(l) BOXES N and O Development and validation of mitigation strategy. Identify and develop a mitigation strategy for the functional failures and adverse effects identified in BOX J. Validation and verification of the mitigation solution should:
NOTE: If the EWIS was the failure cause, the subsequent mitigation strategy developed may introduce new adverse effects not previously identified by the analysis. Check for any new adverse effects and update the aircraft level FHA and other system safety assessments as necessary.
• Determine if initial objective is fully reached.
• Confirm that this mitigation solution is compatible with existing installations and installation criteria.
(m) BOX P Documentation of EWIS safety analysis results.
After mitigation strategies have been validated and verified, document the results of the § 25.1709 analysis. Update as necessary the aircraft level FHA that has been developed in support of certification of the proposed modification, in compliance with § 25.1309, (BOX A).
f. § 25.1711 COMPONENT IDENTIFICATION: EWIS. Section 25.1711 requires applicants to identify EWIS components using consistent methods that facilitate easy identification of the component, its function, and its design limitations. For EWIS associated with flight-essential functions where specific certification requirements are met by redundancy, identification of the EWIS must also include separation requirements. This section requires that the identifying markings remain legible throughout the expected service life of the EWIS component, and that the method used to identify components have no adverse effect on their performance.
(1) § 25.1711(a). This section requires a consistent method of EWIS identification to avoid confusion and mistakes during airplane manufacturing, modification, and maintenance. This means we expect airplane manufacturers to develop an EWIS identification method that facilitates easy identification of the systems that any specific EWIS component supports and use that identification method in a consistent manner throughout the airplane. This consistent identification method must be used for new type certifications and changes to those designs.
(2) § 25.1711(b). Certain airplane systems are installed with redundancy because of certification rules or operating rules or in order to meet the reliability requirements of § 25.1709. For EWIS components associated with these systems, paragraph (b) requires specific identification indicating component part number, function, and separation requirement. This is necessary to prevent 33
modifiers from unintentionally introducing unsafe design or installation features on previously certified airplanes when they install new or modified systems.
Such identification will aid the designers and installers of the new system by alerting them to the presence of these systems. It will enable them to make appropriate design and installation decisions. Component identification will also make those performing maintenance and inspections more aware of what systems are associated with specific EWIS in the areas undergoing maintenance or inspection.
(3) § 25.1711(c). Paragraph (c) requires that identifying markings required by § 25.1711(a) and (b) remain legible throughout the design life of the component.
As most wire installations are designed to remain on the airplane throughout the airplane’s service life, this means the identification marks must be able to be read for the life of the airplane. The method of marking must take into account the environment in which the EWIS component will be installed. The Society of Automotive Engineers (SAE) documents ARP 5607, “Legibility of Print on Aerospace Wire and Cables,” and AS 5942, “Marking of Electrical Insulating Materials,” provide guidance on this subject. Dot matrix and ink-jet marking should only be used when there is no strong need for chemical or mechanical resistance of the ink. The color used for identification should contrast with the wire insulation, sleeve, support material, or other EWIS components.
(4) § 25.1711(d). Paragraph (d) requires that the means used to identify an EWIS component may not have an adverse effect on component performance throughout its design life. The preferred method of EWIS component identification is with dot matrix, ink-jet, or laser marking.
(a) Certain wire marking methods have potential to damage wire insulation. Hot-stamp marking is one such method. According to SAE aerospace information report AIR5575, “Hot Stamp Wire Marking Concerns for Aerospace Vehicle Applications,” the hot-stamp marking method is not well suited for today’s generation of aircraft wiring. As noted in that document, wire insulation has become markedly thinner over the years since the procedure was first introduced in the 1940s. Because of this, problems have arisen over wire damage from excessive penetration by the hot stamp process. The document further states: “The frequent need for adjustments in temperature, pressure, and dwell time inherent to achieving legible hot stamp wire marking provides many opportunities for error. The controls, methods, and guidance necessary to achieve satisfactory performance with hot stamp marking are often not made available to operators in smaller wire shops.”
