Electrical Wiring Interconnection System PDF

Circuit Testing Circuit Tests When electrical circuits are installed, the installations should be inspected and undergo circuit tests to ensure system reliability. The methods of testing and inspection vary with different types of aircraft and equipment fitted; therefore, reference mu...

Circuit Testing Circuit Tests When electrical circuits are installed, the installations should be inspected and undergo circuit tests to ensure system reliability. The methods of testing and inspection vary with different types of aircraft and equipment fitted; therefore, reference must be made to the appropriate maintenance manuals for detailed information. Circuit tests normally include: Bonding testing Continuity testing Insulation resistance testing Functional testing. After completion of all tests, the installations should be inspected to ensure that all connections have been re-made and secured, and that test equipment, tools, etc., have been removed. The circuits should then be proved, as far as the installation permits, by ground-functional checks of the services concerned. MK Test Systems. Portable bond test systems, 2017. Avionics technician performing electrical bond testing 2023-02-06 B-07c Maintenance Practices Page 105 of 255 CASA Part 66 - Training Materials Only  Bonding and Grounding Bonding means electrically connecting two or more conducting objects not otherwise adequately connected. Grounding means electrically connecting a conducting object to a primary structure or earth electrode for return of current. Bonding and grounding connections are made in aircraft for the following purposes: To protect aircraft and personnel against hazards from lightning discharge. To provide power current return paths. To prevent development of RF potentials. To protect personnel from shock hazard. To provide stability and consistency of radio transmission and reception. To prevent accumulation of static charge. To provide fault current return paths. Bonding and grounding The main types of bonding are listed below. 2023-02-06 B-07c Maintenance Practices Page 106 of 255 CASA Part 66 - Training Materials Only  Equipment Bonding Low-impedance paths to aircraft structure are normally required for electronic equipment to provide radio frequency return circuits and for most electrical equipment to facilitate reduction in EMI. The cases of components which produce electromagnetic energy should be grounded to structure. To ensure proper operation of electronic equipment, it is particularly important to conform to the system’s installation specification when interconnections, bonding and grounding are being accomplished. Metallic Surface Bonding All conducting objects on the exterior of the airframe must be electrically connected to the airframe through mechanical joints, conductive hinges or bond straps capable of conducting static charges and lightning strikes. Exceptions may be necessary for some objects, such as antenna elements, whose function requires them to be electrically isolated from the airframe. Such items should be provided with an alternative means to conduct static charges and/or lightning currents as appropriate. Static Bonds All isolated conducting parts inside and outside the aircraft, having an area greater than 3 in.² and a linear dimension over 3 in., that are subjected to appreciable electrostatic charging due to precipitation, fluid or air in motion should have a mechanically secure electrical connection to the aircraft structure of sufficient conductivity to dissipate possible static charges. A resistance of less than 1 Ω when clean and dry generally ensures such dissipation on larger objects. Higher resistances are permissible when connecting smaller objects to the airframe structure. Bond Inspection and Testing The following content outline what to look for when conducting a bonding inspection. If there is evidence of electrical arcing, check for intermittent electrical contact between conducting surfaces that may become a part of a ground plane or a current path. Arcing can be prevented either by bonding or by insulation if bonding is not necessary. The metallic conduit should be bonded to the aircraft structure at each terminating and break point. The conduit bonding strap should be located ahead of the piece of equipment that is connected to the cable wire inside the conduit. Bond connections should be secure and free from corrosion. Bonding jumpers should be installed in such a manner as not to interfere in any way with the operation of movable components of the aircraft. Self-tapping screws should not be used for bonding purposes. Only standard threaded screws or bolts of appropriate size should be used. 2023-02-06 B-07c Maintenance Practices Page 107 of 255 CASA Part 66 - Training Materials Only  Exposed conducting frames or parts of electrical or electronic equipment should have a low- resistance bond of less than 2.5 mΩ to structure. If the equipment design includes a ground terminal or pin which is internally connected to such exposed parts, a ground wire connection to such terminal will satisfy this requirement. Refer to the manufacturer’s instructions. Bonds should be attached directly to the basic aircraft structure rather than through other bonded parts. Bonds must be installed to ensure that the structure and equipment are electrically stable and free from the hazards of lightning, static discharge, electrical shock, etc. To ensure proper operation and suppression of radio interference from hazards, electrical bonding of equipment must conform to the manufacturer’s specifications. Use of bonding testers is strongly recommended. Measurements should be performed after the grounding and bonding mechanical connections are complete to determine if the measured resistance values meet the basic requirements. A high-quality test instrument (such as AN/USM-21A or equivalent) is required to accurately measure the very low resistance values. Another method of measurement is the millivolt drop test. Use appropriate washers when bonding aluminium or copper to dissimilar metallic structures so any corrosion that may occur will be on the washer. Bonding inspection 2023-02-06 B-07c Maintenance Practices Page 108 of 255 CASA Part 66 - Training Materials Only  Bonding Tester The bonding tester is specially designed to measure low resistance and in particular to measure the resistance of the bonding or earthing systems on aircraft. The analogue type bonding tester comprises a meter and two leads, each of specific length: a 60-ft lead with the single probe permits attachment at one point and allows the bonding tester to be carried around to other points on the airframe. The double probe on the end of the 6-ft cable acts as a switch that turns the battery on only when performing a test. The meter inside the tester consists of a ratiometer type ohmmeter and a single 1.2-V nickel-alkaline cell housed in a case. The ratiometer principle is used with two coils mounted at right angles to each other on the movement, which is free to rotate in a magnetic field. One is a control coil and the other carries the current, which passes through the bond or resistance to be measured. They exert opposing torques, and the resultant deflection depends on the resistance under test. Since the resistances to be measured with this instrument are so low, the resistances of the connecting leads are far from negligible, and that is the reason for the otherwise peculiar double 60- ft lead. CAUTION: Cell connected in circuit only when bond is in use. Bonding tester construction If the BOND was a short circuit, all of the circuit current would flow through the control coil and a minimum would flow through the deflection coil. If the BOND resistance was increased, more current would be diverted through the deflection coil, and the deflection would increase. Thus, the larger the BOND resistance, the greater the deflection. 2023-02-06 B-07c Maintenance Practices Page 109 of 255 CASA Part 66 - Training Materials Only  Aviation Australia Bonding tester circuit Any resistance in series with the control coil does not affect the deflection since this deflection depends on the relative currents in the control and deflection coils and any resistance in series with the deflection coil reduces currents in both coils proportionately. For a similar reason, the battery voltage is not vital. The nickel-iron type is used because of its physical and electrical robustness. It can safely be left for long periods and will still be serviceable when required. Bonding Testing Procedure The instrument powered by a 1.2-V alkaline cell can measure from 0 to 0.1 Ω, with the scale graduated in 0.002-Ω divisions. To test the meter: Short the three prongs of both probes together to obtain a zero reading (tests control coil and battery). Momentarily short the two prongs of the double-prong probe to obtain full deflection (tests deflection coil and battery). Note: The resistance figures provided below are for general electrical and RF bonding. Specific requirements detailed in aircraft or component publications should take precedence (for example AC 21-99 Aircraft Wiring and Bonding Sect 2 Chapter 13). To start testing: Remove or pierce protective treatments (i.e. general priming and finishing paints, varnishes, chromic, anodic and phosphate coatings). Cadmium and tin plate should not be removed. Connect the single-prong probe of the 60-ft static lead to an airframe earth point. 