Par 11-68 Page 11-513
FIGURE 11-6. Altitude derating curve.
Par 11-68 Page 11-514 The resultant maximum wire length, after adjusting downward for the added resistance associated with running the wire at a higher temperature, is 15.3 feet, which will meet the original 15-foot wire run length requirement without exceeding the voltage drop limit expressed in figure 11-2
COMPUTING CURRENT CARRYING CAPACITY .
Example 1. Assume a harness (open or braided), consisting of 10 wires, size #20, 200 °C rated copper and 25 wires, size #22, 200 °C rated copper, will be installed in an area where the ambient temperature is 60 °C and the vehicle is capable of operating at a 60,000-foot altitude. Circuit analysis reveals that 7 of the 35 wires in the bundle (7/35 = 20 percent) will be carrying power currents nearly at or up to capacity.
STEP 1: Refer to the “single wire in free air” curves in figure 11-4a. Determine the change of temperature of the wire to determine free air ratings. Since the wire will be in an ambient of 60 ° C and rated at 200 ° C, the change of to temperature is 200 °C - 60 °C = 140 °C.
Follow the 140 °C temperature difference horizontally until it intersects with wire size line on figure 11-4a.
The free air rating for size #20 is 21.5 amps, and the free air rating for size #22 is 16.2 amps.
STEP 2: Refer to the “bundle derating curves” in figure 11-5, the 20 percent curve is selected since circuit analysis indicate that 20 percent or less of the wire in the harness would be carrying power currents and less than 20 percent of the bundle capacity would be used. Find 35 (on the abscissa) since there are 35 wires in the bundle and determine a derating factor of 0.52 (on the ordinate) from the 20 percent curve.
STEP 3: Derate the size #22 free air rating by multiplying 16.2 by 0.52 to get 8.4 amps in-harness rating. Derate the size #20 free airrating by multiplying
21.5 by 0.52 to get 11.2 amps in-harness rating.
STEP 4: Refer to the “altitude derating curve” of figure 11-6, look for 60,000 feet (on the abscissa) since that is the altitude at which the vehicle will be operating. Note that the wire must be derated by a factor of 0.79 (found on the ordinate). Derate the size #22 harness rating by multiply ing 8.4 amps by 0.79 to get 6.6 amps. Derate the size #20 harness rating by multiplying 11.2 amps by 0.79 to get 8.8 amps.
STEP 5: To find the total harness capacity, multiply the total number of size #22 wires by the derated capacity (25 x 6.6 = 165.0 amps) and add to that the number of size #20 wires multiplied by the derated capacity (10 x 8.8 = 88 amps) and multiply the sum by the 20 percent harness capacity factor. Thus, the total harness capacity is (165.0 + 88.0) x 0.20 = 50.6 amps.
It has been determined that the total harness current should not exceed 50.6 A, size #22 wire should not carry more than 6.6 amps and size #20 wire should not carry more than 8.8 amps.
STEP 6: Determine the actual circuit current for each wire in the bundle and for the whole bundle. If the values calculated in step #5 are exceeded, select the next larger size wire and repeat the calculations.
Example 2. Assume a harness (open or braided), consisting of 12, size #12, 200 °C rated copper wires, will be operated in an ambient of 25 °C at sea level and 60 °C at a 20,000-foot altitude. All 12 wires will be operated at or near their maximum capacity.
Par 11-76 Page 11-515 STEP 1: Refer to the “single wire in free air” curve in figure 11-4a, determine the temperature difference of the wire to determine free air ratings. Since the wire will be in ambient of 25 °C and 60 °C and is rated at 200 °C, the temperature differences are 200 °C-25 °C = 175 °C and 200 °C-60 °C = 140 °C respectively. Follow the 175 °C and the 140 °C temperature difference lines on figure 11-4a until each inters ects wire size line, the free air ratings of size #12 are 68 amps and 61 amps, respectively.
