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CHAPTER 5 MAINTENANCE AND TROUBLESHOOTING Aircraft and avionics systems must be in top operating condition to ensure completion of their mission. The effectiveness of avionics systems depends on the technician’s ability to maintain them. You are only as good as the tools and publications you use and as your knowledge of general and specific maintenance procedures. This chapter covers safety, general maintenance procedures, wiring, hardware, printed circuits, and electrostatic discharge. SAFETY LEARNING OBJECTIVES: Identify safety precautions regarding aircraft, personnel, material, and tools. Identify the classes of fire and procedures for extinguishing electrical fires. Identify the dangers of compressed air. Identify the dangers of volatile fluids. A technician will install, maintain, and repair electrical and electronic equipment in confined spaces where dangerously high voltages are present. Among the hazards of this work are injury caused by electric shock, electrical fires, harmful gases, and misused compressed air. Also, you must include improper use of tools among these hazards. Using common sense and carefully following established rules will help produce an accident-free career. When working, there is one rule to stress strongly— SAFETY FIRST. Whether you are working in the shop, on the flight line, or during a flight, you should follow prescribed safety procedures. When you are working on or near aircraft there is the danger of jet blast or of losing your balance or being struck by propeller or rotor blades. Because of these dangers, you need to develop safe and intelligent work habits. You should become a safety specialist, trained in recognizing and correcting dangerous conditions and unsafe acts. Safety is the responsibility of all hands. GENERAL PRECAUTIONS Because of the chance of injury, the danger of fire, and possible material damage, only authorized personnel can repair and maintain electronic and electrical equipment. Some general guidelines for personnel to follow are: • Make sure you get a thorough safety indoctrination from your supervisor. • Report a condition that you believe to be unsafe to your supervisor. • Warn others of an unsafe condition or practice. • Wear personal protective equipment (PPE) as required. • Report all injury and illness immediately. • Administer first aid as required. • DO NOT TAKE AN UNNECESSARY RISK. • Follow each safety precaution carefully. Cooperation and vigilance of personnel will prevent most accidents that occur in non-combat operations. The following is a list of general, common sense safety precautions. Memorize and observe them. • NEVER WORK ALONE. Always work in the presence of another person capable of rendering aid in an emergency (to de-energize equipment or render first aid in case of injury due to electrical shock). • Do not wear loose-fitting clothing while working with mechanical equipment. • Remove all rings, watches, and other metal jewelry prior to working with electrical and electronic equipment. • Ensure all equipment is properly grounded and correct power requirements are satisfied. • Become familiar with the equipment. You must know how to treat burns and how to give artificial respiration to a person suffering from electric shock. In some cases, you may have to perform external heart compression along with artificial ventilation, known as cardiopulmonary resuscitation (CPR). To be qualified to perform CPR, you must take the certified CPR training course. Personal CPR training is available at many Navy medical facilities. It is important to keep 5-1
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your CPR up to date by requalifying in the required time frame. The life of a shipmate could easily depend upon your CPR skills. This is not to say that knowledge of other first aid procedures is less important. You also are responsible for getting first aid training. W ARNING Do not perform CPR unless you have had proper training. The two safety-related publications with which you should be familiar are Navy Occupational Safety and Health (NAVOSH), OPNA VINST 5100.23, and Navy Safety Precautions for Forces Afloat , OPNA VINST 5100.19. These publications deal with a variety of operations; therefore, they are basic and general in nature. An activity refers to these instructions when it establishes specific safety instructions for its particular equipment, weapons system, or locality. Precautions Regarding Aircraft As a technician, you are exposed to flight line hazards. You will be working around moving equipment and aircraft, which is dangerous; therefore, you need to be alert. Always follow your activity’s instruction on the application of external power. The maintenance instruction manual (MIM) for each type of aircraft has an illustration of danger areas, such as that for the F/A-18 aircraft shown in figure 5-1. Study the illustration for each aircraft in your operating area. Most safety instructions require the anti-collision light to be operating whenever the engine or engines are operating. This gives an additional warning so you will be aware of propellers, rotors, or intakes and exhausts. 5-2 70 140 FT 25 FT MAX AB IDLE 50 FT 100 FT 900 FT ELECTROMAGNETIC RADIATION INLET SUCTION- INTERMEDIATE OR AB IDLE POWER 8- FOOT RADIUS TURBINE BLADE FAILURE APU EXHAUST AEf05001 Figure 5-1.—Radiation, intake, exhaust, and turbine blade failure danger areas.
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Precautions Regarding Material and Personnel When repairs on operating equipment are neces- sary, only experienced personnel under supervision should do the work. If possible, repairs should be made on de-energized circuits. When working on electrical equipment, take the following actions: • Open and tag the main supply switches or cut- out switches. The tag should read as follows: “This circuit is open for repairs and shall not be closed except by direct order of ________________(usually the person directly in charge of the repairs).” • Securely cover fuse boxes and junction boxes except when working on them. • Remove and replace fuses only after the circuit is de-energized. If a fuse blows, replace it with a fuse of the same current rating only. When possible, carefully check the circuit before making the replacement, since a burned-out fuse often results from a circuit fault. • Move slowly when working around electrical equipment and maintain good balance. • Make sure there is enough light for good illumination. • Make sure there is insulation for ground by using a suitable non-conducting material. Heed the following cautions: • DO NOT alter or disconnect safety devices, such as interlocks, overload relays, and fuses except when replacing them. • DO NOT change or modify safety or protective devices in any way without authorization. • DO NOT lunge after falling tools. • DO NOT work on electrical equipment if you are mentally or physically exhausted. • DO NOT touch energized electrical equipment when standing on metal, damp, or other well-grounded surfaces. • DO NOT handle energized electrical equipment when wet or perspiring heavily. HIGH-VOLTAGE PRECAUTIONS .—Never work alone near high-voltage equipment. Never measure voltages in excess of 300 volts by probing or holding the test probe in your bare hands. When measurements are necessary on equipment that has a potential in excess of 300 volts, wear rubber gloves if possible. Where rubber gloves cannot be worn, observe the following precautions and procedures: 1 First de-energize the equipment (or circuit). 2. Discharge high-voltage capacitors with a suit- able shorting probe. 3. Attach test leads capable of measuring high-voltage to the desired test points. 4. Have an assistant who is standing by energize the power source for the equipment. Then take the measurement. 5. Have the equipment de-energized prior to re- moving test leads after you have taken the measurement. LOW-VOLTAGE PRECAUTIONS .—Most people never realize the dangers of low-voltage electric shock. Current rather than voltage is the criterion for shock intensity. A potential as low 30 volts can cause a fatal current flow. Observe the following practices: • Work on de-energized equipment when pos- sible. • Do not work alone. • Make measurements using one hand (one-hand rule). ELECTRIC SHOCK .—The amount of current that may pass through a person’s body without causing damage depends on the individual and the current quantity, type, and path in addition to the length of time the current passes through the body. A person’s resistance can vary from 300 ohms to 500,000 ohms depending on the condition of his or her body. For example, if a person’s skin is dry and unbroken, resistance can be as high as 500,000 ohms. However, if his or her skin is moist and broken, cut, or burnt, resistance may be as low as 300 ohms. The following are some examples of the effect of current flow through the body: • At 1 milliamperes (0.001 A), you will feel a shock. • At 10 milliamperes (0.01 A), shock paralyzes your muscles and you may be unable to release the conductor. • At 100 milliamperes (0.1 A), shock is usually fatal if the current causing the shock lasts for 1 second or more. 5-3
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Electric shock produces a jarring, shaking sensation. The victim usually feels like he or she just received a sudden blow. If the voltage and resulting current is high enough, the victim may become unconscious. Severe burns may appear on the skin at the place of electrical contact. Muscular spasm may occur causing the victim to clasp the apparatus or wire that is causing the shock. If this happens, the victim will be unable to release the source of the shock, and you should use the following procedures for rescuing and caring for the shock victim. Remove victim from the source of electrical shock immediately. DO NOT ENDANGER YOURSELF. Remove the victim by throwing the switch if it is nearby, or cut the cable or wires to the apparatus by using an axe with a wooden handle. (Protect your eyes from the flash when you sever the wires.) Also, when you cannot cut the power source off, you can use a non-conductive item to move the electrical source away from the victim or to push or drag the victim to safety. Workbenches should have ropes or wooden canes nearby for such emergencies. When a person is unconscious because of electrical shock, you cannot tell how much current caused the condition and you should begin CPR immediately if you are qualified to do so TOOL SAFETY Tools make a task easier and enable you to work efficiently. If tools are not cared for and used properly, their effectiveness will be lost. A defective tool or improper tool use also increases the possibility of injury to personnel. As a technician you will use a variety of hand tools and power tools. By using each tool correctly you will improve the quality of maintenance and reduce the chance of equipment failure and bodily injury. Always follow the two basic tool safety precautions stated below: • Use the proper tool for its intended function and use it correctly. • Keep all tools in working order and in a safe condition. When using hand tools, observe the following practices: • Sharpen or replace a dulled cutting tool. • Protect a tool from damage while it is in use or in stowage. • If a tool becomes worn, damaged, or broken, turn it in for a replacement. • Return each tool to its proper stowage place. A loose tool can be a major source of foreign object damage (FOD). Nonmagnetic Tools You will use hand tools made of nonmagnetic materials to maintain equipment that can be damaged from magnetized tools. A magnetic-susceptible tool can become magnetized and transfer its magnetic condition to the equipment. When you work near compasses and other components containing permanent magnets, you should always use nonmagnetic tools. Available through normal supply channels, nonmagnetic tools are normally made from beryllium-copper or plastic. They are not as rugged as steel tools and can easily be damaged. If you use nonmagnetic tools properly, they will last longer. W ARNING Due to toxic hazards, do not etch beryllium- copper tools. Insulated Tools Safety considerations require use of insulated hand tools whenever the danger of electrical shock exists. Many types of insulated tools are available directly through supply channels. You should obtain these tools and use them when available. However, many types of insulated tools are not readily available (or are available only at considerable added expense). If a tool is essential, modify the conventional tool or procure the tool by using the following guidance: • Insulated sleeves may be put on the handle of pliers and wrenches and on the shank of a screwdriver. Because of the limitations of the insulating materials used in sleeves, use a tool modified in this manner only for low-voltage circuits. • For high-voltage circuits, use special insulating handles that are available for many of the common types of tools. • When you need a tool that is made of insulating material rather than just using insulating 5-4
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handles, requisition the tool through normal supply channels. Power Tools In working as an electrical or electronics technician, you will use shop machinery such as a power grinder or drill press. Additional precautions to follow when working with machinery are as follows: • NEVER operate a machine with the guard or cover removed. • NEVER operate mechanical or powered equipment unless you know how to operate it. When in doubt, consult the appropriate manual or ask someone who knows. • NEVER plug in electric machinery without knowing that the source voltage is the same as that called for on the nameplate of the machine. • Always make sure that everyone is clear of the equipment before starting or operating mechanical equipment. • Always keep everyone clear of the job site when hoisting heavy machinery or equipment by a chain fall. Guide the hoist with lines attached to the machinery or equipment. • Cut off the source of power before trying to clear jammed machinery. Precautions regarding portable electric power tools are as follows: • Carefully inspect each power tool to be sure the tool is clean, well oiled, and in working order before you use it. For example, the switch should operate normally. • Ground the casing of each electrically driven tool. • Do NOT use a sparking tool in any place where flammable vapors, gases, liquids, or exposed explosives are present. Precautions regarding power cords are as follows: • A cord should be clean and free of defects. • Check to make sure that the cord does not come in contact with sharp objects, have kinks, or is left where it may be run over. • Don’t let a cord come in contact with oil, grease, hot surfaces, or chemicals. • Replace a damaged power cord. • When unplugging a power tool from receptacles, grasp the plug, not the cord. Soldering Irons The soldering iron is a potential fire hazard and source of burns. Observe the following precautions: • Always assume a soldering iron is hot. • NEVER rest the iron anywhere but on a metal surface or rack designed for that purpose. • Keep the iron in the open to reduce the danger of a fire from accumulated heat. • DO NOT shake the iron to get rid of excess solder. The hot solder may strike someone or hit the equipment and cause a short circuit. • Hold a small soldering job with pliers or clamp. • When cleaning an iron, place the cleaning rag on a flat surface and wipe the iron across it. Don’t hold the rag in your hand. • Disconnect the iron when leaving the work area, even for a short period of time; the delay may be longer than planned. GROUNDING A poor safety ground, or one with incorrect wiring, is more dangerous than no ground at all, because it doesn’t offer full protection and it lulls you into a false sense of security. The incorrectly wired ground is a hazard because one of the live wires and the safety ground are transposed. When this happens, the shell of the tool become electrically hot the instant you connect the plug into the outlet, and you will get a shock. A three-wire, standard color-coded cord with a polarized plug and a ground pin should be used with a power tool. In a properly connected tool, the green wire is the safety ground. This wire attaches to the tool’s metal case at one end and to the polarized grounding pin at the other end. The green wire normally carries no current and is in use only when the tool insulation fails. When tool insulation fails, the safety ground short circuits the electricity to ground and protects the user. To check the grounding system resistance, use a low-reading ohmmeter to be certain the safety ground is adequate. If the resistance is greater than 0.1 ohm, you should use a separate ground strap. 5-5