(b) If damage to the insulation occurs during the marking process, it may fail later in service after exposure to the sometimes-harsh environmental conditions of aircraft use. While § 25.1711(d) does not prohibit use of hotstamp marking, its use is not encouraged. To comply with this paragraph, if 34
the hot-stamp marking process is used, the guidelines of SAE recommended practice ARP5369, “Guidelines for Wire Identification Marking Using the Hot Stamp Process” or equivalent should be followed. Other information related to the use of the hot-stamp marking process can be found in SAE AIR 5575 and European Association of Aerospace Industries (AECMA) EN3197.
(c) In some cases it may not be practical to mark an EWIS component directly because of component size or identification requirements. In this case other methods of identification, such as a label or sleeve, should be used.
(5) § 25.1711(e). Paragraph (e) requires that EWIS modifications to the type design take into consideration the identification scheme of the original type design. This is to ensure that the consistency required by § 25.1711(a) is maintained when a modification is installed. The intent of this requirement is to provide continuity for EWIS identification on a particular model. It is not the intent of the requirement to impose on the modifier the exact wire identification methods of the airplane manufacturer. However, since the purpose of § 25.1711 is to make it easy to identify those airplane systems essential to safe operation of the airplane, it is in the best interest of safety that designers of any modifications to the original design consider the approved type design identification methods.
For example it would not be appropriate for a modifier to use purple wire to identify a specific flight critical system when the approved type design used the color green, especially if the type design already uses purple wire to identify non-essential systems. Such a scheme could cause confusion and lead future modifiers or maintainers to believe that the routing of purple wires with green wires (and thus critical systems with non-essential systems) is acceptable. The regulation does not prescribe a particular method for identification but is meant to ensure that consistent identification is maintained throughout the life of the airplane.
(6) Visible Identification of Critical Design Configuration Limitations. Section 25.981(b) states that "...visible means to identify critical features of the design must be placed in areas of the airplane where maintenance actions, repairs, or alterations may be apt to violate the critical design configuration limitations (e.g., color-coding of wire to identify separation limitation)." The design approval holder should define a method of ensuring that this essential information will:
• be communicated by statements in appropriate manuals, such as wiring diagram manuals, and
• be evident to those who may perform and approve such repairs and alterations.
An example of a critical design configuration control limitation that would result in a requirement for visible identification means would be a requirement to 35
maintain wire separation between FQIS (fuel quantity indication system) wiring and other electrical circuits that could introduce unsafe levels of energy into the FQIS wires. Acceptable means of providing visible identification for this limitation would include color-coding of the wiring or, for retrofit, placement of identification tabs at specific intervals along the wiring.
(7) Types of EWIS component identification. There are at least four types of EWIS component identification that are accomplished at different stages. They are listed and described below.
(a) Component manufacturer part number. EWIS components should be identified by their manufacturer in accordance with the International Organization for Standardization document ISO 2574, “Aircraft – Electrical Cables – Identification Marking,” or similar specifications. This identification comprises product part number, manufacturer identification, and, when possible or specifically required, batch identification or year of manufacture. This helps ensure:
• Identification and traceability of the component.
• Verification of compliance with the aircraft certification basis.
• Accuracy in manufacture, maintenance, quality control, storage, and delivery.
• Verification of the use of approved/qualified sourcing.
• Monitoring of the aircraft configuration during the aircraft life.
1 EWIS component manufacturer identification. It is common practice to use the five-digit/letter C.A.G.E. (commercial and government entity) code for manufacturer identification, particularly for wires. Alternatively, for small components whose size may make it difficult to use other forms of clear identification, a logo may be used.
2 Identification intervals. Wires and cables should be identified at intervals of not more than 15 inches (380mm). Exceptions can be made for short runs of wires or cables or when the majority of the wire or cable is installed in a manner that facilitates easy reading of the identification markings.
3 Manufacturer markings. Wire manufacturer markings should generally be green to differentiate them from the black marking typically used by the airplane manufacturer, but other contrasting colors are also acceptable.
4 The component technical specification should include methods used for identification and legibility during the design life of the component.
(b) Airframe manufacturer component function identification number. In addition to the type identification imprinted by the original wire manufacturer, aircraft wire should also contain a unique circuit identification coding that is accomplished at time of harness assembly. This allows existing installed wire to be identified as to its performance capabilities when considering replacement. Inadvertent use of a lower performance and unsuitable replacement wire can thus be avoided.
