AS6826 Tutorial and Learning

This advanced lesson is designed for engineers, certification specialists, fire-test technicians and aerospace programme managers who already understand the basic purpose and scope of SAE AS6826.

The lesson concentrates on four areas that frequently determine whether a fire-test campaign produces usable certification evidence:

1. Burner calibration.

2. Test validity.

3. Certification acceptance.

4. Practical laboratory implementation.

AS6826 was developed to reduce historic variation in aircraft powerplant fire testing. Earlier programmes could differ significantly in burner selection, flame calibration, test-article positioning, boundary conditions and pass/fail interpretation. AS6826 provides a more prescriptive methodology covering the standard flame, calibration equipment, test execution, representative operating conditions and component-specific acceptance criteria.

The central learning principle is:

A component cannot be qualified by an exposure that has not itself been demonstrated to be valid.

This does not mean burner calibration is more important than test-article representativeness. Both are essential. A correctly calibrated burner applied to an unrepresentative article produces weak evidence. Equally, a representative article exposed to an invalid or unverified flame does not complete a valid certification test.

Engineers should therefore treat flame qualification, test-article configuration, boundary conditions, functional monitoring and reporting as one controlled evidence chain.

After completing this lesson, the learner should be able to:

• Explain why flame temperature alone does not define fire-test severity.

• Describe the AS6826 temperature-calibration process.

• Describe the copper-tube heat-transfer-rate calibration method.

• Distinguish between pre-test calibration and post-test validation.

• Identify common conditions that can invalidate a fire test.

• Explain why burner position and orientation must match the calibration setup.

• Identify the minimum evidence expected in a fire-test plan and report.

• Explain the relationship between AS6826, FAA guidance and certification-authority approval.

• Assess whether a failed post-test validation affects the test conclusion.

• Recognise when project-specific authority engagement is required.

A fire-test result is only meaningful when the applied exposure is known, controlled and repeatable.

Historically, different laboratories could generate different thermal exposures while describing each flame as a nominal 2,000°F fire. Differences could arise from:

• Burner type and condition.

• Fuel specification.

• Fuel pressure and flow.

• Combustion-air supply.

• Burner cone geometry.

• Flame stability.

• Temperature instrumentation.

• Heat-transfer measurement.

• Burner orientation.

• Burner-to-test-article distance.

• Local airflow and cross-draughts.

• Soot accumulation.

• Data acquisition and averaging methods.

AS6826 addresses these variables through defined burner, instrumentation, calibration and positioning requirements.

The often-used statement, “the flame must be qualified before the component is qualified”, is useful but incomplete.

The complete principle is:

The burner, exposure geometry, test article, boundary conditions and acceptance criteria must all be controlled before the resulting evidence can support qualification.

An invalid burner calibration can cause:

• Under-testing, where the component receives insufficient thermal exposure.

• Over-testing, where the exposure is more severe than intended.

• Failure to reproduce the result at another laboratory.

• Disagreement over the meaning of the test data.

• Certification-authority rejection.

• Repeat testing.

• Additional cost and schedule delay.

AS6826 defines a standard flame with a minimum temperature of 2,000°F or 1,093°C.

Temperature calibration demonstrates that the flame reaches the required thermal condition across the calibrated region. It also provides evidence that:

• Combustion is stable.

• Fuel and air settings are suitable.

• The burner produces an acceptable temperature distribution.

• The exposure is not locally deficient.

• The selected burner orientation can be reproduced.

Temperature is necessary, but temperature alone does not define the energy delivered to the test article.

A flame can indicate the correct temperature while transferring too little or too much energy.

AS6826 therefore requires a separate heat-transfer-rate calibration using a continuously water-cooled copper-tube apparatus. The pre-test average heat-transfer rate must be at least:

• 4,500 Btu/h, or

• 1,319 W.

This value is a heat-transfer rate measured using the prescribed apparatus. It should not be described as a heat-flux-density value.

The calculation uses:

• Water mass or volumetric flow.

• Water density.

• Water specific heat capacity.

• Inlet water temperature.

• Outlet water temperature.

• The measured water-temperature rise.

The governing relationship is:

Heat-transfer rate = mass flow × specific heat capacity × temperature rise

AS6826 controls the physical relationship between the burner, calibration equipment and test article.

