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 9
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.
NOTE: To avoid contamination from foreign particles, the crimp tool should be dedicated to aluminum wire crimping.
3 Aluminum wire splice. Splices for aluminum wires should be in accordance with the requirements of the airframe manufacturer’s standard practices or SAE AS70991, MS25439, or equivalent specification. You should avoid conditions that result in excessive voltage drop and high resistance at junctions that may ultimately lead to failure of the junction. The preferable location for aluminum splices is in pressurized areas.
(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 10
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.
(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.
NOTE: Additional guidance for selecting wire and other EWIS components can be found in SAE AS50881 and AECMA EN2853 as well as in AC 43-13.1B.
(g) Wire temperature rating. Selection of a temperature rating for wire should include consideration of the worst-case requirements of the application.
You should use caution when locating wires in areas where heat is generated— oxygen generators or lighting ballast units, for example.
1 Wires have a specified maximum continuous operating temperature. For many types, this may be reached by any combination of maximum ambient temperature and the temperature rise due to current flow.
2 In general, it is undesirable to contribute more than 40oC rise to the operating temperature by electrical heating.
3 Other factors to be considered are altitude de-rating, bundle size derating, and use of conduits and other enclosures.
4 Particular note should be taken of the specified voltage of any wire where higher than normal potentials may be used. Examples are discharge lamp circuits and windscreen heating systems.
(h) EWIS components in moisture areas.
1 Severe wind and moisture prone areas. Areas designated as severe wind and moisture prone (SWAMP) areas are different from aircraft to aircraft, but they are generally considered to be such areas as wheel wells, wing folds, pylons, areas near wing flaps, and other exterior areas that may have a harsh environment. Wires for these applications should incorporate design features that address these severe environments.
2 Silver plated conductors. Many high-strength copper alloy conductors and coaxial cables use silver plating. Contamination of silverplated conductors with glycol (de-icing fluid) can result in electrical fire.
Accordingly, you should not use silver-plated conductors in areas where de-icing fluid can be present unless suitable protection features are employed. Silver plated conductors and shields can exhibit a corrosive condition (also known as ‘Red Plague’) if the plating is damaged or of poor quality and is exposed to moisture. Designers should be aware of these conditions.
3 Fluid contamination of EWIS components. Fluid contamination of EWIS components should be avoided as far as practicable. But EWIS components should be designed and installed with the appropriate assumptions about fluid contamination, either from the normal environment or from accidental leaks or spills. Industry standards, such as RTCA DO-160/EUROCAE ED-14, contain information regarding typical aircraft fluids. It is particularly important to appreciate that certain contaminants, notably from toilet waste systems, galleys, and fluids containing sugar, such as sweetened drinks, can induce electrical tracking in already degraded electrical wires and unsealed electrical components.
The only cleaning fluids that should be used are those recommended by the airplane manufacturer in its standard practices manual.
(10) EWIS component selection for future modifications. If a TC includes subpart H in its certification basis, future modifiers of those TCs would be able to substantiate compliance with the subpart H requirements by using the same or equivalent standards and design practices as those used by the TC holder. If modifiers choose to deviate from those standards and design practices, they will have to substantiate compliance independently. They will also have to consider the standards and design practices used by the TC holder in order to justify their own choice of components. However, this does not mean that the airplane manufacturer is required to divulge intellectual property to future modifiers. It is incumbent 13
upon the modifiers to determine what constitutes equivalent standards and design practices.
c. § 25.1705 SYSTEMS AND FUNCTIONS: EWIS. This section requires consideration of EWIS components in showing compliance with the certification requirements for specific airplane systems. Although not all part 25 certification requirements directly address EWIS, many address 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. If a system is required by type certification or by operating rules, EWIS associated with that system must be evaluated as part of showing compliance of the system. Section 25.1705(a) and (b) may seem redundant. Section 25.1705(a) has the general requirement. Section 25.1705(b) identifies specific regulations to which this requirement applies. These specific sections contain requirements that do not lend themselves to creating a separate EWIS-based subpart H requirement. But this is not an exclusive list. If a regulation is absent from 25.1705(b) it must still be considered under the general terms of 25.1705(a). The general requirement of (a) is necessary because there may be other regulations where EWIS must be considered. It also ensures that EWIS is given full consideration for any system-related regulation adopted in the future.
d. § 25.1707 SYSTEM SEPARATION: EWIS.
