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(2) If no color change is observed at the wet-slug tantalum capacitor, remove the indicator solution and any residue with Isopropyl Alcohol, TT-I-735, and an acid brush or a dampened clean cloth. Dry and preserve in accordance with subparagraphs 603i(5) and (7).
(3) If an acid leak has occurred from a wet-slug tantalum capacitor, the solution will change from an amber or blue color to a reddish-purple color. Remove the damaged capacitor and neutralize the contaminated area by applying the following solution:
(a) Dissolve 8 ounces (1 cup) of sodium bicarbonate, conforming to O-S-576, in 1 gallon of fresh water.
(b) Apply the sodium bicarbonate solution to the affected area using an acid brush.
(c) Thoroughly rinse the affected area with deionized or distilled water. Ensure that the rinse water does not contaminate other areas.
(d) Apply another drop of the Thymol Blue Reagent indicator solution to the affected area. Inspect for the reddish-purple color. Repeat steps (b) and (c) above until no color change occurs.
(4) Clean the affected area by scrubbing with a nonabrasive pad. Thoroughly rinse the area with Isopropyl Alcohol, TT-I-735. Wipe the area with a clean cloth or cheesecloth.
(5) Air dry the component.
(6) Install the replacement capacitor.
(7) Preserve the area as follows:
(a) Spray a thin coating of Water-Displacing Corrosion Preventive Compound, conforming to MIL-PRF-81309, Type III, on the capacitor and affected area.
(b) Wipe off excess preservative with a clean cloth or cheesecloth.
(8) Replace the conformal coating in accordance with the OEM’s maintenance instructions or Chapter 5, subparagraph 505e.
j. Aluminum Electrolytic Capacitors. Aluminum electrolytic capacitors that utilize synthetic rubber seals or rubber/plastic combination seals are susceptible to damage from cleaning solutions and processes. Such damage will often result in an acid leak which may cause corrosion in areas adjacent to the damaged capacitor. Capacitors having evidence of leakage should be replaced and all adjacent areas cleaned to prevent further corrosion. The following procedures should apply for the inspection and replacement of aluminum electrolytic capacitors:
(1) Inspect the end seals for deterioration. If the seal looks bulged or uneven, the capacitor should be replaced and all adjacent areas cleaned.
Par 603 Page 139
(2) Clean the affected area by scrubbing with a nonabrasive pad. Thoroughly rinse the area with Isopropyl Alcohol, TT-I-735. Wipe the area with a clean cloth or cheesecloth.
(3) Air dry the component.
(4) Install the replacement capacitor.
(5) Preserve the area as follows:
(a) Spray a thin coating of Water-Displacing Corrosion Preventive Compound, conforming to MIL-PRF-81309, Type III, on the capacitor and affected area.
(b) Wipe off excess preservative with a clean cloth or cheesecloth.
(6) Replace the conformal coating in accordance with the OEM’s maintenance instructions or Chapter 5, subparagraph 505e.
k. Multipin Electrical Connectors Cleaning and Preservation.
(1) General. Multipin electrical connectors require special attention to prevent corrosion and electrical failures, especially when the connectors are in areas that are exposed to harsh environments. The following techniques and associated warnings, cautions, and notes will assist in limiting the corrosion attack.
WARNING: Ensure that all electrical and hydraulic power is removed from the aircraft or component. Install the applicable safety devices. Disconnect all batteries.
CAUTION: Cleaning compounds and solvents identified in Chapter 4 and Appendix 1 may react with some encapsulants or plastics used to form wire harness tubing, wire coatings, conformal coating, gaskets, seals, etc. Test on a small area for softening or other adverse reaction prior to general application. See Chapter 4, Table 4-3 for further restrictions on these materials.
NOTE: Local air pollution regulations may restrict the use of these solvents.
Comply with all local air pollution regulations. A continuity test does not preclude a visual inspection of connectors, because corrosion can still occur outside of pin areas.
(a) Protect open connectors with conductive plastic or metal caps.
Pressure-sensitive tape, conforming to MIL-T-22085, Type II, and as specified in Chapter 4, subparagraph 409e, is an alternate method of sealing connectors if proper caps are not available.
Par 603 Page 140 FIGURE 6-14. MULTIPIN CONNECTOR WITH “DOG BONES” INSTALLED (
b) If connector boots are installed and water intrusion cannot be prevented due to design, a small drain hole (1/4 inch minimum, 3/8 inch maximum) may be incorporated at the lowest point on the connector boot to allow water to drain. This action requires approval of the OEM.
