Question

Based on the practice of EWIS wiring installation practice, describe the technical procedures to satisfy FAA’s requirements.

Answer 1

Following are FAA's guidlines based on the practice of EWIS wiring instalation.

Source:Aircraft Electrical Wiring Interconnect System (EWIS) Best Practices Job Aid Revision: 2.0 by Federal Aviation Administration

(1) § 25.1701(a).
(a) Section 25.1701 defines EWIS for the purposes of complying with the subpart H requirements and other EWIS-related requirements of part 25. Section 25.1701 identifies which wires and components these requirements apply to. Although this definition is located in subpart H to part 25, it applies to all EWIS requirements regardless of location within part 25.
(b) Section 25.1701(a) defines EWIS as any wire, wiring device, or combination of these, including termination devices, installed in any area of the airplane for the purpose of transmitting electrical energy, including data and signals, between two or more intended termination points. The term “wire” means bare or insulated wire used for the purpose of electrical energy transmission, grounding, or bonding. This includes electrical cables, coaxial cables, ribbon cables, power feeders, and databuses.
(c) Paragraph (a) of this rule provides a listing of the component types that are considered part of the EWIS. These component types are listed as items § 25.1701(a)(1) through § 25.1701(a)(14). While these are the most widely used EWIS components, this is not an all inclusive list. There may be components used by an applicant to support transmission of electrical energy that are not listed in § 25.1701(a) but still meet the EWIS definition. These components will be considered as EWIS components and are subject to EWIS-related regulatory requirements.
(2) § 25.1701(a)(14).
(a) Section § 25.1701(a)(14) says that EWIS components located inside shelves, panels, racks, junction boxes, distribution panels, and back-planes of equipment racks (e.g., circuit board back-planes and wire integration units) are covered by the EWIS definition.
(b) These components are included in the EWIS definition because the equipment they are inside of, or part of, is typically designed and made for a particular airplane model or series of models. So the requirements that apply to airplane EWIS components must be applied to the components inside that equipment. These contrast with avionics components that must be sent back to their manufacturer or a specialized repair shop for service. Components inside shelves, panels, racks, junction boxes, distribution panels, and back-planes of equipment racks are maintained, repaired, and modified by the same personnel who maintain, repair, and modify the EWIS in the rest of the airplane. For example, in an electrical distribution panel, system separation must be designed and maintained within the panel just like it must be designed and maintained for the EWIS leading up to that panel. Identification of components inside the panel is just as important as it is outside the panel since the wiring inside the
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panel is treated much the same. Also, while this type of equipment is designed for its intended function and is manufactured and installed to the same standards as other EWIS, it is typically not qualified to an environmental standard such as Radio Technical Commission for Aeronautics (RTCA) document number DO-160.
(3) § 25.1701(b).
(a) There are some exceptions to the EWIS definitions and those are given in §§ 25.1701(b). This paragraph excepts EWIS components inside the following equipment, and the external connectors that are part of that equipment: 1 Electrical equipment or avionics that are qualified to environmental conditions and testing procedures when those conditions and procedures are −
(aa) appropriate for the intended function and operating environment, and
(bb) acceptable to the FAA.
2 Portable electrical devices that are not part of the type design of the airplane including personal entertainment devices and laptop computers. 3 Fiber optics.
(b) The first exception means EWIS components located inside avionic or electrical equipment such as, for example, flight management system computers, flight data recorders, VHF radios, primary flight displays, navigation displays, generator control units, integrated drive generators, and galley ovens, if this equipment has been tested to industry-accepted environmental testing standards. Examples of acceptable standards are RTCA DO-160 and the European Organization for Civil Aviation Equipment (EUROCAE) ED 14, and equipment qualified to an FAA Technical Standard Order (TSO).