Exposure geometry includes:

• Burner orientation.

• Burner-cone dimensions.

• Burner exit-plane position.

• Standoff distance.

• Horizontal buoyancy offset.

• Thermocouple-rake position.

• Copper-tube position.

• Critical flame-impingement location.

• Airflow-hood configuration where used.

• Freedom from unintended obstructions or cross-draughts.

A flame may satisfy temperature and heat-transfer requirements at one position but not at another. The test article must therefore be placed at the same calibrated distance and relevant alignment used during calibration.

The individual temperature requirement applies to the average recorded by each thermocouple over the calibration period. A single instantaneous reading outside the nominal range does not automatically fail the calibration.

An individual average below 1,850°F is unacceptable. An average above 2,150°F may be accepted as conservative, but it represents a more severe exposure than the standard intends.

Before lighting the burner, verify:

• Burner identity and configuration.

• Burner-cone condition and dimensions.

• Fuel type and traceability.

• Fuel and air supply condition.

• Burner orientation.

• Airflow-hood configuration, where applicable.

• Thermocouple-rake condition.

• Copper-tube condition.

• Water system condition.

• Instrument calibration status.

• Data acquisition configuration.

• Burner, rake and copper-tube alignment.

• Unobstructed flame volume.

• Test-rig alignment.

• Safety and emergency-shutdown arrangements.

The burner cone should be clean and free from excessive internal soot. The external copper-tube surface must be cleaned before pre-test calibration.

The burner must operate for at least five minutes without adjustment before calibration.

During stabilisation, confirm:

• Stable ignition.

• Consistent combustion.

• Suitable fuel flow.

• Suitable combustion-air setting.

• No unacceptable flame instability.

• No external airflow disturbing the flame.

• No unplanned change in burner orientation or geometry.

The burner should not be positioned against the copper tube during the flame warm-up period. Unnecessary exposure accelerates soot accumulation and can affect heat-transfer measurement.

AS6826 uses a seven-thermocouple rake.

The thermocouples are:

• Exposed-junction Type K.

• Mineral-insulated and metal-sheathed.

• Nominally 1/16 inch or 1.5 mm sheath diameter.

• Fitted with an exposed tip of approximately 4 to 7 mm.

• Recorded at 0.5 Hz or faster.

The procedure includes:

1. Position the rake in the defined calibration location.

2. Expose the thermocouples for at least two minutes.

3. Record all seven channels for at least three minutes.

4. Calculate the average for each thermocouple.

5. Calculate the average of the five inner thermocouples.

6. Confirm each individual average meets the required band.

7. Confirm the inner-five rake average is at least 2,000°F.

If the burner is adjusted after either temperature or heat-transfer calibration, the affected calibration sequence must be repeated.

The copper-tube apparatus uses continuous water flow and precision temperature measurement.

Key controls include:

• Copper-tube outside diameter of 0.500 inch ±0.004 inch.

• Water flow of 1.00 US gal/min ±0.05 US gal/min.

• Inlet and outlet RTD or approved thermistor measurement.

• Instrument separation and apparatus geometry as defined by the standard.

• Initial temperature offset within the permitted range.

• Three-minute minimum recording duration.

• 0.5 Hz minimum sampling rate.

• Average pre-test heat-transfer rate of at least 4,500 Btu/h.

Thermocouples must not be used to measure the inlet-to-outlet water-temperature difference for the heat-transfer calculation. RTDs or compliant thermistors are required for that measurement.

After successful calibration:

• Do not alter the burner’s physical configuration.

• Do not change the fuel-flow setting.

• Do not change the combustion-air setting.

• Do not shut down the burner.

• Apply the required boundary conditions.

• Start video and data recording.

• Move the burner to the agreed test position.

• Start the test timer as flame application begins.

• Monitor the defined pass/fail criteria.

• Record unexpected behaviour and deviations.

Stage 6: Post-Test Validation

The burner must remain operating until post-test heat-transfer-rate validation is complete.

If the copper tube can be cleaned without shutting down or adjusting the burner:

• The three-minute average must be at least 4,500 Btu/h.

If the tube cannot be cleaned without shutting down the burner:

• The three-minute average must be at least 4,100 Btu/h.

There is no separate post-test temperature validation requirement under AS6826.

