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.
Par 406 Page 63
(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;
Par 407 Page 65
(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;
Par 407 Page 66
(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.
Par 407 Page 67 TABLE 4-4. PRESERVATIVE COMPOUNDS FOR AVIONICS EQUIPMENT Description Characteristics Application Restrictions Corrosion Preventive Compound, Water-Displacing, Ultra-Thin Film, MIL-PRF-81309, Type III, avionics Grade 2.
General preservative for avionics equipment, internal areas of electrical connectors, and solder receptacles and joints. Contains water-displacing properties.
Apply by spraying an even, thin film to the surface. Can be removed with an approved solvent or Dry Cleaning Solvent, MIL-PRF-680, Type II.
Not intended for use on exterior of avionics equipment. Deposits a thin film which must be removed for proper function of contacts points and other electromechanical devices where no slipping or wiping action is involved. Do not use around oxygen, oxygen fittings, or oxygen regulators, since fire or explosion may result.
Corrosion Preventive Compound, Water-Displacing, Ultra-Thin Film, MIL-PRF-81309, Type II.
General preservative for internal and external areas of chassis, equipment covers, hardware, mounting brackets, latches, hinges, terminal boards, busbars, ground straps, and internal/external areas of junction boxes.
Apply by spraying an even, thin film to the surface. Can be removed with an approved solvent or Dry Cleaning Solvent, MIL-PRF-680, Type II.
Do not use on interior surfaces of electrical connectors and receptacles.
Do not use on interior surfaces of coaxial connectors. Do not use around oxygen, oxygen fittings, or oxygen regulators, since fire or explosion may result.
Corrosion Preventive Compound, Water-Displacing, Clear, MIL-DTL-85054.
General preservative for external surfaces exposed to elements and moisture, including: chassis, equipment covers, mounting racks, equipment racks, shelving, brackets, radar plumbing, antenna hardware, latches, terminal boards, busbars, ground straps, junction boxes, fasteners, external connectors, coaxial connectors, and receptacles.
Apply by spraying an even thin film or brush onto the surface. Material presents a thin, non-tacky, clear film.
Can be removed with an approved solvent or dry cleaning solvent, MIL-PRF-680, Type II, Isopropyl Alcohol, TT-I- 735, or by applying Corrosion Preventive Water-Displacing Compound, MIL-DTL-85054, and wiping.
Do not use on interior surfaces of electrical equipment and hardware.
Do not use on moving/sliding surfaces. Do not use on interior surfaces of electrical connectors, coaxial connectors, or receptacles. Do not use around oxygen, oxygen fittings, or oxygen regulators, since fire or explosion may result.
Corrosion Preventive Compound, Solvent Cutback, Cold Application, MIL-PRF-16173, Grade 4.
General preservative for external surfaces exposed to elements and moisture, including: mounting racks, shelving, brackets, radar plumbing, shock mounts, rigid mounts, antenna hardware, general hardware, hinges, fasteners, ground straps, and exterior surfaces of electrical connectors, coaxial connectors, and receptacles.
Apply by brush or spraying an even, thin film to the surface.
Material presents a semi-transparent film.
Can be removed with an approved solvent or Dry Cleaning Solvent, MIL-PRF-680, Type II.
Do not use on interior surfaces of avionics equipment. Do not use on interior of electrical connectors, coaxial connectors, or receptacles.
Do not use around oxygen, or oxygen regulators, since fire or explosion may result.
Par 408 Page 68
408. LUBRICANTS.
a. General. Lubrication of equipment performs several important functions: it prevents wear between moving parts, provides a barrier to corrosion, and is a short-term preservative.
Particular attention should be given to lubrication points around hinges, latches, etc., for prevention of lubrication breakdown. Lubrication breakdown includes caking of the lubricant, an indicator of contamination.
b. Requirements. Maintenance personnel should refer to the applicable maintenance manual for specific lubrication requirements, materials, and frequency.
409. PACKAGING, HANDLING, AND STORAGE.
a. General. An avionics corrosion control program must include procedures for packaging, handling, and storage of avionics equipment and components. These components will be rendered useless if the packaging, handling, and storage procedures are not followed.
Materials used for the packaging, handling, and storage must be compatible with the avionics equipment and the anticipated environment. Figures 4-5 and 4-6 show typical storage and shipping containers. These containers are not intended for storage in the elements.
b. Packaging and Storage Materials Guidelines.