(c) An applicant may use any environmental testing standard if the applicant can demonstrate that the testing methods and pass/fail criteria are at least equivalent to the widely accepted standards of DO-160, EUROCAE ED 14, or a specific TSO. Applicants should submit details of the environmental testing standards and results of the testing that demonstrate the equipment is suited for use in the environment in which it will operate.
b. § 25.1703 FUNCTION AND INSTALLATION: EWIS. Section 25.1703 requires that applicants select EWIS components that are of a kind and design appropriate to their intended function just as § 25.1301 requires this for other pieces of equipment installed on the airplane. Factors such as component design limitations, functionality, and susceptibility to arc tracking and moisture or other known characteristics of the particular component must be considered. The following paragraphs, 5.b.(1) through 5.b.(7), provide guidance in
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showing compliance with the specific provisions of § 25.1703 (a) through (d). Paragraph 5.b.(8) deals with EWIS component selection and paragraph 5.b.(9) has specific guidance on wire selection. Paragraph 5.b.(10) discusses EWIS component selection for future modifications.
(1) § 25.1703(a)(1). This section requires that each EWIS component be of a kind and design appropriate to its intended function. In this context, the requirement means that components must be qualified for airborne use, or otherwise specifically assessed as acceptable for their intended use. To be “appropriate” means that the equipment is used in a manner for which it was designed. For example, a wire rated at 150 degrees Celsius would not be appropriate for installation if that installation would cause the wire to operate at a temperature higher than 150 degrees Celsius. Wire and other components made for household or consumer products use may not be appropriate for airborne use because they are manufactured for the consumer market and not for use in an airborne environment. Other factors that must be considered for EWIS component selection are mechanical strength, voltage drop, required bend radius, and expected service life. Refer to paragraph 5.b.(8)(a) for further explanation of “expected service life.”
(2) § 25.1703(a)(2). This section requires that EWIS components be installed according to their limitations. As used here, limitations means the design and installation requirements of the particular EWIS component. Examples of EWIS component limitations are maximum operating temperature, degree of moisture resistance, voltage drop, maximum current-carrying capability, and tensile strength. EWIS component selection and installation design must take into account various environmental factors including, but not limited to, vibration, temperature, moisture, exposure to the elements or chemicals (de-icing fluid, for instance), insulation type, and type of clamp. In addition, characteristics of both conductor and insulation for wires and cables that are required to regularly flex, such as those in doors and hatches, should also be considered when selecting them for such applications.
(3) § 25.1703(a)(3). This section requires that EWIS function properly when installed. The key word in understanding the intent of this section is “properly,” as that relates to airworthiness of the airplane. For an EWIS component to function properly means that it must be capable of safely performing the function for which it was designed. For example, the fact that an in-flight entertainment (IFE) system fails to deliver satisfactory picture or sound quality is not what the term “properly” refers to, is not a safety issue, and thus is not a certification issue. Failure of an EWIS component has the potential for being a safety hazard whether it is part of a safety-related system or an IFE system. Therefore, EWIS components must always function properly (safely) when installed, no matter what system they are part of, and any malfunction of the EWIS must not degrade the airworthiness of the airplane (refer to § 25.1705 for terminology relating to failure classifications).
(4) § 25.1703(a)(4). This section requires that EWIS components be designed and installed so mechanical strain is minimized. This means the EWIS installation must be designed so that strain on wires would not be so great as to cause the wire or other components to fail. This section requires that adequate consideration be given to mechanical strain when selecting wire and cables, clamps, strain reliefs, stand-offs, and
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other devices used to route and support the wire bundle when designing the installation of these components.
(5) § 25.1703(b). This section requires that selection of wires takes into account known characteristics of different wire types in relation to each specific application, to minimize risk of damage. It is important to select the aircraft wire type whose construction matches the application environment. The wire type selected should be constructed for the most severe environment likely to be encountered in service. This means, for example, that insulation types susceptible to arc tracking should not be used in areas exposed to high vibration and constant flexing in a moisture-prone environment.
(6) § 25.1703(c). This section contains the requirement formerly located in§ 25.869(a)(3) that design and installation of the main power cables allow for a reasonable degree of deformation and stretching without failure. Although now located in § 25.1703(c), the meaning of the requirement has not changed. The reason for this requirement is the same as for § 25.993(f), which requires that each fuel line within the fuselage be designed and installed to allow a reasonable degree of deformation and stretching without leakage. The idea is that the fuselage can be damaged with partial separation or other structural damage without the fuel lines or electrical power cables breaking apart. Allowing for a certain amount of stretching will help to minimize the probability of a fuel-fed fire inside the fuselage. As it is used in this requirement, a “reasonable degree of deformation and stretching” should be about 10% of the length of the electrical cable.