2023-02-06 B-07c Maintenance Practices Page 110 of 255 CASA Part 66 - Training Materials Only  The resistance across a bonding or grounding jumper is required to be 0.1 Ω or less for general electrical bonding, whether using bonding jumpers or where metallic components are directly attached. Where bonding of RF components is required, the resistance should be a maximum 0.0025 Ω (2.5 mΩ; reference – MIL-STD-464 Electromagnetic Environmental Effects Requirements for Systems). Test after the mechanical connection is completed by taking a milliohmmeter reading of the overall resistance between the cleaned areas of the object and the structure. Check that bonding resistance between extremities of the aircraft is within the manufacturer’s limits (not to exceed 0.05 Ω [50 mΩ] on all-metal aircraft). When checking bonding between components and main earth, the resistance should not exceed 0.05 Ω (mid-scale deflection). Bonds to moving parts should be made of braid (maximum length 12 in.) connected by bolts or clamps and with only sufficient slack for working purposes. Flexible fuel pipes are bonded internally. Formation of a leak-proof joint at the end connections gives a good electrical bond and resistance should not exceed 0.025 Ω/ft length or 0.05 Ω (50 mΩ), whichever is greater. Replace or repair the protective finish. A digital version of a bonding tester is also available. Digital bonding tester 2023-02-06 B-07c Maintenance Practices Page 111 of 255 CASA Part 66 - Training Materials Only  Millivolt Drop Test Another method of measuring bonding resistance is the millivolt drop test, as shown. Aviation Australia Millivolt drop test 2023-02-06 B-07c Maintenance Practices Page 112 of 255 CASA Part 66 - Training Materials Only  Continuity Testing Continuity is defined as the condition of being unbroken or uninterrupted. In electrical terms, continuity testing is carried out to verify that electrical connections are configured in accordance with the wiring diagram. A continuity test provides two pieces of information: That there is an electrical link between two or more designated points within a circuit That the resistance of that link is within a specified value. Testing can be performed by using an ohmmeter or a multimeter set to the ohms range. A multimeter for continuity testing WARNING: Never have power applied to a circuit when using continuity test equipment, as this could damage the meter and/or injure personnel. 2023-02-06 B-07c Maintenance Practices Page 113 of 255 CASA Part 66 - Training Materials Only  Continuity Testing Procedure Continuity testing is normally carried out after the bonding test and prior to the insulation test. When performing continuity tests on an aircraft with a metal fuselage, it is acceptable to use the airframe as the return path for the tester. It is always necessary to consult the wiring diagram to ensure that the part of the circuit under test is open-ended and that there is no alternate path to result in a false reading. In the diagram below, an incorrect link joins the two circuits together to create an unintentional live circuit for CB1 when SW2 is closed. Proper circuit isolation and reference to the wiring diagram are necessary to detect the location of the fault and ensure safety is maintained. Aviation Australia No alternate path in continuity testing In many cases, the ohmmeter is not used to measure the resistance of a component, but to simply check the integrity of a connection from one portion of a circuit to another. If there is a good connection, then the ohmmeter will read a near-zero resistance or a short. If the circuit is open or has a very poor connection at some point, like an over-crimped pin in a connector, then the ohmmeter will read infinity or some very high resistance. Keep in mind that while any measurement is being taken, contact with the circuit or probes should be avoided. Contact can introduce another parallel path and provide misleading indications. Continuity testing is also used to verify the serviceability of components such as fuses and light bulbs. These components need to be removed from the circuit to check their condition. 2023-02-06 B-07c Maintenance Practices Page 114 of 255 CASA Part 66 - Training Materials Only  Aviation Australia Continuity testing 2023-02-06 B-07c Maintenance Practices Page 115 of 255 CASA Part 66 - Training Materials Only  Insulation Testing An insulator is a material or device used to prevent the passage of electricity. Insulation testing is the method by which the electrical resistance of an insulator separating conductors is measured. A fault caused by insulation failure to keep the conductor from touching the airframe will most likely trip the circuit breaker, but may result in different outcomes as insulation failure does not only result in a short circuit condition. Minimum values of insulation resistance for electrical wiring are typically 5 MΩ for a single conductor and 2 MΩ for bundled wiring. Insulation resistance tests are normally carried out after the visual, bonding and continuity tests. They are necessary when new circuits have been installed or existing circuits disturbed, when evidence exists that insulation requires checking, or as specified in the aircraft maintenance schedule. There are several types of insulation testers. The one most commonly used on aircraft is known as a Megger. There are 250-V, 500-V and 1000-V Meggers. Two types of Megger are shown: Insulation testers 2023-02-06 B-07c Maintenance Practices Page 116 of 255 CASA Part 66 - Training Materials Only  The basic circuit of a Megger is shown below: Aviation Australia Basic Megger tester construction and circuit When the test resistance is a short circuit, the majority of the current flows in the deflection coil. The movement goes to maximum deflection (i.e. 0 Ω). When the test resistance is an open circuit, all of the current flows through the control coil, giving minimum deflection (i.e. infinity), thus the larger the value of the test resistance, the smaller the current in the deflection coil and the smaller the deflection. A quick way to test the Megger before use is to leave the leads apart. Wind the handle (or press the button) and see that the needle moves to a high value. Short the leads together and repeat the test, noting that the needle goes to 0. CAUTION: When testing a cable for insulation resistance, it is absolutely critical that both ends of the cable be disconnected from the circuit; otherwise, damage to electronic components could result. Connect one lead to the centre conductor and one lead to the aircraft ground, or wire shield if testing shielded cable. Activate the Megger and note the reading. If the Megger is a mechanical hand-wound type, build up to a steady speed of approximately 150 rpm and observe the needle movement. If the insulation is good, the needle will indicate a high value of resistance. If the insulation resistance is bad (that is, it is shorted to earth), the needle will move towards 0 or a very low reading. Stop the handle immediately. 2023-02-06 B-07c Maintenance Practices Page 117 of 255 CASA Part 66 - Training Materials Only  Insulation testing Functional Testing Functional testing is performed to ensure proper functioning of individual and integrated sections of circuits. Levels of tests vary depending on the complexity of a circuit. These range from simple ‘self-tests’ to deeper tests that can only be carried out in the workshop. The B737-600/700/800/900 engine fire detection circuit (AMM 26-11-00) is shown as an example. Aviation Australia Functional testing example 2023-02-06 B-07c Maintenance Practices Page 118 of 255 CASA Part 66 - Training Materials Only  Solderless Terminations and Splices Solderless Termination Lugs Electrical wires are terminated with solderless terminal lugs to permit easy and efficient connection to and disconnection from terminal boards, busbars and other electrical equipment. Solderless splices join electric wires to form permanent continuous runs. Solderless terminal lugs and splices are plated with copper or aluminium and are pre-insulated or uninsulated, depending on the application. Terminal lugs and splices for high-temperature applications are silver- or nickel-plated copper and are insulated with Teflon (or a similar material). Note: Use only copper terminations on copper wire and aluminium terminations on aluminium wire. This helps eliminate the possibility of dissimilar metal corrosion. Only environment-resistant sealed splices must be used as permanent electrical wire splices on aircraft. Solderless terminals in an aircraft Terminal lugs are available in four styles for use under different applications: Straight 90° upright Angle Flag. The illustration shows typical terminal lugs and splices used on aircraft. 2023-02-06 B-07c Maintenance Practices Page 119 of 255 CASA Part 66 - Training Materials Only  Aviation Australia Typical terminal lugs and splices on an aircraft Terminal lugs and splices, and the tools used to install them on wires, are divided into two classes as follows: Class I Terminal Lugs and Splices conform to all the requirements of the applicable specification when installed with the specified crimping tools. Class I Tools meet all the requirements of the applicable specification. Class II Terminal Lugs and Splices are replaceable by Class I terminals and meet the performance requirements of the applicable specification when installed with a tool recommended by the terminal manufacturer. Class II tools crimp terminals to meet the performance requirements of the applicable specification. 