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(4) Inspection for corrosion, deteriorated seals, and proper routing on a scheduled basis;
(5) Routine cleaning of all wiring and the exterior surfaces of components;
(6) Keeping drain holes open;
(7) Early detection and repair of damaged protective coatings;
(8) Prompt corrosion treatment after detection;
(9) Accurate record keeping and reporting of material or design deficiencies; and
(10) Use of appropriate materials, equipment, and technical publications.
b. Corrosion Prevention Philosophy. Corrosion and environmental conditions are natural phenomena that adversely affect avionics equipment. Although they can never be totally eliminated, the problems these phenomena cause can be minimized so that they are more manageable. This can be achieved only by understanding the equipment failure mechanisms, implementing a preventive maintenance program, and using corrosion control techniques and materials.
303. CORROSION-PRONE AREAS.
a. General. There are certain corrosion-prone areas common to all aircraft. For example, bilge areas are particularly susceptible to moisture intrusion/entrapment. The bilge area is where cables run, and where wire bundles, coaxial cables, lights, and antennas are installed. It is almost impossible to prevent moisture intrusion/entrapment in these areas. Other corrosion-prone areas include areas in and around the engine exhaust, battery compartments, lavatories, buffets, galleys, entrances at cargo and passenger doors, and wheel wells and landing gear.
b. Moisture and Other Fluid Intrusion Sources. Moisture, fluid intrusion, and fluid movement within the aircraft are caused by many factors, including:
(1) The flexibility of aircraft structure, which prevents complete and effective airframe sealing at skin joints and around fasteners;
(2) Numerous openings, such as equipment bay doors, access panels, ducts, and static pressure sensors, which can allow moisture intrusion;
(3) Moisture from many sources, such as rain and aircraft wash, and fluids from leaking hydraulic, fuel, oil, and coolant lines; and
(4) Moisture and fluid migration within the aircraft along air condition ducts, fuel and hydraulic lines, and cable and wire bundles. As a result of these conditions, moisture and other fluids can enter, migrate, and be trapped in areas of the aircraft where avionics equipment may be installed, which are normally considered protected.
Par 303 Page 22 FIGURE 3-2. SURFACE CORROSION ON AN AVIONICS MOUNTING RACK
Par 303 Page 23 FIGURE 3-3. CORROSION AND LINT ACCUMULATION ON CARGO DOOR
MICRO-SWITCH
Par 303 Page 24 TABLE 3-1. EFFECTS OF CORROSION ON AVIONICS EQUIPMENT Component Failure Mode Antenna systems Short circuits or changes in circuit constants and structural deterioration.
Chassis, housings, covers, and mount frames Contamination, pitting, loss of finish, and structural deterioration.
Shock mounts and supports Deterioration and loss of shock damping effectiveness.
Control box mechanical and electrical tuning linkage, and motor contacts Intermittent operation and faulty frequency selection.
Water traps Structural deterioration.
Relays and switching systems Mechanical failure, short circuits, intermittent operation, and signal loss.
Plugs, connectors, jacks, and receptacles Short circuits, increasing resistance, intermittent operation, and reduced system reliability.
Multipin cable connectors Short circuits, increasing resistance, intermittent operation, and water seal deterioration.
Power cables Disintegration of insulation and wire/connector deterioration.
Display lamps and wing lights Intermittent operation, mechanical and electrical failures.
Wave guides Loss of integrity against moisture, pitting, reduction of efficiency, and structural deterioration.
Fluid cooling system lines Failure of gaskets, pitting, and power loss.
Printed circuits and microminiature circuits Short circuits, increased resistance, and component and system failures.
Batteries High resistance at terminals, failure of electrical contact points, and structural deterioration of mounting. Erroneous cockpit signals.
Busbars Structural and electrical failures.
Coaxial lines Impedance fluctuations, loss of signal, and structural deterioration of connectors.
c. Structural. Structural parts include housing covers, supports, brackets, cabinets, and chassis, which require structural support and equipment protection. Corrosion on these parts should be cleaned and treated, and the protective finish restored to eliminate long-term deterioration. Severe corrosive damage usually results from damage to the protective finish and subsequent attack to the exposed metal. The protective finish is usually damaged from an environmental attack, handling damage, or microbial growth. Figure 3-2 shows surface corrosion on an avionics mounting rack. Severe corrosion damage requiring major repairs should be accomplished by an authorized repair station. Minor corrosion damage and damaged protective coatings are normally repaired by an authorized certificated person.
