Par 11-76 Page 11-41 SECTION 6. AIRCRAFT ELECTRICAL WIRE SELECTION
11-76. GENERAL. Aircraft service imposes severe environmental condition on electrical wire.
To ensure satisfactory service, inspect wire annually for abrasions, defective insulation, condition of terminations, and potential corrosion. Grounding connections for power, distribution equipment, and electromagnetic shielding must be given particular attention to ensure that electrical bonding resistance has not been significantly increased by the loosening of connections or corrosion.
a. Wire Size. Wires must have sufficient mechanical strength to allow for service conditions. Do not exceed allowable voltage drop levels. Ensure that the wires are protected by system circuit protection devices, and that they meet circuit current carrying requirements. If it is desirable to use wire sizes smaller than #20, particular attention should be given to the mechanical strength and installation handling of these wires, e.g., vibration, flexing, and termination. When used in interconnecting airframe application, #24 gauge wire must be made of high-strength alloy.
b. Installation Precautions for Small Wires. As a general practice, wires smaller than size #20 must be provided with additional clamps, grouped with at least three other wires, and have additional support at terminations, such as connector grommets, strain-relief clamps, shrinkable sleeving, or telescoping bushings. They should not be used in applications where they will be subjected to excessive vibration, repeated bending, or frequent disconnection from screw terminations.
c. Identification. All wire used on aircraft must have its type identification imprinted along its length. It is common practice to follow this part number with the five-digit/letter Commercial and Government Entity (CAGE) code identifying the wire manufacturer. Existing installed wire that needs replacement can thereby be identified as to its performance capabilities, and the inadvertent use of a lower performance and unsuitable replacement wire avoided.
(1) In addition to the type identification imprinted by the original wire manufacturer, aircraft wire also contains its unique circuit identification coding that is put on at the time of harness assembly. The traditional “Hot Stamp” method has not been totally satisfactory in recent years when used on modern, ultra-thin-walled installations. Fracture of the insulation wall and penetration to the conductor of these materials by the stamping dies have occurred. Later in service, when these openings have been wetted by various fluids, serious arcing and surface tracking have damaged wire bundles.
(2) Extreme care must be taken during circuit identification by a hot stamp machine on wire with a 10 mil wall or thinner. Alternative identification methods, such as “Laser Printing” and “Ink Jet,” are coming into increasing use by the industry. When such modern equipment is not available, the use of stamped identification sleeving should be considered on thin-walled wire, especially when insulation wall thickness falls below 10 mils.
11-77. AIRCRAFT WIRE MATERIALS. Only wire specifically designed for airborne use must be installed in aircraft.
Par 11-77 Page 11-42
a. Authentic Aircraft Wire. Most aircraft wire designs are to specifications that require manufacturers to pass rigorous testing of wires before being added to a Qualified Products List (QPL) and being permitted to produce the wire. Aircraft manufacturers who maintain their own wire specifications invariably exercise close control on their approved sources. Such military or original equipment manufacturer (OEM) wire used on aircraft should only have originated from these defined wire mills. Aircraft wire from other unauthorized firms, and fraudulently marked with the specified identification, must be regarded as “unapproved wire,” and usually will be of inferior quality with little or no process control testing. Efforts must be taken to ensure obtaining authentic, fully tested aircraft wire.
b. Plating. Bare copper develops a surface oxide coating at a rate dependent on temperature. This oxide film is a poor conductor of electricity and inhibits determination of wire.
Therefore, all aircraft wiring has a coating of tin, silver, or nickel, that have far slower oxidation rates.
(1) Tin-coated copper is a very common plating material. Its ability to be successfully soldered without highly active fluxes diminishes rapidly with time after manufacture. It can be used up to the limiting temperature of 150 °C.
(2) Silver-coated wire is used where temperatures do not exceed 200 °C (392 °F).