2023-02-06 B-07c Maintenance Practices Page 120 of 255 CASA Part 66 - Training Materials Only  The applicable specification is: SAE AS70991 – Terminals, Lug and Splice, Crimp Style, Aluminium, for Aluminium Aircraft Wire SAE AS7928 – Terminals, Lug, Splices, Conductor: Crimp Style, Copper, General Specification For MIL-S-81824 – Splice, Electric, Permanent, Crimp Style, Copper, Insulated, Environment Resistant MIL-DTL-22520 – Crimping Tools, Wire Termination, General Specification For. Note: Class II tools, terminal lugs and splices should not be used on aircraft. Crimping Tools Terminal lugs and splices are crimped to wires using hand or power crimping tools. Power tools, either portable or stationary (bench-mounted), are usually found in large shops where wire bundles are made up. These tools crimp the barrel to the conductor, and simultaneously crimp the insulation grip to the wire insulation. Strict compliance with the relevant aircraft or equipment manufacturer’s recommendations and instructions is essential when undertaking work of this nature. Aircraft approved crimping tool 2023-02-06 B-07c Maintenance Practices Page 121 of 255 CASA Part 66 - Training Materials Only  Crimped Terminations A typical crimped termination has two principal sections: crimping barrel and tongue. In some types, these sections are accompanied by a pre-insulated copper sleeve which mates with the crimping barrel at one end and is formed, during the crimping process, to grip the cable insulation at the other to give a measure of support. Pre-insulated terminal lug cut-away The barrel is designed to fit closely around the cable conductor so that after pressure has been applied, many points of contact are made. The pressure is applied with a special crimping tool. 2023-02-06 B-07c Maintenance Practices Page 122 of 255 CASA Part 66 - Training Materials Only  EWIS Crimping Procedures Terminating Small Copper Wires Small copper wires (sizes No. 26 through No. 10) are terminated with solderless pre-insulated straight copper terminal lugs conforming to SAE AS7928. The insulation is part of the terminal lug and extends beyond its barrel to cover a portion of the wire insulation; this makes the use of an insulation sleeve unnecessary. In addition, pre-insulated terminal lugs have an insulation support (a metal reinforcing sleeve) beneath the insulation for extra supporting strength on the wire insulation. Most pre-insulated terminals accommodate more than one size of wire. The insulation is colour-coded, and the range of wire sizes is marked on the tongue to identify the wire sizes that can be terminated with each of the terminal lug sizes. For example, terminals with red insulation are used on wire gauge sizes 22 through 18, while blue insulation identifies a terminal used on 16- and 14- gauge wires. If a terminal has yellow insulation, it is used for 12- and 10-gauge wires. Only tools qualified to MIL-DTL-22520 are to be used for crimping terminal lugs. Colour-coded terminal lugs or splices 2023-02-06 B-07c Maintenance Practices Page 123 of 255 CASA Part 66 - Training Materials Only  Hand Crimping Tools All approved hand crimping tools have a self-locking ratchet which prevents the tool from opening until the crimp is complete. This mechanism must never be disassembled since it ensures proper crimping closure. The M22520/5 and M22520/10 tools are approved for crimping sizes 26 through 10 splices and terminal lugs. The M22520/5-01 and M22520/10-01 tools have removable dies for crimping 26 through 10 terminal (wire barrel) sizes. The DMC M22520/5-01 (HX4) crimp tool is shown as an example. M22520/5-01 hand crimp tool Crimping dies are snapped into the tool frame and retained in place by internal spring clips. They may be permanently secured, if desired, by driving a roll pin into the holes provided. Hexagonal crimp dies are available in a variety of combinations with single, double and even triple cavity design. 2023-02-06 B-07c Maintenance Practices Page 124 of 255 CASA Part 66 - Training Materials Only  Aviation Australia Crimping dies Dies and other working parts of crimping tools used for electrical terminations become worn in service and this can result in unsatisfactory terminations. An effective means to ensure a serviceable termination is to monitor the condition of the tool by regular testing. The standard tools are checked by means of a go/no-go gauge for assurance of quality crimps. For good crimping results, gauging should be carried out prior to each series of crimping operations. Replace or repair hand tools which are out of tolerance. The details in the table below are provided for in-service inspection gauging of M22520/5-01 and M22520/10-01 tools and dies. Prior to performing the gauging test, ensure that both the crimping tool jaws and the shafts of the gauge are clean and free of damage. When gauging tools are unavailable, testing can be accomplished by carrying out a millivolt drop and tensile strength test on a completed crimp. Gauging Tools Go/NoGo Gauge used for crimping tools 2023-02-06 B-07c Maintenance Practices Page 125 of 255 CASA Part 66 - Training Materials Only  Aviation Australia Gauging tools M22520 sizing chart Crimping Procedure The following is the crimping procedure for M22520/5 and M22520/10 hand tools. 2023-02-06 B-07c Maintenance Practices Page 126 of 255 CASA Part 66 - Training Materials Only  Hand crimp pre-insulated copper terminal lugs in the Nos. 26–10 wire size range with M22520/5 or M22520/10 hand tools as follows: 1. Strip wire insulation using one of the recommended stripping procedures (stripping lengths are provided in the table below). 2. Check the tool for periodic calibration and proper adjustment. Tools out of adjustment must be returned to the manufacturer for repair. 3. Insert the terminal lug, tongue first, into the wire side of the hand tool barrel crimping jaws until the terminal lug barrel butts flush against the tool stop on the locator (see the illustration for the correct insertion method). 4. Squeeze the tool handles slowly until its jaws hold the terminal lug barrel firmly in place, but without denting it. 5. Insert the stripped wire into the terminal lug barrel until the wire insulation butts flush against the near end of the wire barrel. 6. Squeeze the tool handles until the ratchet releases. 7. Remove the completed assembly and examine it for proper crimp. Wire Size Stripping Length (in Inches) #26 and #24 5/32 #22 through #14 3/16 #12 and #10 9/32 Aviation Australia A typical finished crimp 2023-02-06 B-07c Maintenance Practices Page 127 of 255 CASA Part 66 - Training Materials Only  Terminating Large Copper Wires Copper terminal lugs of two styles (straight and flag) are used to terminate copper wire Nos. 8–4/0. The style used depends on existing space conditions. These terminal lugs are available uninsulated in both types and pre-insulated in the straight type. Straight pre-insulated terminal lugs conform to SAE AS7928. As shown previously, pre-insulated terminal lugs have insulation extending beyond the wire barrel to cover a portion of the wire insulation. This makes the use of a separate insulating sleeve unnecessary. Straight uninsulated terminal lugs conform to SAE AS7928 and MS20659. Flag uninsulated terminal lugs conform to SAE AS7928 and MS25189. Uninsulated straight flag terminal lugs are insulated (after assembly to wire) with heat-shrinkable tubing or with lengths of transparent tubing. These methods of insulation provide electrical and mechanical protection at the connection. When the size of sleeving used is such that it will fit tightly over the terminal lug, the sleeving need not be tied; otherwise, it is to be tied with lacing cord. Insulating sleeves 2023-02-06 B-07c Maintenance Practices Page 128 of 255 CASA Part 66 - Training Materials Only  Crimping Tools for Large Copper Wires Manual and/or power crimping tools are available for crimping M7928/4 and MS25036 insulated terminals, MS20659 uninsulated terminals, and MS25189 flag type terminals. Power crimping tools The tools are the MS25441-5 hydraulic pedal pump and the MS25441-4 electric hydraulic pump, used with the MS25441-1 hydraulic head and the MS25441-3 hose (or an adaption without the control cable), and the proper dies. The MS25441 tools can be checked for proper adjustment. For good crimping results, this must be done before each series of crimping operations. When a tool is adjustable, proper correction must be made; otherwise, the tool must be returned to the manufacturer for repair. Gauge the dies of the MS25441 tool in the closed position with the appropriate go/no-go gauges. When gauging tools are unavailable, testing can be accomplished by carrying out a millivolt drop and tensile strength test on a completed crimp. 2023-02-06 B-07c Maintenance Practices Page 129 of 255 CASA Part 66 - Training Materials Only  Crimping Procedure The following is the crimping procedure for MS25441 tools. Crimp large Military Standard copper terminals as follows: Select the proper die for the terminal and wire size, and install the die in the tool. Caution: Always disconnect power tools from their pressure source before installing or removing dies. Do not use any crimping tool beyond its rated capacity. Strip the wire insulation using recommended practices. Insert the stripped wire into the terminal barrel until the wire insulation butts flush against the end of the barrel. Insert the wire and terminal lug assembly into the die. Actuate the crimp tool. Press the button on the handle of an electrically operated tool. Do not release the button until the dies open automatically. Actuate the handle if using a manual hydraulically operated tool. Remove the crimped assembly and examine it for proper crimp. Terminating Aluminium Wire Aluminium wire is used in aircraft because of its weight advantage over copper. Aluminium, however, has the disadvantage of being softer than copper. Further, bending aluminium wire causes ‘work hardening’ of the metal that makes it more brittle. This results in failure or breakage of strands much sooner than in copper wire. Aluminium also forms a high-resistance oxide film immediately upon exposure to air. To compensate for these disadvantages, it is important to follow the recommended installation procedures carefully. Do not use any aluminium wire which has nicked or broken strands. Damaged strands will fail in service. Only aluminium terminal lugs conforming to SAE AS70991 are used to terminate aluminium wires. Aluminium terminal lugs are available in four types: Straight (MS25435) 90° upright (MS25436) Left angle (MS25437) Right angle (MS25438). 