Par 405 Page 55 immersion or spray, fire extinguishing agents, spilled battery electrolytes, other acids, high pH alkaline cleaners, mercury, and corrosion products during component repair.
e. Precleaning Criteria and Cautions.
(1) Disconnect electrical and other power sources (mechanical/hydraulic).
(2) Ensure the work area and equipment are safe for maintenance.
(3) Ensure drain holes are open.
(4) Remove covers and panels for accessibility.
(5) Disassemble where practicable.
(6) Use only authorized materials.
(7) Ensure compatibility of materials prior to use.
(8) Mask and protect accessories or components to prevent intrusion of water, solvents, and cleaning solutions.
f. Cleaning and Drying Restrictions. Certain circuit components create potential problems during cleaning and drying. These problems can generally be overcome prior to cleaning the equipment by carefully masking those components likely to trap and hold cleaning liquids due to their construction. Table 4-3 lists different components and describes techniques to avoid problems. Mechanical, shock, and heat damage are other types of damage that can occur during cleaning and drying. The following suggestions are provided to avoid trapping water and solvent during cleaning:
(1) Seal small components with pressure-sensitive tape. Ensure tape and any tape residue are removed using an approved solvent prior to drying the component.
(2) Seal large components in static-free plastic bags or other water and vapor-proof barrier material. Place the plastic bag or barrier material around the component and seal with pressure-sensitive tape. Ensure tape and any tape residue are removed using an approved solvent prior to drying the component.
(3) When possible and if authorized, remove subassembly components and clean separately.
Par 405 Page 56 TABLE 4-3. CLEANING AND DRYING RESTRICTIONS Component Problem Solution APC connections (microwave) Shock damage to center conductor Seal and hand clean only Crystal detectors Heat damage from ovens Dry at 130 °F (54 °C) maximum Delay lines Trapped solution in housing Seal or remove Fan motors Trapped solution in housing Seal or remove Gyroscopes Trapped solution in housing Seal Klystron cavity Trapped solution in sockets Remove tube and seal Meters and instrument gauges Trapped solution through open housing Seal Paper capacitors Disintegrate Seal Potentiometers Trapped solution in open housing Seal Printed circuit Trapped solution (when installed) Remove (clean separately) Rotary switches Trapped solution in open housing Seal Sliding attenuators (radio frequency (RF)) Trapped solution in slide housing Seal or remove Sliding cam switches Shock damage to cam only Remove or hand clean Synchros and servos Lubricant removed from bearing Seal or remove Transformers Trapped solution in housing Seal Tunable cavities Trapped solution in cavity area Seal or remove Vacuum tubes Shock damage Remove Variable attenuators (microwave) Trapped solution in housing Seal or remove Wave guide (microwave) Trapped solution in housing (when installed) Seal or remove Wire wrap connections Shock damage Hand clean only
g. Hand Cleaning Methods. Hand methods should be used for cleaning small, delicate, confined surfaces where parts cannot tolerate other cleaning methods. Also, hand cleaning methods should be used when accessories/facilities for other methods are not available. The following list contains some of the equipment that may be utilized for hand cleaning:
(1) Lint-free cloth;
(2) Cheesecloth;
(3) Cotton-tipped applicator;
(4) Acid brush;
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(5) Toothbrush;
(6) Other soft bristle brushes; and
(7) Plastic manual spray bottle.
h. Fingerprint Removal. The salts and oils from human fingerprints are highly corrosive.
The following paragraphs list the material and procedures for their removal:
(1) Apply a mixture of 1 part Isopropyl Alcohol, TT-I-735, or other approved solvent and 1 part distilled water to the affected area with a clean cloth, cheesecloth, acid brush, or toothbrush as appropriate;
WARNING: Do not use a synthetic fiber wiping cloth with Isopropyl Alcohol, TT-I-735, due to its low flash point. A dry fiber wiping cloth will cause a static charge to build up and can result in a fire.
