Appendix B The proposed requirements of § 25.1709(a) were derived from existing § 25.1353(a). While the requirements of § 25.1353(a) are retained, the portion of that requirement applicable to wiring has been moved to the proposed § 25.1709(a). Further clarification of the requirement is also included in the proposal. Section 25.1353(a) states “ … wiring must be installed so that operation of any one unit or system of units….” Proposed section 25.1709(a) expands on the term “operation” to state that it means “operation under norm al and failure conditions as defined by § 25.1309.”
Proposed section 25.1709(b) would require that each EWIS be designed and installed so that any electrical interference likely to be p resent in the airplane will not result in hazardous effects on the airplane or its systems.
This proposed requirement is based on new text recently added to § 25.1353(a) to harmonize part 25 with the existing text of the JAA JAR
25.1353(a)1. The text of JAR 25.1353(a) requires that any electrical inte rference likely to be present in the airplane must not result in hazardous effects on the airplane or it s systems except under extremely remote conditions. The proposed § 25.1709(b) is recognition of the fact that electrical interference can be introduced into airplane systems and wiring by coupling between electrical cables or between cables and coaxial lines, as well as by the other equipment that is the subject of § 25.1353(a). The proposed requirement does not adopt the JAR clause “except under extremely remote conditions.” This is because the intent of the requirement is not to require a num erical probability assessment of the likelihood of electrical interference or its consequences as described previously. Rather it is meant to convey that under failure conditions that may be caused by electrical inference, the resultant effects should not be such as to prevent continued safe flight of the airplane.
Proposed section 25.1709(c) contains the wire-related requirements of the current § 25.1353(b). These requirem ents have been expanded to add that not only wires and cable carrying heavy current are covered, but their associated E WIS components are covered as well. The proposal prescribes that any required physical separation must be achieved either by separation distance or by barrier or other means shown to be at least equivalent to an adequate separation distance.
1 The JAA is the Joint Aviation Authority of Europe and the JAR is its Joint Aviation Requirements, the equivalent of our Federal Aviation Regulations. In the time since these proposals were developed, in 2003, the European Aviation Safety Agency (EASA) was formed.
EASA is now the principal aviation regulatory agency in Europe, and we intend to continue to work with them to ensure our proposal is also harmonized with its Certification Specifications (CS). But since the harmonization efforts involved in developing this proposal occurred before EASA was formed, it was the JAA that was involved with them. So while the JAR and CS are essentially equivalent, and in the future we will be focusing on the CS, it is the JAR that will be referred to in the historical background discussions in this proposal.
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Appendix B
Proposed section 25.1709(d) contains wire-related requirements of existing §§ 25.1351(b)(1) and (b)(2) and would introduce additional requirements. To show compliance with § 25.1709(d), EWIS components associated with the generating system must be considered with the same degree of attention as other components of the system, such as the electrical generators. The proposal prescribes that any required physical separation must be achieved either by separation distance or by a barrier or other means shown to be at least equivalent to an adequate separation distance. Paragraph
(d)(1) would introduce a requirement to prohibit the airplane’s independent electrical power sources from sharing a common ground terminating location. Paragraph (d)(2) would prohibit the airplane’s static grounds from sharing a common ground terminating location with any of the airplane’s independent electrical power sources. These two new requirements would help to ensure the independence of separate electrical power sources and to prevent introduction of unwanted interference into airplane electrical power systems from other airplane systems.