If the applicable post-test heat-transfer-rate threshold is not achieved, the standard requires the fire test to be repeated. Component performance alone does not rescue an exposure whose validity has not been demonstrated.

For a horizontal burner:

• The burner exit plane is positioned 4.0 inches ±0.2 inch from the critical part of the test article.

• The burner centreline is positioned 1.0 inch ±0.2 inch below the critical part to account for flame buoyancy.

For a vertical burner:

• The burner exit plane is positioned 4.0 inches ±0.2 inch below the critical part.

• The burner centreline is aligned with the critical test location.

The normal burner orientations are horizontal or vertical.

If an angled orientation is required for an unusual installation, the applicant should coordinate the proposed orientation and calibration method with the relevant certification authority.

Small changes can affect:

• Local flame temperature.

• Heat-transfer rate.

• Flame coverage.

• Impingement angle.

• Buoyancy effects.

• Heating of joints and penetrations.

• Backside ignition behaviour.

• Repeatability between tests.

Burner position should be measured, recorded and protected against movement caused by vibration, fixture distortion or thermal deformation.

An airflow hood may be used for flammable-fluid conveyance products such as hoses, tubes, couplings and fittings.

Typical AS6826 requirements include:

Where an airflow hood is used for the test, it must also be used during temperature and heat-transfer-rate calibration.

Airflow settings must not be changed after calibration or during the test. The ambient hood velocity is verified before burner operation because the burning flame itself will alter the local flow field.

AS6826 requires a burner capable of producing the defined standard flame.

The standard identifies a modified gun-type oil burner with an appropriate extension cone. Burners derived from FAA Powerplant Engineering Report No. 3A are acceptable because they have demonstrated the ability to meet the required calibration conditions.

Other kerosene-based burners may be used where they meet AS6826 criteria. Examples referenced by the standard include:

• Carlin burners.

• Beckett burners.

• Marlin burners.

• Deatak burners.

• FAA Sonic Burner, subject to suitable configuration and control.

A burner model is not acceptable merely because its name appears in historical guidance. The actual burner configuration must demonstrate compliance with the AS6826 calibration requirements.

Examples of acceptable burner fuels include:

• Jet A or Jet A-1 meeting ASTM D1655.

• JP-8 meeting MIL-DTL-83133.

• Commercial kerosene meeting ASTM D3699 Grade 2.

• Equivalent national fuel specifications where justified.

Fuel identity, specification and batch control should be recorded in the test documentation.

A fire-resistant test requires at least five minutes of flame exposure.

At flame removal:

• The burner remains operating.

• Required fire-intended functions are demonstrated.

• Applicable boundary conditions continue.

• Video and timing continue until residual flames extinguish.

• The backside is monitored for at least two minutes after burner removal.

• Post-test heat-transfer-rate validation is completed before burner shutdown.

A fireproof test requires 15 minutes of continuous flame exposure.

For a typical fireproof sequence:

• Engine-running boundary conditions apply during the first five minutes.

• Required changes to shutdown or windmilling conditions begin no earlier than five minutes.

• The transition must start by five minutes and ten seconds.

• The transition should be completed as quickly as practicable.

• The transition must be complete by six minutes.

• The flame remains applied for the full 15 minutes.

• Applicable shutdown conditions continue after burner removal until residual flames extinguish.

• The backside is monitored for at least two minutes.

• Post-test validation is completed before burner shutdown.

A different time-dependent boundary-condition profile may be used where defined in the test plan and accepted for the certification programme.

A defensible AS6826 result depends on the integrity of the complete chain.

Failure in any critical link can weaken or invalidate the certification evidence.

A small positional error can change the exposure at the critical location.

Control: Use recorded dimensional checks before calibration and testing. Confirm that vibration or thermal distortion cannot change the standoff distance.

Adjusting fuel flow, airflow or burner geometry breaks the link between calibration and test exposure.

Control: Lock settings after successful calibration. If an adjustment is made, repeat the required calibration.

Damaged, oversized, crossed or contaminated thermocouple tips can distort temperature readings.

Control: Inspect each thermocouple before use. Confirm exposed-junction geometry and traceable calibration status.

Individual sample excursions are sometimes confused with averaged acceptance criteria.

Control: Calculate the average for each of the seven channels over the full calibration period and separately calculate the inner-five rake average.

RTDs, thermistors, flow meters and data acquisition channels can drift.