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d. Electromechanical. Motion is an integral function of electromechanical switches, relays, potentiometers, motors, generators, and synchronous components. Storage or non-operation in certain environments tends to promote corrosion of these devices. The principal causes of malfunction are dust, condensation products, resultant corrosion products (oxides), and organic contaminate films. Failure of these devices does not normally occur during operation.
The friction generated during operation usually keeps the critical surfaces clean enough to permit operation. On the other hand, during non-operations, insulating films form, which prevent startup operation of the equipment.
FIGURE 3-4. CORRODED HARDWARE ON TERMINAL BOARD
e. E lectronics. Moisture and contaminates penetrate electronic equipment, causing many detrimental effects, such as corrosion. In most modern electronic systems, circuit areas have been minimized for faster signal processing and higher density. In addition, integrated circuits use low voltage for operation. This means that most circuit paths are thin, or small in a cross-sectional area, and that individual circuit paths are close together. In these systems, trace amounts of moisture and contaminates may cause systems failures. Table 3-1 lists the typical effects of corrosion on avionics equipment.
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f. Special Considerations. The control of corrosion in avionics equipment is like that in the airframe. Procedures used on the airframe are applicable to avionics with appropriate modifications. Figures 3-4, 3-5, and 3-6 show examples of corrosion moisture traps. The general differences in construction and procedures for corrosion control on airframe and avionics are as follow:
(1) Avionics rely on less durable protection systems.
(2) Very small amounts of corrosion can make avionics equipment inoperative, as compared to airframes.
(3) Dissimilar metals are often in electrical contact.
(4) Stray electrical currents can cause corrosion.
(5) Active metals and dissimilar metals in contact are often unprotected.
(6) Vented avionics boxes can be subject to condensation due to normal temperature changes during flight.
(7) Avionics systems have many areas that trap moisture.
(8) Corrosion on internal components is difficult to detect in many avionics systems.
(9) Many materials used in avionics systems are subject to attack by bacteria and fungi.
(10) Organic materials are often used which, when overheated or improperly or incompletely cured, can produce vapors. These vapors are corrosive to electronic components and damaging to coatings and insulators.
Par 303 Page 27 FIGURE 3-5. CORRODED SHOCK MOUNT AND MOISTURE TRAP AREA
Par 303 Page 28 FIGURE 3-6. LINT ACCUMULATION ON AVIONICS RACK COOLING AIR
MANIFOLD
304.
INSPECTION PROCESS.
a. General. Frequent corrosion inspections are essential to the overall success of a corrosion control program. Through detection, identification, and treatment, the costs resulting from corrosion are minimized. Without regular systematic inspections, corrosion will seriously damage avionics equipment. The following paragraphs describe some of the basic aspects of visual inspection for corrosion and the telltale signs associated with various types of corrosion damage.
b. Inspection Factors. A calendar-based check maintenance inspections should be in accordance with the manufacturer’s or operator’s instructions. However, extreme environmental and operating conditions should be considered when determining the frequency of corrosion inspections. The following are factors to consider when developing or using a local inspection interval:
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(1) Operational environment;
(2) Known corrosion-prone areas, such as battery components and compartments, and electrical bonds;
(3) Length of storage time, with respect to the storage environment and the avionics equipment or component;
(4) The amount of time the equipment is nonoperational, especially for low usage aircraft or equipment;
(5) Non-pressurized avionics components and equipment bays;
(6) Antennas and externally mounted avionics packages; and
(7) Avionics equipment mounted in susceptible water entrapment and intrusion entry areas.
FIGURE 3-7. CROWN AREA INSPECTION
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c. General Inspection Procedures. The following general procedures are recommended for an avionics corrosion inspection:
(1) Clean area or component to be inspected.
(2) Visually examine the overall condition of the equipment or component using an appropriate light source. Suspected areas or corrosion-prone areas should be examined using a 10x magnifying glass and an appropriate light source to determine the extent of corrosion or if other damage exists. Miniature or microminiature circuit boards or avionics components should be examined using the appropriate microscope and light source, as necessary.
(3) Refer to the applicable service manual for damage limits.
FIGURE 3-8. CORRODED HARDWARE FOR BONDING STRAP
d. W ater Intrusion Inspection. Maintenance personnel who routinely inspect the aircraft and equipment should inspect the interior of equipment bays for evidence of water intrusion and entrapment. Any evidence of water stains or entrapped water will require a thorough inspection of mounted equipment and the surrounding structures for corrosion. Additionally, the source of water intrusion will need to be determined and eliminated. The following are suggested areas and procedures for a water intrusion inspection:
(1) Verify installation of required fasteners in avionics equipment, airframe, and surrounding structure.