(3) Nickel-coated wire retains its properties beyond 260 °C, but most aircraft wire using such coated strands have insulation systems that cannot exceed that temperature on long-term exposure. Soldered terminations of nickel-plated conductor require the use of different solder sleeves or flux than those used with tin or silver-plated conductor.
c. Conductor Stranding. Because of flight vibration and flexing, conductor round wire should be stranded to minimize fatigue breakage.
d. Wire Construction Versus Application. The most important consideration in the selection of aircraft wire is properly matching the wire’s construction to the application environment. Wire construction that is suitable for the most severe environmental condition to be encountered should be selected. Wires are typically categorized as being suitable for either “open wiring” or “protected wiring” applications. MIL-W-5088L (replaced by SAE AS50881, Wiring Aerospace Vehicle), Appendix A, Table A-I lists wires considered to have sufficient abrasion and cut-through resistance to be suitable for open-harness construction. MIL-W-5088L (replaced by SAE AS50881, Wiring Aerospace Vehicle), Appendix A, Table A-II lists wires for protected applications. These wires are not recommended for aircraft interconnection wiring unless the subject harness is covered throughout its length by a protective jacket. The wire temperature rating is typically a measure of the insulation’s ability to withstand the combination of ambient temperature and current related conductor temperature rise.
e. Insulation. There are many insulation materials and combinations used on aircraft electrical wire. Characteristics should be chosen based on environment; such as abrasion resistance, arc resistance, corrosion resistance, cut-through strength, dielectric strength, flame resistant, mechanical strength, smoke emission, fluid resistance, and heat distortion. An explanation of many of the abbreviations is identified in the glossary.
Par 11-78 Page 11-43
11-78. SUBSTITUTIONS. 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 original aircraft manufacturer (OAM) has approved any substitution. If not, then the OAM must be contacted for an acceptable replacement.
a. MIL-W-5088L (replaced by SAE AS50881, Wiring Aerospace Vehicle) Appendix A lists wire types that have been approved for military aerospace applications in open and protected wiring applications. These wires could potentially be used for substitution when approved by the OAM.
b. Areas designated as severe wind and moisture problem (SWAMP) areas differ from aircraft to aircraft but generally are considered to be areas such as wheel wells, near wing flaps, wing folds, pylons, and other exterior areas that may have a harsh environment. Wires for these applications often have design features incorporated into their construction that may make the wire unique; therefore an acceptable substitution may be difficult, if not impossible, to find. It is very important to use the wire type recommended in the aircraft manufacturer’s maintenance handbook.
c. The use of current military specification, multiconductor cables in place of OEM installed constructions may create problems such as color sequence. Some civilian aircraft are wired with the older color sequence employing “Red-Blue-Yellow” as the first three colors.
Current military specification, multiconductor cables, in accordance with MIL-DTL-27500 (NEMA WC27500), use “White-Blue-Orange” for the initial three colors. Use of an alternative color code during modification without adequate notation on wiring diagrams could severely complicate subsequent servicing of the aircraft.
TABLE 11-1 2. COMPARABLE PROPERTIES OF WIRE INSULATION SYSTEMS 11 -79.–11-84. [RESERVED.]
Par 11-85 Page 11-44 SECTION 7. TABLE OF ACCEPTABLE WIRES
11-85. AIRCRAFT WIRE TABLE. Tables 11-13 and 11-14 list wires used for the transmission of signal and power currents in aircraft. It does not include special purpose wires such as thermocouple, engine vibration monitor wire, fiber optics, data bus, and other such wire designs. Fire resistant wire is included because it is experiencing a wider application in aircraft circuits beyond that of the fire detection systems.
a. All wires in Tables 11-13 and 11-14 have been determined to meet the flammability requirements of 14 CFR part 25, § 25.1713(c) and the applicable portion of part 1 of appendix F part 25.
b. The absence of any wire from Tables 11-13 and 11-14 are not to be construed as being unacceptable for use in aircraft. However, the listed wires have all been reviewed for such use and have been found suitable, or have a successful history of such usage.
c. Explanations of the various insulation materials mentioned in Table 11-13, by abbreviations, can be found in Appendix 1.