(2) Wipe or scrub affected area until contaminants have been removed;
(3) Remove residue by wiping or blotting with clean cheesecloth or cotton-tipped applicator as required. Inspect area for signs of residue or contaminants;
(4) Discard contaminated cheesecloth or cotton-tipped applicators and solvents in an approved container after cleaning operations to avoid contamination of other components; and
(5) Repeat process as required until all evidence of contamination is removed.
i. Cleaning and Removal of Solder Flux Residue. Solder flux residue is present in all soldering operations. This residue will cause corrosion if an electrolyte is present. Use of an approved cleaning solvent or diluted Isopropyl Alcohol, TT-I-735, is required. These solvents should not damage associated wiring circuit components or laminated circuit board coatings. The presence of soldering flux can be detected by using ultraviolet (UV) light. Under the presence of UV light, traces of solder flux appear as a fluorescent yellow to brownish residue. Clean solder flux residue as follows:
WARNING: Lead contained in solder can rub off onto a person’s fingers from a soldered joint. Lead and lead oxide are toxic and cannot be eliminated from the body. This toxic poison will accumulate in the body. Touching solder followed by smoking or eating is a potential means of ingesting trace amounts of lead. Wash hands thoroughly following any soldering or desoldering operation.
(1) Solder flux residues should be removed from circuit boards and circuit components using the general procedures described in subparagraph 405h. Use a solution of 1 part distilled water to 3 parts Isopropyl Alcohol, TT-I-735, to clean the affected area. After a soldering operation, clean the affected area and one inch around the solder point to ensure complete decontamination.
Par 405 Page 58 CAUTION: Prior to using the UV light for inspection to ensure complete removal of soldering flux residue, examine the piece of equipment for erasable programmable read-only memory (EPROM) components. EPROM have windows that are usually covered with an aluminum foil mask or black tape. Visually examine to ensure the tape or foil has not lifted from the window.
NOTE: UV light from an UV light inspection light source is very intense and can degrade EPROM devices.
(2) Visually inspect circuit boards and circuit components for evidence of soldering flux residue using an UV light source. Repeat cleaning process as required until all evidence of solder flux is removed.
j. Cleaning and Removal of Silicone Lubricant. Remove silicone residue from surfaces coated with silicone lubricant as follows:
(1) Wipe or scrub contaminated surface with a cleaning cloth, cheesecloth, acid brush, or cotton-tipped applicator dampened with an approved solvent or Naphtha, TT-N-95B, Type II, until surface is clean;
(2) While surface is still wet, wipe area dry with clean cheesecloth or cotton-tipped applicator as appropriate; and
(3) Discard contaminated cheesecloth, cotton-tipped applicators, or other cleaning devices, and solvents in an approved container after cleaning operations to avoid contamination of other components.
k. Cleaning and Removal of Bacteria and Fungi. Dust, dirt, and other airborne contaminants are leading contributors to bacterial and fungal (microbial) attack. The best defense against this form of attack is to maintain cleanliness and, where possible, low humidity. Remove fungi and bacteria as follows:
(1) Mask air capacitors, relay contacts, open switches, tunable coils, and other components affected by solvents with pressure-sensitive tape; and
(2) Treat affected areas with materials and instructions described in subparagraph 405h.
l. Cleaning and Removal of Dust. Dust, dirt, grease, and oil should be removed from components as follows: (Figures 4-2 and 4-3 show dirt accumulation in an engine compartment.)
(1) For dust and dirt, apply a solution of 1 part Aircraft Cleaning Compound, MIL-PRF-85570, Type II, to 9 parts of distilled water;
(2) For grease and oil, apply a solution of 1 part Aircraft Cleaning Compound, MIL-PRF-85570, Type II, to 4 parts of distilled water;
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(3) Wipe or scrub affected areas or components with cleaning cloth, cheesecloth, acid brush, toothbrush, or cotton-tipped applicator until surface is clean; and
(4) Rinse affected area or component with fresh water and dry with a clean cloth or cheesecloth.
