Sofema Online (SOL) considers key requirements related to Aircraft Material Selection & Flammability Requirements

Introduction

Material selection for Electrical Wiring Interconnection Systems (EWIS) is fundamental to the design and continued airworthiness of both small and large aircraft.

The responsibility lies with the Design Organisation to validate and justify every EWIS material based on functional, environmental, and flammability performance.

This commitment ensures the long-term safety, maintainability, and certifiability of aircraft electrical systems throughout the aircraft lifecycle.

Under the modernized CS-23 (performance-based) and the prescriptive CS-25, compliance requirements related to flammability, durability, environmental protection, and electromagnetic compatibility (EMC) demand careful engineering judgment, backed by robust testing and documentation.

Regulatory Foundations

CS-23 (Small Aircraft) – Performance-Based Approach

With Amendment 5, EASA restructured CS-23 to eliminate many prescriptive design rules, including the former CS 23.1359 (Electrical System Fire Protection) and CS 23.1365 (Electrical Wiring Protection).

Instead, these requirements are now addressed under high-level objectives, such as:

>> CS 23.2510 – Equipment, Systems, and Installations

“The equipment, systems, and installations must be designed and installed to ensure they function as intended under any foreseeable operating condition.”
This includes wiring systems and their resilience to fire, overheating, electromagnetic interference, and mechanical/environmental damage.

Compliance is achieved through adherence to industry standards (e.g., AS50881, FAA AC 23-17C, CS 23.2500) and by leveraging CS-25 guidance where appropriate.

Key Design Considerations Enabled by AS50881 AS50881 provides detailed tables and guidance for: Conductor material types (e.g., copper vs. copper-clad aluminium) Insulation types and temperature ratings Voltage rating and current-carrying capacity Minimum bend radius and mechanical strength This is critical in CS-23/25 compliance, where wire characteristics must support system loads, environmental conditions, and longevity (especially under fatigue or vibration loads in CS-23 commuter aircraft and all CS-25 aircraft).   Updated Clarification – CS-23 and System Safety (Post-2017 Amendment) The traditional CS 23.1309, which mirrored FAA 14 CFR 23.1309, has been removed from CS-23 following the 2017 EASA reorganization of the Certification Specifications for Normal Category Aircraft (now titled: “Normal, Utility, Aerobatic, and Commuter Aeroplanes”).   Current and Applicable Structure – CS-23 Amendment 5 and Onwards As of Amendment 5 (20 July 2017) and subsequent amendments, EASA restructured CS-23 into a performance-based and objective-driven standard. The prescriptive requirements like 23.1309 were removed and replaced with broader safety objectives. Key Replacement Objective:   CS 23.2500 – Safety “The aircraft must be designed so that systems and equipment, considered separately and in relation to other systems, perform their intended function throughout the operating life of the aircraft, without causing unacceptable risk to the aircraft or its occupants.”   This is now the primary safety-driven objective covering what was formerly detailed under CS 23.1309 (system safety assessments, failure classification, etc.).   How AS50881 Supports Compliance with CS-23 Post-Amendment AS50881 remains highly relevant, but it now supports compliance with performance-based objectives rather than prescriptive rules.   AS50881 helps demonstrate: Robust system architecture to avoid unacceptable risks per CS 23.2500. Safe wiring design that supports the functionality, segregation, and fail-safe integrity of critical aircraft systems. Minimized potential for wiring failures that could degrade system safety or interfere with required functionality. Maintainability and accessibility that support continued airworthiness and compliance with CS 23.2600 (Safety Assessment).   For CS-25 Aircraft – CS 25.1309 Remains Valid & To avoid confusion: CS 25.1309 remains active and prescriptive. AS50881 supports compliance with: CS 25.1309 – Equipment, Systems and Installations CS 25.1351–1365 – Electrical Systems CS 25.1701–1729 – EWIS Requirements

 

CS-25 (Large Aeroplanes) – Prescriptive Requirements

CS-25 remains more detailed and includes a dedicated Subpart H – EWIS, encompassing:

 >> CS 25.869 – Fire protection of electrical components

>> CS 25.1701–25.1733 – Full scope of EWIS design, protection, installation, inspection, bonding, and flammability compliance

>> Appendix F to CS-25 – Flammability testing requirements for materials including wiring insulation

These rules mandate material-specific performance validation and apply to both new type designs and major design changes.