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Some old installations do not have receptacles that will accept the grounding plug. If you are assigned to one of these, use one of the following: • Use an adapter fitting. • Use the old type plug and bring the green ground wire out separately. • Connect an independent safety ground wire. When you use an adapter, connect the ground lead extension to a good ground. DON’T use the center screw that holds the cover plate on the receptacle. Always connect the safety ground first and remove it last. Use the following procedures where separate safety ground leads are connected externally: • First connect the safety ground, and then plug in the tool. • Likewise, when disconnecting the tool, first remove the line plug, and then disconnect the safety ground. ELECTRICAL FIRES The four general classes of fires—A, B, C, and D—are defined in most cases by the types of combustible material (or fuel) involved and by the method and agents used to extinguish each. (See table 5-1.) The classes of fires can be defined as follows: Table 5-1 Class of fire Type of fuel A Wood, paper, cotton, wool fabrics, cork, and so forth that leave embers or ashes B Cooking and fuel oils, grease, gasoline, jet fuels, kerosene, paint, turpentine, and so forth C Electrical in origin and may involve fuels from A, B, and D categories D Special metal alloys of magnesium, titanium, zinc and so forth The electronics and electrical technician should be an expert in the extinguishing of class C (electrical) fires. Class C fires are a special situation in that an additional hazard of electric shock is involved and that the fire must be extinguished without further damage to equipment. Carbon dioxide (CO 2) is the preferred extinguishing agent for class C fires. CO 2 does not conduct electricity, evaporates rapidly, and leaves little or no residue. It reduces the possibility of electrical shock to personnel and damage to equipment as a result of contamination. Another choice of extinguishing agent for class C fires is a dry chemical agent known as Purple-K-Powder (PKP). PKP is a nonconductor, which provides protection against electrical shock; however, damage to electrical or electronic parts may result from the use of PKP. When fighting electrical fires, you should use the following general procedures: 1. Promptly de-energize the circuit or equipment affected. 2. Sound the alarm according to station regula- tions or fire bill. 3. Close compartment air vents and windows. 4. Control or extinguish the fire using a CO 2 fire extinguisher. 5. Avoid prolonged exposure to high con- centrations of carbon dioxide in confined spaces. You can suffocate due to the displaced oxygen. 6. Administer artificial ventilation and oxygen to a person overcome by carbon dioxide fumes. CAUTION Never use a solid stream of water to extinguish class C (electrical) fires in energized equipment. Water usually contains minerals that make it conductive. (The conductivity of seawater is many times greater than that of fresh water.) If you must use fresh water or seawater, use a water fog application. When water is broken into small particles as in fog application, the conductivity is greatly reduced. You also must ensure the conductive metal material of the applicator does not come in contact with the energized equipment. All hands need to know the dangers of fire. An unexpected fire aboard a Navy vessel at sea can kill and injure more people and cause more damage than battle. You need to know the type and location of fire-fighting 5-6
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equipment and apparatus in your immediate working and berthing spaces and throughout the ship. VOLATILE LIQUIDS V olatile liquids—liquids that produce vapors or fumes, such as insulating varnish, lacquer, turpentine, and kerosene—are dangerous when used near operating electrical equipment because sparks from the equipment can ignite their vapors. When these liquids are used in compartments containing non-operating equipment, make sure there is enough ventilation to avoid an accumulation of fumes. Also, in com- partments where equipment is to be operated, make sure the space is clear of all fumes before energizing the equipment. Aviation fuels are hydrocarbons. Handling hydrocarbon products is hazardous because of their low flash point. Products such as gasoline, solvents, and most crude oils begin to vaporize at or below 80ºF; their flash point is reached at 80ºF. Their flash point makes them the most hazardous petroleum products to handle. Other petroleum products such as kerosene and lubricating oils have a flash point above 80ºF, making them less hazardous. JP-4 fuel has some of the characteristics of gasoline but has a lower vapor pressure and higher aromatic content (compounds added to increase fuel performance). Handle this fuel very carefully. JP-5 is a kerosene-type of fuel. It has a low vapor pressure (about 0 pounds per square inch [psi]). Its tendency to vaporize is lower than more volatile fuels and the vapor-air mixture above its liquid surface is too lean to ignite. For ignition to occur, the liquid’s surface must reach 140ºF. Nevertheless, handle this fuel with care. Take precautions to prevent personnel from breathing fumes from any fuel. The vapors of petroleum, gasoline, and other petroleum products cause drowsiness when inhaled. Petroleum vapors in concentrations of 0.1 percent can cause dizziness to the point where a person cannot walk a straight line after 4 minutes of exposure. Longer exposure and greater concentrations may cause unconsciousness or death. The first symptoms of exposure to toxic (poisonous) vapors are headaches, nausea, and dizziness. When working in an area where there are possible toxic vapors, stay alert. If you get a headache, become dizzy, or become nauseous, you might be exposed to toxic vapors. You should leave the area and report the condition. You recover from early symptoms of toxic vapors quickly when you move to an area having fresh air. If you find people overcome by vapors, get them medical attention immediately. First aid consists of the prevention of chilling and, if breathing has stopped, artificial respiration. Also, prevent fuel from coming in contact with the skin, especially if the skin has abrasions or sores. Repeated contact with gasoline removes protective oils from the skin, causing drying, roughening, chapping, and cracking, and in some cases, infection. If gasoline remains in contact with your skin, it may irritate the skin, particularly under soaked clothing or gloves. Remove clothing or shoes soaked with gasoline at once. When you remove gasoline-soaked clothes, an arc, caused by static electricity, can cause the fuel to ignite. For this reason, remove fuel-soaked clothes in a running shower. Wash gasoline from your skin with soap and water. If a person swallows gasoline, give first aid immediately, You should give the victim large amounts of water or milk and 4 tablespoons of vegetable oil if available. DO NOT INDUCE VOMITING. GET THE VICTIM MEDICAL ATTENTION IMMEDIATELY . COMPRESSED AIR When using pneumatic tools, see that nearby workers are not in the line of airflow. Compressed air used to power pneumatic tools, when misused, is dangerous and can cause the following injuries: • Injuries from a hose or fitting failure that cause the hose to whip dangerously and to propel fitting parts through the air • Eye injuries from blowing dust and small particles • Internal injury or even death from air under pressure introducing an airstream into body tissue, usually through an existing cut or scratch • Ruptured cell tissues and severe wounds from compressed air that injects minute foreign bodies into the skin from impurities that are always in a shop air supply • Falls from tripping over compressed air hoses thoughtlessly left lying on the floor 5-7
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CAUTION Compressed air is a special tool. Do not use as a substitute for a brush to clean machines, clothing, or your person. When an air hose is essential for blowing out fixtures and jigs, wear eye protection and maintain air pressure below a maximum of 30 psi. It helps to place screens around work to confine the blown particles. The National Safety Council has published the following general safety rules for working with compressed air: • Use only sound, strong hose with secure couplings and connections. • Make sure there aren’t any sharp points or metal hose parts. • Close the control valve in portable pneumatic tools before turning on air. • Turn off air at the control valve before changing pneumatic tools. Never kink a hose to stop the air flow. • Wear suitable goggles, mask, protective cloth- ing, or safety devices. • Never use air to blow dust chips from work clothing or from workbenches. • Never point the hose at anyone. Practical jokes with compressed air have caused painful deaths. Q5-1. Who is responsible for safety? Q5-2. What precaution should you take before re- moving a fuse? Q5-3. What is required for an individual to perform CPR? Q5-4. Where can you find danger areas for a partic- ular aircraft? Q5-5. Who should be standing by while working on energized equipment? Q5-6. What criterion determines shock intensity? Q5-7. Which tools should not be etched? Q5-8. What is the preferred extinguishing agent for Class C fires? Q5-9. What is the maximum psi of compressed air for cleaning equipment? MAINTENANCE CATEGORIES LEARNING OBJECTIVE: Identify the two maintenance categories and types of work done in each category. Maintenance performed on equipment falls into the following two broad categories: • Scheduled maintenance, which consists of actions taken to reduce or eliminate failure and prolong the useful life of the equipment • Unscheduled maintenance, which consists of actions taken when a part or component has failed and the equipment is out of service In maintenance work of any kind, you will need two basic kinds of knowledge. First, you must have specific information that applies to the particular equipment you are repairing or keeping in good condition. Second, you must have certain general skills and knowledge of procedures that apply to many kinds of equipment and types of work assignments. Specific information consists of special procedures and processes and detailed step-by-step directions. This information is approved by the proper authority and recommended for a particular piece of equipment. Information is available in publications or check- lists from the Naval Air Systems Command (NA V AIRSYSCOM), type commanders, or other authorized sources. Thegeneral maintenance skills and knowledge of proceduresare not available in equipment manuals and are the skills and procedures that must be learned during on-the-job training. SCHEDULED MAINTENANCE Scheduled (preventive) maintenance is performed to reduce the likelihood of future troubles or malfunctions. This form of maintenance consists mainly of visually checking the equipment before and during operation, cleaning the equipment and the various components, lubricating, and performing periodic inspections. Visual Checks Before you apply power to equipment, visually check equipment for loose leads, improper con- nections, and damaged or broken components. This type of check applies particularly to new equipment, equipment returned from overhaul, and preserved equipment. Also, it applies to equipment stored for long periods, and equipment that has been exposed to the 5-8
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weather. A close visual inspection of O-rings, gaskets, and other types of seals are necessary when the equipment under check has pressurized components. This visual inspection often reveals easily correctable discrepancies with a minimum amount of labor and parts. Such discrepancies, if left uncorrected, might result in a major maintenance problem. Cleaning Cleaning the equipment and various components consists of removing dust, grease, and other foreign matter from the covers, chassis, and operating parts. Cleaning includes removing corrosion, fungus, and all other types of matter that could cause operating failure of the equipment. The method used to clean the various parts and units will vary, but usually a vacuum cleaner is good for removing the loose dust and foreign matter. Other types of foreign matter can be wiped off using a clean, lint-free cloth. If you need to remove grease or other petroleum deposits, moisten the cloth with alcohol, dry-cleaning solvent, or some other approved degreaser. After removing the grease, wipe the part dry and clean before you apply power to the equipment. For more specific details on corrosion removal, you should refer to Avionics Cleaning and Corrosion Prevention/Control, NA V AIR 16-1-540. Lubrication Lubrication of electronic equipment consists of lubricating the mechanical parts that work with the electronic equipment. Parts, such as unsealed bearings, antenna drives, and waveguide rotating joints may require lubrication as directed by the MIM for the equipment. Using the correct specification number is very important because the viscosity of a lubricant changes with a change in operating temperature. High operating temperatures cause lubricants to become thin, and low operating temperatures cause lubricants to thicken or harden. Therefore, the lubricant for a particular job depends on operating characteristics and temperature. You should pay particular attention to equipment lubrication for aircraft that fly at high altitudes. At high altitudes, aircraft require a special lubricant that will not harden. This reduces physical overload on the drive motors and shafts and electrical overload on the circuits involved. Periodic Inspections Periodic inspections or preventive maintenance services (PMS) ensure aeronautical equipment is maintained throughout its life cycle by controlling degradation resulting from time, operational cycles, use, or climatic exposure. PMS, properly conducted, will ensure equipment receives the necessary servicing, preventive maintenance, and inspections required. Scheduling is the primary factor in the successful and efficient completion of a PMS action. The main tools used to accomplish effective inspections are the following PMS publications: • PMIC-Periodic Maintenance Information Card • MRC-Maintenance Requirement Card • SCC-Sequence Control Card • AESR-Aeronautical Equipment Service Record • SRC-Scheduled Removal Component • EHR-Equipment History Record • ASR-Assembly Service Record UNSCHEDULED MAINTENANCE Unscheduled maintenance is the performance of a repair action without a set interval. Discrepancies found before, during, and after flights or during operational checks fall into this category. When finding defective parts, or if unsatisfactory operation occurs, you must analyze the equipment, determine the defective part or parts, and replace or repair the part. In general, the most effective method for this analysis is a logical step-by-step troubleshooting procedure. Q5-10. What are the two broad categories of mainte- nance? Q5-11. Scheduled maintenance consists of what type of work? Q5-12. What manual should you refer to for avionics corrosion removal? TROUBLESHOOTING LEARNING OBJECTIVE: Identify trouble- shooting techniques for analyzing, detecting, and correcting faults in electrical equipment. Recognize proper aircraft maintenance practices. Recognize proper bench mainte- nance procedures. 5-9