FIGURE 4-2. ENGINE COMPARTMENT ELECTRICAL CONNECTORS
m. Sur face Preparation. The true cleanliness of bare metal surfaces after a cleaning process is very critical to adhesion of any subsequent coating material. Examples of subsequent coatings include: chemical conversion coating (prepaint), paint system, sealant, dry film lubrication, etc. The method generally used to identify a surface that is clean enough for adhesion of any of the subsequent coatings is called the Water Break Test. Figure 4-4 illustrates a water-break surface. Perform a Water Break Test as follows:
(1) Feather edges of any existing paint finish (if applicable) to ensure a smooth overlapping transition between the old and new paint coating. Feathering can be accomplished using 240-grit or 320-grit aluminum oxide abrasive cloth or a fine or medium grade abrasive mat.
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(2) Clean (lightly scrub) the area with very fine or fine abrasive mat saturated with water. Rinse surface with fresh water. Particular attention should be given to fasteners and other areas where residues may become entrapped.
(3) Visually inspect the part after the last rinse; the surface should be water-break free (see Figure 4-4). A surface showing water breaks (water beading or incomplete wetting) is contaminated, usually with grease or oil (fingerprints, etc.). The contaminated surface will not allow proper adhesion with any subsequent protective coating system (conversion coating, primer, topcoat).
(4) If the surface is not water-break free, clean area with a solution of 1 part Aircraft Cleaning Compound, MIL-PRF-85570, Type II, to 9 parts distilled water. Lightly scrub the area with very fine or fine abrasive mat saturated with cleaning solution. Rinse surface with distilled water. Visually inspect the part after the last rinse. The surface should be water-break free.
FIGURE 4-3. ENGINE COMPARTMENT ELECTRICAL CONNECTORS
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n. Special Considerations. Dust, fingerprints, surface oxides, and other contaminants can undo the protection provided by subsequent protective coatings. Specific avionics equipment and components should be cleaned by procedures addressed for the specific equipment in Chapter 5.
o. Post-Cleaning Procedures.
(1) After completion of the cleaning steps, inspect the affected area for signs of residue, surface film, or water.
(2) If inspection reveals the area is not clean, (residue, surface film), repeat the appropriate cleaning procedures. Water displacement, preservation, and lubrication should follow the cleaning and drying steps for completing the preventive maintenance.
406. DRYING EQUIPMENT AND PROCEDURES.
a. General. Drying time depends on the complexity of the equipment/component being dried. The more complex the individual component, the longer the drying time. Another consideration in drying time is the humidity or moisture content of the air where the drying oven is operated. The higher the moisture content of the ambient air, the longer the drying time.
FIGURE 4-4. A WATER-BREAK-FREE SURFACE COMPARED WITH ONE WITH
WATER BREAKS
Par 406 Page 62 CAUTION: Portable air blowers, hot air blowers, hair dryers, and similar drying devices may cause fires when used in or around aircraft. Use only authorized (spark proof) hot air guns in and around aircraft. The motion of air from an aerosol spray, compressed air, and air from dryers can generate static charges that can degrade or destroy ESD-sensitive devices. Care must be exercised during handling, cleaning, and repair of these items. See Chapter 9 for recommended shop practices.
b. Drying Preparation. Prior to placing a component in a drying oven, remove all covers, lids, etc. Ensure any pressure-sensitive tape and protective plastic bags used during the cleaning cycle have been removed.
c. Air Drying. Air drying is usually adequate for housings, covers, and some hardware.
This method is not considered adequate for more complex equipment or components that may contain cavities or moisture traps.
d. Drying with Hot Air Blower. Procedures for the use of hot air blowers are as follows:
(1) Blow off excess water with dry air or dry nitrogen at not more than 10 pounds per square inch (psi) pressure. Deflect air off interior back and sides of enclosure to diffuse air jet.
(2) Dry the equipment with a hot air gun. Surfaces should not be heated with the air gun above 130 °F (54 °C) when drying the equipment.
e. Drying with Circulating Air Oven. The circulating air drying oven is used to dry small electrical and electronic components, such as non-pressurized instruments, control boxes, PCBs, and similar devices. The circulating air drying oven should not be operated above 130 °F (54 °C) when drying avionics equipment or components. Damage may result from overheating of discrete electronic circuits components. Procedures for the operation of the circulating air drying ovens are as follows:
(1) Blow off excess water with dry air or dry nitrogen at not more than 10 psi pressure.