2023-02-06 B-07c Maintenance Practices Page 130 of 255 CASA Part 66 - Training Materials Only  All aluminium terminals have an inspection hole for checking the depth of wire insertion. This hole is sealed with a removable plastic plug, which also serves to retain the oxide-inhibiting compound. Each aluminium terminal lug is marked with the letters AL indicating it is for use with aluminium wire, and also with the wire size it will accommodate. Do not remove the inspection plug until the crimp has been completed and the wire insertion is ready to be inspected. Replace the plug after inspection. The barrels of aluminium terminal lugs are filled with a petroleum-based abrasive compound. By a grinding process during the crimping operation, the compound removes the oxide film from the aluminium. It also prevents oxide from reforming in the completed connection. Use the MS25441 tool to install MS aluminium terminal lugs. Aluminium terminal lugs are not pre-insulated; therefore, it is necessary to insulate them, after assembly, with lengths of transparent flexible tubing or heat-shrink sleeves. The sleeve provides mechanical and electrical protection at the connection. Terminating aluminium wire 2023-02-06 B-07c Maintenance Practices Page 131 of 255 CASA Part 66 - Training Materials Only  Inspection of Crimped Terminations Examine the crimped connection carefully for the following: Crimp indent centred on the terminal lug barrel Indent in line with the barrel Terminal lug not cracked Terminal lug insulation not cracked Insulation grip crimped. Inspection of crimped termination CAUTION: If not properly stripped, some of the smaller gauge, thin-wall wire insulation can be inadvertently inserted and crimped in the terminal wire barrels. This causes a bad electrical connection. Do not use any connection that is found defective during a visual inspection. Cut off the defective connection and remake it using a new terminal lug. 2023-02-06 B-07c Maintenance Practices Page 132 of 255 CASA Part 66 - Training Materials Only  EWIS Splicing Procedures Splicing Small Copper Wires Environmental permanent splices conforming to MIL-S-81824 are used to join small copper wire sizes No. 26 through No. 10. The splice pre-insulation extends over the wire insulation. Each splice size can be used for more than one wire size. Splices are colour-coded in the same manner as insulated copper terminal lugs. The M22520/5 and M22520/10 crimp tools with appropriate dies are the approved tools for crimping these splice sizes. Splices 2023-02-06 B-07c Maintenance Practices Page 133 of 255 CASA Part 66 - Training Materials Only  Splicing Procedure for M81824 Environmental Splices The crimping procedures for M81824 environmental splices vary only slightly from those for pre- insulated copper terminal lugs. Variations are: The crimping operation must be done twice, once for each end of splice. The wire stripping lengths are different and are given in the table below. Insert wires and observe that each stripped wire is visible through the inspection hole in the splice. After crimping, check that wire ends are still visible through the inspection hole. Wire Size Stripping Length (in Inches) #26 and #24 5/32 #22 - #14 7/32 #12 - #10 5/16 Splicing Large Copper Wires Uninsulated splices are used to join large copper wires of sizes No. 8 through No. 4/0. © Aviation Australia Uninsulated splices (top) and insulated splices (bottom) 2023-02-06 B-07c Maintenance Practices Page 134 of 255 CASA Part 66 - Training Materials Only  There is a different splice for each wire size. Uninsulated splices are insulated after assembly with either heat-shrink tubing or transparent flexible sleeving to provide electrical and mechanical protection. If the flexible sleeving method is used, cut the sleeve to the required length. Insulating Sleeve Length for Uninsulated Splices Splice Wire Size Insulating Sleeve Length (in Inches) 8 1-15/16 6 2-1/8 4 2-1/8 2 2-13/32 1 2-13/32 1/0 2-17/32 2/0 2-25/32 3/0 2-13/16 4/0 2-15/16 2023-02-06 B-07c Maintenance Practices Page 135 of 255 CASA Part 66 - Training Materials Only  General Requirements for Splicing Follow these general rules when using splices: Splicing of electrical wire should be kept to a minimum and avoided entirely in locations subject to extreme vibrations. Splicing of individual wires in a group or bundle should have engineering approval and the splice(s) should be located to allow periodic inspection. Many types of aircraft splice connectors are available for use when splicing individual wires. A self-insulated splice connector is preferred; however, a non-insulated splice connector may be used provided the splice is covered with plastic sleeving that is secured at both ends. There should not typically be more than one splice in any one wire segment between any two connectors or other disconnect points, except when attaching to the spare pigtail lead of a potted connector, to splice multiple wires to a single wire, to adjust wire size to fit connector contact crimp barrel size and to make an approved repair. Splices in bundles must be staggered to minimise any increase in the size of the bundle that may prevent the bundle from fitting into its designated space or cause congestion that will adversely affect maintenance. Aviation Australia Staggered splices in wire bundle Splices should not be used within 12 in. of a termination device except when attaching to the pigtail spare lead of a potted termination device, splicing multiple wires to a single wire or adjusting the wire sizes for compatibility with the contact crimp barrel sizes. The proper crimping tool must be selected. 2023-02-06 B-07c Maintenance Practices Page 136 of 255 CASA Part 66 - Training Materials Only  Inspection of Splices Examine the splicing carefully for the following: Crimp indent centred on splice barrels Crimp indent in line with barrel Splice barrel not cracked Splice insulation not cracked Environmental sealing is correct No evidence of overheating Spliced wires are butted against the stop (wire ends are visible through inspection holes). Aviation Australia Inspection of splices 2023-02-06 B-07c Maintenance Practices Page 137 of 255 CASA Part 66 - Training Materials Only  Contact Crimping Tools MIL-DTL-22520/1 Standard crimping tools conforming to MIL-DTL-22520 are used to crimp the contacts. These tools are capable of crimping contact wire barrel sizes 12 to 28 to wire sizes 12 to 32. All MIL-DTL-22520 hand-operated tools are cycle controlled by a ratchet mechanism that will not release until the crimping cycle has been completed. A brief description of each of these tools follows. The basic crimping tool M22520/1-01 accommodates contacts with wire barrel sizes 12 through 20 and has a provision for selecting the proper depth of crimp depending on the contact/wire combination being used. The contact is crimped by the creation of four sets of double impressions caused by the closure of the four indenters. Appropriate turret or positioner heads are installed depending on the application. M22520/1-01 crimping tool 2023-02-06 B-07c Maintenance Practices Page 138 of 255 CASA Part 66 - Training Materials Only  MIL-DTL-22520/2 The basic crimping tool M22520/2-01 accommodates contacts with wire barrel sizes 20 through 28 and has a provision for selecting the proper depth of crimp depending on the contact/wire combination being used. The contact is crimped by the creation of four sets of double impressions caused by the closure of the four indenters. Appropriate positioners are installed depending on the application. M22520/2 crimping tool MIL-DTL-22520/4 The basic tool MS22520/4-01 is used only with the M22520/4-02 single positioner head. This tool– head combination is used to crimp outer pin and socket coaxial contacts to the shielded cables specified for them. The tool, which is not operator adjustable, creates a circular crimp around the contact. For specific tool application, see MIL-DTL-22520/4. M22520/4-02 Head 2023-02-06 B-07c Maintenance Practices Page 139 of 255 CASA Part 66 - Training Materials Only  MIL-DTL-22520/7 The basic crimping tool M22520/7-01 accommodates contacts with wire barrel sizes 16 through 22 and has a provision for selecting the proper depth of crimp depending on the contact/wire combination being used. It is similar to the M22520/2-01 in that it is smaller than the M22520/1-01 and is therefore easier to handle. It is also similar in appearance and has the same method of operation. CAUTION: Do not disassemble any crimping tools. Do not tighten or loosen nuts or otherwise attempt to adjust. Required adjustments should be made only by the manufacturer or by a calibration laboratory. All crimping tools can be checked for proper calibration of the crimping jaws (indenters) using go/no- go inspection gages. If the go gauge does not pass through the indenter tips and/or the no-go gauge passes through the tips, do not use the tool, but return it for repair. The tools should be checked before each series of crimping operations. Do not crimp down on the gauge pins as this will prevent the tool from fully cycling to the ratchet release position. All crimping tools have positioning devices available that are used for locating contacts in the proper relation to the tool indenters. © 2021 Daniels Manufacturing Corporation (DMC) MIL-DTL-22520/7-01 2023-02-06 B-07c Maintenance Practices Page 140 of 255 CASA Part 66 - Training Materials Only  Inspection of Crimp Contacts Note the indentations from the four jaws. Determine the proper length of insulation to be removed: The wire must be visible in the inspection hole. Insulation must be 1/64–1/32 in. from the end of the contact. Inspection of crimp contacts 2023-02-06 B-07c Maintenance Practices Page 141 of 255 CASA Part 66 - Training Materials Only  Contact Retention Systems Connectors Electric connectors are designed in many sizes and shapes to facilitate the installation and maintenance of electric circuits and equipment in aircraft. A connector assembly actually consists of two principal parts, often called the plug and the receptacle. The plug section generally contains the sockets, and the receptacle contains the pins. The pins and sockets are connected to the individual wires that make up the circuit. When the plug and receptacle are assembled together, the pins slip inside the sockets and form the electrical connection. Two basic types of contact retention are used in plug-and-socket connectors in aircraft: Front release Rear release. The front-release contacts are removed from a connector using an extracting tool, which is inserted into the connector from the front or face and pushes the pin or socket out the back of the connector. On a rear-release connector, the contact removal tool must be inserted into the contact cavity from the back or rear of the connector to release the contact retention clip and pull the contact out. Whether rear-release or front-release, contacts are inserted through the rear of the connector. Aviation