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Powerplant fire testing has changed substantially over the past six decades.
Early methods concentrated on reproducing a severe flame environment around fluid-carrying aircraft components. Modern programmes must do much more.
They must control burner performance, thermal severity, test geometry, operating conditions, equipment function, boundary conditions, instrumentation, acceptance criteria and certification evidence.
The journey from FAA Powerplant Engineering Report No. 3A to SAE AS6826 reflects this progression.
It is not simply a history of burner designs.
It is the story of how aerospace fire testing developed from hardware-based instruction into a more repeatable, performance-focused and hazard-led engineering discipline.
| Period | Development | Central contribution |
|---|---|---|
| 1961 | FAA Powerplant Engineering Report No. 3A | Standardised apparatus and hose fire-test procedure |
| Mid-1970s | Original Lennox burner became unavailable | Created a burner-equivalence and repeatability problem |
| 1977 | FAA-RD-76-213 | Characterised burner performance and assessed alternatives |
| 1978 | Revised Engineering Report 3A | Formalised alternative burners and performance controls |
| 1970s onward | SAE AS1055 and AIR1377 | Converted FAA concepts into industry test and equipment standards |
| Later development | ISO 2685 1992 and 1998 | Provided an international aircraft fire-zone test framework |
| Legacy FAA guidance | AC 20-135 and Change 1 | Connected test methods with powerplant certification objectives (SAE AS401B propane burner dropped) |
| 2018 | FAA tasking to SAE | Initiated development of a more harmonised standard |
| April 2025 | SAE AS6826 issued | Established a prescriptive powerplant fire-test framework |
| September 2025 | FAA AC 20-135A issued | Recognised AS6826 as an acceptable means of compliance |
This is not a perfectly linear family tree.
Different documents served different products, authorities and applications. However, the technical continuity is clear. Repeatable thermal severity, representative conditions and defensible evidence became increasingly important.
FAA Powerplant Engineering Report No. 3A introduced a standard fire-test apparatus and procedure for flexible aircraft hose assemblies.
The test was not limited to placing a hose in a flame.
It attempted to represent several conditions that could influence hose performance:
a severe powerplant fire environment
operational fluid pressure
fluid flow
elevated fluid temperature
engine-related vibration
controlled airflow
defined burner position
representative hose mounting
The original procedure used a modified Lennox OB-32 conversion oil burner.
Its central technical features included:
a nominal 2,000°F fire environment
defined burner geometry
controlled hood airflow
vibration during exposure
fluid circulation under specified conditions
thermal calibration using a water-cooled copper-tube device
a minimum heat-transfer-rate requirement of 4,500 Btu/h
These controls represented a major advance.
The test environment was no longer defined only by visible flame or burner identity. It included measurable thermal performance and representative operating stresses.
Flexible hoses and other fluid-system components can become part of the fire hazard.
A failed line may release fuel, oil or hydraulic fluid into an existing fire. The resulting leakage can increase fire intensity, extend the duration or allow the fire to spread.
The safety objective was therefore not merely:
Does the hose material burn?
The more important questions were:
Can the assembly contain its fluid?
Can it maintain the required function?
Does it resist leakage during fire exposure?
Does vibration create a failure path?
Can the system remain safe long enough for isolation or shutdown?
Does the component aggravate the existing hazard?
This is the foundation of hazard-minimisation thinking.
The component is judged by its contribution to aircraft safety, not only by its physical appearance after exposure.
| Classification | Typical minimum exposure | Broad safety intent |
|---|---|---|
| Fire resistant | 5 minutes | Maintain the required function during the period needed to recognise, respond to and isolate the fire hazard |
| Fireproof | 15 minutes | Withstand the longer fire exposure required for components, structures or systems that must continue providing protection |
The classification is not selected by the laboratory.
It follows from:
the applicable regulation
the aircraft or engine installation
the component function
the hazard assessment
the certification basis
the agreed compliance strategy
A five-minute exposure is not automatically suitable because a component is small.
A 15-minute exposure is not automatically required because the fire appears severe.
The applicable safety objective determines the classification. The standard defines how the selected requirement is demonstrated.
The original Report 3A procedure depended on the modified Lennox OB-32 burner.
By the mid-1970s, that burner was no longer commercially available.
