8.1 Aircraft electrical systems comprise those electrical components which generate, distribute, control and protect the supply of AC and/or DC electrical power for other systems, and provide signal and electrical interfaces between aircraft systems. Electrical systems that do not satisfy appropriate design requirements may pose hazards to safe aircraft operations by: providing inadequate or unsuitable power for essential aircraft system loads, increasing the likelihood of fire or explosion, increasing the potential for critical aircraft system failures and so on. To ensure that these hazards are not realised the Authority prescribes design requirements for aircraft electrical systems.
8.2 This Chapter presents the Authority prescribed airworthiness design requirements for Defence aircraft electrical systems and electrical wiring interconnection systems. Importantly, the airworthiness design requirements in this chapter are not suitable for application in isolation from an Airworthiness Code. The Authority's requirements for the application of recognised Airworthiness Codes are defined in Section 1 of this manual.
8.3 This section presents the Authority prescribed electrical systems airworthiness design requirements applicable to Defence aircraft.
8.4 Design Requirement (Essential). The aircraft’s battery capacity must be evaluated to confirm that there is sufficient capacity to supply all essential loads, and to support continued safe flight and landing for the intended Defence operations.
8.5 While the Authority recognised Airworthiness Codes define battery capacity requirements (in terms of the time that the battery must supply essential loads) for emergency operations, the actual battery capacity required to support continued safe flight and landing will be dependent on the intended operations of the Defence aircraft. Given that Defence operations are likely to vary from those assumed within the original aircraft type certification, the battery capacity must be evaluated and confirmed to be adequate for all proposed Defence operations.
8.6 Design Requirement (Essential). Rechargeable Lithium-ion batteries used as a primary power source in Defence aircraft must meet the requirements prescribed in FAA AC 20-184 Guidance on Testing and Installation of Rechargeable Lithium Battery and Battery Systems on Aircraft.
8.7 Design Requirement (Essential). Non-rechargeable Lithium-ion batteries used as a power source in Defence aircraft systems and equipment must meet the requirements prescribed in Annex A - Airworthiness Requirements for Equipment with Non-rechargeable Lithium Batteries.
8.8 Li-ion batteries may pose additional fire hazards not normally present when lead-acid or Ni-Cd batteries are used as main aircraft power sources. Most Authority recognised Airworthiness Codes do not prescribe design requirements tailored to ensure the safety of Li-ion batteries installed in Defence aircraft, and compliance with the requirements prescribed in FAA AC 20-184 for rechargeable Lithium-ion batteries, or Annex A for non-rechargeable Lithium-ion batteries as appropriate, is therefore required.
8.9 Design Requirement (Essential). Electrical loads associated with aircraft design changes must be evaluated and confirmed to be safely accommodated within the aircraft’s electrical power generation capacity.
8.10 Aircraft electrical loads are subject to constant changes as electrical components are replaced with alternatives due to design changes, obsolescence and so on. Failure to appropriately manage the electrical load can lead to overloading the power generation and distribution system, which may pose an airworthiness risk particularly during operations in an emergency.
8.11 Some types of wire insulation may produce toxic fumes in the event of fire, or are known to exhibit properties that notably increase the likelihood of electrical arcing as they degrade over time. The Authority approved Airworthiness Codes may not comprehensively cover the use of these wiring types, or consider the sometimes harsher environments in which this wiring must function in military aircraft. Consequently, the Authority has prescribed additional wiring interconnection system design requirements to ensure that electrical wiring systems are adequately safe for Defence aircraft applications.
8.12 Design Requirement (Essential). Wiring insulation containing PVC must not be used in Defence aircraft electrical wiring interconnection systems.
8.13 Polyvinyl chloride (PVC) releases hydrogen chloride gas when burnt or subject to high temperatures, which transforms into hydrochloric acid on contact with moisture (causing sight and breathing impairment). Consequently, PVC wiring, particularly where it is hidden or inaccessible during flight, presents a hazard to safe aircraft operations and must not be used in any fixed aircraft wiring installations. PVC insulated wiring can often be introduced into Defence aircraft through carriage of Portable Electrical Devices (PED), or use of Commercial off the Shelf (COTS) role equipment. In these circumstances, the Authority prescribed design requirements for PEDs and COTS role equipment in Section 5 Chapter 6 apply.
