ISO 2685 defines the standard fire environment used to assess aircraft equipment, components and structures installed within designated fire zones.
A valid programme requires more than applying a flame. Burner calibration, test-article representativeness, vibration, operating conditions, functional monitoring and acceptance criteria must be agreed before testing begins.
ISO 2685 is an international aircraft fire-test standard for components, equipment and structures installed within designated fire zones. It defines a controlled fire environment and two grades of resistance to fire: fire resistant and fireproof.
The first edition, ISO 2685:1992, was issued on 15 July 1992. The second edition, ISO 2685:1998, was issued on 15 December 1998 as a technical revision that cancelled and replaced the 1992 edition. Both editions remain important because historical qualification reports, customer specifications, OEM procedures and legacy certification programmes may refer to either version.
The standard defines a flame temperature of 1,100°C ±80°C and an absorbed heat-flux density of 116 kW/m² ±10 kW/m². Fire-resistant equipment is assessed for a five-minute exposure. Fireproof equipment is assessed for a 15-minute exposure.
ISO 2685 is not a general cabin-material flammability standard. Its scope concerns equipment, components and structures contained within aircraft designated fire zones. It does not address general flammability requirements, fire resistance outside designated fire zones or combustion-chamber burn-through conditions.
The applicable aircraft or engine regulations determine whether an item must be fire resistant or fireproof. ISO 2685 provides a test method for generating evidence against that requirement. It does not replace the certification basis, customer qualification test procedure or certification-authority agreement.
ISO 2685 is an International Organization for Standardization test procedure titled:
Aircraft, environmental test procedure for airborne equipment, resistance to fire in designated fire zones.
It establishes:
The standard flame.
Fire-resistant and fireproof grades.
Burner arrangements.
Burner-calibration procedures.
General specimen mounting requirements.
Test duration.
Guidance for fluid-system components.
Guidance for electrical cables and connectors.
Guidance for structural components.
Acceptance by analogy in the 1992 edition.
ISO 2685 exists to:
Establish a common aircraft fire-test environment.
Improve comparability between fire-test programmes.
Define controlled flame-temperature and heat-flux conditions.
Support fire-resistant and fireproof demonstrations.
Provide test guidance for equipment installed in designated fire zones.
Support qualification and certification evidence.
Reduce variation in burner selection and calibration.
Encourage representative mounting and operating conditions.
| Programme stage | ISO 2685 application |
|---|---|
| Design | Identify designated fire zones, applicable regulations and fire-protection functions. |
| Development | Evaluate materials, joints, seals, penetrations and component configurations. |
| Qualification | Conduct fire-resistant or fireproof testing under an agreed procedure. |
| Certification verification | Generate test data supporting the approved means of compliance. |
| Design change | Assess whether modified materials, interfaces or operating conditions invalidate previous evidence. |
| Service support | Review legacy qualifications, supplier changes and continued applicability of historical evidence. |
ISO 2685 provides a controlled benchmark for assessing how aircraft equipment behaves in a severe fire environment.
It helps expose risks including:
Burn-through.
Flame penetration.
Structural degradation.
Loss of containment.
Hazardous fluid leakage.
Loss of electrical function.
Seal failure.
Connector failure.
Loss of fire-intended function.
Ignition on the protected side.
ISO 2685 can support compliance where it is accepted within the certification programme. The actual regulatory requirement comes from the aircraft or engine certification basis, not from ISO 2685 alone.
The compliance plan should connect:
The applicable regulation.
The required fire grade.
The component’s fire-intended function.
The selected ISO 2685 test method.
The operating boundary conditions.
The acceptance criteria.
The resulting test evidence.
Repeatability depends on controlling:
Flame temperature.
Absorbed heat-flux density.
Burner geometry.
Burner position.
Specimen mounting.
Flame coverage.
Exposure duration.
Vibration where applicable.
Fluid pressure and flow.
Functional loads.
Instrumentation.
Environmental airflow.
A test that controls only flame temperature does not adequately demonstrate the complete ISO 2685 fire environment.
