Sofema Online (SOL) considers the key drivers for changing the approach to Aircraft Electrical Wiring Interconnect Systems

Introduction - The Legacy View of Aircraft Wiring

Historically, aircraft wiring was largely regarded as a secondary system — a “fit and forget” element. Design Engineers focused on mechanical, hydraulic, and avionics performance, often neglecting wiring integration risks.

The Aviation Industry Lacked Standardised Requirements for:

• Wire separation and segregation

• EMI (electromagnetic interference) protection

• Installation quality assurance

• Wire ageing and degradation assessment

• Human factor considerations during maintenance

The Evolution of EWIS

Reflects a major regulatory learning curve driven by real-world tragedies. Both FAA and EASA have transitioned from passive oversight to active, system-based regulation. EWIS is now a core system with its own design standards, safety framework, and human factor considerations.

A successful EWIS approach now integrates:

• Early design review and hazard identification

• Proactive service data monitoring

• Training for all EWIS stakeholders

• Ongoing engagement between design, maintenance, and safety teams.

This evolution underscores a broader regulatory philosophy shift — from compliance as box-checking to compliance as an enabler of robust safety outcomes.

Catalyst Events: High-Profile Failures That Changed Everything (see separate case studies)

• TWA Flight 800 (1996)  - The Investigation pointed to a short circuit in the centre fuel tank wiring that caused an ignition source.

• Swissair Flight 111 (1998) – The Investigation identified Arcing and thermal events associated with IFE (In-Flight Entertainment) wiring bundles and flammable MPET insulation blankets.

FAA Response: FAR 25 Subpart H & AC 25.1701–25.1733 (Harmonised with JAR 25 later CS 25)

The FAA introduced a dedicated EWIS regulatory structure under 14 CFR Part 25, Subpart H, enacted in 2007:

• Key Provisions:

>> §25.1707: Segregation of EWIS from flammable lines and other critical systems

>> §25.1709: Functional hazard assessment for EWIS failures

>> §25.1711: Identification and labelling of EWIS components

>> §25.1729: Instructions for Continued Airworthiness (ICA) specifically for EWIS

• Advisory Circulars (ACs):

>> AC 25.1701-1 through AC 25.1733-1 provide detailed implementation guidance

>> Emphasise flame resistance, EMI/EMC control, maintainability, and human performance issues

The FAA's response made wiring safety a design consideration from concept to retirement — not just a maintenance concern.

EASA Response: CS-25 Amendments & AMC 20-21 / AMC 20-22

EASA harmonised with FAA by implementing CS-25 Amendment 6 (2009), closely aligning with FAR 25 Subpart H. Key elements include:

• CS 25.1701–25.1733: Mirroring FAA structure with European context

• AMC 20-21 (EWIS Design and Installation):

>> Covers bundling, clamping, routing, grounding, and shielding

>> Requires separation distances and considerations for fluid contamination, chafing, and mechanical stress

• AMC 20-22 (EWIS Training):

>> Introduced the EWIS Target Group Training Framework

>> Recognises that wiring safety is affected by human interaction during design, installation, and maintenance

EASA also mandates that EWIS be integrated into the certification and compliance matrix during initial aircraft design under Part 21 Subpart J, ensuring design organisations address EWIS risks in their Design Risk Assessment (DRA) and integrate feedback from service experience and safety performance data.

Design Accountability: The Role of DOAs and DERs

EASA – Design Organisation Approval (DOA):

• Must embed EWIS into compliance verification, with a specific focus on:

>> Installation architecture decisions (routing, segregation, redundancy)

>> Safety data integration from in-service feedback

>> SMS integration for wiring-specific hazards

>> Human factors engineering — accessibility, error tolerance, inspection feasibility

FAA – Designated Engineering Representatives (DERs):

• Responsible for showing compliance with Subpart H provisions.

• Must provide substantiation of insulation durability, shielding effectiveness, and maintainability.

Cross-Domain Challenges

• Aging Aircraft: Many pre-EWIS regulation aircraft still in service require retrofitting or enhanced maintenance programs.

• Training Gaps: EWIS understanding among engineers and technicians remains uneven. Mandatory training only began in earnest after 2009.

• OEM Oversight: Poor OEM guidance on IFE integration or post-delivery modifications has led to non-conformities.

• Continuing Airworthiness Documentation: Often fragmented, missing specific ICA requirements for wiring systems.

Next Steps

For EWIS Classroom, Webinar or online training, please see Sofema Aviation Services and Sofema Online  or email team@sassofia.com.