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Most of your maintenance time is spent troubleshooting the equipment within your squadron's aircraft. Your job is to maintain several units and systems using both aircraft and bench procedures. Many systems are complex and might seem, at first glance, to be beyond your ability to maintain. However, the most complex job usually becomes much simpler if it is broken down into successive steps. Any maintenance job should be performed in the following order: 1. Analyze the symptom 2. Detect and isolate the trouble 3. Correct the trouble and test the work AIRCRAFT PROCEDURES Aircraft procedures for troubleshooting include tests for continuity, grounded circuits, shorts, and voltage. In troubleshooting, there is no substitute for common sense. Most beginners make a common mistake; they remove major units from the aircraft unnecessarily. The first step you should take when receiving a discrepancy is to determine if the equipment in question is actually faulty. Very often, a preliminary visual check of the system will show a faulty control box, frayed or broken wiring, or corroded or wet connectors. In some cases, you may not find an equipment fault but someone using an improper operating procedure—especially with new equipment. (Improper operating procedures are especially common when the reported discrepancy involves new equipment or when operating personnel are undergoing in- doctrination.) If there is no power present at the input to the equipment, you may assume (temporarily) that the set is not broken. You should check all applicable switch positions, circuit breakers, fuses, and other common problems. Then, check for power at the electrical bus that feeds the equipment. Check the tightness of con- nections and the physical condition of interconnecting cables. Using the wiring diagrams in the applicable manuals, you should check at successive tie points and splices for continuity, short circuits, or grounds. If a circuit breaker trips or if a fuse blows, it indicates a circuit malfunction. Turn off power to the circuit containing the open, and do not reapply power until you locate and correct the malfunction. The most common causes of tripped or blown circuit protectors are short circuits, faulty grounds, or overload con- ditions. However, circuit protectors sometime fail because of age or other conditions. If, after a thorough check, there is no clear reason for the failure, reset the breaker or replace the fuse. Make sure the replacement fuse is the proper size and type, then reapply the power. The analysis may not indicate the existence of a short circuit, faulty ground, or overload condition. If the equipment still does not operate, you should continue to take measurements with power applied. Observe all safety precautions . Systematically take these measurements at progressive checkpoints. Particular faults that can interrupt current through a circuit include broken wiring, loose or faulty terminal or plug connections, faulty relays or switches, and uncoupled splices. Be alert for these conditions! Sometimes, you cannot determine the defective unit while it’s still installed in the aircraft. You may need to turn off the power and replace units, one at a time, with units that operate properly. After replacing each unit, reapply power and check the system for proper operation. If the system operates normally, you have found the faulty unit. You may then take the bad unit to the shop for corrective maintenance. At this stage of the overall maintenance process, you should try to determine the reason for the failure of the unit. If the basic cause has not been corrected, it is possible the new unit also may become damaged. After you have removed the defective unit and further analyzed it, reinstall all other items of the original installation and safety wire. Then, perform a complete operational check. During the operational check, readjust or calibrate as necessary. This should be done before you clear the discrepancy (indicate the unit has been fixed and is in operating condition) on the original maintenance action form. The rules shown here are a guide you can use when making troubleshooting tests: • Always connect an ammeter in series. • Always connect a voltmeter in parallel. • Never connect an ohmmeter to an energized circuit. • Select the highest range first, and then switch to lower ranges, as needed. • When using an ohmmeter, select a scale that will result in a mid-scale reading. • Do not leave the selector switch of a multi-meter in the resistance position when the meter is not 5-10
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in use. The leads may short together and discharge the internal battery. There is less chance of damaging the meter if you leave it on a high ac voltage setting or in the OFF position. Meters that have an OFF position dampen the swing of the needle by connecting the meter movement as a generator. This prevents the needle from swinging wildly when the meter is moved. • View the meter from directly in front to eliminate parallax. • Observe polarity when measuring direct current (dc) voltage. • Do not place meters in the presence of strong magnetic fields. • Never try to measure the resistance of a meter or a circuit with a meter in it. The high current required for ohmmeter operation may damage the meter. This also applies to circuits with low-filament current tubes and some types of semiconductors. • When measuring high resistance, be careful not to touch the test lead tips or the circuit. Your body resistance will shunt the circuit and cause an erroneous reading. • Connect the ground lead of the meter first when making voltage measurements. Work with one hand whenever possible. Continuity Test Open circuits are circuits that interrupt current flow, either from a broken wire, defective switch, or any other means that stops current flow. To check for opens (or to see if the circuit is complete or continuous) you conduct a continuity test. An ohmmeter, which contains its own batteries, is an excellent tool to use when you perform a continuity test. (In an emergency, a flashlight can function as a continuity tester.) Normally, you make continuity checks in circuits where the resistance is very low, such as the resistance of a copper conductor. A very high or infinite resistance indicates an open circuit. Such a condition would be an open conductor. Look at figure 5-2. It shows a continuity test of a cable. When using an ohmmeter, make sure you disconnect both connectors and connect the ohmmeter in series with the conductor under test. The power must be off. When you are checking conductors A, B, and C, the current from the ohmmeter flows through plug 2, the conductor, and plug 1. From this plug, it passes through the jumper to the chassis ground and to the aircraft's structure. The structure serves as the return path of the current to the chassis of unit 2, completing the circuit to the ohmmeter. The ohmmeter will indicate a low resistance. Checking conductor D reveals an open. The ohmmeter indicates maximum resistance because current cannot flow. With an open circuit, the 5-11 RECEPTACLE 1 PLUG 1 PLUG 2 RECEPTACLE 2 JUMPER OPEN AIRCRAFT STRUCTURE RETURN TO OHMMETER DEVICE CHASSIS UNIT 1 UNIT 2 A D B C DEVICE CHASSIS A C D B AEf05002 Figure 5-2.—Continuity test.
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ohmmeter needle is all the way to the left, since it is a series-type ohmmeter (reads right to left). (A digital ohmmeter would read overload or “OL” in its display.) You cannot use the aircraft structure as the return path; use one of the other conductors. For example, to check D, connect a jumper from pin D to pin A of plug 1 and the ohmmeter leads to pins D and A of plug 2. By the process of elimination, this technique will also reveal the open in the circuit. Grounded Circuit Test Grounded circuits may be caused from either direct or indirect contact between some conducting part of the circuit and the metallic framework of the aircraft. Grounds may have many causes. Perhaps the most common cause of a ground is frayed wire insulation that allows the bare wire to come into contact with the metal ground. Grounds are usually indicated by blown fuses or tripped circuit breakers. Blown fuses or tripped circuit breakers, however, also may result from a short other than a ground. A high-resistance ground also may occur where enough current does not flow to rupture the fuse or open the circuit breaker. Ohmmeters provide a good test for grounds. You also may use other continuity testers. By measuring the resistance to ground at any point in a circuit, you can determine if the point is at ground potential. Look at figure 5-2 again. It shows a way to test a cable for grounds. If you remove the jumper from pin D of plug 1, a test for grounds can be made for each conductor of the cable. This is done by connecting one meter lead to ground and the other to each of the pins of one of the plugs. A low-resistance reading on the ohmmeter indicates a grounded pin. You must remove both plugs from their units. If you remove only one plug, a false indication is possible. This false indication occurs because the other conductor receives a ground through the unit. Short Test A short-circuit test is a test to determine whether two conductors have accidentally touched each other, directly or through another conducting element. Two conductors with frayed insulation may touch and cause a short. Too much solder on one pin of a connector may short it to an adjacent pin. In a short circuit, sufficient current may flow to blow a fuse or open a circuit breaker. However, it is entirely possible to have a short between two cables carrying signals and not blow a fuse. The device used to check for a short is the ohmmeter. By measuring the resistance between two conductors, you may detect a short between them. A low-resistance reading usually indicates a short. Look at figure 5-2. You may perform a short test by removing the jumper and disconnecting both plugs. This is done by measuring the resistance between the two suspected conductors. Shorts can occur in many components, such as transformers, motor windings, and capacitors. The major method for testing such components is to take a resistance measurement and then compare the indicated resistance with the resistance given on schematics or in maintenance manuals. You also may make comparisons with identical operational equipment. Voltage Test You make voltage tests with the power applied. Therefore, you must follow the prescribed safety precautions to prevent injury to yourself and others or damage to the equipment. Making voltage tests is an important part of maintenance work. It lets you isolate discrepancies to major components, and you can use these tests in the maintenance of subassemblies, units, and circuits. Before checking a circuit voltage, you should check the voltage of the power source to make sure normal voltage is being input to the circuit. BENCH PROCEDURES When doing bench procedures for troubleshooting, tests can involve signal tracing, test probe substitution, voltage and resistance checks, replacing defective parts, and checking after repair. The visible condition of a unit is usually the first check in any troubleshooting process. If certain parts are obviously not in good condition, correct them before you resume testing. Such faults include burned, loose, disconnected, dented, broken, or otherwise obviously faulty parts. Check the visible condition of a unit before installing and connecting the unit at the test bench. The sense of smell can help pinpoint certain troubles. A part that overheats usually gives off an odor that is readily detectable. However, location of a burned part does not necessarily reveal the cause of the trouble. 5-12
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To determine the cause of the trouble, you should refer to the MIM for the given equipment. The MIM is a source of valuable information for performing main- tenance on electronic equipment. Signal Tracing Signal tracing is a good method for tracing signals in RF receivers and audio amplifiers. However, in radar, the frequencies are higher, the methods of signal application differ, and the output in the final stage is video (viewed). The applicable MIM contains detailed procedures for testing most units or circuits. Signal tracing is a very effective method for locating defective stages in many types of electronic sets. It is especially useful when servicing equipment that normally contains no built-in meters. In signal tracing, a signal voltage, similar to that present under operating conditions, from a signal generator is input to the circuit in question. The signals that result are then checked at various points in the stage by using a high-impedance test instrument. The particular test equipment, such as a vacuum tube voltmeter, an oscilloscope, or an output meter, depends on circuit application and other parameters. The test instrument should have high impedance so that it will not change the operation of the circuit under test. When using the signal tracing to measure ac signals, you should make sure the test instruments are adequately isolated from any dc potential present in the circuit. Some test instruments have special ac probes that incorporate a capacitor in series with the input. Before using any item of test equipment, you must know the characteristics and proper use of the test equipment as well as the equipment under test. By using the signal-tracing method, you can measure the signal gain or loss of amplifiers. You also can locate the points of origin of distortion, hum, noise, and oscillation that occur in the amplifiers. The gain measurement is a good example of an important method in signal tracing. By this procedure, you can quickly isolate a discrepancy to the defective stage. A signal generator, with the output attenuator calibrated to microvolts, and an output meter can measure gain. It is helpful to have data on the normal gain of the various stages of the device. You can find this data in the MIM for the receiver under test. To measure gain, you connect the output meter across the headset (or the voice coil of a speaker) or across the secondary of the output transformer. Connect the output of the signal generator to the grid circuit of the stage under test. Then, adjust the attenuator of the signal generator until the output meter reads a value appropriate to serve as a reference figure. After adjustment, connect the output of the signal generator to the output of the stage under test (or to the input of the next stage). Adjust the attenuator until registering the same reference value on the output meter. To determine the gain of the stage, divide the second value of the signal (taken from the calibrated attenuator) by the value of the signal applied to the input of the stage. For example, suppose the signal generator supplies a voltage of 400 microvolts to the grid of an IF amplifier. This voltage causes the output meter to indicate some value you can use as a reference. When the generator signal is input to the following grid, the signal strength must be increased (4,000 microvolts) to cause the output meter to indicate the same reference value. The gain of the stage is equal to E E in in 2 1 nd stage st stage ; where Ein = voltage of the input, that is, 4 000 400 10, ./c61 If similar measurements made in the remaining stages of the receiver reveal one stage in which the gain is lower than normal or is zero, a faulty stage is indicated. Then, you can check that stage thoroughly by measuring voltage or resistance or by replacing parts until you find the defective one. Test Probe Substitution Do not use a test equipment probe with equipment other than that for which it is designed, as an improper test probe may not have sufficient capacitive adjustment to preserve the waveshape of the observed signal. Any differences in the internal resistance of the probe and input circuitry of the equipment make substitution impossible without calibration. For example, the internal resistance of a 10:1 probe is usually nine times higher than the input circuitry of the equipment. You should note that 2:1, 50:1, and 100:1 probes also are available. Voltage Checks You should make voltage measurements at various points in the stage suspected of being faulty. Compare the observed voltage values with the normal voltage values given in the MIM. When making voltage checks for comparison with a chart, you should use a voltmeter with the proper ohms-per-volt rating (sensitivity). Always connect voltmeters in shunt with the circuit 5-13