Paragraphs (e), (f), (g), and (h) of proposed § 25.1709 contain EWIS- related requirements from § 25.1353(d)(3). These paragraphs contain specific separation requirements for the airplane’s fuel, hydraulic, oxygen, and waste/water systems. They require that EWIS have adequate separation from those systems except to the extent necessary to provide any required electrical connection to them. These paragraphs require that EWIS be designed and installed with adequate separation so a failure of an EWIS component will not create a hazardous condition and any leakage from those systems (i.e., fuel, hydraulic, oxygen, waste/water) onto EWIS components will not create a hazardous condition. The proposed requirements recognize the potential catastrophic hazard that could occur should an arcing fault ignite a flammable fluid like fuel or hydraulic fluid. An arcing fault has the potential to puncture a line associated with those systems if adequate separation is not maintained. If there is leakage from one of those systems and an arcing event occurs, fire or explosion could result. Similarly, leakage from the water/waste system can cause damage to EWIS components and adversely affect their integrity. An EWIS arcing event that punctures a water or waste line could also introduce fluids into other airplane systems and create a hazardous condition.
To prevent chafing, jamming, or other types of interference or other failures that may lead to loss of control of the airplane, EWIS in general and wiring in particular must be physically separated from flight or other control cables. Mechanical cables have the potential to cause chafing of electrical wire if the two come into contact. This can occur either through vibration of the EWIS and/or mechanical cable or because of cable movement in response to a system command. A mechanical cable could also damage other EWIS components, such as a wire bundle support, in a way that would cause failure B-18
Appendix B of that component. Also, if not properly designed and installed, a wire bundle or other EWIS component could interfere with movement of a mechanical control cable by causing jamming or otherwise restricting the cable’s movement. An arcing fault could damage or sever a control cable, or a control cable failure could cause damage to EWIS if not adequately separated. Therefore, proposed paragraph (i) would require an adequate separation distance or barrier between EWIS and flight or other mechanical control systems cables and their associated system components. It would further require that failure of an EWIS component must not create a hazardous condition and that the failure of any flight or other mechanical control systems cables or systems components must not damage EWIS and create a hazardous condition.
EWIS in general and wiring in particular must be routed away from hightemperature equipment, hot air ducts, and hydraulic, fuel, water, and other lines. There must be adequate separation distance in order to prevent damage to the EWIS caused by extreme temperatures and so that an EWIS failure will not damage the equipment, ducts, or lines. High temperatures can deteriorate wire insulation and other parts of EWIS components, and if the wire or component type is not carefully selected, this deterioration could lead to wire or component failure. Similarly, should an arcing event occur, the arc could penetrate a hot air duct or line and allow the release of high pressure, high temperature air. Such a release could damage surrounding components associated with various airplane systems and potentially lead to a hazardous situation. Paragraph (j) would require that EWIS be designed and installed with an adequate separation distance or barrier between the EWIS components and heated equipment, hot air ducts, and lines.
The needed reliability of some airplane systems, such as an autoland system, requires that independent, redundant systems be used. Loss of one channel of a redundant system would not decrease the ability to continue safe operation. However, if both channels of a two-channel system were lost because of a common failure, the results could be catastrophic. To maintain the independence of redundant systems and equipment so that safety functions required for safe operation are maintained, adequate separation and electrical isolation between these systems must be ensured. Paragraph (k) would require that EWIS associated with any system that requires redundancy to meet certification requirements be separated with an adequate separation distance or barrier.
Paragraph (l) of proposed § 25.1709 would require that EWIS be designed and installed so they are adequately separated from aircraft structure and protected from sharp edges and corners. The purpose of this proposal is to minimize the potential for abrasion/chafing, vibration damage, and other types of mechanical damage. Such protection is necessary because over time the insulation on a wire that is touching a rigid object, such as an B-19
Appendix B equipment support bracket, will fail and expose bare wire. This can potentially lead to arcing that could destroy that wire and other wires in its bundle. Depending on the amount of electrical energy being carried by the failed wire, structural damage may also occur.
Section 25.1711 Component identification: EWIS.
Proposed § 25.1711 would require 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, identification of the EWIS separation requirement would also be required.
An important aspect of ensuring safe operation of airplanes is making sure that EWIS components are properly identified. This is necessary so that modification designers, maintenance personnel, and inspectors can easily determine the function of the associated system, together with any associated separation requirements and design limitations. Clear labeling of EWIS components and easy-to-understand identification aids allow installers, inspectors, and maintainers to readily ascertain that correct system components are installed as designed, and allow modifiers to add systems with due regard to the existing protection and separation requirements.