The purpose of testing is not simply to show that a test article remains visibly intact. The relevant safety function may include:
Containing fire.
Preventing flame penetration.
Preventing hazardous leakage.
Maintaining electrical continuity.
Preserving insulation resistance.
Carrying structural load.
Maintaining a closed fluid path.
Remaining attached.
Protecting adjacent equipment.
Supporting safe engine shutdown.
ISO 2685 sits within a wider fire-protection framework that may include:
FAA AC 20-135 and its revisions.
FAA AC 20-135A.
FAA Powerplant Engineering Report No. 3A.
SAE AS1055.
SAE AIR1377.
SAE AS4273.
SAE AS5127/2.
SAE AS8028.
SAE AS6826.
RTCA DO-160 Section 26.
Aircraft or engine OEM specifications.
Customer qualification test procedures.
Applicable CS and 14 CFR fire-protection requirements.
ISO 2685 should not be described as the direct foundation of every later SAE fire standard. A more accurate position is that it forms part of the historical fire-test framework that later standards, including AS6826, sought to clarify or harmonise.
The approved programme may need to address:
Component installation.
Fire-source direction.
Critical flame-impingement location.
Burner orientation.
Number of burners.
Specimen dimensions.
Fixture heat sinks.
Joints and penetrations.
Seal compression.
Fluid type.
Fluid temperature.
Pressure and flow.
Electrical load.
Mechanical load.
Vibration.
Frontside or backside airflow.
Functional monitoring.
Post-test assessment.
Tailoring must not make the test less representative without adequate justification.
Before testing, the applicant should agree:
Applicable ISO 2685 edition.
Fire-resistant or fireproof classification.
Applicable annex or product method.
Qualification test procedure.
Test-article conformity requirements.
Burner type.
Calibration method.
Operating conditions.
Vibration requirement.
Monitoring requirements.
Acceptance criteria.
Required witnesses.
Report and data-package content.
| Area | ISO 2685:1992 | ISO 2685:1998 |
|---|---|---|
| Publication status | First edition | Second edition, technical revision |
| Relationship | Replaced ISO/TR 2685:1984 | Cancelled and replaced ISO 2685:1992 |
| Standard flame | 1,100°C ±80°C and 116 kW/m² ±10 kW/m² | Retained |
| Fire resistant | Five minutes | Retained |
| Fireproof | Fifteen minutes | Retained |
| Burner annexes | Integral | Annexes A and B normative |
| Product-specific annexes | Annexes C, D and E integral | Annexes C, D and E informative |
| General specimen conditioning | Explicit requirement for affected non-metallic specimens | Not retained in the same explicit general form |
| Acceptance by analogy | Included | Reduced or changed from the first-edition approach |
| Burner selection | General critical-area coverage logic | Revised burner-selection provisions |
The 1998 edition states that Annexes C, D and E were temporarily maintained for information pending development of standards for the specific products concerned. This distinction matters when deciding whether a vibration profile or product-specific condition is mandatory, informative or imposed through another controlled requirement.
ISO 2685:1992 required non-metallic components to be conditioned before testing unless evidence showed that aircraft-fluid exposure would not adversely affect fire resistance.
The standard identified 24-hour immersion at ambient temperature as an available conditioning method.
This could affect:
Elastomeric seals.
O-rings.
Gaskets.
Silicone materials.
Sealants.
Fire-barrier materials.
Non-metallic connector components.
Cable insulation.
Hose materials.
Composite or bonded constructions.
The second edition does not retain the same general specimen-conditioning clause found in ISO 2685:1992.
That does not establish a universal “dry test” rule. Fluid conditioning may still be required where:
The customer specification requires it.
The qualification test procedure requires it.
The product annex or related standard requires it.
Service exposure could affect material performance.
The design authority requires comparison with historical evidence.
The certification authority considers it necessary.
Actual ISO 2685 projects continue to use both conditioned and unconditioned specimens depending on the controlled test matrix.
The correct question is not simply:
“Does ISO 2685:1998 mandate fluid immersion?”
The correct question is:
“Could realistic fluid exposure alter the fire performance of this test article, and what does the approved programme require?”