This created a serious practical problem.
New laboratories could not simply purchase the prescribed equipment. Existing facilities faced maintenance and replacement risks. Manufacturers needed confidence that tests conducted using another burner would remain comparable with established certification evidence.
The problem could not be solved by selecting a burner that looked similar.
Different burners can produce different:
flame shapes
temperature distributions
heat-transfer rates
oxygen concentrations
fuel-air ratios
combustion stability
soot deposition
impingement patterns
The real requirement was not to reproduce the appearance of the Lennox burner.
It was to reproduce the fire environment.
FAA-RD-76-213, Re-evaluation of Burner Characteristics for Fire Resistance Tests, was commissioned to characterise the original burner and identify acceptable alternatives.
The investigation assessed measurable characteristics including:
flame temperature
temperature distribution
heat-transfer rate
flame geometry
thermal energy
oxygen concentration
burner stability
The research considered modified versions of burners including:
Carlin 200 CRD
Stewart-Warner HPR-250
Stewart-Warner FR-600
The most important change was philosophical.
The industry moved from asking:
Which burner model is named?
to asking:
Does the burner produce the required thermal environment?
That is the essence of performance-based equipment qualification.
A listed burner model is not automatically compliant. Its modification state, operating settings, geometry and measured output must be controlled.
Similarly, an alternative burner is not automatically unacceptable. It may be suitable if its performance is demonstrated against the governing criteria.
The revised March 1978 Engineering Report incorporated the burner-characterisation work.
It retained the test’s original purpose while providing a route for using alternative modified burners capable of producing the required flame characteristics.
The revision addressed:
acceptable burner arrangements
burner modification
flame-temperature profiles
heat-transfer-rate performance
burner positioning
airflow
vibration
fluid-system operating conditions
test sequence
calibration or standardisation activity
This reduced dependence on one obsolete item of commercial hardware.
More importantly, it preserved the technical intent of the test.
The principle remains relevant:
Certification continuity should be protected by controlling performance, not by preserving obsolete equipment for its own sake.
The Report 3A lineage reveals two related but distinct activities.
This demonstrates that a burner design, modification and setup can reproduce the required flame characteristics.
This confirms that the actual burner achieves the required output before use in a test campaign.
The distinction matters.
Owning a Carlin 200 CRD does not prove that the burner:
has the correct modifications
is correctly maintained
uses the required nozzle
operates at the correct fuel and air settings
produces the required flame profile
meets the heat-transfer requirement
is positioned correctly
remains stable during testing
Burner identity is only one part of test-equipment control.
FAA research and Report 3A practice influenced industry standards addressing hose and fluid-system fire testing.
SAE AS1055 addresses fire testing of:
flexible hose assemblies
tube assemblies
coils
fittings
similar system components
It translated the hose-test philosophy into an industry standard that could be applied through customer specifications, Technical Standard Orders and certification programmes.
SAE AIR1377 addresses fire-test equipment for flexible hose and tube assemblies.
It covers matters such as:
burner designs
airflow hood arrangements
vibration equipment
heat-transfer-rate measurement
burner conversion details
equipment setup
Together, these documents helped laboratories reproduce a recognised fire-test environment without relying solely on informal facility knowledge.
However, they should not be treated as universal methods for every modern powerplant component.
Their relevance depends on the article, applicable standard, certification basis and approved test plan.
ISO 2685 provides an international environmental test procedure for aircraft components, equipment and structures installed in designated fire zones.
Its framework includes:
standard flame requirements
fire-resistant and fireproof classifications
defined exposure periods
burner calibration
test-article configuration
operating conditions
vibration where applicable
functional monitoring
pass criteria
ISO 2685 shares broad technical principles with FAA powerplant fire-testing practice.
These include:
severe controlled flame exposure
measured thermal performance
defined duration
representative mounting
operational conditions
functional requirements
repeatable evidence
However, similar objectives do not establish technical equivalence.
ISO 2685 and FAA or SAE methods can differ in:
burner types
calibration terminology
measurement equipment
heat-transfer definitions
article dimensions
vibration requirements
component-specific methods
acceptance criteria
reporting expectations
ISO 2685 forms part of the wider historical framework. It should not be described as the direct origin of every later SAE requirement.
