Aircraft electrical wires, cables, harnesses and connectors installed in designated fire zones may be required to continue performing defined safety functions while exposed to a representative powerplant fire.
Fire testing evaluates more than whether insulation burns. Depending on the applicable requirement, it can assess conductor continuity, insulation resistance, leakage current, connector functionality, firewall integrity, behaviour under vibration and the ability to support safe engine or auxiliary power unit shutdown
ISO 2685:1992 contains a detailed normative method for single-core, multicore and screened cables. It also defines separate requirements for mated electrical connectors and connector assemblies installed through firewalls. Its electrical method combines a standard flame with vibration and live electrical monitoring.
ISO 2685 was developed to evaluate the ability of aerospace components to withstand exposure to a standardised fire environment representative of an engine fire. For wire harnesses, the objective is not simply to determine whether the insulation burns. The test assesses whether the harness can continue to perform its intended function while subjected to a severe flame exposure.
For manufacturers, this testing provides evidence that critical electrical systems can maintain integrity long enough to support safe shut down of the engine an continued aircraft operation to the certification requirements.
Aircraft wire, harness and connector fire testing is the controlled exposure of representative electrical hardware to a calibrated fire environment while defined electrical and mechanical conditions are applied.
The testing can assess:
Conductor continuity.
Core-to-core insulation.
Core-to-screen insulation.
Leakage current.
Connector contact continuity.
Connector insulation leakage.
Performance under simultaneous vibration.
Resistance to flame penetration through a firewall.
Backside ignition.
Residual flame behaviour.
Continued performance of a defined fire-intended function.
Resonate supports aerospace fire testing programmes globally from our site in the UK.
Our engineers work with customers developing:
Whether supporting a new aircraft programme, engine certification campaign or product qualification activity, our team can assist with test planning, standards review, instrumentation design and test execution.
Aircraft fire testing requirements vary significantly depending on:
Early engineering review helps ensure the selected programme represents the intended installation and generates meaningful certification evidence.
We can review:
Aircraft electrical wires, cable assemblies, harnesses and connector systems installed within designated fire zones must demonstrate the ability to maintain critical safety functions when exposed to a severe powerplant fire environment. Compliance testing is therefore a fundamental requirement for airworthiness certification, system qualification and continued operational safety.
Selecting the Appropriate Aerospace Fire Test Standard
The applicable fire test standard depends on the component, system function, installation environment and certification objectives. Testing may be required for:
What Does Aerospace Fire Testing Evaluate?
Modern aerospace fire testing extends far beyond simple flammability assessment. Depending on the governing specification, testing may evaluate:
The objective is to demonstrate that electrical systems can continue to perform their intended safety functions during and after exposure to a representative aircraft fire environment.
ISO 2685 Fire Testing Requirements
ISO 2685:1998, Aircraft Environmental Test Procedures for Airborne Equipment – Resistance to Fire in Designated Fire Zones, remains one of the most widely referenced standards for evaluating aerospace electrical systems exposed to fire.
The standard provides detailed test methods for:
A key feature of ISO 2685 testing is the combination of a calibrated aviation fire environment with simultaneous vibration loading and live electrical monitoring. This approach provides a realistic assessment of system survivability under conditions representative of an engine nacelle or APU fire zone.
prEN 3475-408 Development of Cable Fire Testing
The draft prEN 3475-408 (April 2023) expands upon established ISO 2685 methodologies and introduces additional requirements intended to improve repeatability and installation representativeness.
Key developments include:
As this document remains a draft standard, it contains work-in-progress provisions and unresolved technical content. Any use of prEN 3475-408 should therefore be agreed between the customer, certification authority and test laboratory before testing commences.
EWIS and Installation-Level Fire Qualification
A critical consideration often overlooked during certification is that component qualification alone may not demonstrate installation compliance.
SAE AS6826 emphasises that qualification to a wire, cable, connector, TSO, military specification or component-level aerospace standard does not automatically qualify the installed system. The complete Electrical Wiring Interconnection System (EWIS) must be assessed in the context of its installed function and environment.
This includes evaluation of:
Consequently, aerospace manufacturers and certification authorities increasingly require installation-level fire testing to demonstrate that the complete EWIS continues to perform its intended function when exposed to powerplant fire conditions
| Standard / Guidance | Primary Focus | Typical Relevance to Testing |
|---|---|---|
| ISO 2685:1998 | Fire resistance of aircraft electrical cables and connectors | ISO 2685:1998 cancelled and replaced the 1992 edition. The 1998 edition retained the common fire environment but changed the status of some product-specific annexes. Annexes C, D and E became informative rather than normative pending separate product standards. This distinction matters. A customer specification, approved test procedure or certification plan may still invoke the product-specific conditions. Their contractual or certification status should be established before testing. The April 2023 prEN 3475-408 draft normatively references ISO 2685:1998 for the fire environment, burner arrangements and calibration methods. |
| prEN 3475-408:2023 | Aerospace cable fire testing | The April 2023 draft specifies a method for testing the fire resistance of fireproof electrical cables. It was intended to supersede EN 3475-408:2005. Its identified significant technical change is the introduction of liquid-burner requirements and the associated test configuration. The reviewed document explicitly states that it: Is not yet a European Standard. Was distributed for review and comment. Was subject to change. Must not be referred to as a completed European Standard. |
| SAE AS6826 | EWIS fire protection guidance | SAE AS6826 standardises powerplant fire-test methods, calibration, boundary conditions and pass/fail criteria. For electrical wires and connectors, it particularly affects: Test-article representativeness. Fire-intended functions. Induced vibration. Standard-flame application. Residual flames. Firewall interfaces. Boundary conditions. Test planning and reporting. Reconciliation between component qualification and installed compliance. AS6826 identifies a non-resonant frequency between 30 Hz and 60 Hz at a minimum acceleration of 4g for electrical cables and connectors. |
| RTCA DO-160 | Environmental qualification | May be referenced as part of broader aerospace qualification programmes |
| Draft SAE ARP8998 | — | The 1 May 2026 draft ARP8998 is intended to provide a consistent approach to engine and APU EWIS fire-protection compliance and to supplement AS6826. It states that where EWIS fire testing is required, the testing must comply with AS6826. Because ARP8998 remains a draft, its content should be treated as developing recommended practice rather than an issued SAE requirement. |
| FAA AC 20-135 Change 1 | Aircraft fire protection compliance | FAA AC 20-135 Change 1 removed the previously listed propane and oxy-acetylene options for powerplant component testing. The FAA concluded that propane does not represent a combustible fluid likely to exist in an engine and that a propane burner may produce a less severe test than a kerosene burner using a representative combustible fluid. This does not automatically invalidate historical approvals. It does affect the burner strategy for new or revised powerplant compliance programmes. |
Why do most modern aircraft still use long-established wire constructions?