This is particularly important for elastomers, silicone compounds, seals, cable insulation and fluid-system components.
The standard flame temperature is:
1,100°C ±80°C
Temperature calibration confirms that the flame achieves the required thermal condition at the defined location.
The required absorbed heat-flux density is:
116 kW/m² ±10 kW/m²
ISO 2685 uses a standardised water-cooled measuring apparatus. Heat-flux density and flame temperature are separate parameters. Both must be controlled.
| Grade | Exposure duration | General purpose |
|---|---|---|
| Fire resistant | Five minutes | Demonstrate limited-duration resistance while meeting the required function. |
| Fireproof | Fifteen minutes | Demonstrate extended fire resistance without unacceptable failure. |
Duration alone does not define success. The specimen must also meet the agreed acceptance criteria.
Identify the certification basis.
Confirm the applicable ISO 2685 edition.
Define the fire grade.
Define the fire-intended functions.
Identify the critical flame locations.
Establish acceptance criteria.
Agree authority and witness requirements.
Prepare the qualification test procedure.
Produce a conforming or representative test article.
Confirm materials and production processes.
Replicate joints, penetrations and fasteners.
Apply required specimen conditioning.
Build the representative fixture.
Install instrumentation.
Configure pressure, flow, load and electrical conditions.
Configure vibration where required.
Complete safety reviews.
Inspect the burner.
Confirm burner identity and maintenance status.
Establish stable operation.
Calibrate flame temperature.
Calibrate absorbed heat-flux density.
Confirm flame coverage.
Record the burner settings.
Prevent unauthorised adjustment after calibration.
Apply required operating conditions.
Start data and video recording.
Position the burner at the controlled test location.
Begin the exposure timer.
Monitor the agreed functions and conditions.
Record leakage, flame behaviour and structural response.
Complete the required five-minute or 15-minute exposure.
Conduct any required post-test burner check.
Inspect for burn-through.
Review flame penetration.
Assess protected-side ignition.
Review leakage.
Assess structural integrity.
Review electrical or functional data.
Examine residual burning.
Review all deviations.
Complete the test report.
Attach calibration data.
Record the achieved test conditions.
Include photographs and video references.
Document conformity.
Record deviations and dispositions.
State results against each acceptance criterion.
Update the compliance matrix.
The programme may need to control:
Working pressure.
Fluid type.
Fluid temperature.
Flow rate.
Hose length and bend geometry.
Fitting position.
Vibration direction.
Leakage detection.
Shut-off conditions.
Post-fire containment.
The programme may include:
Electrical continuity.
Insulation resistance.
Dielectric performance.
Signal integrity.
Crosstalk.
Short-circuit detection.
Connector retention.
Flame propagation.
Vibration between 30 Hz and 60 Hz.
Customer-specific electrical loads.
ISO 2685 Annex D should be treated as a starting point. The qualification test procedure must define the required electrical acceptance criteria. Internal project evidence shows that customers may require vibration, cold-side video, electrical monitoring or fluid-conditioned specimens beyond a basic flame exposure.
Structural tests may need to represent:
Panel dimensions.
Joints.
Fasteners.
Penetrations.
Backing structure.
Pressure differential.
Cooling airflow.
Mechanical load.
Vibration.
Protected-side visibility.
Fire-induced distortion.
A simple flat material coupon may not represent a complete firewall or nacelle assembly.
Vibration can influence fire performance by:
Removing charred or degraded material.
Opening joints or seals.
Changing leakage paths.
Affecting electrical connectors.
Causing relative motion.
Exposing fresh material to the flame.
Changing load distribution.
Product-specific guidance historically includes:
| Test article type | Typical ISO 2685 guidance |
|---|---|
| Fluid-system components | 33 Hz with 1.6 mm minimum amplitude |
| Electrical cables and connectors | 30 to 60 Hz with 4 g minimum acceleration |
| Structural components | 0.4 mm amplitude near 50 Hz at a non-resonant frequency |
These values must be interpreted against the applicable edition, annex status, customer specification and approved test procedure. In ISO 2685:1998, Annexes C, D and E are informative rather than normative. A statement that vibration is always mandatory would therefore be misleading.
