GENERAL. The number and complexity of wiring systems have resulted in an increased use of electrical connectors. The proper choice and application of connectors is a significant part of the aircraft wiring system. Connectors should be kept to a minimum, selected,and installed to provide the maximum degree of safety and reliability to the aircraft. For the installation of any particular connector assembly, the specification of the manufacturer should be followed. SELECTION. The connector used for each application should be selected only after a careful determination of the electrical and environmental requirements. Consider the size, weight, tooling, logistic, maintenance support, and compatibility with standardization programs. For ease of assembly and maintenance, connectors using crimped contacts are generally chosen for all applications except those requiring an hermetic seal. A replacement connector of the same basic type and design as the connector it replaces should be used. With a crimp type connector for any electrical connection, the proper insertion, or extraction tool should be used to install or remove wires from such a connector. Refer to manufacturer or aircraft instruction manual. After the connector is disconnected, inspect it for loose soldered connections to prevent unintentional grounding. Connectors that are susceptible to corrosion difficulties may be treated with a chemically inert waterproof jelly or a environmentally sealed connector may be used. (NOTE: Although not required by AC 43.13-1b, moisture proof connectors should be used in all areas of the aircraft, including the cabin. Service history indicates that most connector failures occur due to some form of moisture penetration. Even in the pressurized environmentally controlled areas of the cockpit and cabin, moisture can occur due to “rain in the plane” type of condensation that generally is a problem in all modern transport category aircraft.)
NAVAIR 01- PAGE 3-2
AC 43.13-1B, Page 11-65
AC 43.13-1B, Page 11-91
AC 43.13-1B, pag.11-51
AC43.13-1B, pag 11.53 There must not be any strain on attachments or excessive slack when parts are fully extended or retracted.
AC43.13-1B, pag 11.57
TERMINATING SHIELDED CABLEShielded cable has a metallic braid over the insulation toprovide a barrier against electro-static interference. Toobtain satisfactory result from shielded cable, the shieldmust be unbroken and must extend to a point as near theend of the conductor as practicable. Shielded cable iseither grounded or dead-ended at each end as required bythe individual installation. STRIPPING JACKET ON SHIELDED CABLESome shielded cable has a thin extruded plastic coatingover the shielding braid. Strip this off as far as necessarywith a hot blade stripper. Length of sfrip depends onmethod of shield termination and type of wire connection.
If no hot blade stripper is available, use plier type handstrippers for sizes No. 22 through No. 10, and a knife forsizes larger than No. 10. Be careful not to damageshielding braid. Extruded jacket of shielded twisted wirescan also be stripped by holding a soldering iron, with tipremoved, against jacket, and pulling off jacket with longnose pliers as iron melts jacket.
PIGTAIL METHOD OF SHIELD TERMINATIONWhen grounding sheath connectors and tools are notavailable, terminate shield for grounding by making apigtail as follows
GROUNDING AND BONDINGOne of the more important factors in the design andmaintenance of aircraft electrical systems is properbonding and grounding.GROUNDING. Grounding is the process of electricallyconnecting conductive objects to either a conductivestructure or some other conductive return path for thepurpose of safely completing either a normal or faultcircuit.a. Types of Grounding. As a minimum, the design shoulduse three ground types:(1) ac returns,(2) Dc returns, and(3) all others.
b. Current Return Paths. The design of the groundreturn circuit should be given as much attention as theother leads of a circuit. A requirement for proper groundconnections is that they maintain an impedance that isessentially constant.c. Heavy-Current Grounds. Power ground connections,for generators, transformer rectifiers, batteries, externalpower receptacles, and other heavy-current, loads mustbe attached to individual grounding brackets that areattached to aircraft structure with a proper metal-to-metalbonding attachment.d. Current Return Paths for Internally GroundedEquipment. Power return or fault current groundconnections within flammable vapor areas must beavoided.
e. Common Ground Connections. The use of commonground connections for more than one circuit or functionshould be avoided except where it can be shown thatrelated malfunctions that could affect more than onecircuit will not result in a hazardous condition:(1) Redundant systems are normally provided with theobjective of assuring continued safe operation in the eventof failure of a single channel and must therefore begrounded at well separated points.(2) The use of loop type grounding systems (severalground leads connected in series with a ground tostructure at each end) must be avoided on redundantsystems.