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elements under test. This results in circuit loading. The sensitivity of the test instrument must be the same as that of the instrument used to make the readings on the chart. This ensures the loading effect will be the same in both cases, and your meter readings should be reliable. Remember, if the meter sensitivity is too low, the loading effect may be so severe that it will prevent proper operation of an otherwise normally functioning circuit. By comparing observed voltages with the voltages given in the MIM, you can often isolate the defect. V oltage checks are most effective when applied within a single stage after you have made checks to localize the defect, because modern electronic equipment is complex, and requires time to check all the voltages present in all the stages. Some electronic sets have built-in meters or plugs for front panel application of meters. These meters usually work with a selector switch and read voltage or current values at set points. Normally, you can isolate a defective stage in this manner. After isolating the defective stage, it becomes a matter of point-to-point checking to isolate the fault within the stage itself. A voltmeter will pinpoint the trouble, but it often becomes necessary to use an ohmmeter to determine the exact cause of trouble; for example, shorted capacitors, open resistors or transformers, or a wire grounded to chassis. Resistance Checks Like voltage measurement, resistance checks are most effective after you isolate the trouble to a particular stage. After isolating the trouble, the ohmmeter is a very useful instrument, and often quickly leads you, the technician, to the cause of the trouble. Resistance checks are made like voltage checks, except you must remove power from the set. You measure resistance and compare your readings to the normal values given in the maintenance publications. Reliance on resistance measurement alone is too time-consuming to be efficient. NOTE: To prevent damage to the ohmmeter, always be sure there are no voltages present in the equipment before beginning the resistance checks. Turn off the power switches, discharge the power supply and other large capacitors, and bleed off any other residual charges in the set. Also, observe proper precautions when connecting or disconnecting the ohmmeter across large inductors. Routine resistance checks on an electrolytic capacitor may be done with an ohmmeter. You make a resistance measurement on the discharged capacitor using the high resistance range of the ohmmeter. When you first apply the ohmmeter leads across the capacitor, the meter pointer rises quickly and then drops back to indicate high resistance. Now, if you reverse the test leads and reapply them, the meter pointer rises again, even higher than before, and again drops to a high value of resistance. The battery of the ohmmeter charges the capacitor and causes the meter to deflect. When reversing the leads, the voltage in the capacitor adds to the applied voltage, resulting in a greater deflection than at first. W ARNING Do not leave the ohmmeter connected across an electrolytic capacitor for any length of time. Electrolytic capacitors are polarity sensitive, and reverse polarity of voltage (even from an ohmmeter) may cause excessive current, which could result in overheating and possible explosion of the capacitor. If the capacitor is open-circuited, no deflection will occur. If the capacitor is short-circuited, the ohmmeter indicates zero ohms. The resistance values registered in the normal electrolytic capacitor result from the slight current leakage between the electrodes. Because the electrolytic capacitor is a polarized device, the resistance is greater in one direction than the other. If a capacitor indicates a short circuit, you must disconnect one end of it from the circuit. Then, take another resistance reading to determine if the capacitor is actually at fault. Unless the ohmmeter has a very high resistance scale, you will not be able to see any meter deflection when you are checking small capacitors. Even a scale of R /c18010,000 is not enough for very small capacitors. The smaller the capacitor, the less leakage across the plates; therefore, the more resistance. When making resistance checks, you need to determine what circuits connect to the checkpoints. The MIM indicates the proper resistance at various checkpoints throughout the set. Also, the MIM contains a complete schematic of the set, as well as a circuit schematic of the stage under test. The schematics may set up conditions for performing voltage and resistance measurements. These conditions may include the positions of switches and control knobs, relays 5-14
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energized or de-energized, and tubes in sockets. These conditions duplicate the initial measurement con- ditions with which you are comparing your readings. Typical instructions might read "Power switch OFF—all controls on the control box full CCW (counterclockwise)." By following these instructions, you should get accurate values to compare with the specified values. Defective Components Before you replace a defective part, determine if such an operation is within your activity's capability. The maintenance that you can perform is a function of your activity's assigned level of maintenance. Because electronic equipment is complex and compact, the trend in the Navy is toward replacement of subassemblies instead of individual parts. This trend stems from the necessity of exact parts replacement and the difficulty of working in small spaces. Even the amount of solder used on a connection is important. However, there are many parts that you may replace at any level of maintenance. The general rule is to replace any defective part with an exact duplicate. You should refer to the specific MIM, illustrated parts breakdown (IPB), and supply publications to help get information (such as stock number and description) about a particular part. The publication that you will use most often when ordering parts for the particular equipment under repair is the IPB. If you consider a substitute part, make sure the substitute part is a proper replacement. (See table 5-2.) For example, one of the most important considerations when you replace a resistor is the wattage value of the resistor. The wattage rating is a measure of the ability of the resistor to dissipate heat. The wattage value is a function of the dimensions of the resistor. Your selection of a resistor with a safe wattage value should be based on a consideration of the working conditions of the resistor in the circuit. Consider the replacement of an 850-ohm resistor with one of equal ohmic value but with a tolerance of 20 percent. Suppose the normal voltage existing across the resistor is 40 volts. Because of the 20 percent tolerance, the actual resistance of the replacement may be as much as 1,020 ohms or as little as 680 ohms. If you choose the resistor with the lesser value (the more unfavorable from a heat-dissipating standpoint), you can find the power that may be developed in the resistor under circuit conditions as follows: W E R /c61 2 Where W = power in watts, E = potential in volts, and R = resistance in ohms. W /c61/c18040 40 680 or W = 2.4 watts, approximately Checking After Repair No repair job is complete until you reinstall the repaired unit or component and check to see that it is operating properly. The component must be bench checked after correcting the trouble. Before completely reassembling the component, you should make any alignment or adjustments that are necessary for the proper operation of the component. After re-assembly of the component, replace dust and shielding covers, install the component in the outer case (and pressurize, if necessary), and perform a final bench operational check. Often, when a shield or plate is installed, the shield or plate will touch a bare wire or make other contact and make the component inoperative or cause substandard operation. It is much better to discover such a fault at the bench than in the aircraft. 5-15 IF the component to be replaced is a … THEN you must … Resistor Match its ohmic value, wattage rating, tolerance, type of construction, and physical dimension. Capacitor Match its capacity, voltage rating, tolerance, temperature coefficient, and physical dimension. Plug or connector Use an exact replacement in most cases, because it is difficult to find an interchangeable item of this type. Table 5-2
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5-16 Step Action 1 Try to locate the trouble by observing the circuit’s faulty operation. Did you find the trouble? If yes, go to step 6. If no, go to step 2. 2 Try to locate the trouble by using eyes and nose. Did you find the trouble? If yes, go to step 6. If no, go to step 3. 3 Localize the trouble at the faulty SECTION by testing techniques. Did you find the trouble? If yes, go to step 6. If no, go to step 4. 4 Localize the trouble at the faulty STAGE by testing techniques. Did you find the trouble? If yes, go to step 6. If no, go to step 5. 5 Localize the trouble at the faulty CIRCUIT or PART by testing techniques and go to step 6. 6 Replace or repair the defective part. 7 Test the circuit’s operation readjust the circuit. Table 5-3A AEf05003 Figure 5-3.—Resistor color codes.
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After installing the component in the aircraft and properly securing it for flight, you must give it a final operational test. You cannot assume that because the component operated properly on the bench it will do so in the aircraft. The most important test is an operational check under exact operating conditions. When the component performs properly in the aircraft and is secure, you may sign off the discrepancy sheet (maintenance action form). Your signature indicates that the electronic component should operate properly under normal flight conditions. The following steps and actions summarize the troubleshooting steps: (See table 5-3A). COLOR-CODE SYSTEMS FOR COMPONENTS As a technician, you need to know the different color codes that identify resistors, capacitors, wiring, and other components. Resistor color codes (fig. 5-3) lets you quickly identify size (in ohms) and tolerances. You can use color codes, along with Resistors, Selection and Use of MIL-HDBK-199 to identify or find suitable replacements. Capacitor color-coding is one of two methods used to identify capacitors. Figures 5-4, 5-5, 5-6, and 5-7 are several examples of capacitor color coding for different 5-17 AEf05004 Figure 5-4.—Six-dot color code for mica and molded paper capacitors.
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5-18 AEf05005 Figure 5-5.—Six-band color code for tubular paper dielectric capacitors.
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5-19 AEf05006 Figure 5-6.—Ceramic capacitor color code.
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5-20 AEf05007 Figure 5-7.—Mica capacitor color code. AEf05008 Figure 5-8.—Semiconductor diode markings and color-code system.
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styles of capacitors. The other method is the typographical method where a number is stamped on the capacitor. Semiconductor diodes and transformers also have color-coding identification. See figures 5-8 and 5-9. To allow a sufficient safety margin, a resistor should be capable of dissipating from 1.5 to 2 times the power it will actually meet. In the above example, this value is not more than 4.7 watts. Since a 5-watt resistor is the next standard size above the 4.7-watt value, this is a desirable wattage rating for the replacement. 5-21 AEf05009 Figure 5-9.—Color codes for transformers.
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Q5-13. What are the two broad categories of mainte- nance? Q5-14. What is the primary purpose of preventive maintenance? Q5-15. What is the first step you should take when receiving a discrepancy? Q5-16. Describe the use of continuity tests. Q5-17. Describe the major method for testing shorts in transformers, motor windings, and capaci- tors Q5-18. What is the effective method of locating defec- tive stages in electronic equipment? Q5-19. Use of improper test probes may result in what? Q5-20. What should be done before making resis- tance measurements? Q5-21. After complete reassembly of equipment, what must be done? Q5-22. What must you consider when substituting a resistor to ensure it is a proper substitution? Q5-23. What is the proper color code for a 100 ohm resistor with a 10 percent tolerance? AIRCRAFT AND EQUIPMENT WIRING LEARNING OBJECTIVES : Identify var- ious wire and cable characteristics. Iden- tify proced ures for installing wiring and cables. An important part of aircraft electrical maintenance is determining the correct wire or cable (fig. 5-10) for a given job, either replacing or installing wire. For electrical installations, the term wire refers to a stranded conductor that is covered with an insulating material. The term cable, as used in aircraft electrical installation, includes the following: • Two or more insulated conductors contained in the same jacket (multiconductor cable) • Two or more insulated conductors twisted together • One or more insulated conductors covered with a metallic braided shield (shielded cable) • A single insulated center conductor with a metallic braided outer conductor (RF cable) WIRE REPLACEMENT After determining the wire or cable size, consider insulation characteristics of the wire or cable. To account for weight discrepancies between the original wire or cable and replacement wire or cable, always refer to NA V AIR 01-1A-505 before selecting the replacement wire or cable. Consider the following factors: • The need for the insulated wire or cable to be used in combination with wire or cable that has other insulating material • The temperature and voltage rating ranges of the wire or cable • The need for the wire or cable to be high-temperature and fire-resistant • The need for the wire or cable insulation to operate efficiently in high ambient temperatures • The need for the insulation design of the wire or cable to assure emergency operation of an elec- trical circuit subject to flaws To find this information for electrical wire, refer to the military standard (a performance and commercial specification and standard) for wire. To identify the part number for the wire, use the same military specification number that is printed or stenciled on the reel, spool, or shipping container along with wire size. Thus, MIL-W-2538-18 refers to number 18 aircraft wire (W) of the military standard 2538 specification. Identify electrical cable in the same manner as wire, only with a C (for cable) instead of a W (for wire). Thus, military standard MIL-C-7078 specification describes three types of electrical cable with twisted, color-coded wires as follows: • Unshielded and unjacketed two or more wires with no overall jacket or shield • Jacketed two or more wires with no overall shield enclosed within a single jacket • Shielded and jacketed one or more wires with an overall shield enclosed within a single jacket Military standard MIL-C-27500 specifies an electrical cable made up of two to seven wires. MIL-C-27500 cable has spirally laid, color-coded wires in the following three configurations: • Unshielded and unjacketed wires without an overall jacket 5-22
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• Jacketed wires with an overall jacket • Shielded and jacketed one to seven wires with one or two shields within an overall jacket Wire and Cable Identification To make aircraft maintenance easier, each connecting wire or cable in an aircraft has identification marked on it. The identification is a combination of letters and numbers. The marking identifies the circuit that the wire or cable belongs to, the gauge size of the wire or cable, and the information that relates the wire or cable to a wiring diagram. This marking uses the wire or cable identification code. You can find details of the wire and cable identification system in Military Specification: Wiring, Aerospace V ehicle , SAE-AS50881. Wiring and Cable Identification Codes The block of wire numbers for each item of equipment starts with the number1 and continues for as many numbers as needed to identify all wires. If a military type designation (AN [Air Force-Navy] nomenclature) is not assigned for a piece of equipment, such as with commercial equipment, get a block of numbers from the procuring activity. To get the wire identification code for equipment with military type (AN nomenclature) equipment designation, use that portion of the equipment designation that follows the slash on AN equipment and exclude the hyphen and 5-23 A. MULTI-CONDUCTOR CABLE/JACKETED/TWISTED B. SHIELDED SINGLE CONDUCTOR CABLE C. SHIELDED MULTI-CONDUTOR CABLE/JACKETED/TWISTED D. UNSHIELDED/UNJACKETE/TWISTED 1 2 3 4 5 6 CONDUCTOR PRIMARY INSULATION WIRE JACKET BRAIDED TINNED COPPER SHIELD PROTECTIVE OUTER JACKET COLOR CODED WIRE (MIL-W-22759, -225030 134 5 4 6 5 2 5 6 NOTE: MAKE AS REQUIRED BY TWISTING SINGLE CONDUCTORS -81044, -81381) AEf05010 Figure 5-10.—Cables commonly used in aircraft.
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suffix letter. The following are wire-coding examples for military type equipment: Equipment Wire Code AN/APS-45 APS45-1A20-APS45-975C22 AN/ARC-52A ARC52-1A22-ARC52-9C22 MX-94 MX94-1A20 NOTE: For an in-depth study of aircraft wiring specifications, limitations, and repair, refer to NA V AIR 01-1A-505. The basic wire identification code for circuits is read from left to right. Refer to figure 5-11. Use prefix unit numbers 1, 2, 3, 4, and so forth where two or more identical items of equipment are in the same aircraft to differentiate between wires or cables for each item of equipment. To make it easier to interchange items, identical wiring or cable is located in left and right wings, nacelles, and major interchangeable structural assemblies without using the unit number. For equipment with circuit function letters R, S, T,o r Y, use the unit number only where complete duplicate equipment is installed. Unit numbers don't 5-24 SUFFIX GROUND, PHASE, OR THERMOCUPLE LETTER WIRE SIZE NUMBER WIRE SEGMENT LETTER WIRE NUMBER CIRCUIT FUNCTION LETTER UNIT NUMBER GROUND, PHASE, OR THERMOCUPLE LETTER WIRE SIZE NUMBER WIRE SEGMENT LETTER WIRE NUMBER CIRCUIT FUNCTION LETTER UNIT NUMBER A - AS APPLIED TO ALL CIRCUIT FUNCTIONS EXCEPT R, S, T, AND Y B - AS APPLIED TOCIRCUIT FUNCTIONS EXCEPT R, S, T, AND Y ALTERNATE METHOD AEf05011 Figure 5-11.—Examples of wire identification coding.