Deflect air off interior back and sides of enclosure to diffuse air jet;
CAUTION: Older circulating air drying ovens may have uncalibrated dials or controls for setting the oven temperature. These uncalibrated ovens should have the dials calibrated, prior to use, by placing a red line on the dial so as not to exceed 130 °F (54 °C).
(2) Set the temperature control at a maximum of 130 °F (54 °C);
(3) Place the component(s) in the oven and close the door. If a timer is available, set the time for approximately 3 to 4 hours; and NOTE: Opening and closing the oven door during the drying cycle will increase the drying time. This is due to the diffusion of hot, dry air in the oven cabinet with cooler, more humid air from the surrounding area.
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(4) Upon completion of the drying cycle, remove the component(s).
f. Drying with Forced Air Oven. The forced air drying oven is the most efficient of the drying ovens. This unit can be used to dry all types and sizes of equipment and components. The procedures for the operation of the forced air drying oven are as follows:
(1) Blow off excess water with dry air or dry nitrogen at not more than 10 psi pressure.
Deflect air off interior back and sides of enclosure to diffuse air jet;
CAUTION: Check the temperature dial (or control calibration) periodically to ensure the temperature setting is correctly calibrated.
(2) Set the temperature control at a maximum of 130 °F (54 °C);
(3) Place the component(s) in the oven and close the door. If a timer is available, set the time for approximately 1 to 2 hours. Opening and closing the oven door during the drying cycle will increase the drying time slightly but not appreciably. This is considered one of the advantages of the forced air drying oven over the circulating air type oven; and
(4) Upon completion of the drying cycle, remove the component(s).
g. Drying with Vented Oven (Bulb-type). Vented drying oven procedures are as follows:
(1) Blow off excess water with dry air or dry nitrogen at not more than 10 psi pressure.
Deflect air off interior back and sides of enclosure to diffuse air jet;
(2) Place the component(s) in the oven and close the door. Dry the component or equipment at a maximum of 130 °F (54 °C). If a timer is available, set the time for approximately 3 to 4 hours; and
(3) Upon completion of the drying cycle, remove the component(s).
h. Drying with Vacuum Oven. Vacuum oven drying procedures are as follows:
(1) Blow off excess water with dry air or dry nitrogen at not more than 10 psi pressure.
Deflect air off interior back and sides of enclosure to diffuse air jet;
(2) Place the component(s) in the oven and close the door. Dry the equipment at approximately 130 °F (54 °C) and 26 inches of mercury (Hg). If a timer is available, set the time for approximately 1 to 2 hours; and
(3) Upon completion of the drying cycle, remove the component(s).
i. Solvent Vapor Drying. Solvent vapor drying is an additional feature of the solvent ultrasonic cleaner. This method of drying is considered the fastest and most efficient method of drying avionics components. The drying time is usually 15 seconds to 3 minutes. Follow the
Par 406 Page 64 equipment manufacturer’s operating procedure for cleaning and degreasing. The procedures for solvent vapor drying are as follows:
CAUTION: Environmental regulations may prohibit the use of the ozone depleting CFCs solvents used in solvent ultrasonic cleaners.
(1) Suspend the component being processed in the solvent vapor cloud for a period of 15 seconds to 3 minutes. Drying time depends on the complexity of the component and the amount of water or solvent present;
(2) Rinse by altering the position of the component in the vapor cloud to drain any trapped water or solvent;
(3) After thorough rinsing, slowly raise (withdraw) the component out of the vapor cloud. This will cause rapid drying of the component; and
(4) Inspect the component for visible signs of water or solvent. If required, repeat the solvent vapor drying process.
407. PRESERVATION.
a. General. Surfaces and components not normally conformal coated or painted need preservation. Cleanliness and elimination of moisture are keys to avoiding corrosion.
Preservation of equipment is essential, since it is nearly impossible to guarantee a dry, moisture-free environment. In today’s avionics systems, miniaturization has resulted in very small electronic circuits. Even a small amount of corrosion can cause the entire system to fail.
Preservation has become an essential part of the repair and maintenance of avionics systems.
b. Why Preserve.