Poor burner calibration, incorrect instrumentation or an unrepresentative flame position can invalidate the result.
A successful laboratory exposure may still be rejected if the setup does not represent the certified installation.
Using ISO 2685:1992 requirements in a programme calling for ISO 2685:1998, or vice versa, can create avoidable disputes.
Dry test articles may overstate performance where service fluids degrade the material.
Omitting vibration without justification can hide leakage, joint or material-degradation mechanisms.
A component may remain physically intact but fail its required electrical, mechanical or containment function.
Missing calibration records, photographs, conformity evidence or deviation dispositions can make the test evidence unusable.
Historical evidence generated using a different burner, test edition or operating condition may not remain directly applicable to a modified product.
State the exact edition and revision in the test plan, quotation, procedure and report.
Replicate production materials, thicknesses, seals, joints, fasteners, penetrations and heat paths.
Avoid vague requirements such as “survive the fire”. State exactly what must remain functional or contained.
Review realistic exposure to fuel, oil, hydraulic fluid, cleaning agents and other aircraft fluids.
Do not treat a correct flame temperature as proof of correct absorbed heat-flux density.
Consider:
Pressure.
Flow.
Fluid temperature.
Airflow.
Electrical load.
Mechanical load.
Vibration.
Differential pressure.
Functional actuation.
Protected-side video can be essential for detecting ignition, penetration or hot-gas effects.
The test laboratory should not be expected to invent certification acceptance criteria after the test.
Resolve uncertainty over edition, burner, vibration, conditioning and equivalence before committing test hardware.
ISO 2685 is an aircraft environmental fire-test procedure for equipment, components and structures installed within designated fire zones.
It covers the standard flame, burner calibration, specimen mounting, exposure duration and fire-test guidance for relevant aircraft equipment.
It is an aircraft area formally identified as requiring specific fire-protection measures because flammable fluids, ignition sources or severe fire hazards may be present.
The standard flame temperature is 1,100°C ±80°C.
The absorbed heat-flux density is 116 kW/m² ±10 kW/m².
Fire-resistant testing uses a five-minute exposure. Fireproof testing uses a 15-minute exposure. The test article must also meet all applicable acceptance criteria.
The 1998 edition technically revised and replaced the 1992 edition. It changed the status of product-specific annexes, revised aspects of burner selection and did not retain the same general specimen-conditioning clause.
It removed the explicit general conditioning clause found in the 1992 edition. However, conditioning may still be required by the product method, customer specification or approved test procedure.
Aircraft fluids can cause swelling, softening, shrinkage, cracking or property changes that affect fire performance.
Examples include firewalls, nacelle structures, hoses, fittings, seals, cables, connectors, fluid components, controls and other fire-zone equipment.
Yes, where the firewall or structure falls within the applicable fire-zone requirement and ISO 2685 is an accepted method.
It depends on the component, applicable edition, annex status, customer specification and approved test procedure.
ISO 2685 is an international fire-zone test procedure. FAA AC 20-135 provides US certification guidance for powerplant fire-protection demonstrations.
AS6826 provides a newer and more prescriptive powerplant fire-test framework covering calibration, boundary conditions and component-specific criteria. ISO 2685 remains important for legacy and customer-specified programmes.
Yes. It remains widely referenced in legacy qualifications, customer procedures, OEM specifications and aircraft fire-zone test programmes.
Potentially, where the FAA accepts the method, configuration and evidence for the specific certification programme.
Potentially, but equivalence must be assessed. Burner type, calibration, test article, operating conditions and acceptance criteria may differ.
Common causes include burn-through, flame penetration, hazardous leakage, loss of function, protected-side ignition, seal failure, fixture movement or invalid burner calibration.
Resonate provides independent support for aircraft fire-test strategy, standards interpretation, test-article review, burner and boundary-condition planning, qualification evidence and certification engagement.
Confirm the applicable edition, installation conditions and acceptance criteria before committing hardware and test time. Early technical review can prevent an expensive test from producing unusable evidence.
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