Most of the currently used wiring was tested using propane burners. These were qualified before FAA AC 20-135 Change 1 (Powerplant Installation and Propulsion System Component Fire Protection Test Methods, Standards, and Criteria), issued 11 October 2018. This change explicitly removed guidance that had previously allowed the SAE 401 propane burner and stated that propane does not represent combustible fluids likely to exist in an aircraft engine fire. The FAA concluded that kerosene burners produce a more severe and representative fire environment. From the EASA side, the corresponding shift occurred through EASA’s acceptance and harmonisation with SAE AS6826.
The fire environment generated by a kerosene burner is extremely severe and wire systems must not only survive direct flame exposure, but also maintain the required electrical performance and system functionality.
Resonate has performed testing of wires of various standards and confirm that it remains extremely challenging to produce conductors that will pass ISO2685 testing with a Kerosine burner. As a result, many engine and airframe and system manufacturers continue to specify proven wire technologies with extensive service history rather than qualifying entirely new wiring systems from new suppliers. For this reason, demonstrating successful ISO 2685 compliance remains a significant milestone for any new aerospace fire zone wire or harness design.
Resonate Testing’s fire laboratory is equipped to perform demanding aerospace fire test programmes, including:
Throughout testing, we can monitor electrical continuity, insulation performance, functionality and other customer-specific parameters to determine how the harness performs during and after flame exposure.
| Specification | Capability |
|---|---|
| Testing Types | Wire, Cable, Harness, Connector & Firewall Testing |
| Monitoring | Continuity, Insulation Resistance, Leakage Current |
| Functional Assessment | Live Electrical Monitoring Available |
| Engineering Support | Test Planning & Programme Development |
| Standards Support | ISO 2685, SAE, FAA, EASA & Customer Requirements |
| Reporting | Detailed Engineering Reports & Traceable Data |
| Integrated Testing | Fire, Environmental, Vibration & Mechanical Testing |
| Project Support | Qualification, Certification & Development Programmes |
– Review standards, requirements, acceptance criteria and intended function.
– Define instrumentation, monitoring methods and test article configuration.
– Prepare wires, harnesses, connectors or firewall assemblies for test.
– Conduct calibrated fire exposure in accordance with the agreed standard.
– Measure continuity, insulation resistance, leakage current and functionality.
– Generate a detailed report including achieved conditions, observations and compliance evidence.
An insulated single metallic conductor without a metallic covering, sheath or shield.
Two or more insulated conductors in a common covering, conductors twisted or moulded together, or an insulated conductor with a metallic shield or outer conductor.
An assembly of wires, electrical or optical cables and terminations designed so that it can be installed and removed as a unit.
Wires, cables or harnesses routed and supported together for part of the installation.
Accurate aerospace fire testing requires careful control and verification of test conditions.
Prior to testing, Resonate works with customers to define:
During testing, systems may be monitored continuously to evaluate key performance parameters such as:
This provides detailed performance data throughout the exposure period rather than relying solely on post-test inspection.
Where required, testing can also be integrated with vibration, environmental or mechanical loading to better represent operational aircraft conditions.
Every aerospace programme has unique certification challenges. Some customers require formal qualification testing against recognised standards, while others need development-level assessments to guide design improvements.
Resonate supports:
Whether assessing a single connector or a complete aircraft wiring system, our engineers help ensure the test programme provides meaningful technical evidence aligned with project objectives.
By combining fire testing expertise with broader environmental, vibration and mechanical testing capabilities, Resonate can support comprehensive aerospace qualification programmes from a single location.
Aircraft Electrical Wiring Interconnection Systems (EWIS) form the backbone of modern aircraft, carrying power, control signals, communications and data throughout the airframe. Where wiring systems pass through engine compartments, APU installations, equipment bays and designated fire zones, they may be required to demonstrate continued functionality during fire exposure.
EWIS fire testing evaluates the ability of wiring systems to maintain critical electrical functions while subjected to representative aerospace fire conditions. Testing may be conducted on individual wires, cable bundles, complete harnesses, connector assemblies or integrated electrical systems depending on programme requirements.
Resonate Testing supports EWIS fire testing programmes for aircraft manufacturers, engine manufacturers, aerospace suppliers and system integrators. Testing can include real-time monitoring of electrical continuity, insulation resistance, leakage current and functional performance throughout the fire exposure period.
Whether you require formal ISO 2685 qualification testing, development trials for a new aircraft electrical system, or support investigating fire protection performance, Resonate Testing can help.
Contact our team to discuss your aircraft wire harness fire testing requirements and develop a programme tailored to your certification and engineering objectives.
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Whether you’re looking to contact us for the first time or have another testing requirement, we’d love to hear from you.