Australia Contact retention systems 2023-02-06 B-07c Maintenance Practices Page 142 of 255 CASA Part 66 - Training Materials Only  Contact Installation and Removal Tools Many different pins and sockets are available in different shapes, lengths, thicknesses, wire gauge sizes, materials, etc. Contact installing and removal tools The contacts have a certain diameter size to fit in the holes of the connector/receptacle. Their diameter sizes are standard and follow the same thickness measurement as wires, American Wire Gauge (AWG). This size is usually referred to as the engaging end of the contact. The other end of the contact is referred to as the crimp barrel. Obviously, the size of the contact (and therefore the size of the holes in the connector/receptacle) decides which contact removal and insert tool can be used for that contact. The removal and insert tools are colour-coded for size, typically: AWG 12 – yellow AWG 16 – blue AWG 20 – red. 2023-02-06 B-07c Maintenance Practices Page 143 of 255 CASA Part 66 - Training Materials Only  Front Release With the front-release system, contact positions can be identified easier from the mating end of the connector for releasing and servicing purposes. The removal tool engages the front portion of each contact, and then by application of axial force, the contact is displaced until it is visible at the wire bundle side. It can then be removed by hand. Care must be exercised with the front-release system, particularly when servicing pin contacts, to avoid canting the removal tool excessively. This action can bend the contacts. Front-release removal tools, as illustrated, typically consist of a handle, a tip and a plunger (shedder) which is sometimes spring-loaded. Front release contacts and removal tools Connectors with front-release type contacts can use a variety of extraction and insertion tools. For insertion, some connectors require use of the tool to disengage the locking mechanism, and others may be manually inserted. 2023-02-06 B-07c Maintenance Practices Page 144 of 255 CASA Part 66 - Training Materials Only  Rear Release Connectors requiring release and removal of the contacts from the rear of the connector almost always have a blue band around the shell (body) of the connector and utilise insert lock retention. With the rear-release system, identifying the wire from the back side of the connector is difficult. Removal tools must be put over the wire and inserted through an insert (grommet) to release the contact; the tool and wire must be pulled together to remove the contacts from the connectors. Contact bend damage while servicing is minimised since the tool does not shroud the contact. Rear release contact Rear-Release Connector Contact Insertion/Removal Tool Rear release pins and sockets are very widely used in aircraft connectors. Plastic and metal/plastic insertion and removal tools are widely used and are inexpensive. Plastic insertion and removal tools 2023-02-06 B-07c Maintenance Practices Page 145 of 255 CASA Part 66 - Training Materials Only  The following photo shows one type of insertion/removal tool, but its use is exactly like other similar types. Colour coding typically reflects AWG sizing and is not the same as typical crimper colour coding. Note these features: White – contact removal – slips around contact and releases locking device Red – contact insertion – locking device secures with forward pressure Conductor channel and serrations – aids in conductor/contact support White end is to remove – white (or light) outside Coloured end is to insert – coloured (or dark) inside Think of a cave – dark is in and light is out. Rear release tool 2023-02-06 B-07c Maintenance Practices Page 146 of 255 CASA Part 66 - Training Materials Only  Unwired Contact Extraction Unwired contacts are inserted to complete connector configuration. To remove the unwired contact, a specially designed unwired contact removal tool, as illustrated, can be used. The tool is designed to be inserted into a contact hole, both releasing the contact and grasping it so that it can be withdrawn. Aviation Australia Removal tools for unwired contacts Maintenance Kit Larger and more modern aircraft, such as the Boeing 777's, 737's, Airbus' etc. have complicated electrical wiring systems containing many different connectors, contacts and terminals, the repair of which requires precision tools. A maintenance kit for electrical connectors and wiring systems contains all of the tooling to connector, contact and terminal cross-reference information needed to support aircraft. In addition, some kits also contain illustrated operating instructions for the required tooling. Image by Red Box Tools - Model / Part: RBI8406T Boeing 737 maintenance kit 2023-02-06 B-07c Maintenance Practices Page 147 of 255 CASA Part 66 - Training Materials Only  Coaxial Cables The Coaxial Cable Coaxial cable (or ‘coax’) is the most common cable used for transmitting radio frequency (RF) signals. The name coaxial refers to the common axis of the two conductors. A typical coaxial cable consists of an inner (centre) conductor separated from the outer conductor, usually called a shield, by an insulating dielectric. The cable is protected against moisture and abrasion by a tough outer jacket (sometimes called a sheath). The inner conductor is usually copper, either solid or stranded, and may be bare, tin-plated or silver- plated. The outer conductor (shield) is usually a copper braid, bare, tin-plated or silver-plated, woven over the dielectric. Some coaxial cables have a double outer conductor (double shield) to provide extra shielding. The dielectric has two functions: (1) it provides low-loss insulation between the inner conductor and the outer conductor and (2) it maintains the relative position of the inner conductor inside the outer conductor and therefore keeps the capacitance between the two at a constant value. The coaxial cable we study here is of the flexible (braided), solid dielectric type, relatively small to medium size. The characteristic impedance of most of the cable is 50 Ω, but some have a characteristic impedance of 48, 53, 75 or 93 Ω. Coaxial cables 2023-02-06 B-07c Maintenance Practices Page 148 of 255 CASA Part 66 - Training Materials Only  Coaxial Cable Connectors Coaxial cable is attached to coaxial cable connectors, often called RF connectors, to form a coaxial cable assembly that carries RF power from one point to another with a known rate of loss. RF connectors are available as plugs, jacks, panel jacks and receptacles. Plugs and jacks are attached to the ends of coaxial cables; panel jacks and receptacles are mounted to panels and chassis and may be either front- or rear-mounted. Plugs always have male contacts; jacks and panel jacks always have female contacts. The common series used in aircraft are: BNC series – a small, lightweight, bayonet type, quick-connect/disconnect connector used with small coaxial cables where peak voltage is not more than 500 V; essentially a miniature version of the C-connector, which was a bayonet version of the N-type connector HN series – a high-voltage (up to 5000 V), threaded coupling connector used with medium- sized coaxial cables N series – a general purpose, threaded coupling connector used with medium-sized coaxial cables C series – a bayonet type, quick-connect/disconnect connector used with medium-sized coaxial cables; electrically similar to the N series Pulse series – a high-voltage connector for pulse or DC applications; designed for use with rubber-dielectric pulse cables, but may be used with equivalent-size cables of other construction where high voltage is not required; with ceramic inserts, peak voltage is 15 000 V at sea level, and with rubber inserts, 5000 V at 50 000 ft, but higher voltages may be used at lower altitudes TNC series – a small, lightweight connector similar to the BNC series, but having a threaded coupling; used where a positive coupling under vibration and a low noise level are desirable SC series – a connector used with medium-sized coaxial cables; similar to the C series, but has a threaded coupling. 2023-02-06 B-07c Maintenance Practices Page 149 of 255 CASA Part 66 - Training Materials Only  Coaxial Cable Assembly Dozens of series of coaxial cable connectors are available today, and each series may develop to various versions. This makes the method of attaching coaxial cable to the connector body different. An improved version of BNC connectors is used as an example below to show the recommended methods for assembling coaxial cable to it. If the vendor provides instructions for installing their connector, that should take precedence over this example. This improved version is a clamp-type connector consisting of a plug or jack body assembled to coaxial cable with a nut, grooved gasket and sleeve clamp. The sleeve clamp has a sharp rear face, which cuts into the grooved gasket and thus makes a tight seal. An insulation bushing is added where assembly is to RC-62/U or 71/U coaxial cables. Plug UG-88E/U and Jack UG-89C/U are typical of this version. Improved version of BNC connector General Coaxial Cable Assembly Procedures Remove 5/16 in. of the outer jacket, exposing the shield. CAUTION: Do not nick the shield. Comb out the shield carefully to avoid breaking shield strands. Strip dielectric to 3/16 in. from the edge of the jacket, exposing the centre conductor. CAUTION: Do not nick the centre conductor. Disassemble the nut, grooved gasket and sleeve clamp from the plug or jack body. Taper the shield towards the centre conductor and wrap a piece of thin pressure tape, wide enough to cover the combed-out shield (one layer is sufficient), around the shielding, forming a cone with the narrow end towards the conductor. Slide the nut and gasket (V-groove away from nut) in that order over the tapered shield and onto the jacket. Slide the sleeve clamp over the tapered shield until the inside shoulder of the clamp butts flush against the cut end of the jacket. 