Identification of EWIS components by the airframe manufacturer helps ensure:
• Identification and inspection of cable runs.
• Accuracy of manufacture, maintenance, quality control, storage, and delivery.
• Verification of the system to which the component belongs.
• Identification of components related to systems required for safe flight, landing, or egress or that have the potential to impact the flightcrew’s ability to cope with adverse operating conditions.
Identification of EWIS components should clearly correspond to aircraft wiring manuals.
(c) Airframe manufacturer routing identification and modification. Electrical drawings should describe wire routings through the entire airplane indicating, for example, incompatibility between routes, minimum distance between routes, and absolute bans on combining bundles. Electrical drawings should be available in the maintenance documentation as required by paragraph H25.5 of Appendix H to part 25.
This information ensures that modification designers and maintenance personnel are aware of the defined physical separation of the different routes of the aircraft model they are working on. Coding for identification of routes or bundles used on aircraft should be displayed by adequate means such as labels, tags, placards, colored ties, or bar-codes. This type of component identification helps insure:
• Identification and inspection of bundles.
• Accuracy of manufacture, maintenance, quality control, storage, and delivery.
• Determination of the type of route, or route function, (feeder power, radio etc.).
• Clear identification of systems that require physical separation (to detect the possible mix of different routes/bundles, the misrouting of a system in an area, etc.).
• Identification of routes taken by systems required for safe flight, landing, egress, or those that have the potential to 37
impact the ability of the flightcrew to cope with adverse operating conditions.
1 Modification and repairs identification, in a form that helps ensure the original airplane manufacturer’s identification scheme, should be maintained throughout the service life of the airplane.
2 Wire and cable routing information should be identified at intervals of preferably not more than 18 inches (460mm) and should not obscure the identification markings of the EWIS component manufacturer or airframe manufacturer component function identification numbers described in paragraphs 5.f.(7)(a) and 5.f.(7)(b). Also, exceptions can be made for short runs of wires or cables or when the majority of the wire or cable is installed in a manner that facilitates easy reading of the identification markings. Coaxial cables may be identified at both ends of the coax if marking the cable at regular intervals would cause crushing of the cable and potentially damage the foam insulation.
Applicants should provide justification for why the chosen identification method would damage the cable.
(d) Identification of user EWIS modification or repair (operator’s identification coding). Repairs or modifications to EWIS should follow the identification guidance given in the above paragraphs for airplane manufacturers. This helps ensure that the original airplane manufacturer’s identification scheme is not compromised by future modifications or repairs and is maintained throughout the service life of the airplane. A temporary repair identification with a non-hydraulic-resistant material could remain in a hydraulic bay for some days, but such material would not be a suitable marking method for the long term.
g. § 25.1713 FIRE PROTECTION: EWIS. The intent of this requirement is to help ensure that the EWIS does not propagate fire and produce hazardous quantities of smoke and toxic fumes.
(1) § 25.1713(a). This paragraph requires that all EWIS components meet the applicable fire and smoke protection requirements of § 25.831(c). This requirement was formerly located in § 25.869(a)(1). After reasonably probable failures or malfunctions, EWIS components should not cause harmful or hazardous concentrations of gases or vapors in excess of the levels prescribed in § 25.831(b)(1) and (2).
(2) § 25.1713(b). This requirement was formerly located in § 25.869(a)(2). It requires that EWIS components that are located in designated fire zones and used during emergency procedures must be at least 38
fire resistant. This requirement is intended to help ensure that emergency services are available in the event of a fire.
(a) The definition of “fire resistant” is found in 14 CFR 1.1 General definitions. As applied to EWIS components it is defined as follows:
To be fire resistant means that EWIS components must have the capacity to perform the intended functions under the heat and other conditions likely to occur when there is a fire at the place concerned.
(b) EWIS components in regions immediately outside fire zones and in engine pod attachment structures should be made of such materials and installed at such a distance from the firewall that they will not suffer damage that could hazard the airplane if the surface of the firewall adjacent to the fire is heated to 1100° C (2012° F) for 15 minutes.
(3) § 25.1713(c). This section was formerly located in § 25.869(a)(4). It requires that insulation on electrical wire and electrical cable installed anywhere in the airplane be self-extinguishing when tested in accordance with the applicable portions of part I Appendix F of part 25.