The current part 25 certification requirement for equipment identification is § 25.1301(b) and it is applicable to “each item of installed equipment.”
This rule is inadequate for EWIS because it does not provide the specific requirements that have been determined necessary for identifying EWIS components. Specific EWIS component identification needs to be done to prevent modifiers from unintentionally introducing unsafe design or installation features on previously certified airplanes when they install new or modified systems. Component identification would also make those performing maintenance and inspections more aware of what systems are associated with specific EWIS in the areas undergoing maintenance or inspection.
When the FAA first certifies an airplane type design, its systems are designed and installed to ensure safe operation of the airplane. Systems essential to that safe operation are often designed and installed to ensure redundancy of the system function. They have two or more circuits, or channels, that can perform the same function in case one of them malfunctions. Separate circuits (channels) typically have their own sensors, wiring, and equipment. This helps ensure that a common failure cannot cause failure of the entire system.
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Appendix B An example of this is the autoland system on modern transport category airplanes. The autoland system allows airplanes to land during adverse weather conditions that would otherwise prevent landing with manual techniques that rely on the flightcrew’s ability to see the runway. Typically the autoland system has three channels that are physically separated and electrically segregated, so if one channel fails, the airplane can safely continue the autoland procedure. The failure of an autoland system at a critical phase of flight can be catastrophic to the airplane and its passengers.
The integrity of an autoland system’s design could be compromised by systems installed after certification of the autoland system. One way to prevent this is to clearly identify EWIS associated with the autoland in a way that makes it easy to see that it is associated with a critical system. Such identification would aid the designers and installers of the new system by alerting them to the presence of the critical system and allow appropriate design and installation decisions, preventing degradation of the safety of the autoland system.
The reverse is also true. For example, suppose an in-flight entertainment system is installed on an airplane and, after that installation, an autoland system is to be installed. The designers and installers of the autoland system would need to be able to identify EWIS associated with the IFE system so they do not mix IFE system EWIS with the autoland system EWIS. The IFE system is a passenger convenience item and its functionality is not important to the continued safe operation of the airplane. When the zone containing the autoland system EWIS is undergoing inspections or maintenance, easy identification of the EWIS will alert inspection or maintenance personnel to use extra caution in the area.
Proposed § 25.1711(a) uses language that is similar to existing § 25.1301(b) but is specifically applicable to EWIS components. The proposal adds the word “consistent” to stress the need for consistency in EWIS identification to avoid confusion and mistakes during airplane manufacturing, modification, and maintenance. This means the FAA expects 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. The consistent identification method must be used for new type certifications and changes to those designs. Proposed § 25.1711(e) would require that modifications to type designs use EWIS identification methods that are consistent with the identification method of the original type design. The proposed requirements of paragraph (e) are discussed later in this document.
Paragraph (b) would impose additional requirements for identification detail, when assessed in accordance with the proposed requirements of § 25.1705, for EWIS components associated with:
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Appendix B
• Systems required for safe flight and landing.
• Systems required for egress.
• Systems with potential to affect the flightcrew’s ability to cope with adverse operating conditions.
Paragraph (c) would require that identifying markings required by paragraphs (a) and (b) of the proposal 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 to support the intended purpose of the markings for the life of the airplane. The method of marking must take into account the environment in which the EWIS component will be installed.
Paragraph (d) would require that the means used to identify an EWIS component does not have an adverse effect on the component’s performance throughout its design life. Certain wire marking methods have the potential to damage the wire’s insulation. Hot-stamp marking is one such method.
According to SAE (Society of Automotive Engineers) aerospace information report AIR5575, “Hot Stamp Wire Marking Concerns for Aerospace Vehicle Applications,” a copy of which is included in the docket, the hot-stamp marking method is not well suited for today’s generation of aircraft wiring.
As noted in the SAE 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 swell 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.”