STEP 2: Refer to the “bundling derating curves” in figure 11-5, the 100 percent curve is selected because we know all 12 wires will be carrying full load. Find 12 (on the absci ssa) since there are 12 wires in the bundle and determine a derating factor of 0.43 (on the ordinate) from the 100 percent curve.
STEP 3: Derate the size #12 free air rati ngs by multiplying 68 amps and 61 am ps by 0.43 to get 29.2 amps and
26.2 amps, respectively.
Par 11-76 Page 11-516 STEP 4: Refer to the “altitude derating curve” of figure 11-6, look for sea level and 20,000 feet (on the abscissa) since these are the conditions at which the load will be carried. The wire must be derated by a factor of 1.0 and 0.91, respectively.
STEP 5: Derate the size #12 in a bundle rati ngs by multiplying 29.2 amps at sea level and 26.6 amps at 20,000 feet by 1.0 and 0.91, respectively, to obtained 29.2 amps and 23.8 amps. The total bundle capacity at sea level and 25 °C ambient is 29.2x12=350.4 amps. At 20,000 feet and 60 °C ambient the bundle capacity is 23.8x12=285.6 amps. Each size #12 wire can carry 29.2 amps at sea level, 25 °C ambient or 23.8 amps at 20,000 feet, 60 °C ambient.
STEP 6: Determine the actual circuit current for each wire in the bundle and for the bundle. If the values calculated in Step #5 are exceeded, select the next larger size wire and repeat the calculations.
11-70.11-75. [RESERVED.]
Par 11-76 Page 11-517 SECTION 6. AIRCRAFT ELECTRICAL WIRE SELECTION
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.
Wire Size. Wires must have sufficient mechanical strength to allow for servi ce 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 ca rrying 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.
Installation Precautio ns for Small Wires. A s 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.
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 C.A.G.E. 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.
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-thinwalled 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.
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.
AIRCRAFT WIRE MATERIALS. Only wire, specifically designed for airborne use, must be installed in aircraft.
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 (Q PL) and being permitted to produce the wire. Aircraft manufacturers who maintain their own wire specifications invariably exercise close control on their approved
Par 11-78 Page 11-518 (and 11-38) 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.
Platings. Bare copper develops a surface oxide coating at a rate dependent on temperature. This oxide film is a poor conductor of electricity and inhibits retermination of wire. Therefore, all airc raft wiring has a coating of either tin, silver, or nick el, that have far slower oxidation rates.
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.
Silver-coated wire is used where temperatures do not exceed 200 °C (392 °F).
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 longterm 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.
Conductor Stranding. Because of fli ght vibration and flexing, conductor round wire should be stranded to minimize fatigue breakage.
Wire Construction Versus Application. T h e m o s t 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 typica lly categorized as being suitable for either “open wiring” or “protected wiring” applications. MIL-W-5088L, 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, Appendix A table A-II lists wires for protected app lications. 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.
Insulations. There are many insulation materials and combinations used on aircra ft electrical wire. An explanation of many of the abbreviations are identified in the glossary.
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.
MIL-W-5088L 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.
Areas designated as severe wind and moisture problem (SWAMP) areas differ
Par 11-78 Page 11-519 (and 11-38) 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 uni que; 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.
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 fi rst three colors. Current military specification, multi-conductor cables, in accordance with MIL-C- 27500, 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 servici ng of the aircraft. At the time of this writing, MIL-C-27500 is being revised to include the older color sequence and could eliminate this problem in the future.
11-79.11-84. [RESERVED.]
Par 11-85 Page 11-520
SECTION 7. TABLE OF ACCEPTABLE WIRES
AIRCRAFT WIRE TABLE. Tables 11-11 and 11-12 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.
All wires in tables 11-11 and 11-12 have been determined to meet the flammability requirements of Title 14 of the Code of Federal Regulation (14 CFR) part 25, section 25.869(a)(4) and the applicable portion of part 1 of Appendix F of part 25.
The absence of any wire from tables 11-11 and 11-12 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.