FAA AC 20-135 expanded the discussion beyond flexible hose assemblies.
It provided guidance for demonstrating fire-protection compliance for powerplant installations and propulsion-system components.
The Advisory Circular addressed matters including:
fireproof and fire-resistant requirements
standard fire severity
burner selection
flame coverage
representative operating conditions
fluid pressure and flow
structural loading
vibration
airflow
component function
installation-specific boundary conditions
acceptance criteria
certification documentation
This was an important development.
Engineering Report 3A principally described apparatus and a test procedure. AC 20-135 connected laboratory activity to the regulatory safety objective.
The central question became:
Does the test provide evidence that the component or installation satisfies the applicable fire-protection requirement?
That connection prevents the test standard from being treated as the certification requirement itself.
Regulations establish the required safety outcome. Advisory material and test standards provide methods for generating evidence. The applicant, certification authority and approved compliance strategy determine whether that evidence is acceptable.
AC 20-135 provided valuable guidance, but programmes could still differ in their interpretation and implementation.
Differences could arise in:
burner configuration
flame calibration
flame positioning
test-article dimensions
fixture design
heat sinks
pressure and flow conditions
vibration requirements
airflow
load application
functional monitoring
pass and fail criteria
post-test burner verification
reporting content
These differences could create technically significant variation between laboratories and certification programmes.
A component tested in one facility might not receive the same thermal exposure, boundary conditions or acceptance assessment in another.
The consequences could include:
repeated testing
additional certification issue papers
disagreement over test validity
difficulty reusing legacy data
increased programme cost
inconsistent severity
delayed certification decisions
This did not mean that historical testing lacked value.
It meant the industry needed a more prescriptive framework capable of reducing avoidable variation while preserving the need for engineering judgement.
In March 2018, the FAA tasked SAE International with developing industry standards to supplement AC 20-135 Change 1.
The work was undertaken through the SAE A-22 Fire Protection and Flammability Testing Committee.
The development involved industry and aviation certification-authority participation.
The objective was not merely to rewrite the existing Advisory Circular.
The new standard needed to address:
burner requirements
flame-temperature control
heat-transfer-rate measurement
calibration equipment
calibration procedures
test-article representativeness
boundary-condition selection
operating conditions
instrumentation
component-specific acceptance criteria
reporting and evidence requirements
This marked another important transition.
Powerplant fire testing was moving from broad guidance and programme precedent towards one more consistent laboratory framework.
SAE AS6826, Powerplant Fire Test Standards, was issued in April 2025.
It provides a structured framework for powerplant and Auxiliary Power Unit components and installations.
Its scope includes hardware such as:
firewalls and shrouds
nacelle and cowling components
fluid-handling components
hoses, tubes and fittings
controls and shut-off valves
fire seals and penetrations
fire-extinguishing lines
electrical wires and connectors
electronic engine controls
mechanical control systems
fire-detection equipment
AS6826 addresses several areas where historical programmes could differ.
The standard flame requires:
a minimum temperature of 2,000°F or 1,093°C
a minimum heat-transfer rate of 4,500 Btu/h or 1,319 W
The 4,500 Btu/h value is a heat-transfer-rate requirement measured using the prescribed water-cooled copper-tube apparatus.
It should not be described as heat-flux density.
The temperature-calibration method uses a defined thermocouple rake and controlled recording requirements.
The purpose is to demonstrate that the burner produces the required temperature distribution rather than one isolated hot point.
The burner must demonstrate the prescribed heat-transfer rate before testing.
Post-test validation then provides evidence that burner performance remained acceptable through the exposure.
The hardware should represent the production design, materials, thicknesses, joints, penetrations, manufacturing features, installation interfaces and relevant heat paths.
Depending on the component, the test plan may need to address:
vibration
structural load
fluid pressure
fluid flow
fluid temperature
differential pressure
airflow
rotational speed
electrical loading
commanded operation
shutdown function
reservoir quantity
mounting orientation
AS6826 does not use one universal pass criterion.
The applicable criteria may include:
burn-through
flame penetration
backside ignition
maintained structural load
leakage
gapping
residual flame
continued fire-intended function
commanded closure
safe engine shutdown
This is a major improvement over treating every component as though the same visual inspection could determine success.