(1) To protect nonmoving parts by filling air spaces, displacing water, and providing coatings;
(2) To protect components such as hinges, control cables, gears, linkages, bearings, etc., from wear by providing lubrication; and
(3) To protect nonoperational or idle equipment.
c. When to Preserve.
(1) After cleaning;
(2) On equipment prior to shipment;
(3) On equipment or components that are not operational, including those awaiting parts and repair;
(4) On parts or components that are normally inaccessible for inspection without disassembly;
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(5) Where additional protection is required for other coating systems;
(6) After immersion and exposure to fresh and saltwater or fire extinguishing agents;
and
(7) When other corrosion protection systems have failed because of in-service use.
d. What to Preserve. Preservatives should be used only where their application and maintenance will not hamper electrical circuits or component operation. Most preservatives form a nonconductive film that acts to insulate mating surfaces from moisture intrusion. The following provides a list of avionics equipment and components that may require preservation and maintenance:
(1) Door latches and hinges;
(2) Electrical connectors (internal and external);
(3) Shock mounts, rigid mounts, and associated attaching hardware and brackets;
(4) Any dissimilar metals not protected by other coating systems;
(5) Antenna mounts, brackets, hardware, and housings;
(6) Fasteners, screws, nuts, and bolts not otherwise covered elsewhere;
(7) Equipment lids on interior or exterior of equipment that is susceptible to moisture;
(8) Solder joints not otherwise protected by other coatings;
(9) Any unprotected surface that will not receive a paint coating system or other coating material (plating or conformal coatings);
(10) External and internal surfaces of coaxial connectors;
(11) External surfaces of cooling system joints; and
(12) Ground straps and wires.
e. What Not to Preserve. The following items should not be preserved or come in contact with preservatives:
(1) Laminated circuit boards that are conformal coated;
(2) Nonmetallic surfaces such as acrylic control box face plates;
(3) Tunable capacitors and inductors;
(4) Internal surfaces of wave guides;
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(5) Internal surfaces of tuned tanks;
(6) Relay and circuit breaker contacts; and
(7) Fuses.
f. Preservative Materials. Preservatives are materials that can take the place of more permanent coating materials such as paint, but require removal and repeated application on a scheduled basis. Some preservatives may also act as water-displacing compounds. Table 4-4 contains a list of preservatives and water-displacing compounds for use on avionics components.
Preservatives must be applied over water-displacing, corrosion preventive compound, MIL-PRF-81309, Type II, to accomplish a complete water-displacing and preservative system on all areas exposed to elements and moisture.
g. How to Preserve. The application of preservatives for specific avionics components is covered in Chapter 5. The following are general application procedures that apply in most cases:
(1) Ensure surface to be preserved is free of dirt, grime, other contaminants, standing water, and corrosion products. Remove corrosion products as outlined in Chapter 5. Clean and dry the surface as appropriate with a cleaner and dryer as outlined earlier in this chapter;
(2) When necessary, apply pressure-sensitive tape to protect areas and components not to be preserved. Subparagraph 407e refers to areas and components not to be preserved;
(3) Water-displacing preservative compounds that conform to MIL-PRF-81309, Type III, (Avionics), MIL-PRF-81309, Type II, and MIL-DTL-85054 (AMLGUARD) should be applied with an even, thin film to the surface. Ensure complete coverage of dissimilar metal contact areas, crevices, and water entrapments areas. Avoid excessive application. When MIL-DTL-85054 is used, apply a second coat after 30 minutes;
(4) Non-water-displacing preservative compound that conforms to MIL-PRF-16173, Grade 4 should receive a film of water-displacing preservative compound as described in subparagraph 407g(3) to the part or component prior to the application of MIL-PRF-16173, Grade 4, to remove any moisture. MIL-PRF-16173, Grade 4, should be applied with a brush or sprayed in a thin, even film to nonmoving external areas. Fasteners may be dipped in MIL-PRF-16173, Grade 4, prior to installation; and NOTE: MIL-PRF-16173, Grade 4, may be thinned for spraying as required with an approved solvent or a general purpose oil.
(5) Remove pressure-sensitive tape as required.