The FAA concurs with this assessment. If damage to the insulation occurs during the marking process, it may fail later in service after it has been exposed to the sometimes-harsh environmental conditions of aircraft use. While the proposed regulation does not prohibit use of hot-stamp marking, its use is not encouraged. To comply with this paragraph, if 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 must be followed. A copy of this document is in the docket.
In some cases it may not be practicable 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 must be used.
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Appendix B
Paragraph (e) would require 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 proposed § 25.1711(a) is maintained when a modification is installed. The intent of this requirement is to provide continuity in the methods used 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 proposed § 25.1711 is to make it easy to identify those airplane systems essential to the 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 is acceptable. This is just an example and should not be construed to say that flight critical systems should use green wire or nonessential systems purple wire. The regulation does not prescribe a particular method for identification, but is meant to ensure that the consistency of the identification method required by paragraph (a) is maintained throughout the life of the airplane.
Section 25.1713 Fire protection: EWIS.
Proposed § 25.1713 would require that EWIS components meet the applicable fire and smoke protection requirements of § 25.831(c). It would further require that EWIS located in designated fire zones be at least fire resistant. Insulation on electrical wires and cables would also be required to be self-extinguishing when tested in accordance with the applicable portions of Appendix F, Part I, of part 25.
During an emergency situation it is important that airplane systems needed by the flightcrew to effectively deal with the emergency be operative.
To help ensure this, § 25.869 requires that electrical systems components meet certain flammability requirements and be designed and installed to minimize probability of ignition of flammable fluids and vapors. Currently, § 25.869(a) is applicable to wiring. The proposal is to move the requirements of § 25.869(a) related to protection of wiring from fire and put them into the proposed § 25.1713. This will allow easy identification of the requirements for fire protection of EWIS, because they will be found in the proposed new subpart H, which is dedicated to EWIS regulations.
Requirements of § 25.869 dealing with isolation from flammable fluid lines have been moved to the new § 25.1709 and requirements for allowance for B-23
Appendix B deformation and stretching have been moved to § 25.1703. As a result, we are amending § 25.869 to accommodate this change.
Section 25.1717 Electrical bonding and protection against static electricity:
EWIS.
Proposed § 25.1717(a) would require that EWIS used for electrical bonding and protection against static electricity meet the requirements of § 25.899. Proposed § 25.1717(b) would require that EWIS components used for any electrical bonding purposes (not just that used for protection against static electricity) provide an adequate electrical return path under both normal and fault conditions.
The buildup and subsequent discharge of static electricity has the potential to create hazardous conditions for both airplane systems and people.
Static electricity can injure people. It can also interfere with installed electrical/electronic equipment and cause ignition of flammable vapors. We are proposing to adopt § 25.899 (as discussed in the section headed “Electrical System Harmonization Rules”) to highlight the importance of considering electrical bonding and static electricity as a full aircraft requirement and to prevent hazardous effects of static electricity. The proper design and installation of EWIS components used to accomplish such protection is critical to ensure the hazardous effects of static discharge are minimized. For example, the cross-sectional area of bonding paths used for primary bonding paths is important in ensuring that a low electrical impedance is obtained, as is the method in which the bonding connection is made to the airplane structure. Thus, EWIS must be fully considered when designing and installing protection from the adverse effects of static electricity. The proposed § 25.1717 highlights the importance EWIS has in providing this protection and requires that EWIS components meet the same requirements as other components used to show compliance with § 25.899.
The ARAC Electrical Systems Harmonization Working Group recommended the adoption of JAR 25.1353(e) as paragraph (e) of § 25.1353.
The JAR requires that electrical bonding provide an adequate electrical return path under both normal and fault conditions on airplanes with grounded electrical systems. ATSRAC recommended that the requirements of JAR 25.1353(e) be moved in their entirety to the proposed subpart H. We agree with that recommendation and, instead of adopting JAR 25.1353(e) as § 25.1353(e), we are proposing to adopt it as § 25.1717(b).