FAA AC 20-135A was issued in September 2025.
It recognises SAE AS6826 as an acceptable means of compliance for applicable powerplant and propulsion-system fire-protection requirements.
It also cancels AC 20-135 Change 1.
This creates an important relationship:
| Document | Role |
|---|---|
| Document Role Airworthiness regulations | Establish the required safety outcome |
| FAA AC 20-135A | Provides FAA guidance and identifies acceptable compliance approaches |
| SAE AS6826 | Provides detailed powerplant fire-test methods and criteria |
| Qualification test plan or procedure | Applies the method to the particular component and installation |
| Test report and evidence package | Records what was tested, how it was tested and what happened |
| Certification finding | Determines whether the evidence supports compliance |
AS6826 is therefore an important modern test standard.
It is not a regulation.
It does not automatically approve a component, engine or aircraft installation.
Certification acceptance still depends on:
the applicable requirements
the approved means of compliance
the test configuration
the boundary conditions
the acceptance criteria
conformity
deviations
authority review
A successful test report supports certification. It does not itself grant certification.
| Area | Earlier approach | Modern AS6826 direction |
|---|---|---|
| Test equipment | Prescribed apparatus and named burner configurations | Controlled burner configuration meeting defined performance requirements |
| Thermal severity | Temperature and water-cooled copper-tube measurement | Detailed temperature and heat-transfer-rate calibration |
| Burner alternatives | Addressed through characterisation and equivalence studies | Burner acceptability tied to defined standard-flame performance |
| Test article | Strong focus on hose assemblies and related hardware | Wider range of powerplant and APU components and installations |
| Operating conditions | Pressure, flow, temperature, vibration and airflow | Structured boundary-condition assessment covering relevant service conditions |
| Acceptance criteria | Product or programme dependent | More detailed component-specific criteria |
| Documentation | Apparatus, procedure and result records | Controlled planning, calibration, conformity, data, deviations and reporting |
| Certification role | Test procedure supporting FAA evidence | Prescriptive method recognised through current FAA guidance |
| Engineering philosophy | Reproduce a severe fire and observe performance | Reproduce a controlled hazard while demonstrating defined fire-intended functions |
The development is significant, but the core engineering objective has remained consistent.
The test must expose representative hardware to a severe and controlled fire environment while applying the operating conditions that could influence safety performance.
A more prescriptive standard does not remove the need for engineering judgement.
The applicant must still determine:
which regulations apply
whether the component must be fire resistant or fireproof
what function must be maintained
which fire source and direction are critical
what installation conditions are representative
which boundary conditions create the worst case
what constitutes failure
what evidence the authority requires
whether legacy data remains applicable
More severe is not automatically more correct.
A test can be excessively severe yet unrepresentative. It can also be insufficiently severe because important operating loads, fluids, airflow or interfaces were omitted.
The correct test is the one that credibly represents the applicable hazard and generates evidence against the required safety objective.
The history of powerplant fire testing provides several practical lessons.
A visible flame is not sufficient evidence of a valid fire environment.
A burner can achieve a high temperature without transferring the required thermal energy to the article.
The temperature distribution and flame coverage affect the component response.
Pressure, flow, vibration, load, airflow, mounting and function can change the failure mechanism.
Pre-test calibration and post-test validation are not administrative details. They support the technical validity of the exposure.
The certification basis and hazard assessment determine what must be demonstrated.
The final evidence must still be accepted within the applicable certification or qualification process.
From Report 3A to AS6826, the equipment, calibration methods and documentation requirements have evolved.
The central safety principle has not.
A powerplant fire test must demonstrate that a component or installation can perform its required safety function during a credible fire hazard without creating an unacceptable additional risk.
That requires more than a burner.
It requires:
a controlled thermal environment
representative hardware
realistic operating conditions
defined fire-intended functions
measurable acceptance criteria
traceable instrumentation
disciplined execution
defensible evidence
The evolution of powerplant fire testing is therefore not simply a progression from one document to another.
It is the progression from prescribed hardware towards controlled performance, representative hazards and evidence that can withstand technical and certification scrutiny.
Understand the hazard. Control the fire environment. Represent the installation. Demonstrate the required function.
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