(a) In addition, to protect against propagation of a fire, EWIS components other than wire and cable should be designed using nonflammable and self-extinguishing materials as tested to the equivalent of part I Appendix F or equivalent.
(b) Maximum physical or spatial separation is especially important above the component, or downstream of any consistent, known airflow.
See paragraph 5.d., System Separation EWIS, of this advisory circular, for guidance on demonstrating compliance with the separation requirements of § 25.1707.
h. § 25.1715 ELECTRICAL BONDING AND PROTECTION AGAINST STATIC ELECTRICITY: EWIS. The build-up and subsequent discharge of static electricity has the potential to create hazardous conditions for both airplane systems and people. It can injure people, interfere with installed electrical/electronic equipment, and cause ignition of flammable vapors. All EWIS components used for bonding and protection against static electricity play a vital role in ensuring the integrity of the bonds.
(1) § 25.1715(a). This section requires that EWIS used for electrical bonding and protection against static electricity meet the requirements of § 25.899. To minimize the hazardous effects of static discharge, EWIS components should be selected, designed, and installed so that the cross-sectional area of bonding paths 39
used for primary and secondary bonding ensure that an appropriately low electrical impedance is obtained and maintained throughout the expected service life of the components.
(a) The maximum resistance for electrical bonds varies depending on the type of bond, e.g., ground stud, between connector shell and structure. A typical value is 1 milliohm, but this can vary from .01 milliohms to 3 milliohms. The airplane manufacturer’s standard wiring practices manual (SWPM) provides guidance on maximum bonding resistance.
(b) The airplane manufacturer’s SWPM provides guidance on proper surface preparation, materials, tooling, maximum torque values, and other variables important to ensuring an adequate bond. One airplane manufacturer’s SWPM, for example, states that steel washers, dyed washers, and other washers with a non-conductive surface should not be used in the conductive path of a bond.
(2) § 25.1715(b). This section requires that EWIS components used for any electrical bonding purposes (not just those used for protection against static electricity) provide an adequate electrical return path under both normal and fault conditions. EWIS components should be selected, designed, and installed so that the cross-sectional area of bonding paths used for primary and secondary bonding paths ensure that an appropriately low electrical impedance is obtained and maintained throughout the expected service life of the components.
j. § 25.1717 CIRCUIT PROTECTIVE DEVICES: EWIS. This section requires that electrical wires and cable be compatible with the circuit protective devices required by § 25.1357. This means that when selecting wire and cables for a specific application, care must be taken to ensure selection of the proper type and rating of the circuit protective device (e.g., circuit breaker) so that the wire and cables are adequately protected from over-current situations. This section is based on the requirements that were formerly found in § 25.1353(d)(1).
k. § 25.1719 ACCESSIBILITY PROVISIONS: EWIS. This section requires that means be provided to allow for inspection of EWIS and replacement of their components as necessary for continued airworthiness.
(1) The intent of § 25.1719 is to ensure that EWIS components are installed so that inspections, tests, repairs, and replacements can be undertaken with a minimum of aircraft disassembly. When adjacent structures and aircraft systems components must be removed to allow access to wire installations, new possibilities for contamination, chafing, and other types of damage are introduced.
(2) As far as practicable, EWIS components should be installed so that inspections, tests, repair, and replacements can be done without undue 40
disturbance to the EWIS installation or to surrounding aircraft systems. During the design phase, consider minimizing the amount of aircraft disassembly required to perform such tasks. For example, wiring inside conduit may incur damage from chafing against the sides of the conduit. If failure of wiring inside a conduit can lead to a hazardous or catastrophic condition, a means should be provided for inspection of those wires. As in the example, there may be other areas of the airplane where human physical access is not possible. In those situations, other types of inspection techniques should be developed. These “other” types of inspection may be by testing or other means acceptable to the Administrator and should be included in the maintenance requirements that are part of the Instructions for Continued Airworthiness.
l. § 25.1721 PROTECTION OF EWIS. The requirements of this section are intended to prevent damage to EWIS by passengers, crew members, baggage or cargo handlers, or maintenance and service personnel, or by movement of cargo or baggage. Section 25.1721(a) is applicable to EWIS located in cargo or baggage compartments, and §§ 25.1721(b) and (c) apply to EWIS located anywhere in the airplane.