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apply to duplicate components within single complete equipment such as duplicate indicators or control boxes. The circuit function letter identifies the circuit function (table 5-3B). When using a wire or cable for more than one circuit function, use the circuit function letter of the predominant circuit. When functional predominance is questionable, use the circuit function letter for the wire or cable having the lowest wire number. Substitute the contractor-assigned “equipment identification” instead of the circuit function letter for equipment that has an R, S, T, or Y circuit function. The wire number identifies the different wires in a circuit and consists of one or more digits. A different number is used for wire not having a common terminal or connection as follows: • Wires with the same circuit function with a common terminal connection or junction have the same wire number but different segment letters. • Assign a number to each wire in numerical sequence beginning with the lowest number when practical. 5-25 Circuit function letter Circuits A Armament B Photographic C Control surface D Instrument (other than instrument & flight) E Engine instrument F Flight instrument G Landing gear, wing folding H Heating, ventilating, and de-icing J Ignition K Engine control L Lighting M Miscellaneous (electrical) P Dc power Q Fuel and oil R Radio (navigation and communication) S Radar (pulse technique) T Special electronic U Miscellaneous (electronic) V Dc power cables and dc control cables for ac systems W Warning and emergency X Ac power Y Armament special systems Z Experimental circuits Table 5-3B.—Wiring Circuit Function Codes
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A wire segment is a conductor between two terminals or connections and identifies different conductor segments in a particular circuit. Use a different letter for wire segments having a common terminal or connection. Wire segment letters are in alphabetical sequence. The letter A identifies the first segment of each circuit starting at the power source. If a circuit contains only one wire segment, this wire segment is A. Do not use the letters I or O as segment letters. Use double letters AA, AB, AC, and so forth when there are more than 24 segments. Two permanently spliced wires do not require separate segment letters if the splice is for modification or repair. The wire size number identifies the size of the wire or cable. Do not include the wire size for coaxial cables and thermocouple wires. For thermocouple wires, use a dash (-) instead of the wire size number. NOTE: Stranded conductor wire is used for flexibility in installation and service. Although aircraft wire sizes approximate American Wire Gage (AWG) wire size, aircraft wire sizes vary enough from AWG wire sizes for it to be improper to refer to aircraft wire size as AWG. Use the ground, phase or thermocouple letter as follows: • Ground cable letter N is a suffix to the wire identification code. It identifies any wire or cable that completes the circuit to the ground network. Such wires and cables connect to the ground network of aircraft electrical systems without causing any circuit malfunctions. For electronic systems with interconnecting ground leads, but only one segment actually grounded to structure, N identifies the segment actually grounded to the structure. • Phase letter A, B,o r C is a suffix on the wire identification code. It identifies the phase or wires in the three-phase power ac distribution systems. The phase sequence is A→B→C. • Phase letter V is a suffix on the cable identification code. It identifies the ungrounded wire or cable in a single-phase system. • For thermocouple wire, use the following suffixes: CHROM—Chromel ALML—Alumel IRON—Iron CONS—Constantan COP—Copper NOTE: Chromel and Alumel are registered trademarks of Hoskins Manufacturing Company. Add the suffix ALUMINUM or ALUM to the identification code (when required) when using aluminum wire. Wire Marking You may stamp the identification code on wires either horizontally or vertically as shown in figure 5-11. Stamping the identification marking directly on the wire or cable with a hot foil-stamping machine is the preferred method. Use this method wherever possible. If the wire insulation or outer covering won't stamp easily, stamp lengths of insulating tubing (sleeves) with the identification marking. Then, install the sleeve on the wire or cable. The following types of wire usually have sleeve identification markings: • Unjacketed shielded wire • Thermocouple wire • Multiconductor cable • High-temperature wire with insulation difficult to mark, such as TFE, fiberglass, and so forth CAUTION DO NOT use metallic markers or bands for iden- tification. DO NOT use any method of marking that will dam- age or deform the wire or cable. Use the following guidance with whatever method you use to mark the wire: • Make sure the marking is legible and the color contrasts with wire insulation or sleeving. • Use black stamping for light-colored back- grounds and white on dark-colored back- grounds. • Make sure that markings are dry so they don't smear. • Stamp wires and cables at intervals of not more than 15 inches along their entire lengths (fig. 5-12). • Stamp wires within 3 inches of each junction (except permanent splices) and at each ending point. 5-26
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• Stamp 3- to 7-inch-long wires in the center. • You don’t need to stamp a wire if it is less than 3 inches long. WIRING INSTALLATION Wiring installation has the following order of precedence: 1. Safety of flight 2. Ease of maintenance 3. Cost effectiveness Safety of flight is always the prime concern in maintenance and must not be compromised by anyone for any reason. Ease of maintenance means that wire should be installed to meet the following criteria: • Maximum reliability • Minimum interference and coupling between systems • Accessibility for inspection • Accessibility for maintenance • Prevention of damage The cost effectiveness pertains to the contractor but also depends upon correct maintenance practices. Routing Wires Install wiring so that it is mechanically and electrically sound and neat in appearance. Route wiring to assure reliability and offer protection from the following hazards: • Chafing • Use as handholds or as support for personal equipment • Damage by personnel • Damage by stowage or shifting of cargo • Damage by battery or acid fumes and fluids • Abrasion in wheel wells where exposed to rocks, ice, mud, and so forth • Combat damage (to the maximum extent pos- sible) • Damage by moving parts • Harsh environments such as swamp areas, high temperatures, or areas susceptible to significant fluid or fume concentration Slack in Wiring Install wiring to provide enough slack to prevent strain on wires and to permit access to equipment during maintenance. When terminating wiring in a connector (excluding RF connectors), provide at least 1 inch of slack for complete connector replacement. Place slack between the connector and the second wiring support clamp. The 1-inch slack requirement means that with the connector unmated and the first wiring support clamp loosened, the wiring will permit the front end of the connector shell to extend 1 inch beyond the point normally required to properly mate the connector. At each end of a wire terminated by a lug, provide a minimum length of slack equal to twice the barrel length of the lug. For copper wire, size 2 AWG and larger, and aluminum wire, size 4 AWG and larger, the minimum length of slack should be equal to 5-27 15" MAX ROTATE 180 BETWEEN IMPRESSIONS IF PRACTICABLE H215A20 H215A20 AEf05012 Figure 5-12.—Spacing of identification stamping on wire and cable.
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one barrel length of the lug. The slack must be in the vicinity of the lug and available for replacement of the lug by maintenance personnel. Tying and Lacing Wire Groups and Bundles A wire group is two or more wires tied or laced together to give identity to an individual system. Awire bundle is two or more wires or groups tied or laced together to provide easier maintenance. Tying is the securing together of a group or bundle of wires by individual pieces of cord tied around the group or bundle at regular intervals. Lacing is the securing together of wires inside enclosures by a continuous piece of cord, forming loops at regular intervals around the wire group or bundle. Tie or lace wire groups and bundles together. This makes it easier to install, maintain, and inspect them. Also, it keeps the cables neatly secured in groups and bundles to help avoid damage from chafing or equipment operation. Wherever possible, use a narrow, flat, non-adhesive tape for tying and lacing. You may use round cord; however, it has a tendency to cut into wire insulation. Use cotton, linen, nylon, or glass fiber cord or tape, according to the temperature requirements. Pre-wax cotton or linen cord or tape to make it moisture- and fungus-resistant. Nylon cord or tape may be waxed or unwaxed. Glass fiber cord or tape is usually not waxed. PRECAUTIONS FOR TYING AND LACING WIRE GROUPS .—When tying and lacing wire groups and bundles, use the following precautions: • Tie or lace bundles tightly enough to prevent slipping, but not so tightly that the cord cuts into or deforms the insulation. This applies especially to coaxial cable, which has a soft dielectric insulation between the inner and outer conductors. • DO NOT place ties on that part of a wire group or bundle located inside a conduit. • Lace wire groups or bundles only inside enclosures, such as junction boxes. • Use double cord on groups or bundles larger than 1 inch in diameter. Use single or double cord for groups or bundles 1 inch or less in diameter. NOTE: Coaxial cables can be damaged from lacing materials or methods of lacing or tying wire bundles that cause a concentrated force on the cable insulation. Elastic lacing materials, small-diameter lacing cord, and excessive tightening deform the inner conductor insulation, which may result in short circuits or impedance changes. Flat, nylon, braided, waxed lacing tape is recommended for coaxial cables. PROCEDURES FOR LACING WITH A SINGLE CORD.— Use the following procedures to lace a wire group or bundle with a single cord: 1. Start the lacing at the thick end of the wire group or bundle with a knot consisting of a clove hitch with an extra loop. See figure 5-13. 2. At regular intervals along the wire group or bundle and at each point where a wire or wire group branches off, continue the lacing with half hitches. Space half hitches so the group or bundle is neat and securely held. 3. End the lacing with a knot consisting of a clove hitch with an extra loop. 4. Trim the free ends of the lacing cord to three-eighths inch minimum. 5-28 PULL TIGHT BEFORE FINISHING KNOT INTERMEDIATE HALF HITCHES STARTING KNOT STARTING KNOT TIGHTENED TRIM TO 3/8" MIN. FIRST PART OF FINAL KNOT TIGHTENED FINAL KNOT AEf05013 Figure 5-13.—Single-cord lacing.
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PROCEDURES FOR LACING WITH A DOUBLE CORD.— Use the following procedures to lace a wire group or bundle with a double cord: 1. Start the lacing at the thick end of the wire group or bundle with a bowline on a bight. See figure 5-14. 2. At regular intervals along the wire group or bundle and at each point where a wire group branches off, continue the lacing with half hitches, holding both cords together. Space half hitches so the group or bundle is neat and securely held. 3. End the lacing with a knot consisting of a half hitch, using one cord clockwise and the other counterclockwise, and then tie the cord ends with a square knot. 4. Trim the free ends of the lacing cord to three-eighths inch minimum. PROCEDURES FOR LACING A BRANCH- ING WIRE GROUP.— Use the following procedures to lace a wire group that branches off the main wire bundle: 1. Start the branch-off by lacing with a starting knot located on the main bundle just past the branch-off point. See figure 5-15. When using single-cord lacing, make the starting knot the same as regular single-cord lacing. When using double-cord lacing, use the double-cord lacing starting knot. 5-29 INTERMEDIATE HALF HITCHES STARTING KNOT- BOWLINE ON A BRIGHT STARTING KNOT TIGHTENED AEf05014 Figure 5-14.—Double-cord lacing. MAIN BUNDLE LACING STARTING KNOT FOR BRANCH-OFF HALF HITCH USED AT BRANCH-OFF POINT BRANCH-OFF LACING AEf05015 Figure 5-15.—Lacing a branch-off.
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2. End the lacing with the regular knot used in single- and double-cord lacing (for single-cord lacing, a clove hitch with a single loop; for double-cord lacing, a half hitch, using one cord clockwise and the other counterclockwise, then tied with a square knot). 3. Trim the free ends of the lacing cord to three-eighths inch minimum. TYING WIRE GROUPS WHEN SUPPORTS ARE MORE THAN 12 INCHES.— Tie all wire groups or bundles (fig. 5-16) when supports are more than 12 inches apart. Space the ties so they are 12 inches or less apart. To make a tie, use the following procedures: 1. Wrap cord around wire group or bundle, as shown in figure 5-16, view A. 2. Make a clove hitch followed by a square knot with an extra loop. 3. Trim free ends of cord to three-eighths inch minimum. TYING SLEEVES TO WIRE GROUPS OR WIRE BUNDLES.— When tying sleeves to wire groups or wire bundles, make the ties the same as for wire groups and bundles (use a clove hitch followed by a square knot with an extra loop). Tape When it is permissible to use tape, use the following method: 1. Wrap tape around the wire group or bundle three times, with a two-thirds overlap for each turn. See figure 5-16, view B. 2. Heat-seal the loose tape end with the side of a soldering iron heating element. DO NOT use tape to secure a wire group or bundle that requires frequent maintenance. USING SELF-CLINCHING CABLE STRAPS.— Self-clinching cable straps are adjustable, lightweight, flat nylon strips. See figure 5-17, view A. They have molded ribs or serrations on the inside surface to grip the wire. You may use them instead of individual cord ties for quickly securing wire groups or bundles. The straps are of two types—a plain cable strap and one that has a flat surface for identification of cables. CAUTION DO NOT use nylon cable straps over wire bundles containing coaxial cable. DO NOT use straps in areas where failure of the strap would allow the strap to fall into movable parts. Installing self-clinching cable straps is done with a military standard hand tool as shown in figure 5-17, view B. Follow the manufacturer's instructions when using the tool. W ARNING Use proper tools and make sure the strap is cut flush with the eye of the strap. This prevents painful cuts and scratches caused by protruding strap ends. DO NOT use plastic cable straps in high-temperature areas (above 250°F). Heat-Shrinkable Tubing Heat-shrinkable tubing is a plastic-like tubing (similar to insulation sleeving) that will shrink to a smaller diameter when heated. Place the tubing over the joint, terminal, or part needing insulation. Now apply heat with a heat gun, oven, or other heat source. When the tubing reaches a specific temperature (shrink temperature depends upon the type of tubing), it quickly shrinks around the object, forming a snug jacket. In addition to being an insulator, the shrinkable tubing helps relieve strain and adds waterproofing. Figure 5-18 gives some of the typical uses of heat-shrinkable tubing. 5-30 WRAP CORD TWICE OVER BUNDLE CLOVE HITCH & SQUARE KNOT HEAT SEAL 2/3 OVERLAP THREE WRAPS (A) (B) AEf05016 Figure 5-16.—Tying groups or bundles.