(3) Electrical power sources must be grounded at separatelocations on the aircraft structure.(4) Bonds to thermally or vibration-isolated structurerequire special consideration to avoid single groundreturn to primary structure.(5) The effect of the interconnection of the circuits whenungrounded should be considered whenever a commonground connection is used. This is particularly importantwhen employing terminal junction grounding modules orother types of gang grounds that have a single attachmentpoint.
f. Grounds for Sensitive Circuits. Special considerationshould be given to grounds for sensitive circuits:(1) Grounding of a signal circuit through a power currentlead introduces power current return voltage drop into thesignal circuit.(2) Running power wires too close will cause signalinterference.(3) Separately grounding two components of a transducersystem may introduce ground plane voltage variations intothe system.(4) Single point grounds for signal circuits, with suchgrounds being at the signal source, are often a good wayto minimize the effects of EMI, lightning, and othersources of interference.
BONDING. The following bonding requirements must beconsidered:a. Equipment Bonding. Low-impedance paths to aircraftstructure are normally required for electronic equipment toprovide radio frequency return circuits and for mostelectrical equipment to facilitate reduction in EMI.b. Metallic Surface Bonding. All conducting objects onthe exterior of the airframe must be electrically connectedto the airframe through mechanical joints, conductivehinges, or bond straps capable of conducting static chargesand lightning strikes.c. Static Bonds. All isolated conducting parts inside andoutside the aircraft, having an area greater than 3 in² and alinear dimension over 3 inches, that are subjected toappreciable electrostatic charging.
d. Bonding jumpers should be installed in such a manneras not to interfere in any way with the operation ofmovable components of the aircraft.e. Self-tapping screws should not be used for bondingpurposes. Only standard threaded screws or bolts ofappropriate size should be used.f. Exposed conducting frames or parts of electrical orelectronic equipment should have a low resistance bond ofless than 2.5 millohms to structure.g. Bonds should be attached directly to the basic aircraftstructure rather than through other bonded parts.h. Bonds must be installed to ensure that the structureand equipment are electrically stable and free from thehazards of lightning, static discharge, electrical shock, etc.
i. Measurements should be performed after thegrounding and bonding mechanical connections arecomplete to determine if the measured resistance valuesmeet the basic requirements.
j. Use appropriate washers when bonding aluminum orcopper to dissimilar metallic structures so that anycorrosion that may occur will be on the washer. BONDING JUMPER INSTALLATIONSBonding jumpers should be made as short as practicable,and installed in such a manner that the resistance of eachconnection does not exceed .003 ohm.a. Bonding Connections. To ensure a low-resistanceconnection, nonconducting finishes, such as paint andanodizing films, should be removed from the attachmentsurface to be contacted by the bonding terminal.
b. Corrosion Protection. One of the more frequent causesof failures in electrical system bonding and grounding iscorrosion. Aircraft operating near salt water areparticularly vulnerable to this failure mode. A suitablenoncorrosive sealant, such as one conforming to MIL-S-8802, should be used to seal dissimilar metals forprotection from exposure to the atmosphere.c. Corrosion Prevention. Electrolytic action may rapidlycorrode a bonding connection if suitable precautions arenot taken. Aluminum alloy jumpers are recommended formost cases; however, copper jumpers should be used tobond together parts made of stainless steel, cadmiumplated steel, copper, brass, or bronze. Tables 1 through 3and figures 1 through 3 show the proper hardwarecombinations for making a bond connection.
TABLE 1: Stud bonding or grounding to flat surface.
TABLE 2: Plate nut bonding or grounding to flat surface.
TABLE 3: Bolt and nut bonding or grounding to flat surface.
FIGURE 3: Aluminum jumper connection to tubular structure.d. Bonding Jumper Attachment. The use of solder toattach bonding jumpers should be avoided.e. Ground Return Connection. When bonding jumperscarry substantial ground return current, the current ratingof the jumper should be determined to be adequate andthat a negligible voltage drop is produced.
CREEPAGE DISTANCE. Care should be used in theselection of electrical components to ensure that electricalclearance and creepage distance along surfaces betweenadjacent terminals, at different potentials, and betweenthese terminals and adjacent ground surfaces are adequatefor the voltages involved.LIGHTNING PROTECTION FOR ANTENNAS.Antennas are mounted on exterior surfaces withinlightning strike zones should be provided with a means tosafely transfer lightning currents to the airframe, and toprevent hazardous surges from being conducted into theairframe via antenna cables or wire harnesses.