(c) Special attention should be given to connectors using replaceable pins. These connectors use a self-sealing gasket that seals the connector against water intrusion. “Dog bones” (plastic inserts) are used to fill unused connector (pin) cavities. Figure 6-14 shows “dog bones” installed in a multipin connector. The self-sealing gasket may lose its effectiveness to seal against water intrusion with repeated removal and replacement of connector pins or omission of the “dog bones.” The use of potting compounds mentioned in Chapter 5, subparagraph 505b may be required to prevent water intrusion in extreme cases where the connector cannot be replaced.
(d) Connectors that are susceptible to the same environment as the aircraft wire harness connectors should be treated with the same corrosion removal and preservation techniques. Mounting plates normally contain a gasket that acts as a watertight seal. These gaskets should be inspected each time a connector is dismantled for cleaning or repair.
(2) External Corrosion Inspection, Removal, and Cleaning. Inspection, removal, and cleaning of corrosion on the exterior of electrical connectors (see Figures 6-15 and 6-16) should be performed as follows:
Par 603 Page 141
(a) Disassemble the connector backshell, if possible, and visually inspect all parts for evidence of corrosion. Extensive corrosion damage may require the replacement of the connector.
(b) Remove corrosion by scrubbing with a nonabrasive pad or an abrasive nylon mat, conforming to A-A-58054, Type I, as appropriate. Ensure connector mating surface threads, shell, and mounting plate (if used) are cleaned.
(c) Wipe off residue with a clean cloth or cheesecloth.
(d) Apply Isopropyl Alcohol, TT-I-735, with a typewriter brush or toothbrush.
Scrub connector mating areas, threads, shell, and mounting plates.
(e) Remove solvent and residue with a clean cloth or cheesecloth. Allow all parts to air dry.
FIGURE 6-15. EXTERNAL MULTIPIN CONNECTOR CORROSION
Par 603 Page 142
(3) Internal Corrosion Inspection, Removal, and Cleaning. Inspection, removal, and cleaning of corrosion on the interior of electrical connectors should be as follows:
NOTE: On most connectors it is difficult to clean and remove corrosion from the receptacle (female) contacts. If corrosion is noted, the most practical solution is to replace the pin.
(a) Visually inspect all areas of the connector for evidence of corrosion. Extensive corrosion damage may require the replacement of the pin(s) or the connector.
(b) Clean internal areas of the connector, wiring, and pins with Isopropyl Alcohol, TT-I-735, and an acid brush.
(c) Wipe excess solvent and residue with a clean cloth or cheesecloth. Use a pipe cleaner, as required, to remove solvent from the pin area.
FIGURE 6-16. EXTERNAL MULTIPIN CONNECTOR CORROSION
Par 603 Page 143
(4) Sealing Connector Backshell. Moisture intrusion into a connector often occurs by way of the backshell. This problem is particularly acute after damage to the seal occurs during pin replacement. The backshell may be sealed as follows:
(a) Verify that sealing plugs (“dog bones”) are installed in unused contact cavities.
(b) Remove the retainer ring and Mylar tape (if present) from the back of the electrical connector. Slide the backshell and retainer ring in back of the electrical connector up the electrical wire bundle (see Figure 6-17).
(c) Tie back shielded wire pigtails, where applicable.
(d) Apply RTV sealing compound, conforming to MIL-A-46146, by inserting the sealant applicator nozzle approximately halfway into the wire bundle at the back of the connector (see Figure 6-17). Inject RTV sealing compound by squeezing the applicator tube while slowly withdrawing the nozzle from the wire bundle at the back of the connector. Repeat as many times as necessary at different locations around the connector to achieve a sealing compound thickness of 1/16 inch across the entire rear face of the connector. Position the connector in the vertical position until the sealing compound sets. The sealing compound will self-level in approximately 15 minutes.
FIGURE 6-17. CONNECTOR SEALING PROCEDURE Procedures:
1. Remove retainer ring.
2. Remove Mylar tape.
3. Peel back the shielded pigtails.
4. Remove the backshell to expose wiring.
5. Seal between wires.
Par 603 Page 144 NOTE: For those connectors exposed to fluids (dielectric, coolant, turbine oil, etc.) that adversely affect RTV sealant, conforming to MIL-A-46146, apply Sealing Compound, Synthetic Rubber, Accelerated, conforming to MIL-PRF-8516, to the connector backshell and wire bundle where temperatures will not exceed 250 °F (121 °C).