GENERAL GUIDANCE FOR § 25.1707
(1) Summary. The continuing safe operation of an airplane depends on the safe transfer of electrical energy by the EWIS. If an EWIS failure occurs, the separation that the EWIS has from other EWIS, systems, or structure plays an important role in ensuring that hazardous effects of the failure are mitigated to an acceptable level.
Section 25.1707 requires applicants to design EWIS with appropriate separation to minimize the possibility of hazardous effects upon the airplane or its systems. As used in § 25.1707, the term “separation” is a measure of physical distance. The purpose of separation is to prevent hazards of arcing between wires in a single bundle, between two or more bundles, or between an electrical bundle and a non-electrical system or structure.
(2) Separation by physical distances versus separation by barrier. Section
25.1707 states that adequate physical separation must be achieved by separation distance or by a barrier that provides protection equivalent to that separation distance.
The following should be considered when designing and installing EWIS:
(a) In most cases, physical distance is the preferred method of achieving the required separation. This is because barriers themselves can be the cause of EWIS component damage (e.g., chafing inside of conduits) and can lead to maintenance errors, such as barriers removed during maintenance and inadvertently left off.
They can also interfere with visual inspections of the EWIS.
(b) If a barrier is used to achieve the required separation, § 25.1707 requires that it provide at least the same level of protection that would be achieved with physical distance. That means that when deciding on the choice of the barrier, factors such as dielectric strength, maximum and minimum operating temperatures, chemical resistivity, and mechanical strength should be taken into account.
(c) In addition to the considerations given in paragraph (b) above, when wire bundle sleeving (or tubing) is used to provide separation, designers should consider that the sleeving itself is susceptible to the same types of damage as wire insulation.
The appropriate type of sleeving must be selected for each specific application and design consideration must be given to ensuring that the sleeving is not subjected to damage that would reduce the separation it provides.
(3) Determination of separation. Determining the necessary amount of physical separation distance is essential. But because each system design and airplane model can be unique, and because manufacturers have differing design standards and installation techniques, § 25.1707 does not mandate specific separation distances. Instead it requires that the chosen separation be adequate so that an EWIS component failure will not create hazardous effects on the airplane or its systems. The following factors should be considered when determining the separation distance:
(a) The electrical characteristics, amount of power, and severity of failure condition of the system functions performed by the signals in the EWIS and adjacent EWIS.
(b) Installation design features, including the number, type, and location of support devices along the wire path.
(c) The maximum amount of slack wire resulting from wire bundle build tolerances and other wire bundle manufacturing variabilities.
(d) Probable variations in the installation of the wiring and adjacent wiring, including position of wire support devices and amount of wire slack possible.
(e) The intended operating environment, including amount of deflection or relative movement possible and the effect of failure of a wire support or other separation means.
(f) Maintenance practices as defined by the airplane manufacturer’s standard wiring practices manual and the ICA required by § 25.1529 and § 25.1729.
(g) The maximum temperature generated by adjacent wire/wire bundles during normal and fault conditions.
(h) Possible EMI, HIRF, or induced lightning effects.
(4) Cases of inadequate separation. Some areas of an airplane may have localized areas where maintaining the minimum physical separation distance is not 15
feasible. This is especially true in smaller transport category airplanes. In those cases, other means of ensuring equivalent minimum physical separation may be acceptable, if testing or analysis demonstrates that safe operation of the airplane is not jeopardized.
The testing or analysis program should be conservative and consider the worst possible condition not shown to be extremely improbable. The applicant should substantiate to the ACO that the means to achieve the necessary separation provides the necessary level of protection for wire related failures. Electro-magnetic interference (EMI) protection must also be verified.
(5) Meaning of the term “hazardous condition” as used in § 25.1707.
(a) The term “hazardous condition” in § 25.1707 is used in a different context than it is used in§ 25.1709. In the EWIS System Safety rule, § 25.1709, the applicant must show that each EWIS system is designed and installed so that each hazardous failure condition is extremely remote. The definition of a hazardous failure condition is the same as it is for section § 25.1707 (see Table 1). In § 25.1709, however, a numerical probability is required to demonstrate that the possibility for such an occurrence is extremely remote.
