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.
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(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
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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).
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(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.
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(6) Radar Dish Antenna Corrosion Prevention Procedure. One of the primary problems related to dish antennas is the integrity of the protective finish. The protective finish is subject to scratches and chipping from normal maintenance and handling. The metal used in antenna construction is usually aluminum or magnesium. Both metals are anoxic to the attached hardware and subject to galvanic corrosive around the hardware. The dish and remainder of the mount are subject to surface corrosion when the finish is damaged. Spot touch up, as required, the paint finish in accordance with the instructions of Chapter 5, paragraphs 502 and 503.
Complete paint stripping, cleaning, and refinishing is normally justified if 20 percent of the paint finish is damaged. There are some radar antennas that have a protective finish or covering of Mylar over the base aluminum or magnesium. For these types of antenna dishes, refer to the OEM’s maintenance instructions.
(7) Temporary Dish Protection. This procedure is appropriate for line maintenance personnel to repair limited, minor surface damage of the finish to the dish. The procedure is also appropriate as an alternate procedure to paint touch-up until paint can be applied.
(a) Remove surface corrosion 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.
(b) Wipe off corrosion products using a clean cloth or cheesecloth dampened with Isopropyl Alcohol, TT-I-735. Allow the surface to air dry.
(c) Spray a coat of Water-Displacing Corrosion Preventive Compound, conforming to MIL-DTL-85054, on the affected area(s).
NOTE: Unless the area is subject to a significant amount of abrasion from operation or handling, MIL-DTL-85054 can provide up to a year of protection.
(8) Radar Antenna Hardware. For protection of the radar antenna hardware, such as nuts, bolts, screws, washers, and clamps, utilize the following procedure:
(a) Clean hardware with a clean cloth or cheesecloth dampened with Isopropyl Alcohol, TT-I-735. Allow surfaces to air dry.
NOTE: Replace nuts, bolts, screws, washers, and clamps which have worn off their protective coatings (normally cadmium).
(b) If corrosion is present but replacement is not practical, the corrosion may be removed using 320-grit abrasive cloth, conforming to A-A-1048, or an abrasive nylon mat, conforming to A-A-58054, Type I. Repeat step (a) following corrosion removal.
(c) Spray a coat of Water-Displacing Corrosion Preventive Compound, conforming to MIL-DTL-85054, on the affected hardware after installation.
(9) Antenna Connectors. Antenna connectors require special procedures to avoid moisture entry and corrosion damage. Corrosion is by far the principal cause of antenna
Par 603 Page 122 performance deterioration. Clean and preserve antenna connectors, both multipin and coaxial, in accordance with subparagraphs 603k(2) through (5).
(10) Ultrahigh Frequency (UHF)/Very High Frequency (VHF)/Automatic Direction Finder (ADF) Antenna Sealing. The location of UHF/VHF/ADF antennas is normally on the lower fuselage. Figure 6-10 shows a corroded antenna mounting area on a removed lower fuselage skin. These mounting locations are particularly susceptible to corrosion because of fluid entrapment in bilge areas. This fluid entrapment is the principal reason for additional maintenance requirements for these types of antennas. The following preventive measures should be utilized to minimize this problem.
(a) With the antenna removed, clean the antenna and the mounting location on the aircraft of grease, oil, and dirt using a clean cloth or cheesecloth dampened with an approved solvent or the cleaning procedures described in Chapter 4, paragraph 405.
(b) 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.
(c) Wipe clean corrosion removal areas on the antenna and aircraft skin/structure with a clean cloth or cheesecloth dampened with Isopropyl Alcohol, TT-I-735. Allow surfaces to air dry.
(d) Install the UHF/VHF/ADF antennas per the OEM’s maintenance instructions.
The following procedures may be used as an alternative.
(e) (Alternate installation procedures.) Normally, antennas are assembled with the dustcover mated to the bottom of the antenna casting. A plate called the antenna element mates to the antenna cavity flange under a plastic plate. During reassembly of the antenna components, discard the extruded rubber dust cover channel which fits between the dustcover edge and the antenna casting (this area will be sealed with sealant). Wipe the circumference of the mated dustcover and antenna casting clean, using a clean cloth or cheesecloth dampened with Isopropyl Alcohol, TT-I-735. Additionally, clean around the circumference of the plastic plate which mates to the antenna cavity and the attached fastener heads. Allow all surfaces to air dry.
(f) Mask as required, using pressure-sensitive tape.
(g) Seal the junction of the dustcover, antenna cavity, and antenna casting outer edge with Polysulfide Sealing Compound, conforming to MIL-PRF-81733, Type II. Mix sealant in accordance with the manufacturer’s instructions (see Chapter 6, subparagraph 603a(6)). Apply sealant with a spatula or extrude a bead of sealant using a sealant gun. Additionally, seal around the circumference of the plastic plate, antenna cavity, and the attach fastener heads for the dustcover. Remove any masking tape within the working time of the sealant.