LACING AND TIESTies, lacing, and straps are used to secure wire groups orbundles to provide ease of maintenance, inspection, andinstallation. Braided lacing tape per MIL-T-43435 issuitable for lacing and tying wires.In lieu of applying ties, straps meeting SpecificationMS17821 or MS17822 may be used in areas where thetemperature does not exceed 120 C.Straps may not be used in areas of SWAMP such as wheelwells, near wing flaps or wing folds and where they couldbe exposed to UV light, unless the straps are resistant tosuch exposure.
a. Lacing. Lace wire groups or bundles inside junctionboxes or other enclosures. Single cord-lacing method, andtying tape, meeting specification MIL-T-43435, may beused for wire groups of bundles 1-inch in diameter or less:
Use the double cordlacing method on wire bundles 1-inchin diameter or larger:
b. Tying. Use wire group or bundle ties where the supportsfor the wire are more than 12 inches apart.
c. Plastic Ties. Plastic tie-down straps (MS3367, Type I,Class 1):
SPLICINGSplicing is permitted on wiring as long as it does notaffect the reliability and the electromechanicalcharacteristics of the wiring. Splicing of power wires,coaxial cables, multiplex bus, and large gauge wire musthave approved data.a. Splicing of electrical wire should be kept to aminimum and avoided entirely in locations subject toextreme vibrations.b. Many types of aircraft splice connectors are availablefor use when splicing individual wires.c. There should not be more than one splice in any onewire segment between any two connectors or otherdisconnect points.
d. Splices in bundles must be staggered so as to minimizeany increase in the size of the bundle:e. Splices should not be used within 12 inches of atermination device, except for paragraph f below.f. Splices may be used within 12 inches of a terminationdevice when attaching to the pigtail spare lead of a pottedtermination device, or to splice multiple wires to a singlewire, or to adjust the wire sizes so that they are compatiblewith the contact crimp barrel sizes.
ConnectorsConnectors provide means of quickly connecting and disconnectingwires to simplify installation and maintenance of electric andelectronic equipment.A connector set consists of two parts: a plug assembly and areceptacle assembly. The receptacle is usually the “fixed” part ofthe connector, attached to a wall, bulkhead or equipment case. Theplug is a removable part of the connector usually attached to acable.When the two parts are joined, the electric circuit is made by pin-and-socket contacts inside the connector.
Cont.Many types, however crimped contacts generally used Circular type Rectangular Module blocksThe proper choice and application of connectors is asignificant part of the aircraft wiring system.Selected to provide max. degree of safety and reliabilitygiven electrical and environmental requirements Use environmentally sealed connectors to prevent moisture penetration Plug unused pins
a. Circular connectors. Military Specifications cover thecircular connectors most commonly used in aircraft: MIL-C-5015, MIL-C-26482, MIL-C-26500, MIL-C-38999,MIL-C-81511, MIL-C-81703, and MIL-C-83723.Connectors manufactured to these specifications haveeither solder type or crimp type contacts and cover a rangeof contact sizes from 0 to 22.Military connector marking. Each connector is markedon the shell or coupling ring with a code of letters andnumbers giving all the information necessary to identifythe connector. A typical code is as follows:
b. Rectangular Connectors. The rectangular connectorsare typically used in applications where a very largenumber of circuits are accommodated in a single matedpair. They are available with a great variety of contacts,which can include a mix of standard, coaxial, and largepower types.c. Module Blocks. These junctions accept crimpedcontacts similar to those on connectors. Some use internalbusing to provide a variety of circuit arrangements. Theyare useful where a number of wires are connected forpower or signal distribution. When used as groundingmodules, they save and reduce hardware installation onthe aircraft. Standardized modules are available with wireend grommet seals for environmental applications and aretrack-mounted.
Connectors must be identified by an original identificationnumber derived from MIL Specification (MS) or OAMspecification. Several different types are shown in the nextfigures:
Environmental Classes. Environment-resistantconnectors are used in applications where they willprobably be subjected to fluids, vibration, thermal,mechanical shock, corrosive elements, etc.• Firewall class connectors• Hermetic connectorsVoltage and Current Rating. Selected connectors mustbe rated for continuous operation under the maximumcombination of ambient temperature and circuit currentload. Hermetic connectors and connectors used in circuitapplications involving high-inrush currents should bederated.
SPARE CONTACTS (Future Wiring). To accommodatefuture wiring additions, spare contacts are normallyprovided. Locating the unwired contacts along the outerpart of the connector facilitates future access.A good practice is to provide: Two spares on connectorswith 25 or less contacts; 4 spares on connectors with 26 to100 contacts; and 6 spares on connectors with more than100 contacts.Connectors must have all available contact cavities filledwith wired or unwired contacts. Unwired contactsshould be provided with a plastic grommet sealing plug.