(e) After the connector has self-leveled for the required time, visually inspect the rear of the connector to ensure a complete seal. If areas are void of sealing compound, add additional sealing compound to entirely seal the back of the connector. The maximum depth of the sealing compound should not exceed 1/8 inch. The connector should be kept in the vertical position for a minimum of 30 minutes as the sealing compound cures to the tack-free condition.
After the initial 30-minute curing time, the connector may be moved around as required. Curing of the sealing compound will continue for approximately 24 hours.
(f) If a subsequent repair action requires the replacement of a contact (pin), inject a small amount of sealing compound around the replacement contact to restore the watertight seal.
Position the electrical connector in the vertical position for 30 minutes to allow the sealing compound to self-level and cure to a stable condition.
(g) Connectors that are exposed to severe environmental conditions, such as externally mounted connectors and those in wheel wells, bilges, etc., should be taped using an electrical insulating tape. After wrapping the connector and wire bundle with electrical tape, RTV sealing compound, conforming to MIL-A-46146, should be brushed over the tape.
(5) Water-Displacement and Treatment. After corrosion removal and cleaning, or any time connectors, plugs, or receptacles are disconnected for maintenance, treat as follows:
(a) Apply Isopropyl Alcohol, TT-I-735, liberally to internal and external sections of male and female connectors using an acid brush. Mate and unmate connectors several times to clean. Thoroughly rinse the connector with Isopropyl Alcohol, TT-I-735. Shake out excess solvent and wipe the connector with a clean cloth or cheesecloth. Allow the connector to air dry.
(b) Spray a thin coating of Water-Displacing Corrosion Preventive Compound, conforming to MIL-PRF-81309, Type III, on the internal sections of the connectors, plugs, and receptacles. Avoid excessive application or overspray of preservative.
(c) If possible, tilt or rotate the connector down and around to drain excess preservative. Wipe off any additional preservative with a clean cloth or cheesecloth.
(d) Prior to connecting the threaded sections of the connector, plug, or receptacle backshell, treat threaded areas with Water-Displacing Corrosion Preventive Compound, conforming to MIL-PRF-81309, Type III.
(e) Mate connector sections. Wipe off excessive preservative with a clean cloth or cheesecloth.
(f) Apply a film of Water-Displacing Corrosion Preventive Compound, conforming to MIL-DTL-85054, to the exterior surfaces of connector plugs and receptacles.
Par 603 Page 145 CAUTION: Corrosion Preventive Compound, MIL-DTL-85054, cures to a hard, permanent finish. Once cured, it is difficult to remove with Isopropyl Alcohol, TT-I-735, or other approved solvents. Repeated application of MIL-DTL-85054 should be avoided. For connectors requiring frequent disconnecting and connecting, use Corrosion Preventive Compound, MIL-PRF-81309, Type III, in lieu of MIL-DTL-85054.
l. Coaxial Connectors. Coaxial connectors require special steps in order to avoid water intrusion. In most cases, water intrusion in the fuel/oil quantity indicator and similar capacitance type indicating system connectors will cause erroneous quantity indications on cockpit instruments. Antenna coaxial connectors can generate similar erroneous signals when water intrusion in those connectors occurs. Coaxial connectors should be inspected, cleaned, and treated in accordance with subparagraphs 603k(2), (3), and (5).
m. Wire Harnesses and Cables. When corrosion is found at the pin-to-wire connection on electrical connectors, plugs, and receptacles, the wire harnesses and cables should be inspected for corrosion, cracking, and other damage to the wire insulation. Wire harnesses and cables should be inspected, cleaned, and treated as follows:
(1) If corrosion is apparent at the back of the connector, it may be necessary to remove an inch or two of the wire harness cable cover to inspect for corrosion.
(2) Visually inspect the rear of the connector at the wire-to-pin connection for corrosion damage. Inspect the wire harness cover for tears and separations, and wiring and wiring insulation for cracks, corrosion, and burn indications.
(3) Apply Isopropyl Alcohol, TT-I-735, with a typewriter brush or toothbrush. Scrub the affected area until contaminants are loosened. Reapply Isopropyl Alcohol, TT-I-735, as required, to flush the area of the contaminants.
(4) Shake excess solvent from the wire harness and wipe dry with a clean cloth or cheesecloth.