2023-02-06 B-07c Maintenance Practices Page 150 of 255 CASA Part 66 - Training Materials Only  Remove the tape from the shield, comb the shield back smoothly over the sleeve clamp and trim it to 3/32 in. with scissors. Trim the dielectric to 1/8 in. from the shield, and cut the centre conductor to 1/8 in. from the edge of the dielectric. Tin the centre conductor and the inside of the contact (male or female). Slip the contact over the centre conductor so that it butts flush against the dielectric. For RC-62/U and 71/U, add a bushing. Solder using a clean, well-tinned soldering iron – the contact must still be flush against the dielectric after the solder has cooled; if it is not, remake the joint. CAUTION: Make sure the correct contact is used; a male contact always goes into a plug body, and a female contact always goes into a jack body. Push the cable assembly into the connector body as far as it will go. Make sure the gasket is properly seated, with the sharp edge of the sleeve clamp entering the gasket groove. Slide the nut into the connector body and fasten it in the vice. Start the nut by hand and tighten it with an end wrench until enough pressure is applied to make a good seal by splitting the gasket. Aviation Australia General coaxial cable assembly procedures A coaxial cable can get damage at some distance from the sending end of the line. The damage may alter the electrical properties or the shape of the coaxial cable, causing the impedance at the fault location to differ from its characteristic impedance. Some instruments can be used to detect that damage, but the time-domain reflectometer is the most comprehensive. As previously introduced in this module, a time-domain reflectometer (TDR) is an electronic instrument used to characterise and locate faults on a transmission line, including coaxial cables, twisted wire pairs and parallel conductors. A TDR works much like radar, sending an electrical pulse onto the cable segment under test, then displaying the echo of the pulse. The shape of the echo, the direction of the echo pulse (whether the pulse goes upwards or downwards on the screen), and the location of the pulse on the screen provide a good deal of information. 2023-02-06 B-07c Maintenance Practices Page 151 of 255 CASA Part 66 - Training Materials Only  A metal time-domain reflectometer (TDR) TDR screen shots of various coax faults 2023-02-06 B-07c Maintenance Practices Page 152 of 255 CASA Part 66 - Training Materials Only  Coaxial Cable Assembly General Precautions A good connection depends on holding coaxial cable and connectors to the design dimensions. Any change in these dimensions causes added losses to the RF power being carried, and may cause radiation interference. The assembly directions given for each connection must be followed carefully to avoid trouble. The following precautions are common to all assemblies of the coaxial cable and RE connectors. When working with coaxial cable, never step on the cable, set anything heavy on it or bend it sharply. This flattens the cable and changes its electrical characteristics. Handle coaxial cable carefully at all times. Anything which damages it, or which might lead to its being damaged later, reduces the efficiency of the system. Do not use pliers to assemble or disassemble RF connectors. Contacts for RF connectors are usually packed unassembled. Do not misplace them. When attaching connectors to coaxial cable having a double shield, make sure both shields are soldered together at the connector. Use care in starting the nut into a plug or jack body in order to prevent cross-threading. Keep the soldering iron clean, smooth and well tinned at all times. Coax cable 2023-02-06 B-07c Maintenance Practices Page 153 of 255 CASA Part 66 - Training Materials Only  Coaxial Cable Installation Precautions The installation and maintenance of coaxial cable are critical because of the possibility of causing erroneous electronic equipment indications and interference. Despite the shielding of coaxial cable, cable runs should be as direct and as short as possible to further minimise radio frequency interference (RFI) hazards. The braided outer conductor may be grounded at only one end or at both ends to protect the electronic circuitry. It depends on the installation. Cable ties are not permitted to be used with coaxial cable – they can crush the dielectric and damage the cable. When installing coaxial cable, if the cable must be routed around corners, its minimum bend radius should be 6 times the diameter of the cable to prevent kinking. If the cable becomes kinked, dented or crimped, the reduced distance from the centre conductor to the shielding may cause electrical interference. Some coaxial cables have a strictly defined length; do not change the length during maintenance. To prevent damage, the cable should be supported at a minimum of 24-in. intervals and located in a position that precludes it from being stepped on or used as a handhold. Coaxial cable minimum bend radius 2023-02-06 B-07c Maintenance Practices Page 154 of 255 CASA Part 66 - Training Materials Only  Wires Wire Types The electrical wiring in an aircraft must be properly selected, installed and maintained in order to ensure the safety of the aircraft. Several conditions must be considered when choosing an aircraft electrical wire, including design temperature, flexibility requirements, abrasion resistance, strength, insulation, electrical resistance, weight and applied voltage and current flow. These factors determine the type of conductor and insulation necessary for a given installation. Any type of single conductor surrounded by insulation is usually referred to as a wire. Most aircraft wire is made with a stranded copper conductor, either seven or 19 strands for small wire and 19 or more for larger wire. The use of stranded, or twisted, wire increases the flexibility of the conductor, thus decreasing the chance of fatigue failure. Flexible wire is made of several small strands; less flexible wire is made of fewer, coarser strands. Solid wire (a single strand) is very inflexible and may only be used in limited areas of the aircraft. In most cases, the wiring is coated with tin, silver or nickel to help prevent oxidation. Where large amounts of current must be carried for long distances, aluminium wire is often used. Approved wire insulations include polyvinylidene fluoride (PVF), fluorinated ethylene propylene (FEP), polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene copolymer (ETFE) and glass braids. The TFE type insulations are often sold under the trade name Teflon or Tefzel. In any case, the type of insulation is governed by operating temperature, insulation resistance, abrasion resistance, chemical resistance and strength. Design engineers determine the requirements and specify the type of wire that meets the requirements for each circuit. A cable is any group of two or more conductors separately insulated and grouped together by an outer sleeve. One cable can be routed through the aircraft and used for several circuits. A cable’s primary disadvantage is created by the inability to repair or replace a single wire. 2023-02-06 B-07c Maintenance Practices Page 155 of 255 CASA Part 66 - Training Materials Only  Wire Size Aircraft wire is measured by the AWG system, with the larger numbers representing the smaller wires. The smallest size wire normally used in aircraft is 22-gauge wire, which has a diameter of about 0.025 in. Each gauge size is related to a specific cross-sectional area of wire. The amount of current a wire is capable of carrying is determined by its cross-sectional area, usually expressed in circular mil sizes. Aviation Australia Circular mil and square mil A circular mil (cmil) is the standard measurement of a round conductor’s cross-sectional area. Since 1 mil is equivalent to 0.001 in., a wire diameter of 0.125 is expressed as 125 mils. To find the cross- sectional area of a round conductor in circular mils, square the conductor’s diameter. For example, if a round wire has a diameter of 3/8 in., or 375 mils, its circular area is 140 625 cmil (375 × 375 = 140 625). The square mil is the unit of measure for square or rectangular conductors such as bus bars. To determine the cross-sectional area of such a conductor in square mils, multiply the conductor’s thickness by its width. For example, the cross-sectional area of a copper strip 400 mils thick and 500 mils wide is 200 000 sq mils. Note that 1 cmil is 0.7854 of 1 sq mil. 2023-02-06 B-07c Maintenance Practices Page 156 of 255 CASA Part 66 - Training Materials Only  Wire Gauge A wire gauge is used to determine the wire size of conductors. The picture shows a typical American Standard wire gauge ranging in size from No. 0 to No. 36. A wire gauge The measurement of a bare conductor is taken in the slot, not in the circular area at the bottom of the slot. Each slot, being a specific size, represents a given wire gauge. The stripped portion of a wire is inserted into a slot which fits snugly around the conductor. The wire size is marked adjacent to that slot. Wire Specifications 2023-02-06 B-07c Maintenance Practices Page 157 of 255 CASA Part 66 - Training Materials Only  To ensure safety, all electrical wires used in aircraft must meet one or more of the applicable specifications. The current wire specifications include: Major Original Aircraft Manufacturers (OAMs) standards, such as the Boeing Material Standard (BMS) Military Specifications (Mil-Spec) or Defence Standard (Def-Stan) Wire manufacturers’ specifications Specifications given by industry standards bodies such as the SAE (Society of Automotive Engineers – Aerospace), NEMA (National Electrical Manufacturers Association), UL (Underwriters Laboratories), ASTM (American Society for Testing and Materials), AECMA (European Association of Aerospace Industries), etc. Governing bodies’ standards. 