CONNECTORS INSTALLATIONa. Redundancy. Wires that perform the same function inredundant systems must be routed through separateconnectors.b. Adjacent Locations. Mating of adjacent connectorsshould not be possible. In order to ensure this, adjacentconnector pairs must be different in shell size, couplingmeans, insert arrangement, or keying arrangement.c. Sealing. Connectors must be of a type that excludemoisture entry through the use of peripheral and interfacialseal that are compressed when the connector is mated.
d. Drainage. Connectors must be installed in a mannerwhich ensures that moisture and fluids will drain out of andnot into the connector when unmated.e. Wire Support. A rear accessory backshell must be usedon connectors that are not enclosed.f. Slack. Sufficient wire length must be provided atconnectors to ensure a proper drip loop and that there is nostrain on termination after a complete replacement of theconnector and its contacts.g. Identification. Each connector should have a referenceidentification that is legible throughout the expected life ofthe aircraft.
Pin Arrangement e b g f a b f a b e n a c d h c d m c l e i f d j k A B C a, b: 115 Vac, 400 Hz, galley power c, d: 5 Vdc, discrete weight on wheels signal e, f: 28 Vdc, backup power
Wiring Routing• Wire bundles must be routed in accessible areas that areprotected from damage from personnel, cargo, andmaintenance activity.• Wiring must be clamped so that contact with equipmentand structure is avoided.• Advantageous to have a number of wire groupsindividually tied within the wire bundle for ease ofidentification at a later date.
Wiring Routing, cont. Eliminate potential for chafing against structure or other components Position to eliminate/minimize use as handhold or support Minimize exposure to damage by maintenance crews or shifting cargo Avoid battery electrolytes or other corrosive fluids
Wiring Routing, cont. Route wires above fluid lines, if practical Path of exposed end Broken wire shall not make contact with fluid line (fail-safe concept)
Wiring Routing, cont.Use drip loops to control fluids or condensed moisture Drainage hole in low point of tubing.
Wires improperly tied,riding on hydraulic lines,contaminated with caustic fluid
Wiring Routing, cont. Keep slack to allow maintenance and prevent mechanical strain Appropriate slack
Wiring Routing, cont.Wiring should be installed with sufficient slack so thatbundles and individual wires are not under tension.Wire groups or bundles should not exceed 1/2-inchdeflection between support points.
Wiring Routing, cont.Have sufficient length to allow full travel withouttension on the bundle in wires connected to movable orshock-mounted equipment or terminal lugs orconnectorsSufficient slack should be provided at each end to: a. Permit replacement of terminals. b. Prevent mechanical strain on wires. c. Permit shifting of equipment for maintenance purposes.
Wiring Routing, cont.Power Feeders: Easily inspected or replaced. Special protection to prevent potential chafing Power cables riding on structure can cause damage to the power cables A B
Wires Riding on Other Wires Wire bundles that cross should be secured together to avoid chafing
Wires Riding on Other Wires Wire bundles that cross should be secured together to avoid chafingA B
Wiring Routing, cont. RF Cable: a. Never kink coaxial cable. b. Never drop anything on coaxial cable. c. Never step on coaxial cable. d. Never bend coaxial cable sharply. e. Never loop coaxial cable tighter than the allowable bend radius. f. Never pull on coaxial cable except in a straight line. g. Never use coaxial cable for a handle, lean on it, or hang things on it (or any other wire).
• MOISTURE PROTECTION, WHEEL WELLS, AND LANDING GEAR AREAS:a. Wires located on landing gear with protective tubingb. Wires should be routed so that fluids drain away fromthe connectors.• PROTECTION AGAINST PERSONNEL AND CARGO• HEAT PRECAUTIONS• MOVABLE CONTROLS WIRING PRECAUTIONS.
• FLAMMABLE FLUIDS AND GASES Separation of wires from plumbing lines
CLAMPINGWires and wire bundles must be supported by usingclamps meeting Specification MS-21919 or plastic cablestraps• Constructed of materials that are compatible with their installation and environment in terms of: Temperature, Fluid resistance, Exposure to ultraviolet (UV) light, Wire bundle mechanical loads. Spaced at intervals not exceeding 24 inches. Selected so that they have a snug fit without pinching wires.
Y Type Wire Bundle BreakoutsFigure 8 loop maybe located before Wire bundleor after Afte breakouttail of Y Be Be r ffo orre e Wire Head of strap shall not bundles be located in this area or touching anything to cause chafing Plastic mechanical strapping
T Type Wire Bundle BreakoutsHead of strap shall not be located in Wire bundle breakout this area ortouching anything to cause chafing Wire Plastic mechanical strapping bundle