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(2) Inspect condition of gaskets, form-in-place seals, and pre-formed seals.
(3) Verify drain holes are open and clear in avionics equipment and airframe structure.
(4) Inspect condition of doors, electronic rack drip shields, and covers and panels for material condition, especially warping.
(5) Prepare the compartment for a water intrusion test by installing witness material, such as blotter paper, paper towels, etc. The witness material should be placed in all suspected areas of water intrusion in a manner that will indicate the leakage path.
(6) Close or secure compartment doors, panels, or covers.
CAUTION: Do not direct the stream of water at components with bearings and shaft seals.
(7) Apply fresh water with a hose in a stream to the exterior surface of the airframe for approximately five minutes.
(8) Allow approximately three to five minutes for the water to penetrate and drain.
(9) Open or remove compartment doors, panels, or covers.
(10) Visually examine witness material for signs of water and source of water intrusion.
(11) Accomplish repairs in accordance with applicable repair service manual.
(12) Close or secure compartment doors, panels, or covers.
(13) Repeat water intrusion test (subparagraph d(5) and subsequent), along with the appropriate repairs until water intrusion leaks are no longer observed.
305. RECOGNIZING AVIONICS CORROSION.
a. General. Recognizing the appearance of corrosion or corrosion products for specific metals is an important part of an avionics corrosion prevention and control program. Metals are susceptible to corrosion because all metals have a tendency to return to their natural forms. For example, iron tends to return to iron oxide (rust). Avionics systems make use of many metals not normally considered for airframe structures. Some of the rarer metals are found in avionics components, in transistors, miniature and microminiature circuits, and integrated circuits.
Table 3-2 lists the metals most often used in electronics and avionics components. In addition to recognizing the appearance of corrosion or corrosion products for specific metals, maintenance personnel must be able to recognize a corrosive attack from solder flux, microbes, insects, and animal attack. Table 3-3 describes the appearance of corrosion for specific metals and different corrosive attacks.
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b. Corrosion Effects on Metals. Deterioration (corrosion) of a metal is caused by a chemical reaction with its environment. The corrosive effect can be accelerated by many of the factors that were discussed in Chapter 2. No metal can have a perfect environmental integrity, and therefore will corrode. Corrosion of a specific metal will take on many different forms as the corrosive attack progresses. The following paragraphs provide a description of corrosion with respect to the most commonly used metals in avionics systems.
(1) Iron and Steel. Iron and steel are used in avionics components as leads, magnetic shields, transformer cores, brackets, racks, and general hardware. Some of these components are plated with nickel, tin, or cadmium. Corrosion of steel is easily recognized because the corrosion product is red or black iron oxide (rust). When iron-based alloys corrode, dark corrosion products usually form first. This material will promote further attack by absorbing moisture from any water source, including ambient air. The most practicable means of controlling corrosion on non-plated steel or iron is complete removal of the corrosion product (rust) by the least harsh method. Iron and steel surfaces are normally protected by applying a plating system, paint system, or application of a preservative compound.
(2) Corrosion Resistant Steel. Stainless steel is used for mountings, racks, brackets, and hardware in avionics systems. Stainless steel is not readily susceptible to corrosion because of a tough chromium oxide film on the surface. However, exposure to a saltwater environment will cause pitting. Stainless steel is corrosion resistant, but is susceptible to crevice corrosion.
The corrosion product of stainless steel is a roughened surface with a red, brown, or black stain.
Corrosion treatment for stainless steel to remove the red, brown, or black stain should be limited to cleaning with stainless steel wool or a stainless steel brush.
(3) Aluminum Alloys. Aluminum and aluminum alloys are widely used in avionics systems for electrical connectors and back shells, cabinets, housings, chassis, structures, and mounting fixtures.
Corrosion of aluminum and aluminum alloys is indicated by a white or gray powder (aluminum oxide).
In most environments, especially moist salt-laden air, aluminum alloys are subject to many types of corrosive attack and therefore require protection. Aluminum surfaces are protected by an entire paint coating system composed of a chemical conversion coat, primer, and topcoat. Painted aluminum surfaces, even with the recommended protection system, tend to mask any corrosion attack to the aluminum. Corrosion damage to the aluminum surface will show up in the paint coating as filiform corrosion showing signs of blistering, flaking, chipping, lumping, or other irregularities.