(7) § 25.1703(d). This section requires that EWIS components located in areas of known moisture build-up be adequately protected to minimize moisture’s hazardous effects. This is to ensure that all practical means be considered and the most appropriate method used to address potential damage from fluid contact with EWIS components. Wires routed near a lavatory, galley, hydraulic lines, severe wind and moisture problem areas such as wheel wells and wing trailing edges, and any other area of the airplane where moisture collection could be a concern must be adequately protected from possible adverse effects of exposure to moisture.
(8) EWIS component selection.
(a) Expected service life. Expected service life should be considered in selecting EWIS components to use. Expected service life means the expected service lifetime of the EWIS. This is not normally less than the expected service life of the aircraft structure. If the expected service life requires that all or some of the EWIS components be replaced at certain intervals, then these intervals must be specified in the ICA as required by § 25.1529.
(b) Qualified components. EWIS components should be qualified for airborne use or specifically assessed as acceptable for the intended use and be appropriate for the environment in which they are installed. Aircraft manufacturers list approved components in their manuals, such as the standard wiring practices manual (ATA Chapter 20). Only the components listed in the
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applicable manual or approved substitutes should be used for the maintenance, repair, or modification of the aircraft. EWIS modifications to the original type design should be designed and installed to the same standards used by the original aircraft manufacturer or other equivalent standards acceptable to the FAA. This is because the manufacturer’s technical choice of an EWIS component is not always driven by regulatory requirements alone. In some cases, specific technical constraints would result in the choice of a component that exceeds the minimum level required by the regulations.
(c) Mechanical strength. EWIS components should have sufficient mechanical strength for their service conditions.
1 The EWIS should be installed with sufficient slack so that bundles and individual wires are not under undue tension.
2 Wires connected to movable or shock-mounted equipment should have sufficient length to allow full travel without tension on the bundle to the point where failure of the EWIS could occur.
3 Wiring at terminal lugs or connectors should have sufficient slack to allow for two re-terminations without replacement of wires, unless other design considerations apply. This slack should be in addition to the drip loop and the allowance for movable equipment.
4 In order to prevent mechanical damage, wires should be supported by suitable clamps or other devices at suitable intervals. 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 as per Society of Automotive Engineers (SAE) AS50881 or equivalent standard. In-service experience has revealed abrasion and chafing of wires contained in troughs, ducts, or conduits, so the design should mitigate possibilities for this occurring. This may require additional support or other design strategies.
(d) Minimum bend radius. To avoid damage to wire insulation, the minimum radius of bends in single wires or bundles should be in accordance with the wire manufacturer’s specifications. Guidance on the minimum bend radius can be found in the airplane manufacturer’s standard wiring practices manual. Other industry standards such as the European Association of Aerospace Industries’ document AECMA EN3197 or SAE AS50881 also contain guidance on minimum bend radius. For example, SAE AS50881b states:
For wiring groups, bundles, or harnesses, and single wires and electrical cables individually routed and supported, the minimum bend radius shall be ten times the outside diameter of the largest included wire or electrical cable. At the point where wiring breaks out from a group, harness or bundle, the minimum bend
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radius shall be ten times the diameter of the largest included wire or electrical cable, provided the wiring is suitably supported at the breakout point. If wires used as shield terminators or jumpers are required to reverse direction in a harness, the minimum bend radius of the wire shall be three times the diameter at the point of reversal providing the wire is adequately supported.
(e) Coaxial cable damage. Damage to coaxial cable can occur when the cable is clamped too tightly or bent sharply (normally at or near connectors). Damage can also be incurred during unrelated maintenance actions around the coaxial cable. Coaxial cable can be severely damaged on the inside without any evidence of damage on the outside. Installation design should minimize the possibility of such damage. Coaxial cables have a minimum bend radius. SAE AS50881b states: “The minimum radius of bend shall not adversely affect the characteristics of the cable. For flexible type coaxial cables, the radius of bend shall not be less than six times the outside diameter. For semi-rigid types, the radius shall not be less than ten times the outside diameter.”