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5-31 INSTALLING SELF-CLINCHING CABLE STRAPS CONFIGURATION MAY VARY INDEX LINE (SEE NOTE) TENSION LOCATOR (LOCATION OPTIONAL) TENSION ADJUSTMENT (KNOB SHAPE OPTIONAL) SPRING RETURN HANDLE SELF-CLINCHING CABLE STRAP TOOL THIS TOOL FOR INSTALLING MS3367 AND MS3368 PLASTIC TIE DOWN STRAPS MS 90367-1(TYP) INCREASETENSION (A) (B) AEf05017 Figure 5-17.—Installing self-clinching straps. COAXIAL CABLE CONNECTOR SLEEVING FOR MOISTURE-PROOFING AND STRAIN RELIEF. TOUGH, SEMI- RIGID HEAT-SHRINKABLE TUBING PROVIDES STRAIN RELIEF BY TRANSFERRING THE FLEXING STRESS FROM THE WIRE INSULATION DIRECTLY TO THE CONNECTOR PIN, TERMINAL OR COMPONENT BODY. THE STRESS ON THE BARE CONDUCTOR JOINT IS THUS RELIEVED AND THE CONNECTION MADE RELIABLE. BUSBAR INSULATION TERMINAL INSULATION CLEAR SLEEVING FOR INSULATION IDENTIFICATION AND INSPECTION ARE NOT HINDERED. INSULATION, COLOR CODING AND IDENTIFICATION. HARNESS JACKET AND CONNECTOR BOOT. BOTH SHRINK TO PROVIDE TIGHT COVERING FOR WIRE BUNDLE AND MOISTURE-PROOFING AND STRAIN RELIEF FOR CONNECTOR TERMINALS WITHOUT POTTING. DISCONNECT INSULATION AND MARKING. AEf05018 Figure 5-18.—Typical heat shrinkable tubing.
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Cable Stripping Nearly all wire and cable electrical conductors have some type of insulation. When making electrical connections with wire, you must remove a part of this insulation, leaving the end of the wire bare. To help you remove insulation, use a wire and cable-stripping tool similar to the one shown in figure 5-19. The operation of this basic tool is efficient and simple. To operate it, insert the wire end in the proper direction and depth to be stripped. Now position the wire so it rests in the groove for the size wire being stripped and squeeze. Q5-24. Where should you first look for wire replace- ment information? Q5-25. What type wire should you use to carry 600-1000 volts with a temperature rating be- tween 302°F to 500°F? Q5-26. When stamping wire identification numbers, at what interval should you stamp the wire? Q5-27. In wire identification numbers, what does the suffix N mean? Q5-28. When wiring is terminated with a connector, what is the minimum slack that should be provided? Q5-29. Why is flat, nylon, braided, waxed lacing tape preferred for tying coaxial cables? Q5-30. What is the purpose of heat shrink tubing? AIRCRAFT AND EQUIPMENT HARDW ARE LEARNING OBJECTIVES: Recognize air- craft hardware and equipment hardware and their uses in aircraft maintenance. The hardware you should use when installing electrical equipment in aircraft is specified in the applicable MIM. Some of the types of hardware discussed in this section include electrical connectors, conduit and fittings, junction boxes, safety wiring, shock mounts, and bonding. You should always use proper parts, but you shouldn't always use the same mounting parts that you removed from the installation. Before reusing a part, inspect it to make sure it isn't defective or damaged. Also check the instructions; some parts can't be reused. If you need to substitute a part, make sure the substitute part is satisfactory. Aircraft Structural Hardware for Aircraft Repair, NA V AIR 01-1A-8, and Installation Practices, Aircraft Electric and Electronic Wiring , NA V AIR 01-1A-505, are sources of detailed informa- tion. If you can't get a mounting part specified by the IPB for the electrical equipment, you may make a temporary installation using a suitable substitute part. Replace the part with the item specified by the IPB as soon as you receive it. Always check with your work center supervisor before you make a substitution. When making part substitutions, you should give special con- sideration to the factors in table 5-4. 5-32 SELECT CORRECT HOLE TO MATCH WIRE GAGE BLADES REMAIN OPEN UNTIL WIRE IS REMOVED BE CAREFUL NOT TO NICK OR CUT STRANDS AEf05019 Figure 5-19.—Wire stripping method.
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ELECTRICAL CONNECTORS In this section, the word connector is used in a general sense. It applies to electrical connectors with Air Force-Navy (AN) numbers and those with military specification (MS) numbers. The Air Force and Navy formerly used AN numbers for all supply items cataloged jointly. Many items, especially those of older design, still carry the AN designator. The supply system is shifting to MS numbers. Connector Function Electrical connectors provide detachable coupling between major components of electrical and electronic equipment. These connectors are built to withstand the extreme operating conditions imposed by airborne service. They must make and hold electrical contact without excessive voltage drop despite extreme vibration, rapid shifts in temperature, and changes in altitude. Moisture-proof Connector Use Present Navy practice is to use potted connectors (moisture-proof or environment-proof connectors). All jet- and carrier-type aircraft have potted connectors. Other aircraft need moisture-proofing sealant on electrical connectors in areas where a chance of failure exists. Connectors in wheel wells, wing fold areas, 5-33 FACTORS CONSIDERATIONS Corrosion • Pay attention to the chemical or metallic composition of the part. • Choose a part that doesn't contribute appreciably to the danger of corrosion. Strength • The strength of the substitute part must be the same or greater than the one prescribed. (When determining the strength, consider the tensile, compression, and shear strength, as applicable to the specific use.) Size • Substitute nuts, bolts, and screws should be the same size as the prescribed item. • Washers must have the same inner diameter as the prescribed item. A different outer diameter or thickness is acceptable. Length • The length of substitute screws or bolts must be enough for the particular installation. However, length can't be long enough to interfere with any moving part. • Hardware shouldn't come in contact with other aircraft items, such as electrical wiring, hydraulic lines, and so forth. Magnetic properties • Equipment installed in specific areas of the aircraft shouldn't cause distortion of the magnetic fields of the area. Examples of such items include the magnetic compass, magnetic anomaly detection equipment, radio direction finder, or gyros. In areas containing these types of equipment, any substitute part must have the same magnetic properties and characteristics as the one prescribed. Style • Most items of mounting hardware are available in various styles. It is usually easy to find screws and bolts that are the same in all respects except the type head. Use these parts as substitutes when they have the required special features. Special features • If a bolt requires torque to a given value, a suitable torque wrench for that type part must be available. • If the MIM calls for lock wire, the part must have suitable provisions for lock wire. Lubrication or coating • If specific instructions call for lubrication or coating of the parts, follow those instructions for the substitute part as well as for the prescribed part. Table 5-4.—Factors To Consider When Making Part Substitutions
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engine areas, engine nacelles, or cockpit decks have a high chance of failure and are sealed in addition to connectors that interconnect flight and basic navigation equipment. Moisture proofing reduces electrical connector failures by reinforcing the wires against vibration and lateral pressure at the solder cup. The sealing compound also protects electrical connectors from corrosion and contamination by excluding metallic particles, water moisture, and aircraft liquids and results in better dielectric characteristics that reduce chance of arc-over between pins. Connector Design Connectors consist of two portions—the fixed portion, called the receptacle, and the movable portion, called the plug. Plug assemblies may be straight or angled (usually 90 degrees). Receptacle assemblies may be of the wall-mounted, box-mounted, or integral-mounted types. MS numbers and letters identify the type, style, and arrangement of a connector. Connectors vary widely in design and application. A coupling nut or ring holds the two assemblies firmly together. The assembly consists of an aluminum shell containing an insulating insert, which holds the current-carrying contacts. The plug usually attaches to the cable end and is the part of the connector on which the coupling nut mounts. The receptacle is the half of the connector to which the plug connects. The receptacle is usually mounted on a part of the equipment. In naval aircraft, connectors with crimp-type contacts are widely used. Maintenance is easier because you can remove the contact from the connector. If the connector is damaged, you can remove the contacts and replace the connector shell. If just a connector pin is damaged, you can remove and replace the pin. This is a considerable advantage over the solder-type connector, both in convenience and timesavings. A discussion of the special tools you need to remove and insert crimped contacts is contained in Installation Practices, Aircraft Electric and Electronic Wiring, NA V AIR 01-1A-505. Some common types of subminiature connectors are shown in figure 5-20. They are used on instruments, switches, transformers, amplifiers, relays, and so forth. CONDUIT AND FITTINGS In many aircraft, the use of conduit (pipe or tubing) for electrical wire or cable is limited. This practice saves weight and ensures wide separation of wire or cables. The separation of the electrical system makes it less vulnerable to gunfire. However, some current aircraft, especially those with limited space for wire routing, use conduit. Some examples of conduit and fittings are shown in figure 5-21. Conduit comes in two basic types—flexible and rigid. Its chief functions are to act as radio shielding and as a support and protection for wires. Conduit fittings attach conduit to junction boxes and other equipment, and usually include ferrules and coupling nuts. Various forms of both are in use along with special designs of locknuts, box connectors, and coupling adapters. Couplings for conduit are straight or angular in design. A ferrule is a bushing or flange applied to the end of conduit to give greater strength and support to the coupling nuts. A bushing or flange is crimped or swaged on conduit with a crimping or swaging tool. 5-34 PLUG WITH SOCKET INSERT PLUG WITH PIN INSERT RECEPTACLE WITH INSERT AEf05020 Figure 5-20.—Subminiature connectors.
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SUPPORT CLAMPS Clamps provide support for conduit and open wiring and serve as lacing on open wiring. A clamp usually has a rubber cushion or is of all-plastic construction. When used with shielded conduit, the clamp is of the bonded type (fig. 5-22), that is, there is a provision for electrical contact between the clamp and conduit. Use unbonded clamps for the support of open wiring. Long cable runs between panels need the use of a strap-type clamp (fig. 5-23, view A) or an AN 742 cable clamp (fig. 5-23, view B). The preferred method for supporting cable runs of all types is using AN 742 cable clamps. MS 25281D plastic clamps are for use where the maximum temperature does not exceed 250°F. When using the strap-type clamp, you must make sure the clamps hold the cable firmly away from lines, surface control cables, pulleys, and all movable parts of the aircraft. Use these clamps only as a temporary measure. Replace with a permanent installation as soon as possible. When cables pass through lightening holes, the installation should conform to the examples shown in figure 5-24. In each case, the AN 742 cable clamp holds the cable firmly. Route the cable well in the clear of the edges of the lightening hole to avoid chance of chafing the insulation. If wires are closer than one-fourth inch to the edge of the lightening hole, use a grommet (a rubber cushion) to protect the wires. Protect wire bundles from the following: • High temperature • Battery acid fumes, spray, or spillage • Solvents or fluids • Abrasion in wheel wells where exposed to rocks, ice, or mud • Damage due to personnel using the wire bundle as handholds or footsteps • Damage due to shifting cargo 5-35 A B AEf05021 Figure 5-21.—Conduit and fittings. CUSHION BONDING STRIP AEf05022 Figure 5-22.—Bonded-type cable clamp. MS 25281D STRAP TYPE CLAMP AN742 CLAMP (A) (B) AEf05023 Figure 5-23.—(A) Strap type cable clamp and (B) AN 742 cable clamp. AN 743 BRACKET AN 742 CABLE CLAMP AN 742 CABLE CLAMP AN 742 CABLE CLAMP AEf05024 Figure 5-24.—Routing cables through lightening holes.
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Never support any wire or wire bundle from a plumbing line carrying flammable fluids or oxygen. Use clamps on these lines only to ensure separation of the wire bundle from the plumbing line. Whenever possible, route wires and bundles parallel with or at right angles to the stringers or ribs of the area involved, as seen in figure 5-25. Don't install single wires or wire bundles with excessive slack. Slack between support points, such as cable clamps, should not normally exceed one-half inch. (This is the maximum you should be able to deflect the wire with moderate hand force.) You may exceed this slack if the wire bundle is thin and the clamps are far apart. The slack must never be so large that the wire bundle can touch any surface. Allow a sufficient amount of slack near each end for the following reasons: • To permit ease of maintenance • To allow replacement of terminals at least twice • To prevent mechanical strain on the wires, cables, junctions, and supports • To permit free movement of shock- and vibration-mounted equipment • To permit shifting of installed equipment for purposes of maintenance TERMINALS Since most aircraft have stranded wires, you use terminal lugs to hold the strands together and make it easier to fasten wires to terminal studs. The types of terminals used in electrical wiring are either soldered or crimped. Terminals used in repair work must be the size and type specified on the electrical wiring diagram for the model being maintained. You may use soldered- and crimped-type terminals interchangeably, but both must have the same amperage capacity and the same size hole in the lug. The increased use of crimp-on terminals is, to a large degree, due to the limitations of soldered terminals. The quality of soldered connections depends upon the operator's skill. Such factors as temperature, flux, cleanliness, oxides, and insulation damage caused by heat contribute to defective connections. The crimp-on solderless terminals require relatively little operator skill. Another advantage is that the use of a crimping tool eliminates the necessity of supplying power to a soldering iron. This allows installing terminals in an aircraft with a minimum of time and effort. The connections are made more rapidly, are cleaner, and are more uniform. Because of the pressures exerted and the materials used, the crimped connection or splice (when properly made) has an electrical resistance that is less than that of an equivalent length of wire. 5-36 CORRECT: BUNDLE IS AT RIGHT ANGLES TO RIB STRUCTURE CORRECT: BUNDLE IS PARALLEL TO RIB STRUCTURE INCORRECT: BUNDLE ANGLES ACROSS RIB STRUCTURE AEf05025 Figure 5-25.—Routing cables.