TABLE 11-13. OPEN WIRING Document Voltage Rating (maximum) Rated Wire Temperature (°C) Insulation Type Conductor Type M22759/1 600 200 Fluoropolymer insulated TFE and TFE coated glass Silver coated copper M22759/2 600 260 Fluoropolymer insulated TFE and TFE coated glass Nickel coated copper M22759/3 600 260 Fluoropolymer insulated TFE-glass TFE Nickel coated copper M22759/4 600 200 Fluoropolymer insulated TFE-glass FEP Silver coated copper M22759/5 600 200 Fluoropolymer insulated extruded TFE Silver coated copper M22759/6 600 260 Fluoropolymer insulated extruded TFE Nickel coated copper M22759/7 600 200 Fluoropolymer insulated extruded TFE Silver coated copper M22759/8 600 260 Fluoropolymer insulated extruded TFE Nickel coated copper M22759/9 1000 200 Fluoropolymer insulated extruded TFE Silver coated copper
Par 11-85 Page 11-45 Document Voltage Rating (maximum) Rated Wire Temperature (°C) Insulation Type Conductor Type M22759/10 1000 260 Fluoropolymer insulated extruded TFE Nickel coated copper M22759/13 600 135 Fluoropolymer insulated FEP PVF2 Tin coated copper M22759/16 600 150 Fluoropolymer insulated extruded ETFE Tin coated copper M22759/17 600 150 Fluoropolymer insulated extruded ETFE Silver coated high-strength copper alloy M22759/20 1000 200 Fluoropolymer insulated extruded TFE Silver coated high-strength copper alloy M22759/21 1000 260 Fluoropolymer insulated extruded TFE Nickel coated high strength copper alloy M22759/34 600 150 Fluoropolymer insulated cross-linked modified ETFE Tin coated copper M22759/35 600 200 Fluoropolymer insulated cross-linked modified ETFE Silver coated high strength copper alloy M22759/41 600 200 Fluoropolymer insulated cross-linked modified ETFE Nickel coated copper M22759/42 600 200 Fluoropolymer insulated cross-linked modified ETFE Nickel coated high strength copper alloy M22759/43 600 200 Fluoropolymer insulated cross-linked modified ETFE Silver coated copper MIL-W-25038/3/2 600 260 See specification sheet * See specification sheet MIL-W-81044/6 600 150 Cross-linked polyalkene Tin coated copper MIL-W-81044/7 600 150 Cross-linked polyalkene Silver coated high-strength copper alloy MIL-W-81044/9 600 150 Cross-linked polyalkene Tin coated copper MIL-W-81044/10 600 150 Cross-linked polyalkene Silver coated high-strength copper alloy * Inorganic Fibers Glass TFE
Par 11-85 Page 11-46 TABLE 11-14. PROTECTED WIRING Document Voltage Rating (maximum) Rated Wire Temperature (°C) Insulation Type Conductor Type M22759/11 600 200 Fluoropolymer insulated extruded TFE Silver coated copper M22759/12 600 260 Fluoropolymer insulated extruded TFE Nickel coated copper M22759/14 600 135 Fluoropolymer insulated FEP-PVF2 Tin coated copper M22759/15 600 135 Fluoropolymer insulated FEP-PVF2 Silver plated high-strength copper alloy M22759/18 600 150 Fluoropolymer insulated extruded ETFE Tin coated copper M22759/19 600 150 Fluoropolymer insulated extruded ETFE Silver coated high-strength copper alloy M22759/22 600 200 Fluoropolymer insulated extruded TFE Silver coated high-strength copper alloy M22759/23 600 260 Fluoropolymer insulated extruded TFE Nickel coated high-strength copper alloy M22759/32 600 150 Fluoropolymer insulated cross-linked modified ETFE Tin coated copper M22759/33 600 200 Fluoropolymer insulated cross-linked modified ETFE Silver coated high-strength copper alloy M22759/44 600 200 Fluoropolymer insulated cross-linked modified ETFE Silver coated copper M22759/45 600 200 Fluoropolymer insulated cross-linked modified ETFE Nickel coated copper M22759/46 600 200 Fluoropolymer insulated cross-linked modified ETFE Nickel coated high-strength copper alloy MIL-W-81044/12 600 150 Cross-linked polyalkene-PVF2 Tin coated copper MIL-W-81044/13 600 150 Cross-linked polyalkene-PVF2 Silver coated high-strength copper alloy
Par 11-85 Page 11-47 Document Voltage Rating (maximum) Rated Wire Temperature (°C) Insulation Type Conductor Type MIL-W-81381/17 600 200 Fluorocarbon polyimide Silver coated copper MIL-W-81381/18 600 200 Fluorocarbon polyimide Nickel coated copper MIL-W-81381/19 600 200 Fluorocarbon polyimide Silver coated high-strength copper alloy MIL-W-81381/20 600 200 Fluorocarbon polyimide Nickel coated high-strength copper alloy MIL-W-81381/21 600 150 Fluorocarbon polyimide Tin coated copper
11-86. OPEN AIRFRAME INTERCONNECTING WIRE. Interconnecting wire is used in point-to-point open harnesses, normally in the interior or pressurized fuselage, with each wire providing enough insulation to resist damage from handling and service exposure. (See Table 11-13.) Electrical wiring is often installed in aircraft without special enclosing means. This practice is known as open wiring and offers the advantages of ease of maintenance and reduced weight.
