Sofema Aviation Services (SAS) considers key aspects related to EWIS Design Considerations
Electrical Wiring Interconnection Systems (EWIS) are a critical safety component of modern aircraft. Historically overlooked or treated as secondary to other systems, poor EWIS design has contributed to multiple catastrophic failures (TWA 800 – Swissair 111 –United Airlines (UAL) Flight 811 usually with devastating consequences.
Engineers must now prioritise EWIS from the earliest stages of aircraft design to ensure compliance, reliability, and above all, safety.
EWIS is the silent risk that, when neglected, has proven fatal. Engineers must accept that every decision—from routing and separation to documentation and maintainability—can either mitigate or magnify that risk.
Prioritising EWIS from the design phase is not only a regulatory obligation—it is a professional duty grounded in tragic aviation history.
History Shows That Poor EWIS Design Leads to Catastrophe
Numerous fatal accidents have demonstrated that design shortfalls around EWIS have deadly consequences:
- TWA Flight 800 (1996): A fuel tank explosion believed to be caused by arcing in degraded wiring.
- Swissair Flight 111 (1998): A fire above the cockpit ceiling spread rapidly due to arcing in IFE system wiring, igniting flammable insulation materials. The crew lost control due to the loss of systems and visibility, and the aircraft crashed with 229 fatalities.
- United Airlines (UAL) Flight 811 (February 24, 1989) experienced an explosive decompression at about 22,000 feet after the forward lobe cargo door suddenly opened in flight
These are not isolated cases; the link between poor EWIS design and catastrophic outcomes is well established.
Design Compliance Is Mandatory – CS 25 Subpart H
EASA mandates that all EWIS must meet strict certification standards, including:
- Physical separation from fuel, hydraulic, oxygen, and hot air systems.
- Electrical isolation between power sources.
- Mechanical protection to prevent abrasion, vibration-induced damage, or contamination.
- No shared ground points for independent power systems.
These requirements under CS 25.1701 to CS 25.1733 are not optional. A poor design that fails to meet them can result in the denial of certification and significant safety risks.
Design Shortfalls Can Undermine Safety Even if the Component Itself Is Compliant
As identified by ATSRAC, CS 25.1309 focuses on the reliability of components, but did not originally consider the wiring connecting them.
- This oversight contributed to ageing and degraded wiring systems going undetected until failure occurred. EWIS must now be assessed with the same rigour as any other flight-critical system.
Ageing and Maintenance-Induced Failures Begin at the Design Table
Wiring is subject to:
- Environmental degradation (heat, moisture, contamination).
- Collateral maintenance damage due to poor accessibility or routing.
- Vibration and chafing, especially near sharp edges or clamps.
These issues often arise from poor initial routing, inadequate protection, or a lack of maintainability considerations during design. Design decisions must account for the entire lifecycle of the aircraft—not just initial installation.
Instructions for Continued Airworthiness (ICA) Depend on Good Design
Under CRI H-01, designers must provide:
- Zone-specific EWIS inspection tasks.
- Material identification for combustible risks.
- Maintenance procedures to protect wiring from contamination and damage.
Without proper design inputs, ICA development becomes flawed, leaving operators exposed to undetected failure risks.
EZAP and Safety Management Must Be Built-In
The Enhanced Zonal Analysis Procedure (EZAP) is a tool to mitigate EWIS risks, but it only works if the original design:
- Identifies risk zones.
- Segregates wiring from ignition sources.
- Facilitates clear inspections.
Furthermore, design organisations under Part 21 Subpart J must integrate EWIS into the Safety Management System (SMS), proactively identifying and managing EWIS hazards throughout the aircraft lifecycle.
Human Factors Must Inform EWIS Design
Poor EWIS design contributes to maintenance errors:
- Needling of wires due to a lack of access.
- Incorrect routing and clamping due to unclear instructions.
- Missed damage due to lack of visibility or protection.
Designs that anticipate human interaction reduce risks, promote “protect and clean as you go” practices, and support safer maintenance environments.
Next Steps
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Sofema Aviation Services Provides Specialised EWIS Training for Design Organisations – Electrical Wiring Interconnection System (EWIS) and FAA-EASA Part 23-25 Certification – 4 Days. For details, please see SAS Website or email [email protected].
Tags:
Electrical Wiring Interconnect Systems (EWIS, Human Factors, Safety, SAS blogs, Instructions for Continued Airworthiness (ICA), Enhanced Zonal Analysis Procedure (EZAP), CS 25 Subpart H, Design Engineers, Design Phase, United Airlines (UAL), Design Compliance, IFE system wiring, Ageing and Maintenance
