Aerospace Fluid Fitting Fire Testing
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Fluid fittings are among the most critical fire-protection elements within aircraft powerplant installations.
A single fitting failure during an engine-compartment fire can turn a contained event into a continuing release of flammable fluid. This can increase fire severity, extend its duration and threaten adjacent systems.
A valid fitting fire test must therefore assess more than whether the metal body survives.
It must determine whether the complete assembly maintains containment and its required safety function while exposed to a controlled fire environment, representative fluid conditions and relevant mechanical loading.
The historical technical framework is built around four important references:
SAE AS1055, covering fire testing of flexible hose, tube assemblies, coils, fittings and similar system components
SAE AIR1377, covering fire-test equipment for flexible hose and tube assemblies
FAA Powerplant Engineering Report No. 3A, covering the standard apparatus and procedure originally developed for flexible hose assemblies
FAA AC 20-135 Change 1, providing historical powerplant fire-protection guidance
For current FAA programmes, AC 20-135A and SAE AS6826 must also be considered.
These documents are related, but they do not perform the same function.
Your Test Facilitator. Not simply a test facility.
Fluid fittings connect:
flexible hoses
rigid tubes
valves
pumps
heat exchangers
reservoirs
actuators
fire-extinguishing pipes
engine accessories
aircraft and engine interfaces
Within designated fire zones, these assemblies may carry:
fuel
lubricating oil
hydraulic fluid
pneumatic media
fire-extinguishing agents
A fitting may appear mechanically simple, but its fire performance can depend on several vulnerable features:
threaded joints – Loosening due to vibration, thread stripping, fatigue cracking, galling, leakage past threads
sealing faces – Leakage due to surface damage, scratches, erosion, distortion, insufficient contact pressure
ferrules – Slippage, pull-off, cracking, over-crimp damage, fatigue failure
olives – Relaxation, creep, cracking, incorrect deformation, loss of sealing force, leakage
brazed joints – Cracking, lack of braze contact area, void formation, corrosion, fatigue failure, leakage
welded interfaces – Weld cracking, lack of fusion, porosity, heat-affected zone failure, fatigue cracking, leakage
mechanical retainers
protective sleeves
adjacent hose material
differential thermal expansion
These features can create leakage paths before the main fitting body loses structural integrity.
The relevant question is not simply:
Did the fitting melt?
The stronger engineering questions are:
Did the fitting leak?
Did the seal relax or extrude?
Did the joint loosen under vibration?
Did the fitting maintain pressure?
Did fluid spray into the flame?
Did leakage aggravate the fire?
Did the assembly maintain its required function?
Did the failure occur within the required exposure period?
A fitting that remains visibly intact can still fail the safety objective through leakage.
Historical FAA guidance commonly describes two fire-performance classifications.
| Classification | Typical exposure period | Broad objective |
|---|---|---|
| Fire resistant | At least 5 minutes | Maintain the required function or containment during the period needed to recognise and respond to the fire |
| Fireproof | At least 15 minutes | Maintain the required protection or function during the longer defined fire exposure |
The associated fire environment has historically been based on a nominal 2,000°F or approximately 1,093°C flame.
The classification is not selected by the laboratory.
It is determined through:
the applicable aircraft or engine regulation
installation location
designated fire-zone status
fluid type
shut-off arrangements
continued fluid supply
component function
hazard assessment
certification basis
agreed means of compliance
A fuel fitting may have a different safety case from an oil fitting.
A fuel supply may be isolated after engine shutdown. An oil system may continue feeding fluid during windmilling or rundown. A hydraulic fitting may remain pressurised while an essential function is being completed.
The required classification must therefore follow the installation hazard.
The test standard provides the method. It does not decide the requirement.
The exact conditions must be taken from the controlled standard revision and approved test procedure.
A typical AS1055-style programme considers the following:
| Parameter | Typical consideration |
|---|---|
| Flame temperature | Approximately 2,000°F with the tolerance defined by the applicable revision |
| Heat-transfer rate | Verified using the specified water-cooled copper-tube device |
| Airflow | Controlled airflow through the test hood |
| Specimen length | Sufficient assembly length to represent the hose, tube and fitting |
| Bend configuration | Representative bend, commonly including a 90-degree hose bend where applicable |
| Flame coverage | Fitting and adjacent hose or tube exposed within the defined fire zone |
| Vibration | Applied to the fitting or assembly during exposure |
| Fluid temperature | Elevated to the required operating range |
| Fluid flow | Set from component type, internal diameter and applicable requirement |
| Pressure | Working or specified system pressure |
| Duration | Five minutes or 15 minutes, depending on classification |
| Acceptance | No prohibited rupture, leakage or loss of required function |
An older AS1055 revision specifies a test assembly of at least 24 inches, horizontal mounting, a 90-degree hose bend, a vibrated fitting within the flame and exposure of the fitting plus a defined length of adjoining hose or tube.
These details matter because a straight, unloaded fitting on an unpressurised line may not reproduce the relevant failure mechanism.
A fitting can be more vulnerable than the adjoining hose or tube.
Potential failure mechanisms include:
seal degradation
loss of preload
differential thermal expansion
braze deterioration
local softening
joint relaxation
firesleeve termination failure
leakage at the hose-to-fitting interface
mechanical loosening under vibration
If the burner primarily heats the middle of a hose while the end fitting remains outside the severe zone, the test may miss the critical failure location.