(1) § 25.1721(a). The requirements of this paragraph, as they pertain to EWIS, were formerly located in § 25.855(e). Under section 25.1721(a), EWIS cannot be located in cargo or baggage compartments if its damage or failure may affect safe operation unless it cannot be damaged by movement of cargo or baggage in the compartment and unless its breakage or failure will not create a fire hazard.
This means that any EWIS located in a cargo or baggage compartment must be protected against damage. It further means that EWIS must be designed and installed so that if it fails or breaks, for whatever reason, such as failed wire bundle support devices, damage to cargo liners, or chafed wires, such failure will not create a fire hazard. It does not matter if the EWIS located in the compartment is associated with a flight critical or essential system or a passenger convenience system, such as an IFE system. Failure of an EWIS component, no matter what system it is associated with, could cause a fire or other type of damage to aircraft systems or structure. EWIS in general and wiring in particular should be installed so the structure affords protection against its use as a handhold and damage from cargo. Wires and wire bundles should be routed or otherwise protected to minimize the potential for maintenance personnel stepping, walking, or climbing on them. Wire bundles should be routed along heavier structural members whenever possible. If the structure does not afford adequate protection, other protection means such as a mechanical guard should be provided. When EWIS is close to sharp metal edges, the edges should be protected by grommets to prevent chafing. Additionally, wires should not be routed between aircraft skin and fuel lines within the same plane.
(2) § 25.1721(b). This paragraph requires that EWIS be designed and installed to minimize its damage and risk of damage by movement of people in the airplane during all phases of flight, maintenance, and servicing. Some examples 41
of areas of concern are the flight deck, passenger compartment, crew rest area, wheel wells, and wing leading and trailing edges.
(a) Special consideration should be given to EWIS that are routed to and on passenger seats. It should be protected so that passengers cannot damage it with their feet or access it with their hands.
(b) EWIS located in the lavatories should not be readily accessible by passengers or aircraft cleaners. It should be designed and installed so that it cannot be damaged by the removal and replacement of items such as trash containers.
(c) EWIS located in the galleys should not be readily accessible by cabin crew, aircraft cleaners, or passengers. It should be designed and installed so that galley equipment, including galley carts, cannot come into contact with it and cause damage. The design and installation of EWIS around and in galley areas should be such that galley equipment, such as chiller units, can be removed and reinstalled without coming into contact with EWIS components and damaging them.
(d) As with EWIS located in baggage and cargo compartments, EWIS in areas such as landing gear bays, the APU compartment, and electrical and electronic bays should be designed and installed to minimize potential for maintenance personnel stepping, walking, or climbing on them. Where the structure does not afford adequate protection, other protection such as a mechanical guard should be provided.
(3) § 25.1721(c). This paragraph requires that EWIS be designed and installed to minimize its damage and risk of damage by items carried onto the aircraft by passengers or cabin crew. This is intended to protect EWIS from items such as baggage that is carried on board by passengers and cabin crew and stowed beneath passenger seats or other places where luggage is likely to be stowed.
m. § 25.1723 FLAMMABLE FLUID FIRE PROTECTION: EWIS.
(1) This section requires that EWIS located in areas where flammable fluid or vapors might escape must be considered to be a potential ignition source. As a result, these EWIS components must meet the requirements of § 25.863. Section
25.863 requires that efforts be made to minimize the probability of ignition of fluids and vapors, and the hazards if ignition does occur. See § 25.1707 for the separation requirements between EWIS and flammable fluids. Paragraph 5d of this AC contains the advisory material for § 25.1707.
(2) The airplane manufacturer defines fuel vapor zones. EWIS components located in fuel vapor zones should be qualified as explosion proof (when applicable OR when appropriate) in accordance with Section 9 of RTCA 42
Document DO160 or EUROCAE ED-14, "Environmental Conditions and Test Procedures for Airborne Equipment," latest approved revision, or other equivalent approved industry standard.
(3) The possibility of contamination with flammable fluids due to spillage during maintenance action should also be considered.
n. § 25.1725 Powerplants: EWIS.