Control: Maintain calibration traceability and complete system checks. Where practicable, calibrate temperature probes with the associated acquisition system.

Pressure instability and inconsistent fuel condition can change burner output.

Control: Monitor the supply, document the fuel and maintain stable burner settings.

Cross-draughts can deflect the flame and alter calibration.

Control: Protect the calibration and test area from uncontrolled airflow. Where a hood is used, verify and lock the defined airflow.

Soot and surface deposits affect heat transfer.

Control: Clean the external tube before pre-test calibration. Inspect and maintain the internal water path. Check tube diameter at the required maintenance interval.

Describing 4,500 Btu/h as heat flux creates technical ambiguity.

Control: Use “heat-transfer rate” for the copper-tube measurement. Use heat-flux density only where the referenced method genuinely measures energy per unit area.

A technically successful exposure may be unusable if the evidence is incomplete.

Control: Record setup, calibration, configuration, boundary conditions, data, observations, deviations, photographs, video and witnesses.

AS6826 provides methodologies and pass/fail criteria developed with industry and certification-authority participation. Current FAA guidance should be checked for the exact status of AS6826 within the applicable certification programme.

Use of AS6826 does not remove the applicant’s responsibility to:

• Identify applicable regulations.

• Propose the means of compliance.

• Establish representative boundary conditions.

• Define component-specific criteria.

• Control conformity.

• Resolve deviations.

• Obtain certification-authority acceptance.

AS6826 provides a common technical framework that may support FAA, EASA and other certification programmes. However, test-data acceptance remains programme specific.

The applicant should not assume that one authority’s acceptance automatically binds another authority.

Historical fire-test evidence may remain useful where:

• The tested configuration is sufficiently similar.

• The historical method is understood.

• Calibration evidence is available.

• Boundary conditions remain representative.

• Differences are assessed.

• The certification authority accepts the proposed use.

A historical pass result should not be treated as automatically equivalent to an AS6826-compliant test.

AS6826 is an SAE Aerospace Standard providing powerplant fire-test methods, burner-calibration requirements, boundary-condition guidance and component-specific pass/fail criteria.

It was developed to reduce variation in powerplant fire testing and provide a more harmonised, repeatable and certification-focused methodology.

The standard was developed by the SAE A-22 Fire Protection and Flammability Testing Committee following FAA tasking initiated in 2018. Industry representatives and aviation certification authorities contributed to its development.

The published standard was issued in April 2025.

It covers powerplant and APU components and installations, including firewalls, nacelle hardware, fluid systems, controls, fire detectors, extinguishing lines, electrical equipment and related assemblies.

No. AS6826 addresses powerplant fire-protection testing under a severe calibrated flame. It should not be confused with cabin-material flammability methods.

It brings standardised calibration, setup, boundary-condition and acceptance methods into one structured framework intended to reduce variation between laboratories and certification programmes.

AS6826 provides a structured method for generating fire-test evidence for applicable powerplant and propulsion-system fire-protection requirements. The current FAA advisory material and project certification basis must be checked to confirm the accepted means of compliance.

Yes, it can support an EASA certification programme where the proposed compliance method and evidence are accepted by EASA or the relevant approved organisation.

No. Certification acceptance remains dependent on the applicable regulations, agreed means of compliance, configuration, boundary conditions, deviations and authority review.

A typical evidence package includes:

• Approved or agreed fire-test plan.

• Applicable regulations and compliance matrix.

• Test-article conformity records.

• Representativeness justification.

• Burner and fuel records.

• Temperature-calibration data.

• Heat-transfer-rate calibration data.

• Instrumentation and calibration records.

• Boundary-condition data.

• Test data.

• Photographs and video.

• Post-test inspection records.

• Post-test heat-transfer validation.

• Deviation records.

• Final test report.

• Witness record.

• Authority dispositions where required.

Potentially, but acceptance is not automatic. The applicant should align the test strategy with all intended authorities before testing.

The deviation must be recorded and its effect on test validity assessed. Certification-authority disposition may be required before the evidence can be accepted.

Conformity demonstrates that the test article and, where required, the setup match the approved configuration. Without configuration control, the relationship between the test result and the certified design may be unclear.

A fireproof test applies the standard flame for at least 15 minutes while the test article satisfies all applicable functional, structural, containment, leakage and ignition criteria.