11-87. PROTECTED WIRE. Airborne wire that is used within equipment boxes, or has additional protection, such as an exterior jacket, conduit, tray, or other covering is known as protected wire. (See Table 11-14.)
11-88. SEVERE WIND AND MOISTURE PROBLEMS (SWAMP).
a. Areas such as wheel wells, wing fold and pylons, flap areas, and those areas exposed to extended weather shall dictate selection and will require special consideration. Insulation or jacketing will vary according to the environment. Suitable wire types selected from MIL-W-22759 (M22759) shall be used in these applications. (See Table 11-13.)
b. Suitable wire types selected from MIL-W-22759 are preferred for areas that require repeated bending and flexing of the wire. Consideration should be made to areas that require frequent component removal or repair. (See Table 11-13.)
11-89. SHIELDED WIRE. With the increase in number of highly sensitive electronic devices found on modern aircraft, it has become very important to ensure proper shielding for many electric circuits. Shielding is the process of applying a metallic covering to wiring and equipment to eliminate interference caused by stray electromagnetic energy. Shielded wire or cable is typically connected to the aircraft’s ground at both ends of the wire, or at connectors in the cable.
Electromagnetic Interference (EMI) is caused when electromagnetic fields (radio waves) induce high-frequency (HF) voltages in a wire or component. The induced voltage can cause system inaccuracies or even failure, therefore putting the aircraft and passengers at risk. Shielding helps to eliminate EMI by protecting the primary conductor with an outer conductor. Refer to
Par 11-89 Page 11-48 MIL-DTL-27500 (NEMA WC27500), Cable, Power, Electrical and Cable Special Purpose, Electrical Shielded and Unshielded General Specifications.
11-90.–11-95. [RESERVED.]
Par 11-96 Page 11-49 SECTION 8. WIRING INSTALLATION INSPECTION REQUIREMENTS
11-96. GENERAL. Wires and cables should be inspected for adequacy of support, protection, and general condition throughout. The desirable and undesirable features in aircraft wiring installations are listed below and indicate conditions that may or may not exist. 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.
a. Wires and cables are supported by suitable clamps, grommets, or other devices at intervals of not more than 24 inches, 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.
b. Metal stand-offs must be used to maintain clearance between wires and structure.
Employing tape or tubing is not acceptable as an alternative to stand-offs for maintaining clearance.
c. Phenolic blocks, plastic liners, or rubber grommets are installed in holes, bulkheads, floors, or structural members where it is impossible to install off-angle clamps to maintain wiring separation. In such cases, additional protection in the form of plastic or insulating tape may be used.
d. Wires and cables in junction boxes, panels, and bundles are properly supported and laced to provide proper grouping and routing.
e. C lamp retaining screws are properly secured so that the movement of wires and cables is restricted to the span between the points of support and not on soldered or mechanical connections at terminal posts or connectors.
f. Wire and cables are properly supported and bound so that there is no interference with other wires, cables, and equipment.
g. Wires and cables are adequately supported to prevent excessive movement in areas of high vibration.
h. Insulating tubing is secured by tying, tie straps, or with clamps.
i. Continuous lacing (spaced 6 inches 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.
j. 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.
Par 11-96 Page 11-50
k. 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; the use of insulating tubing for support of wires and cable in lieu of stand-offs is prohibited.
l. Do not use moisture-absorbent material as “fill” for clamps or adapters.
m. Ensure that wires and cables are not tied or fastened together in conduit or insulating tubing.
n. Ensure cable supports do not restrict the wires or cables in such a manner as to interfere with operation of equipment shock mounts.
o. Do not use tape, tie straps, or cord for primary support.
p. Make sure that drain holes are present in drip loops or in the lowest portion of tubing placed over the wiring.
q. Ensure that wires and cables are routed in such a manner that chafing will not occur against the airframe or other components.
r. Ensure that wires and cables are positioned in such a manner that they are not likely to be used as handholds or as support for personal belongings and equipment.
s. Ensure that wires and cables are routed, insofar as practicable, so that they are not exposed to damage by personnel moving within the aircraft.
t. Ensure that wires and cables are located so as not to be susceptible to damage by the storage or shifting of cargo.
u. Ensure that wires and cables are routed so that there is not a possibility of damage from battery electrolytes or other corrosive fluids.