8.14 Design Requirement (Recommended). Polyimide wiring insulation should not be used in Defence aircraft electrical wiring interconnection systems.
8.15 Polyimide (Kapton) wiring is susceptible to hydrolysis, characterised by breakdown and cracking of the insulation material, wet arc tracking and dry arc tracking. Typically the use of polyimide wiring insulation should be avoided, or restricted from areas that expose the wire to caustic fluids or gases, where repeated flexing of the wire is required or where repeated exposure to water is expected, especially in the presence of ultraviolet light. Where the use of polyimide wiring insulation is unavoidable, appropriate management procedures should be implemented.
8.16 Design Requirement (Essential). To ensure interconnection system integrity is maintained in-service, electrical wiring interconnection system ICA must be defined and promulgated as prescribed in FAR/CS 25 Appendix H.
8.17 In the past, Defence has experienced severe degradation of interconnection systems due to inadequate maintenance and/or poor husbandry. To ensure that interconnection systems integrity is maintained throughout the aircraft’s service life, electrical wiring interconnection system ICA must developed (as required) and include: mandatory replacement times for interconnection system components, enhanced zonal inspection requirements, tasks and intervals to reduce the likelihood of ignition sources, cleaning of interconnection system components, protection and caution information to prevent contamination of interconnection systems and so on.
8.18 Design Requirement (Essential). Where polyimide wiring insulation is used in Defence aircraft interconnection systems, requirements to ensure that polyimide wiring integrity is maintained in-service must be defined and promulgated.
8.19 Polyimide wiring insulation has historically been a challenge for Defence to manage, often due to the harsher environments in which Defence aircraft operate. This has resulted in degradation of the wiring insulation and loss of interconnection system integrity. The unique features of polyimide wiring insulation may demand different maintenance regimes to those promulgated in ICA for other wiring insulation. Therefore, the need for specific ICA for polyimide insulated wiring installed in Defence aircraft must be considered, and developed and promulgated where required.
8.20 Design Requirement (Essential). In-service management requirements must be defined and promulgated, for any lead-free solder used in Defence aircraft electrical systems or electrical/electronic equipment.
8.21 The use of lead-free solder in aircraft applications has highlighted a number of potential hazards related to the solder’s ability to maintain an effective electrical joint over time. Nevertheless, EU legislation has resulted in the adoption of lead free solder in many aircraft components, including wiring interconnection systems. Recent FAA research has highlighted the risks and challenges associated with management of lead-free solder, including reliability of solder joints, tin whisker susceptibility, maintenance/repair requirements, configuration control and obsolescence. Provided that lead free solder is appropriately managed, its use on Defence aircraft does not pose an airworthiness risk. Therefore, appropriate in-service management requirements must be defined and promulgated for any lead-free solder use on Defence aircraft.
8.22 Aircraft electrical power demands do not remain constant over the aircraft’s service life. All aircraft are subject to design changes and these changes often result in increased power requirements. To avoid costly retrofit of increased power generation capacity, potential future power requirements should be considered during initial aircraft design or major changes to the electrical supply and interconnection system, and a determination made whether the power generation and distribution systems provide an acceptable level of growth capacity. DEF STAN 00-970 requires an additional power generation capacity of at least 50% greater than the maximum continuous demand, and 10% growth capacity for interconnection systems. While not an Authority prescribed design requirement, these additional generating capacity and interconnection system requirements represent a reasonable benchmark for Defence aircraft.
8.23 Capability Design Requirement. Additional power generation and distribution capacity should be considered during initial aircraft electrical power system design, or major changes to the electrical supply system, to accommodate future potential growth in power demand.
8.24 Capability Design Requirement. Additional interconnection system capacity should be considered during initial electrical wiring interconnection system design, or major changes to the interconnection system, to accommodate future potential growth.
8.25 Further guidance on implementing the aircraft electrical systems design requirements prescribed in this chapter can be provided by the chapter sponsor.
A. Airworthiness Requirements for Equipment with Non-rechargeable Li-Ion Batteries