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The basic types of terminals are shown in figure 5-26. View A shows the straight type, view B the right-angle type, view C the flag type, and view D the splice type. There are also variations of these types. Variations may include the use of a slot instead of a terminal hole, three- and four-way splice-type connectors, and others. Since present-day aircraft have both copper and aluminum wiring, both copper and aluminum terminals are necessary. There are various size terminal and stud holes for each of the different wire sizes. A further refinement of the solderless terminals is the insulated type, where insulation encloses the barrel of the terminal. The crimping process compresses the insulation along with the terminal barrel but does not damage it in the process. This eliminates the need for taping or tying an insulating sleeve over the joint. Terminal Blocks Terminal blocks, made from an insulating material, support and insulate a series of terminals from each other, as well as from ground. They provide a way to install terminals within junction boxes. The two methods of attaching cable terminals to terminal blocks are shown in figure 5-27. View A uses a standard nonlocking nut. In this installation method, the use of a lock washer is necessary. View B shows the preferred method. When using an anchor nut, or self-locking nut, you omit the lock washer. The use of anchor nuts is especially desirable in areas of high vibration. In both installations, you must use a flat washer, as shown in the drawing. The letters TB followed by the number of the individual board identify each terminal board in the aircraft electrical system. A number identifies each stud on the terminal board. The lowest number in the series starts at the end nearest the terminal board identification number. The identification number is on the structure to which the terminal board attaches. It mounts on an identification strip cemented to the structure under the terminal board. When replacing a terminal board, don't remove the identification marking. If the identification marking is damaged, replace the marking with one that is the same as the original. Junction Boxes Junction boxes accommodate electrical terminals or other equipment. Individual junction boxes are named according to their function, location, or equipment with which they are associated. Junction boxes have a drain hole (except boxes labeled vapor tight) at the lowest point to drain water, oil, condensate, or other liquids. Figure 5-28 shows a representative junction box for housing and protecting several terminal blocks. 5-37 TONGUE BARREL TERMINAL HOLE (A) (B) (C) (D) AEf05026 Figure 5-26.—Types of solderless terminals. AN 340 OR AN 345 AN 935 AN 960 AN 365 OR AN363 AN 935 AN 960 (A) (B) AEf05027 Figure 5-27.—Installation of cable terminals on terminal block. AEf05028 Figure 5-28.—Aircraft junction box.
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When you install a junction box, make sure the screw or bolt heads are inside the box. Don't install attaching hardware so the threaded part of the screw or bolt protrudes inside the junction box. The sharp thread edges of protruding hardware may damage wire insulation. SAFETY WIRING Some equipment parts require a positive safety-locking device. The use of safety wire is one accepted method of providing this safety measure. Two of the most common types of safety wiring are lock wire and shear wire. Lock wire and shear wire serve two complete and distinctly different purposes. Lock wire secures nuts, bolts, screws, and connectors from movement. Shear wire ties electrical switching devices in the OFF position. W ARNING Loss of life may occur when lock wire is used instead of shear wire. Under no circumstances shall lock wire and shear wire procedures and materials be mixed or interchanged. Lock Wire Lock wire, also referred to as safety wiring, is used to prevent accidental loosening of aircraft equipment and connectors due to vibration. You should always use new safety wire on every job. Be careful to use pliers only on the ends of the wire so you don't nick the wire. If safety wire becomes nicked, discard it and use a new piece. NEVER back off or over-torque to align holes for safety wiring. DOUBLE-TWIST METHOD.— The most com- mon method of safety tying nuts, bolts, and screws is the double-twist method. You can do this by hand or with special safety wire pliers. (See fig. 5-29) If you make the twists in safety wire by hand without pliers, use pliers to make the final few twists so there is enough tension to secure the ends of the wire properly. Install and twist safety wire so that the loop around the head stays down and does not tend to come up over the bolt head. When you twist the safety wire together, be extremely careful to ensure it is tight, but do not overstress it to the point where the wire will break under a slight load or vibration. After you make the final twists of safety wire with pliers, cut off the nicked loose ends and bend the end of the wire around the bolt or screw head. This will protect personnel from the sharp ends. SINGLE-WIRE METHOD.— You may use the single-wire methodof safety wiring (fig. 5-30) on small screws in a closely spaced area provided the screws form a closed geometrical pattern. Note that any loosening tendencies will pull against the tension of the wire. SAFETY-WIRING CONNECTORS.— Secure an electrical connector with safety wire only when specified on engineering drawings or when experience has shown that the connector will not stay tight. Electric connectors are usually safety-wired in engine nacelles, in areas of high vibration, and in locations not readily accessible for periodic maintenance inspection. When you must safety-wire electrical connectors, you should use 0.032-inch-diameter safety wire wherever possible. On a small part with a 0.045-inch-nominal-diameter hole or smaller, use 0.020-inch-diameter safety wire. Sometimes the connector to be safety-wired does not have a wire hole. If there is no wire hole, remove the coupling nut and drill a No. 56 (0.045-inch-diameter) hole diagonally 5-38 NOTE: SAFETY METHODS SHOWN ARE FOR RIGHT HAND THREADS LEFT HAND OPPOSITE BOLT HEADS CASTLE NUTS SCREW HEADS DOUBLE-TWIST METHOD AEf05029 Figure 5-29.—Double-twist method of safety tying. SMALL SCREWS IN CLOSELY SPACED CLOSED GEOMETRICAL PATTERN SINGLE-WIRE METHOD AEf05030 Figure 5-30.—Single-wire method of safety wiring.
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through the edge of the nut. Figure 5-31 shows a safety-wired connector. Shear Wire Shear wire is used for emergency device protection against accidental activation. An example of safety-wiring a guarded switch is shown in figure 5-32. You can see that the wire is not twisted tightly. Use very soft wire; the wire may be either aluminum or copper. Shear wire lets the operator break the wire easily when necessary to engage the switch. SHOCK MOUNTS Electronic equipment is sensitive to mechanical shock and vibration. Therefore, units of electronic equipment are normally shock mounted to provide some protection against in-flight vibration and against launching and landing shock. The specific type of prescribed shock mount will be in the MIM for the specific aircraft, and you should not use a substitute. Shock mounts require periodic inspections. Replace any defective mounts as soon as possible. In the inspection, you should check for chemical decay of the shock-absorbing material, stiffness and resiliency of the material, and overall rigidity of the mount. If the mount is too rigid, it may not provide adequate protection against the shock of launching and landing. If it is not rigid enough, it may permit prolonged vibration following an initial shock. When determining the limits of rigidity and resiliency, you should consider the weight of the mounted unit as well as the possible amounts of positive and negative acceleration the unit may receive. Shock-absorbing materials commonly used in shock mounts are usually electrical insulators. For 5-39 SAFETY WIRE FOR SPLIT SHELL SEE INSERT SEE INSERT SAFETY WIRE FOR COUPLING NUT BEND PIGTAIL AROUND SCREW TO PROTECT PERSONNEL AEf05031 Figure 5-31.—Safety-wiring a connector. BONDING STRAPSHOCK MOUNTS RUBBER MOUNTSAEf05033 Figure 5-32.—Shear wire on a switch guard.
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safety, each electronic unit mounted in this manner is electrically bonded to a structural member of the aircraft. Examples of two types of shock mounts are shown in figure 5-33. The inspection of the shock mounts should include the bonding straps. Replace or redo any defective or ineffective bonds straps. BONDING A bond is any fixed union between two metallic objects that results in electrical conductivity between them. Such a union results either from physical contact between conductive surfaces of the objects or from the addition of a firm electrical connection between them. Aircraft electrical bonding is the process by which the necessary electrical conductivity between the component and metallic parts of the aircraft is gotten. An isolated conducting part of an object is one that is physically separate (by intervening insulation) from the aircraft structure and from other conductors bonded to the structure. A bonding connector provides the necessary electrical conductivity between metallic parts in an aircraft where electrical contact is insufficient. Reasons for Bonding An aircraft can become highly charged with static electricity while in flight. In an improperly bonded aircraft, all metal parts will not have the same amount of charge, and a difference of potential will exist between various metal surfaces. Charges flowing through paths of variable resistance, such as moving control surfaces, will produce electrical disturbances (noise) in the radio receiver. If the resistance between isolated metal surfaces is large enough, charges can accumulate until the potential difference becomes high enough to cause a spark, creating a fire hazard. If lightning strikes an aircraft, a good conducting path is necessary for the heavy current. This reduces severe arcs and sparks, which would damage the aircraft and possibly injure its occupants. The aircraft structure is also the ground for the radio. For the radio to function properly, a proper balance between the aircraft structure and antenna is required. This means the surface area of the ground must be constant. Control surfaces, for example, may at times become partially insulated from the remaining structure because of a film of lubricant on the hinges. This will affect radio operation if the condition is not taken care of by bonding. 5-40 BONDING STRAPSHOCK MOUNTS RUBBER MOUNTSAEf05033 Figure 5-33.—Typical shock mounts.
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Bonding also provides the necessary low-resistance return path for single-wire electrical systems. This low-resistance return path also aids the effectiveness of the shielding and provides a means of bringing the entire aircraft to the Earth's ground potential. In summary, aircraft are electrically bonded for the following reasons: • To reduce radio and radar interferences by equalizing static charges that accumulate • To eliminate a fire hazard by preventing static charges from accumulating between two isolated members and creating a spark • To reduce lightning damage to the aircraft and injury to its occupants • To provide the ground for proper functioning of the aircraft radio • To provide a low-resistance return path for single-wire electrical systems • To aid in the effectiveness of the shielding • To provide a means of bringing the entire aircraft to the Earth's potential, and keeping it that way while it is grounded to the Earth Bonding Methods Bonding connections are made so vibration, expan- sion or contraction, or relative movement incidental to normal service use will not break the bonding con- nections. Bonding should not loosen to such an extent that the resistance would vary during the movement. The bonding of most concern is the bonding jumpers that go across shock mounts used to support electronic equipment. Examples of bonding connectors are bond- ing jumpers and bonding clamps. See figure 5-34. Since a primary aim of bonding is to provide an electrical path of low dc resistance and low radio frequency (RF) impedance, the jumper should be a good conductor of ample size for the current-carrying capacity, have low resistance, and be as short as possible. If practical, you should bond parts directly to the basic aircraft structure rather than through other bonded parts. Install bonding jumpers so they do not interfere with the operation of movable components of the aircraft. Contact of dissimilar metals in the presence of an electrolyte, such as salt water, produces an electric action (battery action) that causes a pitting in one of the metals. The intensity of this electric action varies with the kinds of metals. Frequently, bonding involves the direct contact of dissimilar metals. In such cases, the 5-41 PLANO HINGE BOND LINE BOND HINGE BOND-FLEXIBLE CLAMPS SOLDERED T0 THIMBLES JUMPER TAPE TO TURNBUCKLE BARREL BOND ACROSS TURNBUCKLE WITH SPLICED CABLE ENDS. CABLES WHICH ARE SWAGED INTO TURNBUCKLES ENDS DO NOT REQUIRE THIS BOND. AEf05034 Figure 5-34.—Bonding methods.