(h) Allow the sealant to cure (tack-free) as stated in the manufacturer’s instructions.
Install the antenna in accordance with the OEM’s maintenance instructions.
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(i) Spray a coat of Water-Displacing Corrosion Preventive Compound, conforming to MIL-DTL-85054, on the antenna mount.
(j) Clean and preserve the electrical connectors in accordance with the instructions of subparagraphs 603k(2) through (5).
(k) Mask as required the antenna and aircraft surfaces using pressure-sensitive tape.
(l) Mix sealant in accordance with the manufacturer’s instructions. Apply sealant to the circumference of the antenna and aircraft skin/structure with a spatula or extrude a bead of sealant from a sealant gun (see Chapter 6, subparagraph 603a(6)). Smooth sealant using a spatula. Within the working time of the sealant, remove any masking tape. Allow the sealant to cure prior to flight. Figures 6-11 and 6-12 show examples of properly mounted and sealed blade mounted antennas.
(11) Long Wire and Direction Finder (DF) Sense Antenna Corrosion Prevention Procedures. The cleaning and preservation procedures for this type of antenna are as follows:
NOTE: If the long wire is the stranded type, and the corrosion is more extensive than light surface corrosion, it is normal practice to replace the wire. Stranded wire has the propensity of wicking corrosives through capillary action, causing extensive damage that may not be readily apparent.
(a) Cleaning and corrosion procedures for individual parts prior to mounting the antenna are described in subparagraph 603a(2), while antenna mounting procedures are described in subparagraph 603a(3).
(b) Wipe clean all antenna parts with a clean cloth or cheesecloth dampened with Isopropyl Alcohol, TT-I-735. Allow surfaces to air dry.
(c) Assemble long wire or DF antenna parts and install in accordance with the OEM’s maintenance instructions.
(d) After installing and tensioning the antenna, spray a coating of Water-Displacing Corrosion Preventive Compound, conforming to MIL-DTL-85054, over the attached hardware.
If a new bare wire (no nylon sleeve) was installed, wipe the wire with a clean cloth or cheesecloth soaked with Water-Displacing Corrosion Preventive Compound, conforming to MIL-DTL-85054. This will allow the Corrosion Preventive Compound to penetrate around the individual wire strands.
NOTE: It is preferable to apply Water-Displacing Corrosion Preventive Compound, conforming to MIL-DTL-85054, to a new bare wire after the antenna is installed and tensioned. If access to the wire is impractical after installation, wet-wipe the wire with MIL-DTL-85054 prior to installation. In such cases, minimize flexing, coiling, or abrading the wire after the Corrosion Preventive Compound has been applied and before the wire is installed and tensioned.
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(e) To prevent moisture intrusion into the insulator, fill the space around the wire where it enters the insulator with room temperature vulcanizing (RTV) sealant, conforming to MIL-A-46146, Type I; RTV 3140 (clear) is recommended. Use the cleaning and application procedures of Chapter 5, subparagraphs 505d(8) and (9). Seal both ends of the insulator in this manner.
(12) Bonding/Ground Connection Electrical Resistance Test.
NOTE: Select a scale on the milliohm meter that will allow a reading of
2.5 milliohms around mid-scale. This will ensure maximum instrument accuracy. Proper torque on the connections, good resistance readings, and complete sealing of the antenna installation are all essential to ensure the antenna will function properly in service.
(a) The electrical resistance test is performed after an antenna base is mounted or ground installation is assembled. The test should take place prior to the application of sealant.
The test uses a milliohm meter to obtain resistance readings between the antenna base or grounded portion of the antenna and the aircraft skin/structure. It is essential that the test probe be placed against bare metal when taking the readings.
(b) The maximum allowable resistance reading between the antenna base or grounded portion of the antenna and the aircraft skin/structure is 2.5 milliohms.
b. Avionics Test Equipment. Precision measurement and test equipment is required for testing, troubleshooting, and repairing avionics components and systems. This makes the reliability of this equipment in any environment critical to safe aircraft flight and system functions. Aircraft operational requirements often result in short troubleshooting and repair times for malfunctioning avionics systems. Valuable maintenance time is lost if this equipment is not functioning properly. A major source of test equipment malfunction is corrosion on contacts.
Corrosion sources that are particularly detrimental to avionics test equipment include moisture and fluid intrusion (rain, condensation, fuel, hydraulic fluid, etc.), corrosive elements in the surrounding atmosphere, malfunctioning or inadequate shop environmental control systems, and malfunctioning or inadequate built-in filter systems.
NOTE: In this section, use of the term “avionics test equipment” shall refer to all aircraft electrical and electronic system test equipment. This includes support equipment, oscilloscopes, signal generators, meters, Automatic Test Equipment (ATE), and any other equipment used to perform measurements on test, or troubleshoot avionics equipment.