Explanations of the various insulation materials mentioned in table 11-11, by abbreviations, can be found in the glossary.
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 11-11.) 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.
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 11-12.)
SEVERE WIND AND MOISTURE PROBLEMS (SWAMP). 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 jacking will vary according to the environment. Suitable wire types selected from MIL-W-22759 shall be used in these applications. (See table 11-11.)
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 11-11.)
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 MIL-DTL-27500, Cable, Power, Electrical and Cable Special Purpose, Electrical Shielded and Unshielded General Specifications.
Par 11-85 Page 11-521 TABLE 11-11. Open Wiring.
Document Voltage rating (maximum) Rated wire temperature (°C) Insulation Type Conductor type MIL-W-22759/1 600 200 Fluoropolymer insulated TFE and TFE coated glass Silver coated copper MIL-W-22759/2 600 260 Fluoropolymer insulated TFE and TFE coated glass Nickel coated copper MIL-W-22759/3 600 260 Fluoropolymer insulated TFE -glass- TFE Nickel coated copper MIL-W-22759/4 600 200 Fluoropolymer insulated TFE -glass- FEP Silver coated copper MIL-W-22759/5 600 200 Fluoropolymer insulat ed extruded TFE Silver coated copper MIL-W-22759/6 600 260 Fluoropolymer insulated extruded TFE Nickel coated copper MIL-W-22759/7 600 200 Fluoropolymer insulat ed extruded TFE Silver coated copper MIL-W-22759/8 600 260 Fluoropolymer insulated extruded TFE Nickel coated copper MIL-W-22759/9 1000 200 Fluoropolymer insulat ed extruded TFE Silver coated copper MIL-W-22759/10 1000 260 Fluoropolymer insulated extruded TFE Nickel coated copper MIL-W-22759/13 600 135 Fluoropolymer insulated FEP PVF2 Tin coated copper, MIL-W-22759/16 600 150 Fluoropolymer insulated extruded ETFE Tin coated copper, MIL-W-22759/17 600 150 Fluoropolymer insulated extruded ETFE Silver coated high strength copper alloy MIL-W-22759/20 1000 200 Fluoropolymer insulated ex truded TFE Silver coated high strength copper alloy MIL-W-22759/21 1000 260 Fluoropolymer insulated extruded TFE Nickel coated high strength copper alloy MIL-W-22759/34 600 150 Fluoropolymer insulated crosslinked modified ETFE Tin coated copper MIL-W-22759/35 600 200 Fluoropolymer insulated crosslinked modified ETFE Silver coated high strength copper alloy MIL-W-22759/41 600 200 Fluoropolymer insulated crosslinked modified ETFE Nickel coated copper MIL-W-22759/42 600 200 Fluoropolymer insulated crosslinked modified ETFE Nickel coated high strength copper alloy MIL-W-22759/43 600 200 Fluoropolymer insulated crosslinked modified ETFE Silver coated copper MIL-W-25038/3/2/ 600 260 See specification sheet See specification sheet MIL-W-81044/6 600 150 Crosslinked polyalkene Tin coated copper MIL-W-81044/7 600 150 Crosslinked polya lkene Silver coated high strength copper alloy MIL-W-81044/9 600 150 Crosslinked polyalkene Tin coated copper MIL-W-81044/10 600 150 Crosslinked polya lkene Silver coated high strength copper alloy
Par 11-89 Page 11-522 (and 11-42) TABLE 11-12. Protected wiring.