2023-02-06 B-07c Maintenance Practices Page 158 of 255 CASA Part 66 - Training Materials Only  Wire Military Specifications Military Specifications cover the installation of wiring in aircraft. Typical specification numbers for approved aircraft wire are MIL-W-5086, MIL-C-7078, MIL-W-22759, MIL-W-81381, MIL-C-27500 and MIL-W-81044. These standards specify the type of conductor, the conductor plating (if any) and the type of insulation. They give specific operating temperature ranges, voltage levels and general application guidelines. All aircraft wire must meet the appropriate standard to be considered airworthy. Always install wire according to current manufacturers’ data. The specification number for aluminium aircraft wire is MIL-W-7072. When copper wire is replaced with aluminium wire, the aluminium wire should be two sizes larger than the copper wire because of its greater resistance. Aluminium wire smaller than No. 6 must not be used on aircraft. Characteristics of some typical aircraft wires are as follows: MIL-W-22759/1 – silver-coated copper conductor, TFE fluorocarbon and glass insulator, 600 V, 260 °C MIL-W-22759/41 – nickel-coated copper conductor, ETFE dual insulator, 600 V, 105 °C MIL-W-22759/16 – tin-coated copper conductor, ETFE insulator, 600 V, 150 °C MIL-W-22759/19 – silver-plated high-strength copper alloy conductor, ETFE insulation, 600 V, 150 °C MIL-W-8 1044/9 – tin-coated copper conductor, polyalkene-reinforced PVF insulation, 600 V, 150 °C. An important reason for these specifications is that ensuring uniformity of sizes reduces the danger of fires caused by improper selection of wire sizes. Wires can carry only a limited amount of current safely. If the current flowing through a wire exceeds the wire’s current-carrying capacity, excess heat is generated. This heat may be great enough to burn off the insulation around the wire and start a fire. Wiring Inspection Requirements Wires and cables should be inspected for adequacy of support, protection and general condition throughout. Accordingly, aircraft wiring must be visually inspected for the following requirements. CAUTION: For personal safety, and to avoid the possibility of fire, turn off all electrical power prior to starting an inspection of the aircraft electrical system or performing maintenance. 2023-02-06 B-07c Maintenance Practices Page 159 of 255 CASA Part 66 - Training Materials Only  Support Wires and cables are supported by suitable clamps, grommets or other devices at intervals of not more than 24 in., except when contained in troughs, ducts or conduits. The supporting devices should be of a suitable size and type, with the wires and cables held securely in place without damage to the insulation. Wire and cables are properly supported and bound so that there is no interference with other wires, cables and equipment. Wires and cables in junction boxes, panels and bundles are properly supported and laced to provide proper grouping and routing. Wires and cables are adequately supported to prevent excessive movement in areas of high vibration. Ensure the supports do not restrict the wires or cables in a manner that interferes with operation of equipment shock mounts. Metal stand-offs must be used to maintain clearance between wires and structure. Tape or tubing is not acceptable as an alternative to stand-offs for maintaining clearance. Insulating tubing is secured by tying, tie straps or clamps. Insulating tubing must be kept at a minimum and must be used to protect wire and cable from abrasion, chafing, exposure to fluid and other conditions which could affect the cable insulation. However, use of insulating tubing to support wires and cable in lieu of stand-offs is prohibited. 2023-02-06 B-07c Maintenance Practices Page 160 of 255 CASA Part 66 - Training Materials Only  Protection Ensure that wires and cables are routed so that chafing will not occur against the airframe or other components. Ensure that wires and cables are positioned so that they are not likely to be used as handholds or as support for personal belongings and equipment. Ensure that wires and cables are routed, insofar as practicable, so that they are not exposed to damage by personnel moving within the aircraft. Ensure that wires and cables are located so as not to be susceptible to damage by the storage or shifting of cargo. Ensure that wires and cables are routed so that there is no possibility of damage from battery electrolytes or other corrosive fluids. Ensure that wires and cables are adequately protected in wheel wells and other areas where they may be exposed to damage from impact by rocks, ice, mud, etc. If rerouting is not practical, protective jacketing may be installed. This type of installation must be held to a minimum. Other Conditions Ensure that unused wires are individually dead-ended, tied into a bundle and secured to a permanent structure. Each wire should have strands cut even with the insulation and a pre-insulated closed end connector or a 1-in. piece of insulating tubing placed over the wire with its end folded back and tied. Ensure that all wires and cables are identified properly at intervals of not more than 15 in. Coaxial cables are identified at both equipment ends. Do not use tape, tie straps or cord for primary support. Do not use tapes (such as friction or plastic tape) which will dry out in service, produce chemical reactions with wire or cable insulation, or absorb moisture. Do not use moisture-absorbent material as ‘fill’ for clamps or adapters. Continuous lacing (spaced 6 in. apart) is not used except in panels and junction boxes, where this practice is optional. When lacing is installed in this manner, outside junction boxes should be removed and replaced with individual loops. 2023-02-06 B-07c Maintenance Practices Page 161 of 255 CASA Part 66 - Training Materials Only  Wiring Damage Tolerance In the repair and modification of existing aircraft, when a replacement wire is required, the maintenance manual for that aircraft must first be reviewed to determine if the OAM has approved any substitution. If not, then the OAM should be contacted for an acceptable replacement. Wiring must be replaced with equivalent wire when found to have any of the following defects: Wiring that has been subjected to chafing or fraying, has been severely damaged, or contains primary insulation suspected of being penetrated Wiring on which the outer insulation is brittle to the point that slight flexing causes it to crack Wiring having weather-cracked outer insulation Wiring that is known to have been exposed to electrolyte or on which the insulation appears to be, or is suspected of being, in an initial stage of deterioration due to the effects of electrolyte Wiring that shows evidence of overheating (even if only to a minor degree), depending on the cause of the overheating Wiring on which the insulation has become saturated with engine oil, hydraulic fluid or another lubricant Wiring that bears evidence of having been crushed or severely kinked Shielded wiring on which the metallic shield is frayed and/or corroded; cleaning agents or preservatives should not be used to minimise the effects of corrosion or deterioration of wire shields Wiring showing evidence of breaks, cracks, dirt or moisture in the plastic sleeves placed over wire splices or terminal lugs Sections of wire in which splices occur at less than 10-ft intervals, unless specifically authorised due to parallel connections, locations or inaccessibility. When replacing wiring or coaxial cables, identify them properly at both equipment power source ends. Wiring identification 2023-02-06 B-07c Maintenance Practices Page 162 of 255 CASA Part 66 - Training Materials Only  EWIS Wiring Protection Techniques Cable Looming With so many wires used in aircraft, it helps to bundle them into a loom and anchor them at suitable points. A wire bundle consists of a quantity of wires fastened or secured together and all travelling in the same direction. Wire bundles may consist of two or more groups of wires. It is often advantageous to have a number of wire groups individually tied within the wire bundle for ease of identification at a later date. Group and bundle ties (left) and combing tool (right) Good practice and most approved installations generally limit the size of a wire bundle to 75 wires or 5 cm (2 inches) diameter, whichever is smaller. Comb the wire groups and bundles so that the wires lie parallel to each other and minimise the possibility of insulation abrasion. A combing tool similar to that shown may be made from any suitable insulating material, taking care to ensure all edges are rounded to protect the wire insulation. 2023-02-06 B-07c Maintenance Practices Page 163 of 255 CASA Part 66 - Training Materials Only  Bend Radii The minimum radius of bends in wire groups or bundles must not be less than 10 times the outside diameter of the largest wire or cable, except at the terminal strips where wires break out at terminations or reverse direction in a bundle. Where the wire is suitably supported, the radius may be 3 times the diameter of the wire or cable. Minimum bend radius Where it is not practical to install wiring or cables within the radius requirements, the bend should be enclosed in insulating tubing. The radius for thermocouple wire is 20 times the diameter (this is very delicate wire). Ensure that RF cables, e.g. coaxial and triaxial, are bent at a radius of no less than 6 times the outside diameter of the cable. 2023-02-06 B-07c Maintenance Practices Page 164 of 255 CASA Part 66 - Training Materials Only  Slack Wiring should be installed with sufficient slack so that bundles and individual wires are not under tension. Wires connected to movable or shock-mounted equipment should have sufficient length to allow full travel without tension on the bundle. Wiring at terminal lugs or connectors should have sufficient slack to allow two re-terminations without replacement of wires. This slack should be in addition to the drip loop and the allowance for movable equipment. Normally, wire groups or bundles should not exceed 1.2 cm (0.5 inch) deflection between support points, as shown in the illustration. This measurement may be exceeded provided there is no possibility of the wire group or bundle touching a surface that may cause abrasion. Sufficient slack should be provided at each end to: Permit replacement of terminals Prevent mechanical strain on wires Permit shifting of equipment for maintenance purposes. © Aviation Australia Slack between supports In some cases, a loop in the wiring is the preferred method for providing adequate slack to enable equipment removal and replacement. 