• CS 23.2510 (Amendment 5 – 2020) – Equipment, systems, and installations
• CS 23.2515 – Electrical and electronic system lightning protection
• CS 25.1309 – Equipment, systems and installations
• CS 25.1353 – Electrical equipment and installations
• CS 25.1707 / CS 25.1711 – EWIS-related physical separation and protection requirements
• AMC 25.1709 & AMC 25.1309 – EWIS failure assessment and safety analysis
>> Critical systems (e.g., flight control wiring, essential power) must be physically separated to avoid single-point failures.
>> CS-25.1707 requires adequate physical separation of EWIS elements based on system criticality.
>> For CS-23 (post-Amendment 5), the focus is performance-based, requiring that installations do not jeopardise continued safe flight.
>> Electrical arc tracking, insulation deterioration, connector degradation, and wire chafing are all potential failure modes.
>> Degradation over time due to environmental exposure (moisture, heat, vibration) must be considered in material selection and routing strategy.
>> For CS-25 aircraft, compliance with 25.1309(a)–(d) necessitates systematic analysis of failure effects, including cascading failures triggered by shared routing.
>> Incorrect bundling or proximity to flammable fluid lines, oxygen systems, or control cables can create hazardous interactions.
>> CS-25.1707(c) mandates routing decisions that prevent simultaneous damage to redundant systems from localised hazards (e.g., overheat or hydraulic burst).
>> No single failure must result in a catastrophic condition.
>> Failure propagation due to wiring faults must be prevented via isolation devices, circuit protection, and independent routing.
Fault Tolerance Defined:
Fault tolerance refers to a system’s ability to continue safe operation despite failures within individual components or subsystems, especially wiring systems.
CS-25.1309 & AMC 25.1309:
This is the cornerstone regulation for assessing fault-tolerant architecture in CS-25 aircraft:
• Requires systematic safety assessments.
• Fault-tolerant strategies must reduce the probability of hazardous or catastrophic conditions to acceptable levels:
>> Major: ≤ 10⁻⁵
>> Hazardous: ≤ 10⁻⁷
>> Catastrophic: ≤ 10⁻⁹ per flight hour
CS-23 has moved from prescriptive rules to a performance-based approach.
• CS 23.2510 requires systems to perform their intended function under all foreseeable operating conditions and not to introduce unacceptable risk.
• Fault tolerance is expected via system architecture, protective devices, and failure containment.
>> Routing redundant EWIS paths through spatially segregated zones.
>> Independent power sources, breakers, and fuses prevent common-mode failure.
>> Circuit breakers and arc fault detection limit fault propagation.
>> Use of shielding and grounding to prevent EMI-induced faults.
>> Facilitates troubleshooting and reduces human-induced secondary failures.
>> Fault-tolerant designs support easier inspection and less intrusive maintenance actions.
>> Incorporating conservative design margins (e.g., current carrying capacity well below max limits) improves long-term reliability.
>> Insulation and wire types selected for thermal, mechanical, and electrical robustness.
Design Phase Challenges:
• Under CS-23, performance-based rules may create ambiguity in demonstrating compliance for fault tolerance without clear AMC guidance.
• For CS-25, demonstrating system-level compliance with 25.1309 often requires exhaustive Functional Hazard Assessments (FHA), Failure Modes and Effects Analysis (FMEA), and Fault Tree Analysis (FTA).
Wiring Degradation:
• Ageing aircraft pose major compliance and safety risks due to insulation degradation or unknown modifications.
• Fault tolerance can degrade with time, necessitating lifecycle assessments and predictive maintenance.
Software & System Integration Risk:
• EWIS design interacts with integrated modular avionics (IMA), which increases potential for fault propagation if not managed through zoning and isolation principles.
Approval Considerations:
• Authorities require documented demonstration of compliance:
>> Testing for EMI, flammability, fault isolation.
>> Evidence of segregation and independence in the System Safety Assessment (SSA).
• In CS-23, lacking detailed AMC can result in delays unless supported by robust design assurance processes.
Summary Takeaways:
• System segregation and EWIS failure prevention are critical to the airworthiness of CS-23 and CS-25 aircraft.
• CS-25 takes a more prescriptive and rigorous approach, while CS-23 relies on performance-based compliance, requiring strong design rationale and safety analysis.
• Fault-tolerant design decisions must address both physical separation and functional independence to prevent hazardous failure propagation.
• Design Organisations must maintain a system-wide perspective, integrating EWIS early in the architecture phase and supporting the design with full lifecycle assurance documentation.
For EWIS Classroom, Webinar or online training, please see Sofema Aviation Services and Sofema Online or email team@sassofia.com.