(1) Certain packaging materials containing wood, cotton, foam, and paper are susceptible to mold and fungal attack. (See Chapter 3, paragraph 308.) These materials and other items such as shredded newspaper, excelsior, and cardboard may give off sulfurous or acidic vapors that can promote a corrosive attack on electronic components.
(2) Use only metal or preserved wooden shelves for storing avionics equipment and components. Inadequately preserved wood can produce corrosive vapors.
(3) Provide closed-cell polyethylene foam 1/2 inch thick as a cushioning for equipment on shelves, pallets, etc. Do not use an open-cell foam or sponge rubber, horse hair, or similar material that will hold moisture.
(4) Use conductive plastic or metal caps for electrical connector protection.
(5) Use cellular plastic film cushioning material (bubble wrap) for short-term protection during transportation of equipment and to protect against handling and shock damage.
(6) Never place bubble wrap in direct contact with ESD-sensitive devices. The electronic device or component should first be placed in a conductive bag. For equipment requiring ESD protection, see Chapter 9.
(7) Use plastic bags for short-term protection of noninstalled small components and microminiature PCBs against moisture and contamination.
(8) Use unicellular polypropylene packing foam and water vapor proof packaging material for long-term protection of miniature and microminiature circuit components, laminated
Par 409 Page 69 circuit boards, and critical avionics components against moisture and contamination. Figures 4-5 and 4-6 show typical shipping containers for avionics equipment.
c. Handling. Damage can occur to avionics equipment because of incorrect packaging methods and rough handling between the manufacturer, aircraft, and avionics repair facilities.
The best method of avoiding handling damage when transporting equipment is to use the specially designed cushioned shipping container from the OEM. A method that can be used if the original shipping container is not available is to use cellular plastic film cushioning material (bubble wrap). Bubble wrap is primarily used to absorb shock and is not intended as a water vapor proof packaging material. Bubble wrap should be placed around the component to be protected in two layers, with the second layer rotated 90 degrees from the original layer. The corners should be left open to allow the component and packing material to breath and avoid condensation and water entrapment. Secure the alternating layers of bubble wrap using masking tape or other pressure-sensitive tape.
d. Package and Storage. Packaging of avionics equipment is normally a function of a supply or receiving and shipping department. In many cases, the packaging of avionics equipment is accomplished by aircraft or avionics maintenance personnel. Packaging methods employed by these personnel are an important consideration for the equipment due to the length of time in storage awaiting repair or transit between aircraft and repair stations.
(1) Proper packaging should include provisions for the length of time the equipment will be in storage. Equipment should be packaged for long-term storage if the length of storage time is not known.
(2) Proper packaging materials, when specifically designed shipping containers are not available, should include: 6 mm thick (minimum) anti-static zip lock plastic-type bags. These bags are adequate for short-term storage and protection from moisture and contamination. They should be used during maintenance or repair operations on laminated circuit boards and small electrical/electronic components. They should also be used as part of a longer storage packaging and sealing system. For long-term storage, water vapor proof, heat-sealed barrier material provides excellent protection against moisture and contamination. Cushioning material such as unicellular polypropylene packaging foam should be used in conjunction with the plastic bags and water vapor proof, heat-sealed barrier material when protection from shock and handling is required.
(3) For equipment requiring ESD protection, see Chapter 9.
e. Electrical Connector and Wave Guide Caps. Metal caps and blank off plates, preferably moisture and vapor proof, are the preferred methods of protecting electrical connectors and wave guides from contamination and damage. Conductive plastic caps are another acceptable method of protection. Use only plastic caps which cover by surrounding the connectors or wave guide. Do not use push-in type caps or covers. These type caps or covers can easily be overlooked during assembly or become foreign object debris (FOD). When metal or conductive plastic caps and blank off plates are not available, electrical connectors and wave guides may be capped off with pressure-sensitive tape conforming to MIL-T-22085, Type II.
Other type tapes should not be used to seal or cap-off electrical connectors or wave guides.
Par 409 Page 70
f. Desiccants. Desiccants are normally packaged with equipment packaged for storage or shipment. The purpose of the desiccant is to absorb moisture and lower the relative humidity.
The following describes some of the problems and considerations for using desiccants.