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Appendix B Section 25.1719 Systems and functions: EWIS.
Proposed § 25.1719 would require that EWIS components be considered in showing compliance with the certification requirements of specific airplane systems. Many of the current part 25 sections contain system specific requirements that apply to EWIS in an indirect way. The EWIS associated with such systems play an integral role in ensuring the safe operation of the system and of the airplane. In general, the EWIS associated with any airplane system needs to be considered an integral part of that system and must be given the same design and installation attention as the rest of the system. The proposed § 25.1719(a) contains this general requirement, while paragraph (b) of the proposal identifies specific sections of part 25 that are associated with airplane systems where wire and its associated components play an important part in ensuring safety. These specific part 25 sections contain requirements that do not lend themselves to creating a separate EWIS-based Subpart H requirement.
It is the intent of the proposed § 25.1719 to require that EWIS be designed and installed to support systems required for type certification or by operating rules, including those systems addressed by the regulations specifically listed in paragraph (b) of the proposal. They must be considered part of those systems, and be given the same design and installation considerations as the rest of the system. While paragraphs (a) and (b) may seem redundant, we have listed specific sections in (b) to ensure that applicants are aware of the need to give EWIS associated with those systems the same consideration as the other components of those systems. We consider the general requirements of (a) necessary because there may be other regulations where EWIS must be considered in showing compliance with those regulations. It also ensures that EWIS is given full consideration for any system-related regulation adopted in the future.
Section 25.1721 Circuit protective devices: EWIS.
Proposed § 25.1721 would require that electrical wires and cable be compatible with the circuit protective devices required by § 25.1357.
We recently adopted § 25.1353(d)(1) based on recommendations of ARAC, as part of the effort to harmonize the requirements of JAA JAR 25 and FAA 14 CFR part 25. Paragraph (d)(1) requires that electrical cables be compatible with the circuit protection devices required by § 25.1357, so that a fire or smoke hazard cannot be created under temporary or continuous fault conditions. That requirement would be moved from § 25.1353(d)(1) into the proposed § 25.1721 in its entirety. The proposal also adds the word “wire” to the requirement. This is because this requirement applies to all sizes of wire, not just heavy-current-carrying cables.
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Appendix B
Section 25.1723 Instruments using a power supply: EWIS.
The proposed § 25.1723 would require that EWIS components associated with flight and navigation instruments using a power supply be designed and installed so that compliance with § 25.1331 is ensured.
Section 25.1331 requires that flight and navigation instruments using a power supply must, in the event of the failure of one power source, be supplied by another power source. No change is proposed to the wording of that section.
Section 25.1725 Accessibility provisions: EWIS.
The proposed new § 25.1725 would require that means be provided to allow for inspection of EWIS and replacement of their components as necessary for continued airworthiness.
Currently, § 25.611 requires that means must be provided to allow inspection, replacement of parts, adjustment, and lubrication as necessary for principal structural elements and control systems. While wiring systems are not specifically referred to in the existing rule, the “accessibility” concept is easily applied to EWIS. Many of the wiring systems on airplanes today are very difficult to access and inspect. We now have an increased awareness of the importance of inspecting wiring for separation and for contamination and damage in order to ensure proper functioning, maintenance, and safety. We also know that when adjacent structures must be removed to allow access to wire installations, new possibilities for contamination, chafing, and other types of damage are introduced. Section 25.611 would be amended to specify that EWIS must meet the accessibility requirements of § 25.1725.
The intent of proposed § 25.1725 is to ensure that EWIS components be installed so that inspections, tests, repairs, and replacements can be undertaken, and that these can be carried out with a minimum of aircraft disassembly. This proposal would facilitate the proposed implementation of the new wiring inspection programs developed under proposed § 25.1739 and the operating rules contained in this proposal.
Section 25.1727 Protection of EWIS.
Proposed § 25.1727 would require that cargo or baggage compartments not contain any EWIS whose failure would adversely affect safe operation.