v. Ensure that wires and cables are adequately protected in wheel wells and other areas where they may be exposed to damage from impact of rocks, ice, mud, etc. (If rerouting of wires or cables is not practical, protective jacketing may be installed). This type of installation must be held to a minimum.
w. Where practical, route electrical wires and cables above fluid lines and avoid routing any wiring though fuel tanks to prevent an ignition hazard. If unavoidable, provide a 6-inch separation from any flammable liquid, fuel, or oxygen line, fuel tank wall, or other low-voltage wiring. Where 6-inch spacing cannot practically be provided, a minimum of 2 inches must be maintained between wiring and such lines, related equipment, fuel tank walls and low-voltage wiring that enters a fuel tank. Such wiring should be closely clamped and rigidly supported and tied at intervals such that contact between such lines, related equipment, fuel tank walls, or other wires would not occur, assuming a broken wire and a missing wire tie or clamp.
Par 11-96 Page 11-51
x. Ensure that a trap or drip loop is provided to prevent fluids or condensed moisture from running into wires and cables dressed downward to a connector, terminal block, panel, or junction box.
y. Wires and cables installed in bilges and other locations where fluids may be trapped are routed as far from the lowest point as possible or otherwise provided with a moisture-proof covering.
z. Separate wires from high-temperature equipment, such as resistors, exhaust stacks, heating ducts, etc., to prevent insulation breakdown. Insulate wires that must run through hot areas with a high-temperature insulation material such as fiberglass or polytetrafluoroethylene (PTFE). Avoid high-temperature areas when using cables having soft plastic insulation such as polyethylene, because these materials are subject to deterioration and deformation at elevated temperatures. Many coaxial cables have this type of insulation.
aa. 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 that 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. 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 should be done in accordance with the manufacturer’s recommendation and shall be sufficient to avoid excess losses or damage to the cable.
bb. 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.
cc. Ensure that wires and cables, that are attached to assemblies where relative movement occurs (such as at hinges and rotating pieces; particularly doors, control sticks, control wheels, columns, and flight control surfaces), are installed or protected in such a manner as to prevent deterioration of the wires and cables caused by the relative movement of the assembled parts.
dd. Ensure that wires and electrical cables are separated from mechanical control cables.
In no instance should wire be able to come closer than 1/2 inch to such controls when light hand pressure is applied to wires or controls. In cases where clearance is less than this, adequate support must be provided to prevent chafing.
ee. Ensure that wires and cables are provided with enough slack to meet the following requirements:
(1) Permit ease of maintenance.
(2) Prevent mechanical strain on the wires, cables, junctions, and supports.
(3) Permit free movement of shock- and vibration-mounted equipment.
(4) Allow shifting of equipment, as necessary, to perform alignment, servicing, tuning, removal of dust covers, and changing of internal components while installed in aircraft.
Par 11-96 Page 11-52 ff. 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-inch piece of insulating tubing placed over the wire with its end folded back and tied.
gg. Ensure that all wires and cables are identified properly at intervals of not more than 15 inches. Coaxial cables are identified at both equipment ends.
11-97. WIRING REPLACEMENT. Wiring must be replaced with equivalent wire (see paragraph 11-78) when found to have any of the following defects:
a. Wiring that has been subjected to chafing or fraying, that has been severely damaged, or that primary insulation is suspected of being penetrated.
b. Wiring on which the outer insulation is brittle to the point that slight flexing causes it to crack.
c. Wiring having weather-cracked outer insulation.
d. 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.
e. Check wiring that shows evidence of overheating (even if only to a minor degree) for the cause of the overheating.
f. Wiring on which the insulation has become saturated with engine oil, hydraulic fluid, or another lubricant.
g. Wiring that bears evidence of having been crushed or severely kinked.
h. Shielded wiring on which the metallic shield is frayed and/or corroded. Cleaning agents or preservatives should not be used to minimize the effects of corrosion or deterioration of wire shields.
i. Wiring showing evidence of breaks, cracks, dirt, or moisture in the plastic sleeves placed over wire splices or terminal lugs.
j. Sections of wire in which splices occur at less than 10-foot intervals, unless specifically authorized, due to parallel connections, locations, or inaccessibility.
k. When replacing wiring or coaxial cables, identify them properly at both equipment and power source ends.
l. Wire substitution: In the repair and modification of existing aircraft, when a replacement wire is required, the maintenance manual for that aircraft should first be reviewed to determine if the original aircraft manufacturer (OAM) has approved any substitution. If not, then the OAM should be contacted for an acceptable replacement.