(1) § 25.1725(a). This paragraph requires that the failure of EWIS components must be considered when demonstrating compliance with the requirements of § 25.903(b). That means that when a powerplant fails or malfunctions, an EWIS component may not fail in such a way as to prevent continued safe operation of the remaining powerplants or require immediate action by any crewmember for continued safe operation.
(2) § 25.1725(b). This paragraph requires that design of EWIS must consider their damage from a powerplant rotor failure or from a fire originating in the powerplant that burns through the powerplant case. The design of EWIS must minimize hazards to the airplane when these events occur.
o. § 25.1727 Flammable fluid shutoff means: EWIS. This section requires that EWIS associated with each flammable fluid shutoff means and control be fireproof or be located and protected so that any fire in a fire zone will not affect operation of the flammable fluid shutoff means, in accordance with the requirements of § 25.1189.
p. § 25.1729 INSTRUCTIONS FOR CONTINUED AIRWORTHINESS:
EWIS. Section 25.1729 requires applicants to prepare Instructions for Continued Airworthiness (ICA) in accordance with sections H25.4(a)(3) and H25.5. The guidance for those sections of Appendix H applicable to EWIS ICA are contained in paragraph 5.s. of this AC.
q. § 25.1731 POWERPLANT AND APU FIRE DETECTOR SYSTEMS:
EWIS. To minimize occurrences of nuisance fire warnings, consider separately routing fire detection system wiring within the fire zone to allow optimal routing and ease of replacement. Exercise particular care regarding the environmental qualification of the system connectors (fire resistance, resistance to moisture and fluids, etc.).
r. § 25.1733 FIRE DETECTOR SYSTEMS, GENERAL: EWIS. The environmental qualification of the fire detector system components should include the use of fire resistant EWIS components and/or design to ensure that the system will detect fire if exposed to it.
s. APPENDIX H TO PART 25 INSTRUCTIONS FOR CONTINUED AIRWORTHINESS REQUIREMENTS FOR EWIS. Following is a discussion of the requirements for incorporating EWIS component information in the ICA for aircraft. Including EWIS in the ICA will promote proper maintenance, repair, and modification of EWIS components. Improper maintenance, repair, and modification often hastens the “aging” of EWIS. To properly maintain, repair, and modify airplane EWIS, certain information needs to be available to people who design, modify, install, and maintain it. Sections H25.4(a)(3) and H25.5 require applicants for both type certificates and supplemental type certificates to prepare ICA for EWIS. The EWIS-related ICA must be approved by the cognizant FAA office and be in the form of a document easily recognizable as EWIS ICA. The following paragraphs, 5.s.(1) through 5.s.(7), provide specific guidance on acceptable compliance methods for EWIS ICA.
(1) H25.4(a)(3). This section requires the applicant to include in the Airworthiness Limitations section of the Instructions for Continued Airworthiness any mandatory replacement times for EWIS components. EWIS components are those defined by § 25.1701. Generally, EWIS components are designed and selected to last for the service life of the airplane. Any EWIS component that must be replaced at regular intervals to maintain the airworthiness of the associated system or airplane must be specified, with its required replacement interval, in the Airworthiness Limitations section of the ICA.
(2) H25.5(a). This section requires applicants to prepare ICA applicable to EWIS. The ICA must cover all EWIS components as defined in § 25.1701. The rule requires that the FAA approve the EWIS ICA.
(3) H25.5(a)(1). This paragraph requires applicants to prepare EWIS maintenance and inspection requirements using an enhanced zonal analysis procedure (EZAP). An EZAP is a specifically wire-focused version of the zonal analysis procedure widely used to analyze an airplane’s physical areas or zones.
It is used for developing maintenance tasks. One version of an EZAP is described in AC 25-27 Development of Transport Category Airplane Electrical Wiring Interconnection Systems Instructions for Continued Airworthiness Using an Enhanced Zonal Analysis Procedure
(4) H25.5(a)(2). This paragraph requires applicants to document EWIS maintenance practices in a standard format. This is typically accomplished with publication of a standard wiring practices manual (SWPM). The rule is not intended to require that every manufacturer’s SWPM be identical. The intent is to enable people performing EWIS maintenance and repairs to find needed information in the SWPM more quickly and easily, regardless of what airplane model they are currently working on. Standard wiring practices include procedures and practices for the installation, repair, and removal of EWIS components, including information about wire splices, methods of bundle 44