(1) Cleaning Versus Calibration. A problem common to all automatic/manual test equipment is the effect of dirt, dust, lint, etc., on equipment calibration. A large quantity of test equipment, particularly older equipment, is possibly not maintained (cleaned, calibrated, etc.) on a scheduled basis. This lack of maintenance allows contaminants to collect on the components and become an integral part of the circuit, thus altering circuit parameters. These contaminant-induced changes are compensated for during each re-calibration process and can limit the equipment’s peak operating efficiency over time. Additionally, the calibration of
Par 603 Page 125 equipment can shift in service if some of the contaminants become dislodged. Because of the effect that contaminants have on electrical characteristics, immediate cleaning and preservation is mandatory after exposure of the equipment to any of the following conditions:
(a) External exposure to wet weather conditions.
(b) Internal exposure to water or any other fluids.
(c) Internal or external exposure to fire extinguishing agents.
(d) Internal or external exposure to electrolytes or corrosive deposits from batteries.
(2) Support Equipment and General Purpose Test Equipment Covers and Housings. The inspection, cleaning, corrosion removal, and preservation of support equipment and general purpose test equipment covers and housings should be performed as follows:
NOTE: Prior to cleaning covers and housings, remove the operator’s panel, electrical and electronic components, harnesses, and connectors.
(a) Inspect, remove corrosion, clean, treat, paint, and preserve in accordance with subparagraph 602b.
(b) Hardware associated with test equipment housings and covers that are not normally painted should be preserved in accordance with subparagraph 602e(4).
(c) Equipment covers with hinges and latches should be lubricated and preserved in accordance with subparagraphs 602k(6) and (7).
(3) ATE Cabinets, Doors, and Panels. Inspection, cleaning, corrosion removal, and preservation of ATE cabinets, doors, and panels should be performed as follows:
NOTE: The use of dissimilar metals in the selection of screws, washers, and nuts should be eliminated whenever possible. See Chapter 7 for information on bonding and grounding hardware.
(a) Inspect hardware and electrical bonding locations for signs of galvanic corrosion. Pay particular attention to dissimilar metal couples.
(b) Remove corrosion, clean, treat, and paint in accordance with subparagraph 602b.
(c) Hardware associated with ATE cabinets, doors, and panels that are not normally painted should be preserved as often as required in accordance with subparagraph 602e(4). Do not preserve water seal gaskets.
(d) Equipment covers with hinges and latches should be lubricated and preserved in accordance with subparagraphs 602k(6) and (7).
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(4) Battery Compartments. Some support equipment and general purpose test equipment contain internal batteries. Clean, neutralize, and preserve the battery compartment as follows:
(a) Clean and neutralize electrolyte spills in accordance with subparagraphs 602d(1) through (6).
(b) Inspect, clean, treat, and paint the battery compartment in accordance with Chapter 5, subparagraphs 504a through 504f.
(c) When painting of the battery compartment is not practical, temporarily preserve the area in accordance with subparagraph 602e(4).
(5) Meters. Voltmeters, ammeters, and multimeters are usually constructed of high-impact plastic or acrylic. Normally these meters are not disassembled for cleaning. Clean the exterior surfaces as follows:
(a) Clean metal hardware and metal surfaces with cotton swabs and a clean cloth or cheesecloth dampened with Isopropyl Alcohol, TT-I-735. Allow the surface to air dry.
(b) Apply a coating of Water-Displacing Corrosion Preventive Compound, conforming to MIL-DTL-85054, on metal hardware. Take the necessary steps to avoid getting preservative on acrylic plastic face plates.
(c) Clean toggle, rotary, and push-button switches in accordance with subparagraph 603g.
(d) Clean meter face plates in accordance with subparagraph 602g(9).
(e) Clean high-impact plastic or acrylic housings with a 1 ounce solution of nonionic liquid detergent, conforming to MIL-D-16791, Type I, in 1 gallon of water. Clean the surface with a clean cloth or cheesecloth wet with the detergent solution. When the surface is clean, remove the detergent solution by wiping the surface with a clean cloth or cheesecloth that has been dampened with clean water. Rinse the cloth frequently.
(6) Operator and Instrument Panels. Support equipment, general purpose test equipment, and ATE operator instrument panels should be inspected, cleaned, treated, painted, and preserved as follows:
(a) Visually inspect operator and instrument panels for corrosion and contaminants.
Pay particular attention to switches, dials, knobs, and hardware.
(b) Remove corrosion using an abrasive nylon mat, conforming to A-A-58054, Type I.
(c) Clean metal surfaces with cotton swabs and a clean cloth or cheesecloth dampened with Isopropyl Alcohol, TT-I-735. Allow the surface to air dry.