U.S. Department of Transportation Federal Aviation Administration
Advisory Circular
Subject: Certification of Electrical Wiring Interconnection Systems on Transport Category Airplanes Date: 12/4/07 Initiated by: ANM-100 AC No. 25.1701-1
1. PURPOSE. This Advisory Circular (AC) provides guidance for certification of electrical wiring interconnection systems (EWIS) on transport category airplanes in accordance with 14 CFR part 25, subpart H, sections 25.1701 through 25.1733 and sections H25.4 and H25.5 of Appendix H to part 25.
2. APPLICABILITY.
a. The guidance provided in this document is applicable to transport category airplane manufacturers, modifiers, foreign civil aviation authorities, Federal Aviation Administration (FAA) transport airplane type certification engineers, designees, and FAA Flight Standards personnel.
b. This guidance can be used to meet the regulatory requirements for development of EWIS maintenance and inspection procedures as required by 14 CFR sections 25.1729 and
26.11. These maintenance and inspection procedures are required to be part of the instructions for continued airworthiness (ICA) specified by 14 CFR part 25, section H25.5(a)(1).
c. This material is neither mandatory nor regulatory in nature and does not constitute a regulation. It describes acceptable means, but not the only means, for demonstrating compliance with the applicable regulations. We will consider other methods of demonstrating compliance that an applicant may elect to present. While these guidelines are not mandatory, they are derived from extensive FAA and industry experience in determining compliance with the relevant regulations. On the other hand, if we become aware of circumstances that convince us that following this AC would not result in compliance with the applicable regulations, we will not be bound by the terms of this AC, and we may require additional substantiation or design changes as a basis for finding compliance.
d. This material does not change or create any additional regulatory requirements nor does it authorize changes in or permit deviations from existing regulatory requirements.
e. Terms such as “shall” or “must” are used in this AC only in the sense of ensuring applicability of this particular method of compliance when the acceptable method of compliance described herein is used.
3. HOW THIS INFORMATION WAS DERIVED.
a. The National Transportation Safety Board (NTSB) recommended that we address all wiring issues identified in our Aging Systems Plan, either through rulemaking or through other means. To accomplish this we established, in 1998, the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC). The ATSRAC provides a forum for airlines, manufacturers, and other regulatory authorities to make recommendations to us based on the Aging Systems Plan. Recommendations have addressed EWIS certification issues, development of standard wiring practices manuals, enhanced EWIS maintenance requirements, and EWIS training.
b. The guidance in this AC is based on recommendations submitted to us by the ATSRAC and by the Aviation Rulemaking Advisory Committee (ARAC). It is derived from best practices identified through extensive research by ATSRAC Industry Working Groups 6, 7, and 9, the ARAC Electrical Systems Harmonization Working Group, United States and European standard aircraft industry practice, and the results of various EWIS-related research and development programs. These recommendations and best practices are based on treating EWIS as an aircraft system, and the belief that EWIS should be designed and installed with the same level of diligence as any other essential or critical system in the aircraft. We endorse these best practices.
4. COMPLIANCE METHODS.
a. The guidance in this AC describes means of compliance with subpart H of part 25 and supplements similar guidance provided in other advisory material and policy memoranda concerning EWIS requirements for airplane systems. A list of related regulations, ACs, and policy memoranda is in Appendix A of this AC.
b. To fully realize the objectives of the guidance contained in this AC, type certificate (TC) holders, supplemental type certificate (STC) holders, maintenance providers, repair stations, and persons performing modifications or repairs may need to adjust their ii
approach to designing and modifying EWIS. They may need to be aware of the history of the EWIS in a particular aircraft and the role it plays in the safe operation of that aircraft. People who design and modify aircraft EWIS should be aware that it should be designed and installed with the same level of diligence given to any other essential or critical system in the aircraft.
c. The guidance in this AC is organized by subpart H section.
/s/Ali Bahrami Ali Bahrami Manager Transport Airplane Directorate Aircraft Certification Service
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CONTENTS
§ 25.1701 Definition3
§ 25.1703 Function and installation: EWIS4
§ 25.1705 Systems and functions: EWIS14
§ 25.1707 System separation: EWIS14
§ 25.1709 System safety: EWIS20
§ 25.1711 Component Identification: EWIS33
§ 25.1713 Fire protection: EWIS38
§ 25.1715 Electrical bonding and protection against static electricity: EWIS39
§ 25.1717 Circuit protective devices: EWIS40
§ 25.1719 Accessibility provisions: EWIS40
§ 25.1721 Protection of EWIS41
§ 25.1723 Flammable fluid fire protection: EWIS42
§ 25.1725 Powerplants: EWIS43
§ 25.1727 Flammable fluid shutoff means: EWIS43
§ 25.1729 Instructions for Continued Airworthiness: EWIS43
§ 25.1731 Powerplant and APU fire detector system: EWIS43
§ 25.1733 Fire detector systems, general: EWIS43
H25.4 & H25.5 - Instructions for Continued Airworthiness44
Appendix A – Related Regulations and Documents Appendix B – NPRM Subpart H and Appendix H Preamble Discussion
5. SPECIFIC COMPLIANCE GUIDANCE BY SUBPART H SECTION
a. § 25.1701 DEFINITION.
(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 3
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 4
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 5
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 6
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 ha rnesses, and single wires and electrical cables individually routed and supported, the m inimum bend radius shall be ten tim es the outside diam eter of the largest included wire or electrical cable. At the point where wiring breaks out from a group, harne ss or bundle, the m inimum bend 7
radius shall be ten times the diameter of the largest included wire or electrical cable, provided th e wiring is suitably supported at the breakout point. If wires used as shield terminators or jumpers are required to reverse d irection in a harness, the m inimum bend radius of the wire shall b e 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 hightemperature equipment and lines to prevent deterioration of insulation (reference § 25.1707(j)).