(5) Where applicable, repair the wire harness and cable covering.
(6) Treat, as required, connecting electrical connectors and plugs in accordance with subparagraph 603k.
n. PCBs. Edge connectors (and mating plugs) used in miniature and microminiature PCBs should be cleaned and preserved in accordance with subparagraph 603h.
o. Filters. The cleaning of air filters is essential to maintaining the cleanliness and reliability of avionics and test equipment. The frequency of cleaning and method used will normally be specified in the OEM’s maintenance instruction manual. The frequency of cleaning may need to be increased if local environmental conditions dictate. Filters may be cleaned as follows:
Par 603 Page 146
(1) Place the filter in a deep sink and flush thoroughly with fresh water.
(2) Scrub the rigid or metal filters with a cleaning brush to remove dirt, grime, and lint.
(3) If oil or grease is present in the filter, clean the filter with a solution of 9 parts fresh water to 1 part Cleaning Compound, conforming to MIL-PRF-85570, Type II. Scrub the rigid or metal filters with a cleaning brush. Rinse thoroughly with fresh water.
(4) Blow off excess water with dry air or dry nitrogen at not more than 10 pounds per square inch (psi) pressure.
(5) Allow filter to air dry.
(6) Do not preserve filters.
p. Sensitive Internal Metal Surfaces. Metal surfaces such as resonant cavities, tube covers, and other delicate metal components should be visually inspected for signs of corrosion.
Remove corrosion, clean, and preserve as follows:
(1) Remove dirt and contaminants with a nonabrasive cleaning cloth. Scrub the affected area until all contaminants are dislodged.
(2) Remove corrosion and tarnish with a typewriter eraser or a pencil eraser. Care should be taken not to remove the thin plating from the surfaces.
(3) Clean the part of residue with Isopropyl Alcohol, TT-I-735. Apply with an acid brush or a typewriter brush.
(4) Remove residue with a clean cloth.
(5) Rinse the affected area with Isopropyl Alcohol, TT-I-735, and wipe dry with a clean cloth. This step will assist in removing any water.
(6) Air dry or dry with a hot air gun as described in Chapter 4, subparagraph 406d.
CAUTION: Exercise care when using a hot air gun near plastic materials.
Excessive heat may decompose the plastic and change its electrical characteristics.
(7) Preserve where circuit function will not be affected as follows:
(a) Spray a thin coating of Water-Displacing Corrosion Preventive Compound, conforming to MIL-PRF-81309, Type III, on all metal surfaces. Avoid excessive application or overspray of preservative.
(b) Remove excess preservative by wiping with a clean cloth.
q. Static Discharge Wicks. Corrosion, deterioration, or structural damage to the static discharge wicks can result in poor performance from aircraft radios and communication systems,
Par 603 Page 147 erratic operation of instruments, and potential electrical shock to personnel. When damaged or corroded static discharge wicks are found, replace as follows:
(1) Remove and discard old static discharge wicks.
(2) Remove corrosion and contaminants from the mounting area with a nonabrasive pad. Scrub the affected area until all corrosion and contaminants are loosened.
(3) Clean the mounting area with Isopropyl Alcohol, TT-I-735, and an acid brush.
(4) Rinse the mounting area with Isopropyl Alcohol, TT-I-735, to flush out remaining residue.
(5) Wipe dry with a clean cloth. Allow the area to air dry.
(6) Chemically treat the mounting surface as described in Chapter 5, paragraph 503.
(7) Install the replacement static discharge wicks in accordance with the OEM’s maintenance instructions.
(8) Spray a thin coating of Water-Displacing Corrosion Preventive Compound, conforming to MIL-DTL-85054, on all metal surfaces and attachment points. Avoid excessive application or overspray of preservative.
604 through 700. RESERVED.