2023-02-06 B-07c Maintenance Practices Page 165 of 255 CASA Part 66 - Training Materials Only  Cable Loom Support Bind and support wire and wire bundles to meet the following requirements: Prevent chafing of cables Prevent mechanical strain that could break the conductors and connections Prevent arcing or overheated wires from damage to mechanical control cables Prevent interference between wires and other equipment Prevent excessive movement in areas of high vibration Secure wires and wire bundles routed through bulkheads and structural members Fasten wires in junction boxes, panels and bundles for proper routing and grouping Facilitate re-assembly to equipment and terminal boards Permit replacement or repair of individual wires without removing the entire bundle. Cable loom support 2023-02-06 B-07c Maintenance Practices Page 166 of 255 CASA Part 66 - Training Materials Only  Lacing and Tying Wire groups and bundles are laced or tied to ease installation, maintenance and inspection. Lacing is securing a group or bundle of wires, installed inside enclosures, with a continuous cord forming loops at regular intervals around the group or bundle. Except for enclosures, wire groups or bundles should not be laced. Tying is securing a group or bundle of wires with individual ties at regular intervals around the group or bundle. A wire group is two or more wires tied or laced together to give identity to an individual system. A wire bundle is two or more wire groups tied or laced together because they are going in the same direction at the point where the tie is located. Lacing and tying General Precautions When lacing or tying wire groups or bundles, observe the following precautions: Lace or tie bundles tightly enough to prevent slipping, but not so tightly that the cord or tape cuts into or deforms the insulation. Be especially careful when lacing or tying coaxial cable, which has a soft dielectric insulation between the inner and outer conductors. Caution: Do not use round cord for lacing or tying coaxial cable or bundles that contain coaxial cable. Do not use ties on wire groups or bundles located inside a conduit. When tying wire bundles behind connectors, start ties far enough back from the connector to avoid splaying of contacts. 2023-02-06 B-07c Maintenance Practices Page 167 of 255 CASA Part 66 - Training Materials Only  Lacing Continuous lacing may be used only on wire groups or bundles that are to be installed in panels or junction boxes. Use double-cord lacing on groups or bundles larger than 2.5 cm (1 inch) in diameter. Use either single- or double-cord lacing on groups or bundles 2.5 cm (1 in) or less in diameter. FAA Single cord lacing FAA Double cord lacing 2023-02-06 B-07c Maintenance Practices Page 168 of 255 CASA Part 66 - Training Materials Only  Tying Wire groups or bundles where supports are more than 30cm (12 inches) apart use tying to maintain the form and structure of the loom. The distance between the spot ties is dependent on the wiring loom diameter (size). The following points can be used as a guide to determine the spot tie spacing. If the loom is 0.6 cm (0.25 inch) or less then a spot tie is placed every 1.2 cm (0.5 inch) If the loom is 1.2 cm (0.5 inch) or less then a spot tie is placed every 4 cm (1.5 inches) If the loom is 2.5 cm (1 inch) or less then a spot tie is placed every 5 cm (2 inches.) If the loom is greater than 2.5 cm (1 inch) then a spot tie is placed every 7.5 cm (3 inches.) Tying Making Ties Make ties as follows: Wrap the cord around the wire group or bundle. Make a clove hitch, followed by a square knot with an extra loop. Trim the free ends of the cord to 1 cm (3/8 of an inch) minimum. Self-Clinching Cable Straps Self-clinching cable straps are adjustable, lightweight, flat plastic straps used for tying and supporting cable assemblies and wire bundles. They have moulded ribs or serrations on the inside surface to grip the wire. They may be used instead of individual cord ties for securing wire groups or bundles quickly. These straps are of two types: MS3367 cable-securing straps are plain cable straps. MS3368 identification and securing straps have a flat surface for identifying the cables. These straps are available in either black or natural colour. 2023-02-06 B-07c Maintenance Practices Page 169 of 255 CASA Part 66 - Training Materials Only  They may be used instead of individual cord ties to quickly secure wire groups or bundles. Do not use cable straps over wire bundles containing coaxial cable. Cable straps should not be used in the following situations: Where total temperature (ambient plus rise) exceeds 85 °C Where failure of the strap would permit movement of the wiring against parts which could damage the insulation or foul mechanical linkages Where failure would permit the strap to fall into moving mechanical parts In high-vibration areas Outside the fuselage In wheel wells Where exposure to ultraviolet (UV) light might exist, unless the straps are resistant to such exposure, i.e. black straps To tie wire groups or harnesses within bundles Installing self-clinching cable straps is done with a Military Standard hand tool, as shown in the photo. To use the tool, follow the manufacturer's instructions. Tool tension must be set for typical wire bundle applications. Use proper tools and make sure the strap is cut flush with its eye. This prevents painful cuts and scratches caused by protruding strap ends. Self-clinching cable straps and a tool 2023-02-06 B-07c Maintenance Practices Page 170 of 255 CASA Part 66 - Training Materials Only  Cable Clamps Wire bundles must be securely clamped to aircraft structure using cable clamps. Clamps and other primary support devices must be constructed of materials that are compatible with their installation and environment, in terms of temperature, fluid resistance, exposure to UV light and wire bundle mechanical loads. They should be spaced at intervals not exceeding 60 cm (24 inches.) Clamps lined with non-metallic material should be used to support the wire bundle along the run. Tying may be used between clamps, but should not be considered a substitute for adequate clamping. The back of the clamp, whenever practical, should be rested against a structural member. Mount cable clamps directly to Z members of the structure. Use an angle bracket with two mounting screws if the structural member is angled. Cable clamps Clamps on wire bundles should be selected to achieve a snug fit without pinching wires. Clamps on wire bundles should not allow the bundle to move through the clamp when a slight axial pull is applied. Clamps on RF cables must fit without crushing and must be snug enough to prevent the cable from moving freely through the clamp, but may allow the cable to slide through the clamp when a light axial pull is applied. The cable or wire bundle may be wrapped with one or more turns of electrical tape when required to achieve this fit. CAUTION: The use of metal clamps on coaxial RF cables may cause problems if the clamp fit distorts the RF cable's original cross-section. 2023-02-06 B-07c Maintenance Practices Page 171 of 255 CASA Part 66 - Training Materials Only  When a wire bundle is clamped into position, if there is less than 1 cm (3/8 of an inch) clearance between the bulkhead cut-out and the wire bundle, a suitable grommet should be installed. The grommet may be cut at a 45° angle to facilitate installation, provided it is cemented in place and the slot is located at the top of the cut-out. Aviation Australia Mount cable clamps Clamps must be installed with their attachment hardware positioned above them, wherever practicable, so that they are unlikely to rotate as the result of wire bundle weight or wire bundle chafing. Aviation Australia Cable clamp installation 2023-02-06 B-07c Maintenance Practices Page 172 of 255 CASA Part 66 - Training Materials Only  Wiring Protection Install and route wires and wire bundles to protect against: Chafing or abrasion - Wiring must be installed in a way that avoids friction, which can damage wire insulation. High temperatures - Wiring must be routed away from high-temperature equipment and lines to prevent deterioration of insulation. Personnel and cargo - Wiring must be installed so the structure affords protection against use as a handhold and damage from cargo. Where the structure does not afford adequate protection, conduit must be used or a suitable mechanical guard must be provided. Battery acids - Never route any wires below a battery. Inspect wires in battery areas frequently. Replace any wires that are discoloured by battery fumes. Solvents and fluids - Avoid areas where wires will be subjected to damage from fluids. Wires and cables installed in aircraft bilges must be installed at least 6 in. from the aircraft centreline. If wire without a protective outer jacket may be soaked in any location, use plastic tubing to protect it. This tubing should extend past the wet area in both directions and be tied at each end if the wire has a low point between the tubing ends. The lowest point of the tubing should have a 1/8-in. drainage hole. Punch the hole in the tubing after the installation is complete and the low point definitely established. Use a hole punch to cut a half circle. Be careful not to damage any wires inside the tubing when using the punch. 2023-02-06 B-07c Maintenance Practices Page 173 of 255 CASA Part 66 - Training Materials Only  Wires and wire bundles protection Wire Protection in Wheel Wells Wires located in wheel wells are subject to many additional hazards in service, such as exposure to fluids, pinching and severe flexing. Make sure all wire bundles are protected by sleeves of flexible tubing securely held at each end. There should be no relative movement at points where tubing is secured. Inspect these wires and the insulating tubing carefully at very frequent intervals. Replace wires and/or tubing at the first sign of wear. There should be no strain on attachments when parts are fully extended, but slack should not be excessive. 2023-02-06 B-07c Maintenance Practices Page 174 of 255 CASA Part 66 - Training Materials Only  Separation From Plumbing Lines When wiring must be routed