Document Voltage rating (maximum) Rated wire temperature (°C) Insulation Type Conductor type MIL-W-22759/11 600 200 Fluoropolymer insula ted extruded TFE Silver coated copper MIL-W-22759/12 600 260 Fluoropolymer insulated extruded TFE Nickel coated copper MIL-W-22759/14 600 135 Fluoropolymer insulated FEP-PVF2 Tin coated copper MIL-W-22759/15 600 135 Fluoropolymer insulated F EP-PVF2 Silver plated high strength copper alloy MIL-W-22759/18 600 150 Fluoropolymer insulated extruded ETFE Tin coated copper MIL-W-22759/19 600 150 Fluoropolymer insulated extr uded ETFE Silver coated high strength copper alloy MIL-W-22759/22 600 200 Fluoropolymer insulated extr uded TFE Silver coated high strength copper alloy MIL-W-22759/23 600 260 Fluoropolymer insulated extruded TFE Nickel coated high strength copper alloy MIL-W-22759/32 600 150 Fluoropolymer insulated crosslinked modified ETFE Tin coated copper MIL-W-22759/33 600 200 Fluoropolymer insulated crosslinked modified ETFE Silver coated high strength copper alloy MIL-W-22759/44 600 200 Fluoropolymer insulated crosslinked modified ETFE Silver coated copper MIL-W-22759/45 600 200 Fluoropolymer insulated crosslinked modified ETFE Nickel coated copper MIL-W-22759/46 600 200 Fluoropolymer insulated crosslinked modified ETFE Nickel coated high strength copper alloy MIL-W-81044/12 600 150 Crosslinked polyalkene Tin coated copper MIL-W-81044/13 600 150 Crosslinked polyalkene Silver coated high strength copper alloy MIL-W-81381/17 600 200 Fluorocarbon polyi mide Silver coated copper MIL-W-81381/18 600 200 Fluorocarbon pol yimide Nickel coated copper MIL-W-81381/19 600 200 Fluorocarbon polyimide Silver coated high strength copper alloy MIL-W-81381/20 600 200 Fluorocarbon polyi mide Nickel coated high strength copper alloy MIL-W-81381/21 600 150 Fluorocarbon polyimide Tin coated copper
11-90.11-95. [RESERVED.]
Par 11-96 Page 11-523
SECTION 8. WIRING INSTALLATION INSPECTION REQUIREMENTS
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.
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.
Metal stand-offs must be used to maintain clearance between wires and structure. Employing tape or tubing is not acceptable as an alte rnative to stand-offs for maintaining clearance.
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.
Wires and cables in junction boxes, panels, and bundles are properly supporte d and laced to provide proper grouping and routing.
Clamp 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.
Wire and cables are properly supported and bound so that there is no interference with other wires, cables, and equipment.
Wires and cables are adequately supported to prevent excessive movement in areas of high vibration.
Insulating tubing is secured by tying, tie straps or with clamps.
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 boxe s should be removed and replaced with individual loops.
Do not use tapes (such as friction or plastic tape) which will dry out in service, produ ce chemical reactions with wire or cable insulation, or absorb moisture.
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.
Do not use moisture-absorbent material as “fill” for clamps or adapters.
Ensure that wires and cables are not tied or fastened together in conduit or insulating tubing.
Ensure cable supports do not restrict the wires or cables in such a manner as to interfere with operation of equipment shock mounts.
Do not use tape, tie straps, or cord for primary support.
Make sure that drain holes are present in drip loops or in the lowest portion of tubing placed over the wiring.
Ensure that wires and cables are routed in such a manner that chafing will not occur against the airframe or other components.
Par 11-105 Page 11-524 (and 11-48) 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.
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 not a 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 of rocks, ice, mud, etc.
(If re-routing of wires or cables is not practical, protective jacketing may be installed). This type of installation must be held to a minimum.
Where practical, route wires and cables above fluid lines. Wires and cables routed within 6 inches of any flammable liquid, fuel, or oxygen line should be closely and rigidly supported. A minimum of 2 inches must be maintained between wiring and such lines or related equipment, except when the wiring is positively clamped to maintain at least 1/2-inch separation or when it must be connected di rectly to the fluid-carrying equipment.
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.
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.
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 PTFE. Avoid hightemperature areas when usi ng cables having soft plastic insulation such as polyethylen e, because these materials are subject to deterioration and deformation at elevated temperatures. Many coaxial cables have this type of insulation.
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 br eak out at terminations or reverse direction in a bundle. Where the wire is