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metals used produce a minimum amount of corrosion. The connections also are made so that if corrosion does occur, it will be in replaceable elements, such as jumpers, washers, or separators, rather than the bonded or bonding members. Thus, use washers made of the same material as the structural member against the structural member. Also, use washers of the same material as the bonded member that is in contact with that item. Self-tapping screws should not be used for bonding purposes, nor should jumpers be compression-fastened through plywood or other nonmetallic material. When performing a bonding operation, you should remove contact surface films before assembly, and then refinish the completed assembly with a suitable protective finish. For more detailed information about bonding methods, you should refer to Installation Practices, Aircraft Electric and Electronic Wiring , NA V AIR 01-1A-505. Q5-31. What publication contains information on mounting hardware for aircraft parts? Q5-32. When substituting hardware, what factors should you consider before making the substitution? Q5-33. What is the only reason for clamping a wire bundle to a plumbing line? Q5-34. What letters identify a terminal board? Q5-35. What is the purpose of lockwire? REPLACEABLE ASSEMBLIES LEARNING OBJECTIVES: Recognize terms that are applicable to replaceable assemblies and subassemblies. Recognize tools to determine the maintenance level for repair of equipment. Aircraft systems today are designed with a replaceable unit concept. Once the malfunction is isolated to the defective unit, simply replace the defective unit with a good one from supply. This design gives speed and economy to maintenance, as well as saving space and weight. WEAPONS AND SHOP REPLACEABLE ASSEMBLIES Weapons replaceable assembly (WRA) is the term given to replaceable assemblies of an avionics system that is installed in an aircraft, with the exception of cables, mounts, and fuse boxes or circuit breakers. Figure 5-35 is an example of how WRAs may be installed in the aircraft. Shop replaceable assembly (SRA) is the term that includes all the assemblies within a WRA. SRAs also may have replaceable subassemblies. SRAs also may be referred to by other terms. The following are a few you should be aware of: • PCB’s- Printed Circuit boards • EMs- Electronic Modules • CCAs- Circuit Card Assemblies In figure 5-36 are a few examples of the types of SRAs you may see. REPAIR PROCEDURES Once the organizational level (O-level) activity or squadron has determined the defective unit in the aircraft, the defective unit is sent to next higher maintenance level for repair. Normally this will be the intermediate level (I-level) activity or aircraft intermediate maintenance department (AIMD). The maintenance level for repair of a particular WRA or SRA can be determined by the source, maintenance and recoverability (S, M, & R) code. However, repair capabilities of an intermediate maintenance activity (IMA) is dependent on overcoming the following obstacles: • Lack of skills • Lack of equipment or tools • Lack of facilities • Lack of personnel • Lack of technical data • Lack of parts The depth of repair for IMA also is dependent on the IMA’s ability to perform 2M (miniature/microminiature) repair. 2M repair involves the removal and replacement of discrete components (transistors, resistors, capacitors, etc.) along with intricate soldering repair. 2M repair should only be done by certified 2M technicians. You can find detailed information on the 2M program in the Naval Aviation Maintenance Program (NAMP), OPNA VINST 4790.2 (series), V ol. 5, chapter 23. 5-42
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5-43 BATT SWBATT SW FCS HOT FCS HOT R GENR GEN L GENL GEN READY DISCH FIRE EXTGH A/A A/A A/G A/G GND PWR EXT PWR RESET BATT ON 1 AO N AO N AO N BO N BO N BO N 2 AO N BO N 3 4 A U T O A U T O A U T O A U T O 0FF 0FF ORIDEN O R M WRA WRA WRA WRA WRA WRA COCKPIT E M E R G PARK AEf05035 Figure 5-35.—Examples of WRA installation. AEf05036 Figure 5-36. —Examples of SRAs.
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Q5-36. What is the meaning of the term WRA? Q5-37. Who is authorized to make soldering repairs? Q5-38. What should you use to determine the mainte- nance level for repair of a defective as- sembly? ELECTROSTATIC DISCHARGE (ESD) LEARNING OBJECTIVES: Recognize the cause and effect of static electricity. Recognize the hazards of ESD to components. Recognize control and elimination procedures of ESD. The sensitivity of electronic devices and components to electrostatic discharge (ESD) has recently become clear through use, testing, and failure analysis. The construction and design features of current microtechnology have resulted in devices being destroyed or damaged by ESD voltages as low as 20 volts through improper handling. The trend is toward greater complexity, increased packaging density, and thinner dielectrics between active elements, which will result in devices even more sensitive to ESD. Your knowledge of ESD cause and effects, hazards, control, elimination and repair of damage will decrease maintenance costs and time. STATIC ELECTRICITY Static electricity is electrical energy at rest. Some substances readily give up electrons while others accumulate excessive electrons. When two substances are rubbed together, separated, or flow relative to one another (such as a gas or liquid over a solid), one substance becomes negatively charged and the other positively charged. An electrostatic field or lines of force radiate between a charged object to an object at a different electrostatic potential (such as more or less electrons) or ground. Objects entering this field will receive a charge by induction. The capacitance of the charged object relative to another object or ground also has an effect on the field. If the capacitance is reduced, there is an inverse linear increase in voltage, since the charge must be conserved. As the capacitance decreases, the voltage increases until a discharge occurs via an arc. Causes Of Static Electricity Generation of static electricity on an object by rubbing is known as the triboelectric effect. Table 5-5 lists substances in the triboelectric series. Table 5-5.—Triboelectric Series POSITIVE (+) ACETATE GLASS HUMAN HAIR NYLON WOOL FUR ALUMINUM POLYESTER PAPER COTTON WOOD STEEL ACETATE FIBER NICKEL, COPPER, SILVER BRASS, STAINLESS STEEL RUBBER ACRYLIC POLYSTYRENE FOAM POLYURETHANE FOAM SARAN POLYETHYLENE POLYPROPYLENE PVC (VINYL) KEL F TEFLON NEGATIVE (-) NOTE: THE TRIBOELECTRIC SERIES IS AR- RANGED IN SUCH AN ORDER THAT WHEN ANY TWO SUBSTANCES IN THE LIST CONTACT ONE ANOTHER AND ARE SEPARATED, THE SUB- STANCE HIGHER ON THE LIST ASSUMES A POSITIVE CHARGE. The size of an electrostatic charge on two different materials is proportional to the separation of the two materials. 5-44
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Electrostatic voltage levels generated by nonconductors can be extremely high. However, air slowly dissipates the charge to a nearby conductor or ground. The more moisture in the air, the faster a charge dissipates. Table 5-6 shows typical measured charges generated by personnel in a manufacturing facility. You can see that the generated voltage decreases with an increase in humidity levels of the surrounding air. Component Susceptibility and Failure Various devices and components are susceptible to damage by electrostatic voltage levels commonly generated in production, test, operation, and by maintenance personnel. These devices and components include the following: • All microelectronic and most semiconductor devices, except for various power diodes and transistors • Thick and thin film resistors, chips and hybrid devices, and crystals All subassemblies, assemblies, and equipment containing these components/devices without adequate protective circuitry are ESD-sensitive (ESDS). You can protect ESDS items by implementing simple, low-cost ESD controls. Lack of im- plementation has resulted in high repair costs, excessive equipment downtime, and reduced equip- ment effectiveness. ESD overstress can produce a dielectric breakdown of a self-healing nature when the current is unlimited. When this occurs, the device may retest as good. However, it contains a hole in the gate oxide. With use, metal will eventually migrate through the puncture, resulting in a shorting of this oxide layer. Another structure mechanism involves highly limited current dielectric breakdown from which no apparent damage is done. However, this reduces the voltage at which subsequent breakdown occurs to as low as one-third of the original breakdown value. ESD damage can result in a lowered damage threshold at which a subsequent lower voltage ESD will cause further degradation or a functional failure. ESD CONTROL AND ELIMINATION The heart of an ESD control program is the ESD-protected work area and ESD-grounded work station. When you handle an ESD-sensitive (ESDS) device outside of its ESD protective packaging, you need to provide a means of reducing generated electrostatic voltages below the levels at which the item is sensitive. The greater the margin between the level at which the generated voltages are limited and the ESDS item sensitivity level, the greater the probability of protecting that item. 5-45 MEANS OF STATIC GENERATION VOLTAGE LEVELS @ RELATIVE HUMIDITY LOW-10-20 % HIGH-65-90% W ALKING ACROSS CARPET 35,000 1,500 W ALKING OVER VINYL FLOOR 12,000 250 WORKER AT BENCH 6,000 100 VINYL ENVELOPES FOR WORK INSTRUC- TIONS 7,000 600 COMMON POLY BAG PICKED UP FROM BENCH 20,000 1,200 WORK CHAIR PADDED WITH URETHANE FOAM 18,000 1,500 Table 5-6.—Typical Measured Electrostatic Voltages
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PRIME GENERATORS Look at table 5-7. It lists ESD prime generators. All common plastics and other prime generators of static electricity should be prohibited in the ESD-protected work area. Carpeting also should be prohibited. If you must use carpet, it should be of a permanently anti-static type. Perform weekly static voltage monitoring where carpeting is in use. PERSONAL APPAREL AND GROUNDING An essential part of the ESD program is grounding personnel and their apparel when they handle ESDS material. Smocks Personnel handling ESDS items should wear long-sleeve ESD-protective smocks, short-sleeve shirts or blouses, and ESD-protective gauntlets banded to the bare wrist and extending toward the elbow. If these items are not available, use other anti-static material (such as cotton) that will cover sections of the body that could contact an ESDS item during handling. Personnel Ground Straps Personnel ground straps should have a minimum resistance of 250,000 ohms. The wrist, leg, or ankle bracelet end of the ground strap should have some 5-46 WORK SURFACES • FORMICA (W AXED OR HIGHLY RESISTIVE) • FINISHED WOOD • SYNTHETIC MATS FLOORS • W AX FINISHED • VINYL CLOTHES • COMMON CLEAN ROOM SMOCKS • PERSONNEL GARMENTS (ALL TEXTILES EXCEPT VIRGIN COTTON) • NON CONDUCTIVE SHOES CHAIRS • FINISHED WOOD • VINYL • FIBERGLASS PACKAGING AND HANDLING • COMMON POLYETHYLENE—BAGS, WRAPS, ENVE- LOPES • COMMON BUBBLE PACK, FOAM • COMMON PLASTIC TRAYS, PLASTIC TOTE • BOXES, VIALS ASSEMBLY , CLEANING, TEST AND REPAIR AREAS • SPRAY CLEANERS • COMMON SOLDER SUCKERS • COMMON SOLDER IRONS • SOLVENT BRUSHING (SYNTHETIC BRISTLES) • CLEANING, DRYING • TEMPERATURE CHAMBERS Table 5-7.—Typical Charge Generators
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metal contact with the skin. Bracelets made completely of carbon-impregnated plastic may burnish around the area in contact with the skin, resulting in too high an impedance to ground. To ensure personnel grounding straps are safe, periodic maintenance and preoperational checks of ESD work areas should be performed per the following publications: • NA V AIR 17-600-141-6-1 • NA V AIR 17-600-141-6-2 • NA V AIR 17-600-193-6-2 ESD-protective Materials There are two basic types of ESD-protective materials—conductive and anti-static. Conductive materials protect ESD devices from static discharges and electromagnetic fields. Anti-static material is a nonstatic-generating material. Other than not generating static, anti-static material offers no other protection to an ESD device. CONDUCTIVE ESD-PROTECTIVE MATE- RIALS.— Conductive ESD-protective materials consist of metal, metal-coated, and metal-impregnated materials (such as carbon particle impregnated, conductive mesh or wire encased in plastic). The most common conductive materials used for ESD protection are steel, aluminum, and carbon-impregnated polyethylene and nylon. The latter two are opaque, black, flexible, heat sealable, electrically conductive plastics. These plastics are composed of carbon particles, impregnated in the plastic, which provides volume conductivity throughout the material. ANTI-STATIC ESD PROTECTIVE MATE- RIALS.— Anti-static materials are normally plastic materials (such as polyethylene, polyolefin, polyurethane, nylon) that are impregnated with an anti-static substance. This anti-static substance migrates to the surface and combines with the humidity in the air to form a conductive sweat layer on the surface. This layer is invisible and, although highly resistive, is conductive enough to prevent the buildup of electrostatic charges by triboelectric (or rubbing) methods in normal handling. Simply stated, the primary asset of an anti-static material is that it will not generate a charge on its surface. However, this material won't protect an enclosed ESD device if it comes into contact with a charged surface. Anti-static material is tinted pink, a symbol of its being anti-static. Anti-static materials are used for inner-wrap packaging. However, anti-static trays, vials, carriers, boxes, etc., are not used unless components and/or assemblies are wrapped in conductive packaging. HYBRID ESD-PROTECTIVE BAGS.— Hybrid ESD-protective bags are a laminate of different ESD-protective materials. They are made from conductive and anti-static materials. The hybrid ESD-protective bag provides the advantages of both types of materials in a single bag. ESDS Device Handling and Packaging The following are general guidelines that you should follow when handling ESDS devices: • Ground all containers, tools, test equipment, and fixtures used in ESD-protective areas before and/or during use, either directly or by contact with a grounded surface. • Avoid physical activities around ESDS items that are friction-producing; for example, removing or putting on smocks, wiping feet, sliding objects over surfaces, etc. • Wear cotton smocks and/or other anti-static treated clothing. • Avoid the use or presence of plastics, synthetic textiles, rubber, finished wood, vinyls, and other static-generating materials, especially when handling ESDS out of their ESD-protective packaging. • Place the ESD protective material containing the ESD item on a grounded work bench surface to remove any charge before opening the packaging material. • Attach personnel grounding strap before removing ESDS items from their protective packaging. • Remove ESDS items from ESD-protective packaging only after grounding, and place on the ESD-grounded work bench surface. • Make periodic electrostatic measurements at all ESD-protected areas. This assures the ESD-protective properties of the work station and all equipment contained there have not degraded. 5-47
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• Perform periodic continuity checks of personnel ground straps (between skin contact and ground connection), ESD-grounded work station surfaces, conductive floor mats, and other connections to ground. Perform this check with a megohmmeter to make sure grounding resistivity requirements are met. Before an ESDS item leaves an ESD-protected area, ensure shorting bars, clips, or noncorrective conductive materials are inserted correctly in or on all terminals or connectors. A list of approved ESD anti-static protective materials for packaging can be found in OPNA VINST 4790.2 series, V ol. 5, chapter 22. Q5-39. ESD-sensitive devices can be damaged by electrostatic voltages as low as__________? Q5-40. When handling ESDS devices, personnel and their apparel should be connected to_____? Q5-41. What is the minimum resistance for personnel ground straps? Q5-42. What color is material that is antistatic? 5-48