(b) In the System Separation rule, § 25.1707, separation distances or a barrier that provides protection equivalent to the separation distance must be used to ensure that none of the types of failures described in the rule will create a situation that would fit the definition of a hazardous condition. The operative term in this rule is that such failures will not create a hazardous condition. To show that a given failure will not create a hazardous condition, the applicant must perform a qualitative design assessment of the installed EWIS. This assessment involves the use of reasonable engineering and manufacturing judgment and assessment of relevant service history to decide whether an EWIS, system, or structural component could fail in such a way as to create a condition that would affect the airplane’s ability to continue safe operation. However, the requirements of § 25.1707 do not preclude the use of valid component failure rates if the applicant chooses to use a probability argument in addition to the design assessment to demonstrate compliance. It also does not preclude the FAA from requiring such an analysis if the applicant cannot adequately demonstrate that hazardous conditions will be prevented solely by using the qualitative design assessment. Also note that a numerical probability assessment may still be required by § 25.1709 if the airplane-level functional hazard assessment identifies EWIS failures that could affect safe operation of the airplane.
(c) To illustrate the type of assessment required by § 25.1707, consider the following simple example involving the use of wire bundle clamps. Clamps are used to secure a wire bundle to structure in order to hold the bundle in place and route it from one location to another along a predetermined path. An airplane manufacturer, using the criteria contained in paragraph 5.d.(3) above, determines that a 2-inch separation between a certain wire bundle and some hydraulic lines is necessary. The manufacturer further decides that one clamp every 10 inches is needed to maintain that separation. However, there is one localized area where a single clamp failure would potentially create a hazard. This is because the area in 16
question is a high vibration, high temperature area, subject to exposure to moisture. So the clamp in this particular area is exposed to severe environmental conditions that could lead to accelerated degradation. The manufacturer decides that using just a single clamp every 10 inches in this area would not suffice to preclude a hazardous event. The manufacturer prescribes use of double clamps every 10 inches in that area.
SECTION-BY-SECTION GUIDANCE FOR § 25.1707
(6) Specific guidance by section of § 25.1707
(a) § 25.1707(a). Paragraph 25.1707(a) requires that adequate physical separation be achieved by separation distance or by a barrier that provides protection equivalent to that separation distance. Refer to paragraphs 5.(d)(2), (3), and (4) for further guidance regarding separation distances and the use of barriers.
(b) § 25.1707(b).
1 This section requires that each EWIS be designed and installed so that any electrical interference likely to be present in the airplane will not result in hazardous effects on the airplane or its systems.
2 One type of electrical interference is electromagnetic interference (EMI).
Electromagnetic interference can be introduced into airplane systems and wiring by coupling between electrical cables or between cables and coaxial lines or other airplane systems. The design should not allow system function to be affected by EMI generated by adjacent wire. EMI between its source wiring and wire susceptible to it increases in proportion to the length of parallel runs. It decreases with greater separation. Wiring of sensitive circuits that may be affected by EMI should be routed away from other wiring that may cause such interference, or it should be provided with sufficient shielding to avoid system malfunctions under operating conditions. The design should limit EMI to negligible levels in wiring related to systems necessary for continued safe flight, landing, and egress. The following sources of interference should be considered:
(aa) Conducted and radiated interference caused by electrical noise generation from apparatus connected to the bus bars.
(bb) Coupling between electrical cables or between cables and aerial feeders.
(cc) Malfunctioning of electrically-powered apparatus.
(dd) Parasitic currents and voltages in the electrical distribution and grounding systems, including the effects of lightning currents or static discharge.
(ee) Different frequencies between electrical generating systems and other systems.
(c) § 25.1707(c). This section contains the wire-related requirements formerly located in § 25.1353(b). Coverage is expanded beyond wires and cable carrying heavy current to include their associated EWIS components as well. This means that all EWIS components, as defined by § 25.1701, that are associated with wires and cables carrying heavy current must be installed in the airplane in such a way that damage to essential circuits will be minimized under fault conditions.
(d) § 25.1707(d). Section 25.1707(d) contains wire-related requirements from §§ 25.1351(b)(1) and (b)(2) and introduces additional requirements. As used in this section, “independent airplane power source” means a general source of power for the whole of the airplane or for major subsystems (such as flight controls – as, for instance, permanent magnet generators on the engines for the fly-by-wire system). Examples include engine- or APU-driven generators, batteries, and ram air turbines.