A fire-resistant test applies the standard flame for at least five minutes while the test article satisfies the applicable fire-intended functions and acceptance criteria.

Firewalls are normally required to be fireproof. The applicable regulatory requirement and approved compliance strategy govern the final test definition.

No. The required duration depends on the system architecture, shut-off-valve position, function and applicable regulation. Some fluid-carrying components may require 15-minute performance.

Potentially, where it executes an approved safe shutdown and continues to satisfy its fire-intended function. The acceptance criteria must be defined before testing.

Burn-through is physical damage, melting or deformation that creates a hole through the test article.

Flame penetration is passage of flame through an opening or path in the test article or installation. It is not identical to burn-through.

No. Residual flames may be acceptable where they remain limited, do not propagate, do not compromise the fire-intended function and self-extinguish within two minutes.

No. Flashing or sparks on the backside of a firewall are considered an ignition or combustion event and are unacceptable.

AS6826 primarily uses a modified gun-type liquid-fuel burner capable of producing the defined standard flame. Other accepted burner configurations may be used where they satisfy the standard’s requirements.

It is a calibrated flame with a minimum temperature of 2,000°F or 1,093°C and a minimum heat-transfer rate of 4,500 Btu/h or 1,319 W.

Temperature calibration confirms that the burner produces the required thermal condition across the calibrated flame region.

Temperature alone does not establish how much energy is transferred. Heat-transfer-rate calibration confirms that the flame delivers at least the required thermal power to the prescribed apparatus.

A seven-channel exposed-junction Type K thermocouple rake is used. Data is recorded for at least three minutes at 0.5 Hz or faster.

A continuously water-cooled copper-tube apparatus is exposed to the flame. Water flow and inlet-to-outlet temperature rise are used to calculate heat-transfer rate.

No. It is a heat-transfer-rate value measured using the prescribed copper-tube apparatus.

It confirms that burner output did not deteriorate during the fire test.

The average must be at least 4,500 Btu/h where the tube is cleaned. If the tube cannot be cleaned without shutting down the burner, the permitted threshold is 4,100 Btu/h.

No. AS6826 requires post-test heat-transfer-rate validation, not a separate post-test temperature validation.

No. After successful calibration, the burner remains operating with unchanged settings through testing and post-test validation.

The standard identifies:

• Induced vibration.

• Loading.

• Differential pressure.

• Backside airflow.

• Commanded or controlled function.

• Fluid pressure.

• Fluid flow.

• Fluid temperature.

• Fluid-reservoir quantity.

• Component rotational speed.

• Test fluid.

No. Every listed condition must be considered, but only applicable conditions are applied. The rationale for applicability or non-applicability should be documented.

Not necessarily. The applicant must assess the normal operating ground and flight envelope and determine the worst-case steady-state value for each applicable boundary condition.

Induced vibration is expected for many test articles. It may be omitted where all defined exclusion criteria are met or where the certification authority accepts a justified alternative.

Vibration can remove charred or degraded material, expose fresh material to the flame, affect leakage, loosen moving parts and change seal or composite behaviour.

Yes. A fireproof test may transition from engine-running conditions to shutdown or windmilling conditions after the first five minutes.

Potentially. However, combining separate worst cases can create an unrealistically severe configuration or alter the failure mechanism. The engineering rationale must be reviewed carefully.

ISO 2685 provides aircraft fire-resistance test methods for components, equipment and structures in designated fire zones.

AS6826 provides a more prescriptive powerplant-focused framework covering:

• Burner requirements.

• Temperature calibration.

• Heat-transfer-rate calibration.

• Post-test validation.

• Test-article representativeness.

• Worst-case boundary-condition selection.

• Fire-intended functions.

• Component-specific pass/fail criteria.

• Fire-test planning and reporting.

The applicable certification basis determines which standard or combination of methods should be used.

FAA AC 20-135 historically provided broad guidance on powerplant fire-protection testing.

AS6826 adds a more detailed methodology for:

• Burner configuration.

• Calibration equipment.

• Data acquisition.

• Temperature averaging.

• Heat-transfer-rate calculation.

• Post-test validation.

• Boundary-condition determination.

• Residual-flame assessment.

• Component-specific acceptance.

• Documentation.