(1) Desiccants may be ineffective for the following reasons:
(a) Moisture may condense as water when the desiccant becomes saturated.
(b) Desiccant is not placed in the proper location.
FIGURE 4-5. TYPICAL SHIPPING CONTAINER FOR AVIONICS EQUIPMENT (
c) Rapid changes in ambient temperature may produce precipitation before the desiccants can react.
(d) Not enough desiccant was used for the area to be protected.
(2) The following considerations apply to desiccants:
Par 409 Page 71 CAUTION: Do not use loose desiccants in packing of avionics equipment.
Loose desiccant may contaminate and cause damage to the packaged equipment.
(a) Desiccant material should be contained in rupture-resistant sturdy bags.
(b) Desiccant bags should be secured to prevent movement.
(c) Desiccant bags should not be placed on or permitted to come in contact with unprotected surfaces.
(d) Desiccants should be reactivated prior to reuse.
(e) Desiccant bags should not be removed from their sealed container until ready for use.
FIGURE 4-6. TYPICAL SHIPPING CONTAINER FOR AVIONICS EQUIPMENT
Par 409 Page 72
(3) If a desiccant bag should break open during transit, clean the avionics equipment immediately upon discovery. Do not turn moving parts any more than absolutely necessary until all desiccant particles have been removed. Work out the desiccant particles with a clean acid brush and not more than 10 psi dry air pressure. An alternate method is to use a clean acid brush and a vacuum cleaner. For equipment with ESD protection, see Chapter 9.
g. Humidity Indicators. Humidity indicators should be placed in containers when desiccants are used. A humidity indicator is used to determine if a desiccant is sufficiently active to maintain an acceptable relative humidity within the container.
CAUTION: Do not place humidity indicator in direct contact with metal.
Chemicals used in the indicator may cause corrosion.
410 through 500. RESERVED.
Par 501 Page 73 CHAPTER 5. CORROSION REMOVAL, SURFACE TREATMENT, PAINTING, AND
SEALING
501. GENERAL. This chapter outlines the materials, equipment, and techniques involved in corrosion control. Maintenance personnel should analyze each corrosion problem and select the correct corrosion removal and preservation materials. Where possible, follow-up actions should be conducted to ensure that all corrosion has been removed and proper protection has been applied. An avionics corrosion control program is an important function in maintaining any aircraft and aircraft component. The program requires knowledge of the science and technology of avionics corrosion control. Preventive maintenance must occur as part of all maintenance functions performed on avionics systems. Whenever equipment is removed from the aircraft for bench check or repair, covers and housings should be inspected and treated for corrosion.
Avionics technicians must ensure that corrosion, repair, treatment, and preventive maintenance becomes a normal part of their maintenance and repair procedure.
502. CORROSION REMOVAL MATERIALS AND EQUIPMENT.
a. General. Avionics technicians and repair stations should review Appendix 1 of this AC to convert military specification corrosion repair materials and equipment mentioned in these chapters to the commercial equivalent.
b. Corrosion Removal. When corrosion is detected, corrective action is required. When the corrosion is within repairable limits specified in the applicable Original Equipment Manufacturer’s (OEM) manual or other directive, corrective action should be initiated. This should consist of paint removal (as required), cleaning, corrosion removal, treatment, and the application of protective coatings and preservation. The mildest method of corrosion removal should always be used. The following paragraphs list approved methods for use on avionics equipment.
WARNING: Prolonged breathing of vapors from organic solvents or materials containing organic solvents is dangerous. Refer to the appropriate material safety data sheet (MSDS) for warnings and required protective gear. Chemical paint removers are toxic to the skin, eyes, and respiratory tract. Avoid breathing the vapors. Use only with adequate ventilation. Avoid skin and eye contact. Wear gloves and goggles while handling. If eye contact is made, wash immediately with large amounts of water. If skin contact is made, wash immediately with soap and water.
CAUTION: Epoxy paint removers are harmful to rubber and plastic products, including wiring insulation. Exercise care to avoid contact with such surfaces. Mask those adjacent areas which are not to be stripped with pressure-sensitive tape.
(1) Chemical Paint Removers. Epoxy paint removers conforming to QPL-81294 may be used to chemically remove paint from metal surfaces. This paint remover should be applied by brush. Care must be observed to ensure that the paint remover does not contact any part of the body. Observe all cautions and warnings. Whenever a chemical paint remover has been used,