1 Paragraph (d) requires that EWIS components associated with the generating system receive the same degree of attention as other components of the system, such as the electrical generators.
2 Paragraph (d)(1) prohibits airplane independent electrical power sources from sharing a common ground terminating location. Paragraph (d)(2) prohibits airplane static grounds from sharing a common ground terminating location with any airplane independent electrical power sources. The reason for these paragraphs is twofold:
• to help ensure the independence of separate electrical power sources so that a single ground failure will not disable multiple power sources; and
• to prevent introduction of unwanted interference into airplane electrical power systems from other airplane systems.
(e) §§ 25.1707(e), (f), (g), (h). These paragraphs contain specific separation requirements for the fuel, hydraulic, oxygen, and waste/water systems. They require adequate EWIS separation from those systems except to the extent necessary to provide any required electrical connection to them. EWIS must 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 situation.
1 Under fault conditions and without adequate EWIS separation, a potential catastrophic hazard could occur if an arcing fault ignited a flammable fluid like fuel or hydraulic fluid. Also, 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 damage 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.
2 In addition to the required separation distance, the use of other protection means such as drip shields should be considered to prevent fluids from leaking onto EWIS.
(f) § 25.1707(i). 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 control or other types of 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 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 jamming or otherwise restricting the cable’s movement. Without adequate separation, an arcing fault could damage or sever a control cable. A control cable failure could damage EWIS. Therefore, paragraph (i) requires an adequate separation distance or barrier between EWIS and flight or other mechanical control systems cables and their associated system components. It also requires 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. Clamps for wires routed near moveable flight controls should be attached and spaced so that failure of a single attachment point cannot interfere with flight controls or their cables, components, or other moveable flight control surfaces or moveable equipment.
(g) § 25.1707(j). This section requires that EWIS design and installation provide adequate physical separation between the EWIS components and heated equipment, hot air ducts, and lines. Adequate separation distance is necessary to prevent EWIS damage from extreme temperatures and to prevent an EWIS failure from damaging 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.
(h) § 25.1707(k). Reliability for some airplane systems is so critical that independent, redundant systems are required. The autoland system is an example.
If one channel of a redundant autoland system is lost, the airplane can continue to operate safely. But 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 are maintained, adequate separation and electrical isolation between these systems must be ensured.
Paragraph (k) requires that EWIS associated with any system requiring redundancy to meet certification requirements have an adequate separation distance or barrier.
1 EWIS of redundant aircraft systems, as required by certification rules, operating rules, or as a result of the assessment required by § 25.1709, should be routed in separate bundles and through separate connectors to prevent a single fault from disabling multiple redundant systems. Separation of functionally similar EWIS components is necessary to prevent degradation of their ability to perform their required functions.
2 Power feeders from separate power sources should be routed in bundles separate from each other and from other aircraft wiring in order to prevent a single fault from disabling more than one power source.
3 Wiring that is part of electro-explosive subsystems, such as cartridgeactuated fire extinguishers and emergency jettison devices, should be routed in shielded and jacketed twisted-pair cables, shielded without discontinuities, and kept separate from other wiring at connectors.
(i) § 25.1707(l). This section requires that EWIS be designed and installed so they are adequately separated from aircraft structure and protected from sharp edges and corners. This is to minimize the potential for abrasion and chafing, vibration damage, and other types of mechanical damage. This protection is necessary because over time the insulation on a wire that is touching a rigid object, such as an equipment support bracket, will fail and expose bare wire. This can lead to arcing that could destroy that wire and other wires in its bundle. Structural damage could also occur, depending on the amount of electrical energy the failed wire carries.
e. § 25.1709 SYSTEM SAFETY: EWIS. Section 25.1709 requires applicants to perform a system safety assessment of the EWIS. The analysis required for compliance with § 25.1709 is based on a qualitative and quantitative approach to assessing EWIS safety, as opposed to a purely numerical, probability-based quantitative analysis. The safety assessment must consider the effects that both physical and functional failures of EWIS would have on airplane safety. That safety assessment must show that each EWIS failure considered hazardous is extremely remote. It must show that each EWIS failure considered to be catastrophic is extremely improbable and will not result from a single failure.
(1) Objective. The objective of § 25.1709 is to use the concepts of § 25.1309 to provide a thorough and structured analysis of aircraft wiring and its associated 20