AS6826 supplements certification guidance. It does not supersede applicable regulations.

Not in every certification programme. It may be used as an accepted means of compliance where recognised by the applicable authority. Alternative methods may be proposed, but they require adequate technical justification and authority acceptance.

Potentially. Their acceptability depends on the applicable regulations, historical evidence, configuration similarity, method equivalence and certification-authority agreement.

It provides a current, repeatable and certification-focused method that addresses known weaknesses in historical variation between laboratories and programmes.

A representative firewall panel completes a 15-minute fire exposure without burn-through, flame penetration or backside ignition.

After the exposure, the post-test heat-transfer-rate validation is below the applicable AS6826 acceptance threshold.

Has the firewall passed the certification fire test?

No valid pass has been demonstrated.

The firewall’s observed performance is not sufficient on its own. The validity of the fire exposure must also be demonstrated.

If the applicable post-test heat-transfer-rate threshold is not achieved, the burner output may have deteriorated during the exposure. The test cannot establish that the firewall received the required standard flame for the complete duration.

Under AS6826, an unacceptable post-test heat-transfer-rate validation requires the fire test to be repeated.

A successful-looking test article does not compensate for an invalid test environment.

One thermocouple records several individual samples below 1,850°F, but its three-minute average is 1,930°F. Has that channel failed?

Answer: Not solely because of the individual samples. The acceptance criterion applies to the average over the three-minute period. The complete rake requirements must still be satisfied.

The burner is adjusted after temperature calibration to increase heat-transfer rate. Can the earlier temperature calibration still be used?

Answer: No. Burner adjustment can affect both temperature and heat transfer. The relevant calibrations must be repeated.

A component passes a five-minute exposure, but the required operational function was not monitored. Is the evidence complete?

Answer: No. Where the fire-intended function forms part of the acceptance criteria, failure to monitor or demonstrate it leaves an evidence gap.

The test article moves during vibration and reduces the burner standoff distance. Is the result automatically conservative?

Answer: No. The change breaks the controlled relationship between calibration and testing. It may create local effects not represented by the approved method and must be treated as a deviation.

The copper tube cannot be cleaned after the fire test without shutting down the burner. What validation threshold applies?

Answer: The uncleaned-tube average threshold of 4,100 Btu/h applies, provided the prescribed procedure is followed.

A firewall has no burn-through, but brief sparks are observed on the backside. Has it passed?

Answer: No. Backside flashing or sparks constitute an ignition or combustion event and are unacceptable.

A project uses a unique hybrid-electric propulsion configuration. Can AS6826 be applied without authority discussion?

Answer: It may provide a useful technical basis, but the unique configuration and any conditions outside the standard’s normal scope should be coordinated with the certification authority.

• Burner calibration and test-article representativeness are both essential.

• Temperature does not define fire severity by itself.

• AS6826 uses a prescribed heat-transfer-rate calibration.

• The 4,500 Btu/h value is not a heat-flux-density value.

• Calibration geometry must be reproduced during testing.

• Burner settings must remain unchanged after successful calibration.

• Post-test heat-transfer-rate validation is a test-validity requirement.

• An unacceptable post-test validation requires repeat testing.

• Boundary conditions must be based on the justified worst case, not merely typical operation.

• Fire-intended functions must be defined before the test.

• Pass/fail criteria are component specific.

• Complete documentation is part of the compliance evidence, not an administrative afterthought.

• Certification-authority acceptance remains project specific.

This tutorial was developed using the following enterprise source material:

• SAE AS6826, issued April 2025.

• AS6826 Powerplant Fire Test Standards_2024-08-23 (1).docx. This was treated as developmental reference material and checked against the issued standard.

• AS6826 Powerplant.pptx. Technical statements were corrected where the training draft used “heat flux” for the AS6826 copper-tube heat-transfer-rate measurement.

• AS6826 Fire Testing vs AC 20-135.docx.

• SAE-AS6826.aspx. This page is currently identified in the source as draft content, so it was used only as supporting context rather than as the primary technical authority.

• Aircraft-Fire-Testing-to-ISO-2685,-AC-20-135-&-RTCA-DO-160-Section-26.aspx.

• FAA AC 20-135 Change 1.

• ISO 2685:1992.

• FAA Powerplant Engineering Report No. 3A, revised March 1978.

• SAE AIR1377.