasteners per the OEM’s maintenance instructions. Mask, as required, using pressure-sensitive tape adjacent to the seam with two parallel strips of tape. Apply a smooth seam of Polysulfide Sealing Compound, MIL-PRF-81733 or AMS-3276, within the two parallel strips of tape around the periphery of the antenna cover and mated aircraft structure/skin in accordance with Chapter 5, subparagraph 505d. Ensure sealant is also applied over the fastener heads to form a watertight seal. Remove the tape within the working time of the sealant. Allow the sealant to cure prior to flight.