Par 701 Page 148 CHAPTER 7. CORROSION CONTROL MEASURES FOR ELECTRICAL
BONDING/GROUNDING
701. ELECTRICAL BONDING/GROUND CONNECTIONS.
a. General. Electrical bonding provides a low resistance electrical path between two or more conductive units or components. Grounding is a form of bonding that utilizes the primary structure as a portion (return path) of the electrical circuit. Bonding may serve as one or all of the following functions:
(1) Provide a common ground for the proper electrical functioning of the units involved;
(2) Provide a path to minimize lightning strike damage;
(3) Prevent the buildup of static potentials that could result in a spark discharge;
(4) Minimize static and stray currents in units involved;
(5) Prevent a unit from emitting electromagnetic energy that would interfere with itself or other units (see Chapter 8); and
(6) Shield equipment from outside electromagnetic interference (EMI) sources.
b. Bimetallic Junctions. The connection of two or more diverse electrical objects often results in a bimetallic junction that is susceptible to galvanic corrosion. This type of corrosion can rapidly destroy a bonding connection through physical corrosion damage and the loss of the low resistance electrical path if suitable precautions are not observed. Refer to AC 43.13-1, Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair. Aluminum alloy jumpers (bonding straps) are used in many bonding situations. Copper, tin-plated copper, and stainless-steel jumpers are most often used to bond together aircraft and components parts made of stainless steel, cadmium-plated steel, aluminum, brass, or other metals. Where contact between dissimilar metals cannot be avoided, the choice of bonding material and associated attach hardware is important. When selecting materials for the bonding installation, the material(s) that is the most prone to corrosion (anode) should be the easiest and least expensive to replace. At bimetallic junctions, where finishes are removed to provide a good electrical connection, a preservative or sealant should be applied to the completed connection to prevent corrosion. This chapter describes and illustrates the procedures for the assembly and preservation of bonding or grounding connections. This includes special emphasis on techniques to minimize galvanic corrosion.
c. Hardware Selection. When repairing or replacing existing bonding or grounding connections, follow the Original Equipment Manufacturer’s (OEM) parts and maintenance instructions or use the same kind of bonding material and associated attach hardware as the original installation. The bonding material and associated attach hardware have been selected by the OEM for their mechanical strength, electrical requirements, corrosion resistance, and ease of installation. However, when a bonding or grounding connection installation displays evidence of galvanic corrosion after proper assembly, the installation of a sacrificial washer made of an
Par 701 Page 149 anoxic material between the dissimilar materials will allow that anoxic washer to corrode.
Replacement of the corroded washer is easy and the least expensive way of repairing the bonding or grounding connection. Figures 7-1 through 7-6 and their corresponding tables show proper assembly configurations and list hardware and materials in the order of assembly depending on the particular metal(s) of the structural and bonding or grounding connection. For example, a proper installation for the aluminum terminal and jumper configuration in Figure 7-1 shows a bolt secured as a mounting stud for a bonding or grounding connection through a flat structural surface. The structure in this case is an aluminum alloy and the bonding or grounding jumper, as mentioned, is also aluminum. The attaching hardware are cadmium-plated steel bolts or screws, aluminum washers, and cadmium-plated lockwashers and nuts.
702. BONDING/GROUNDING SURFACE PREPARATION.
a. Surface Preparation. Procedures for the preparation of metallic surfaces before mating electrical conductor(s) are as follows:
(1) Remove all dirt, oil, grease, and other contaminants from an area slightly larger than the bonding or grounding connection. The area to be cleaned should be a minimum of 1 1/2 diameters the size of the bonding or grounding connection. Use a clean cloth or cheesecloth dampened with an approved solvent or Dry Cleaning Solvent, MIL-PRF-680, Type II.
NOTE: Local air pollution regulations may restrict the use of this solvent.
Comply with all local air pollution regulations.
(2) If more vigorous contaminant removal is required, scrub with an acid brush and an approved solvent or Dry Cleaning Solvent, MIL-PRF-680, Type II.
(3) Wipe area dry with a clean cloth or cheesecloth.
(4) Remove, as required, paint, anoxic coating or conversion coating film, and any corrosion from the attachment area using an abrasive nylon mat, conforming to A-A-58054, Type I. Do not exceed the maximum depth allowed by the OEM maintenance instructions.
(5) Wipe area clean with a clean cloth or cheesecloth dampened with Isopropyl Alcohol, TT-I-735. Allow area to air dry.
(6) Apply chemical conversion coat to aluminum or magnesium as described in Chapter 5, paragraph 503.
(7) Remove all dirt, oil, grease, and other contaminants from the bonding cable terminal with a clean cloth or cheesecloth dampened with Isopropyl Alcohol, TT-I-735.
(8) If more vigorous contaminant removal is required, scrub with an acid brush and Isopropyl Alcohol, TT-I-735.
(9) Assemble bonding or grounding connection(s) and torque in accordance with the OEM’s maintenance instructions, or use Figures 7-1 through 7-6 as a guide.