(f) Wire bundle adhesive clamp selection. Certain designs use adhesive means to fasten bundle supports to the aircraft structure. Service history shows that these can become loose during aircraft operation, either as a result of improper design, or because of inadequate surface preparation. You should pay particular attention to the selection of such means and to the methods used for affixing this type of wire bundle support.
(g) Wire bundle routing. Following are some considerations that should go into the design of an EWIS installation.
1 Wire bundles should be routed in accessible areas that are protected from damage from personnel, cargo, and maintenance activity. As far as practicable they should not be routed in areas where they are likely to be used as handholds or as support for personal equipment or where they could become damaged during removal of aircraft equipment (reference §§ 25.1719 and 25.1721).
2 Wiring should be clamped so that contact with equipment and structure is avoided. Where this cannot be accomplished, extra protection, in the form of grommets, chafe strips, etc., should be provided. Wherever wires cannot be clamped, protective grommets should be used, in a way that ensures clearance from structure at penetrations. Wire should not have a preload against the corners or edges of chafing strips or grommets.
3 As far as practicable, wiring should be routed away from high-temperature equipment and lines to prevent deterioration of insulation (reference § 25.1707(j)).
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4 Wiring routed across hinged panels should be routed and clamped so that the bundle will twist, rather than bend, when the panel is moved. When this is not possible, the bending radius must be in accordance with the acceptable minimum bundle radius.
(h) Conduits. Conduits should be designed and manufactured so that potential for chafing between the wiring and the conduit internal walls is minimized.
1 Non-metallic conduit. Insulating tubing (or sleeving) is sometimes used to provide additional electrical, environmental, and limited additional mechanical protection or to increase the external wire dimension. Insulating tubing should not be considered as the sole mechanical protection against external abrasion of wire because it does not prevent external abrasion. At best, it provides only a delaying action against the abrasion. The electrical and mechanical properties of the tubing should be appropriate for the type of protection the designer intends it to be used for. Additional guidance on the use of tubing or sleeving is given in paragraph 5. d(2)(c) of this AC.
2 Metallic conduit. The ends of metallic conduits should be flared and the interior surface treated to reduce the possibility of abrasion.
(i) Connector selection. The connector used for each application should be selected only after a careful determination of the electrical and environmental requirements.
1 Additional scrutiny should be given to any use of components with dissimilar metals, because this may cause electrolytic corrosion.
2 Environment-resistant connectors should be used in applications that will be subject to fluids, vibration, temperature extremes, mechanical shock, corrosive elements, etc.
3 You should use sealing plugs and contacts in unused connector cavities. In addition, firewall class connectors incorporating sealing plugs should be able to prevent the penetration of fire through the aircraft firewall connector opening and continue to function without failure for the period of time that the connector is designed to function when exposed to fire.
4 When electromagnetic interference and radio frequency interference (EMI and RFI) protection is required, you should give special attention to the termination of individual and overall shields. Back shell adapters designed for shield termination, connectors with conductive finishes, and EMI grounding fingers are available for this purpose.
(j) Splice selection. Environmentally sealed splices should be used in accordance with the requirements of the airframe manufacturer’s standard wiring practices or SAE AS81824/1, or equivalent specification, particularly in
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un-pressurized and severe wind and moisture prone (SWAMP) areas. However, the possibility of fluid contamination in any installation needs to be considered.
1 Splices in pressurized areas. In pressurized areas, pre-insulated splices conforming to SAE AS7928 or equivalent specification may be used if these types of splices are listed as acceptable for use by the manufacturer in its standard wiring practices manual. If these are not included in the standard wiring practices manual, then you should get approval from your Aircraft Certification Office to use them. In any case, you need to show that the possibility of fluid contamination has been adequately considered.
2 Mechanically protected splices. Mechanical splices give maintenance personnel an alternative to using a heat gun for splices in fuel vapor areas on post-delivery aircraft. The generally available environmental splices use heat shrink material that require heat application. Most of these heat sources cannot be used in flammable vapor areas of an aircraft without proper precautions. Mechanical splices are acceptable for use in high temperature and fuel vapor areas, provided the splice is covered with a suitable plastic sleeve, such as a dual wall shrink sleeve or high temperature tape, such as Teflon, wrapped around the splice and tied at both ends. If high temperature tape is used, it should be permanently secured at both ends. Mechanical splices should be installed according to the airframe manufacturer’s standard practices, or equivalent specification. The manufacturer’s standard wiring practices manual should provide part number detail and best practices procedures for mechanical splices. It should also detail the applicability of each of the recommended splices for all required critical airplane installations.