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Appendix B It would also require that all EWIS be protected from damage by movement of people.
Section 25.855(e) requires that no cargo or baggage compartments may contain any controls, wiring, lines, equipment, or accessories whose damage or failure would affect safe operation of the airplane unless they are protected so that they cannot be damaged by movement of cargo in the compartment and their breakage or failure will not create a fire hazard. The proposed regulations would remove the word “wiring” from the current language and move those requirements, as they apply to EWIS, to the proposed § 25.1727(a). Proposed § 25.855(j) would mandate that cargo or baggage compartment EWIS components must meet the requirements of § 25.1727(a).
The proposed § 25.1727(b) and (c) are new EWIS requirements that currently don’t exist in part 25. Paragraph (b) would require that EWIS be designed so that damage and risk of damage from movement of people in the airplane during all phases of flight, maintenance, and service, be minimized.
Paragraph (c) would require designers to minimize damage and risk of damage to EWIS by items carried onto the airplane by passengers, cabin crew, and flightcrew. These two new requirements are justified by service experience that shows wires can easily be damaged by movement of people on the airplane and by items carried on board.
Paragraph (b) would require that EWIS designers and installers consider such things as the routing of wires that could be damaged by personnel in the cargo compartments. For example, EWIS would have to be designed and installed in ways that prevent their use as hand- or footholds as much as practicable. It would further require that EWIS be protected from damage by people in the cabin or flight deck. More and more wiring is being routed to passenger seats to support increasingly complex passenger convenience features. If an airplane is equipped with seat-back monitors, for example, the electronic components necessary to support the monitor are typically mounted underneath the seat. This requires wire routing to the seats, usually through the seat tracks (structural channels used to fasten the seats to the floor) or from the side wall directly next to the seat. Many wires mounted on or under the seats have been damaged by passengers. In one case an airplane was operated with wires lying on the floor in the area where a passenger would put his feet. The wires had become dislodged from the seat track.
This not only exposed the wires to damage but also posed a potential electrical shock risk to the passenger. In other cases, wires have been routed to the seats through holes cut into the cabin side wall, exposing them to damage from both passengers and carry-on items stored beneath the seat or between the side wall and seat.
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Appendix B
Section 25.1729 Flammable fluid fire protection: EWIS.
The proposed § 25.1729 would require that EWIS components be considered a potential ignition source in each area where flammable fluid or vapors might escape by leakage of a fluid system and must meet the requirements of § 25.863.
The current § 25.863 mandates that, in each area where flammable fluids or vapors might escape by leakage of a fluid system, there must be means to minimize the probability of ignition, and resultant hazards if ignition does occur. Possible ignition sources, including overheating of equipment, malfunctioning of protective devices, and electrical faults must be considered in showing compliance with this rule. Many types of electrical faults could cause ignition. Among them are sparks emitting from an avionics component, overheated electrical component surfaces, and arcing from electrical wiring. The wording of § 25.863 would not change.
Section 25.1731 Powerplants: EWIS.
The proposed § 25.1731 specifies that EWIS associated with any powerplant must be designed and installed so that failure of an EWIS component will not prevent continued safe operation of the remaining powerplants or require immediate action by any crewmember for continued safe operation, in accordance with § 25.903(b). It would also mandate that design precautions be taken to minimize hazards to the airplane because of EWIS damage in the event of a powerplant rotor failure or a fire originating in the powerplant that burns through the powerplant case, in accordance with § 25.903(d)(1). The purpose of this section is to ensure proper consideration of EWIS in evaluating powerplant installation designs.
The current § 25.903(b) requires, among other things, that powerplants be arranged and isolated from each other to allow operation, in at least one configuration, so that failure or malfunction of any engine, or of any system that can affect the engine, will not prevent continued safe operation of the remaining engines or require immediate action by any crewmember for continued safe operation. Section 25.901(d)(1) requires that design precautions be taken to minimize hazards to the airplane in the event of an engine rotor failure or a fire originating within the engine that burns through the engine case.
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