Core EWIS Material Selection Criteria

Flammability Resistance

Materials must:

>> Self-extinguish and not propagate flame

>> Limit smoke and toxic gas emission

>> Resist arc tracking, burn-through, and flame spread

>> Pass tests under Appendix F, Part I(a)(3) (12-second vertical burn test for wire insulation)

Applicability to CS-23 Aircraft After the 2017 performance-based rewrite of CS-23: There is no direct equivalent to CS 25.1707. However, under CS 23.2500 (Safety) and CS 23.2600 (Electrical Systems), the design must demonstrate an equivalent level of safety. In practice, EASA DOAs use Appendix F Part I(a)(3) as a de facto standard to satisfy safety expectations, especially for commuter-class aircraft using complex EWIS.

This is especially important for high-power circuits and areas with fire exposure risk.

Environmental and Chemical Resistance

Materials must be:

>> Resistant to hydraulic fluids, fuels, oils, and cleaning agents

>> Able to operate across a wide temperature range (−55°C to +200°C, depending on application)

>> Resilient against moisture, UV, ozone, and corrosive atmospheres

Qualification data must reflect installation zones—e.g., pressurized cabin, unpressurized bays, or near engines.

Mechanical and Physical Performance

Designers must ensure that materials:

>> Maintain flexibility and tensile strength throughout their service life

>> Are abrasion-resistant and do not suffer cold flow or brittle cracking

>> Tolerate vibration, bending, and repeated movement

Harnesses must be protected with clamps, grommets, and wear-resistant coverings at penetration and interface points.

1. d) Electromagnetic Compatibility (EMC)

Material properties affect:

>> Shielding attenuation against EMI and HIRF (High-Intensity Radiated Fields)

>> Grounding and bonding effectiveness

>> Crosstalk prevention in signal and power wire bundles

EMC requirements are particularly critical in digital fly-by-wire and avionics-intensive aircraft.

1. e) Aging and Long-Term Durability

Long-term performance must be proven through:

>> Accelerated aging tests and historical service experience

>> Evaluation of insulation breakdown, thermal degradation, and chemical interaction

>> Resistance to UV exposure, thermal cycling, and humidity ingress

Certification and Compliance Responsibilities

Design Organisations approved under Part 21 Subpart J must:

>> Define material specifications in the Type Design, referencing qualified materials (e.g., AS22759 for wiring)

>> Include flammability qualification reports in the EWIS Compliance Plan (especially for CS-25)

>> Apply Design Risk Assessment (DRA) and Functional Hazard Assessment (FHA) methods to identify and mitigate EWIS-specific risks

>> Incorporate guidance from AMC 20-21 regarding wiring installation, protection, and separation

>> Align CS-23 performance objectives with CS-25 best practices for flammability and fire resistance where prescriptive rules are lacking

Common EWIS Material Compliance Pitfalls

>> Substituting non-qualified components (e.g., automotive or consumer-grade wiring)

>> Failing to verify batch-to-batch flammability consistency

>> Overlooking installation environment limitations (temperature or chemical exposure)

>> Using outdated or unsupported material test data

>> Inadequate documentation of wiring insulation specifications or EMI testing results

Best Practices

To achieve and sustain EWIS compliance:

>> Adopt aerospace-qualified materials (e.g., AS22759, MIL-W-5088 equivalents)

>> Work with suppliers who provide EASA Form 1 or FAA 8130-3 traceability

>> Use digital harness design and routing software to simulate clearances, thermal loads, and separation

>> Retain flammability and material compliance documentation in the compliance archive

Link material performance data with safety reporting and continuous improvement loops

Next Steps

For EWIS Classroom, Webinar or online training please see www.sasssofia.com and www.sofemaonline.com or email team@sassofia.com