(9) Wire selection.
(a) Installation environment.
1 Careful attention should be applied when deciding on the type of wire needed for a specific application. You should consider whether the wire’s construction properly matches the application environment. For each
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installation, you should select wire construction type suitable for the most severe environment likely to be encountered in service. As examples, use a wire type suitable for flexing for installations involving movement; and a wire type with a high temperature rating for higher temperature installations.
2 When considering the acceptability of wire, you should refer to the industry standards defining acceptable test methods for aircraft wire, including arc tracking test methods (e.g. European Norm (EN) 3475, SAE AS4373, or alternative manufacturer standards).
3 Wires in such systems as fire detection, fire extinguishing, fuel shutoff, and fly-by-wire flight control that must operate during and after a fire must be selected from wire types qualified to provide circuit integrity after exposure to fire for a specified period.
(b) Wire insulation selection. Wire insulation type should be chosen according to the environmental characteristics of wire routing areas. One wire insulation characteristic of particular concern is arc tracking. Arc tracking is a phenomenon in which a conductive carbon path forms across an insulating surface. A breach in the insulation allows arcing and carbonizes the insulation. The resulting carbon residue is electrically conductive. The carbon then provides a short circuit path through which current can flow. This can occur on either dry or wet wires. Certain types of wire insulation, such as wire insulated with aromatic polyimide, are more susceptible to arc tracking than others. Although there are new types of aromatic polyimide insulated wire, such as hybrid constructions (e.g., the aromatic polyimide tape is the middle layer, and the top and bottom layer is another type of insulation such as Teflon tape) wire insulated with only aromatic polyimide tape is more susceptible to arc tracking than other types of commonly used wire insulation. Therefore, its use should be limited to applications where it will not be subjected to high moisture, high vibration levels, or abrasion, and where flexing of the wire will not occur.
(c) Mechanical strength of wire. Wires should be sufficiently robust to withstand all movement, flexing, vibration, abrasion, and other mechanical hazards to which they may be subjected on the airplane. Generally, conductor wire should be stranded to minimize fatigue breakage. Refer to AS50881 and
European Association of Aerospace Industries (AECMA) EN3197 for additional guidance. Additionally, wires should be robust enough to withstand the mechanical hazards they may be subjected to during installation into the aircraft.
(d) Mixing of different wire insulation types. Different wire types installed in the same bundle should withstand the wire-to-wire abrasion they will be subject to. Consideration should be given to the types of insulation mixed within wire bundles, especially if mixing a hard insulation type with a relatively softer type, and particularly when relative motion could occur between the wires. Such relative motion between varying wire insulation types could lead to accelerated abrasion and subsequent wire failure.
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(e) Tin plated conductors. Tin plated conductors may be difficult to solder if not treated properly, so preparation of the conductor is necessary to ensure a good connection is made.
(f) Wire gage selection. To select the correct size of electrical wire, the following should be considered:
1 The wire size should be matched with the circuit protective device with regard to the required current.
2 The wire size should be sufficient to carry the required current without overheating.
3 The wire size should be sufficient to carry the required current over the required distance without excessive voltage drop (based on system requirements).
4 Particular attention should be given to the mechanical strength and installation handling of wire sizes smaller than AWG 22 (e.g., consideration of vibration, flexing, and termination). Use of high-strength alloy conductors should be considered in small gauge wires to increase mechanical strength.

FOR FURTHER GUIDLINE PLEASE REFER: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwj6l7LrnOjoAhUP6nMBHXxsB-4QFjAAegQIAxAB&url=https%3A%2F%2Fwww.faa.gov%2FdocumentLibrary%2Fmedia%2FAdvisory_Circular%2FAC_25_1701-1.pdf&usg=AOvVaw2WfStlHw2vryve1YLjL46B