For fitting-centred qualification, the flame must challenge:
the fitting body
the fitting-to-hose or fitting-to-tube interface
sealing features
the relevant firesleeve termination
the most vulnerable installation orientation
The target position must be documented before testing.
“Put the fitting somewhere in the flame” is not an adequate qualification instruction.
SAE AIR1377 addresses the equipment used for flexible hose and tube fire testing.
Its technical importance lies in controlling the apparatus rather than defining the complete certification requirement.
AIR1377 covers areas such as:
burner configuration
burner conversion details
burner extension geometry
airflow hood
exhaust arrangement
vibration equipment
thermocouple positioning
flame-temperature measurement
water-cooled heat-transfer equipment
equipment setup
calibration procedure
The central principle is repeatability.
A fitting should not pass in one laboratory and fail in another simply because the burners create materially different thermal environments.
AIR1377 therefore helps define the equipment needed to reproduce a recognised fire exposure.
The airflow hood is an important part of the historical hose and fitting test arrangement.
It serves two principal purposes:
It controls airflow around the test specimen.
It helps contain and manage leakage if a flammable-fluid component fails.
The Report 3A and AS1055 lineage uses a hood with controlled air movement around the assembly. The modern AS6826 framework retains an airflow-hood arrangement for flammable-fluid conveyance products and identifies an airflow of approximately 400 ft/min, subject to the specified tolerance.
Airflow affects:
flame shape
combustion
heat transfer
flame attachment
fluid-vapour behaviour
leakage movement
secondary burning
test repeatability
The burner must be calibrated in the configuration used for the test.
If the airflow hood is present during the test, it must also be present during the applicable calibration activity.
Calibrating in still air and testing inside a high-flow hood may not reproduce the same exposure.
A flame can reach the required temperature while transferring insufficient energy to the fitting.
Conversely, a flame can create excessive local heating despite an apparently acceptable temperature reading.
A defensible fire test must control both:
flame temperature
thermal energy transferred to the calibration device
The Engineering Report 3A and AIR1377 lineage uses a water-cooled copper-tube device to assess heat-transfer rate.
The historical target is approximately 4,500 Btu/h, equivalent to about 1,319 W.
This value is a heat-transfer rate measured by the prescribed apparatus.
It should not be casually described as heat-flux density.
Temperature and heat transfer answer different questions:
| Measurement | Question answered |
|---|---|
| Flame temperature | Is the flame sufficiently hot across the required region? |
| Heat-transfer measurement | Is the flame delivering the required thermal energy to the calibrated device? |
Both are needed to characterise the exposure properly.
FAA Powerplant Engineering Report No. 3A introduced a coordinated test environment combining:
a modified oil burner
flame-temperature control
heat-transfer measurement
fluid circulation
internal pressure
elevated fluid temperature
assembly vibration
controlled airflow
defined burner position
representative specimen mounting
The original reference burner was a modified Lennox OB-32.
When that commercial burner became unavailable, the FAA assessed alternatives, including modified Carlin and Stewart-Warner configurations.
This created an important engineering principle:
The burner model alone does not define compliance.
A listed burner must still have:
the correct modification state
the correct nozzle
controlled fuel delivery
controlled airflow
correct extension geometry
stable combustion
verified temperature performance
verified heat-transfer performance
controlled burner-to-specimen geometry
Owning the named burner does not prove that it produces the required fire environment.
Fire alone does not fully represent the aircraft environment.
Vibration can expose failure mechanisms such as:
fitting loosening
seal movement
fretting
cracked joints
firesleeve movement
loss of brittle char
opening of thermally induced gaps
leakage through a heat-damaged interface
Historical AS1055 and AIR1377 methods apply vibration at approximately 33 Hz with the displacement defined by the applicable revision.
The vibration may be applied laterally or longitudinally.
The purpose is not to perform a complete aircraft vibration qualification at the same time.
It is to ensure that the fitting and assembly remain mechanically challenged while their materials and interfaces are being degraded by fire.
The approved test procedure must define:
vibration frequency
displacement or acceleration
direction
measurement location
control method
tolerances
fixture interface
start and stop sequence
A nominal shaker setting without evidence at the relevant control point is weak qualification evidence.
Fluid conditions can determine whether a fitting passes or fails.
Internal pressure challenges:
seals
threaded interfaces
braze joints
tube connections
hose-to-fitting attachments
thermally weakened walls
A depressurised test may miss a pressure-dependent leakage path.
Fluid flow can cool the fitting and adjacent hose.
High flow may therefore make the component easier to pass. Low flow can create a more severe thermal condition.
The selected flow must follow the applicable standard, component classification and approved test plan.
A hot fluid reduces the cooling margin within the assembly.
It can also affect:
seal properties
viscosity
vapour generation
leakage behaviour
ignition potential
internal heat transfer
Historical AS1055 methods commonly use elevated oil temperatures within the range defined by the standard revision.
The actual operating fluid can influence the test outcome through:
boiling point
vapour pressure
heat capacity
viscosity
flammability
chemical compatibility
seal interaction
Modern AS6826 guidance states that the system operating fluid should normally be used for components carrying liquids. SAE No. 20 oil substitution is retained for conveyance products historically tested under AS1055, subject to the programme requirements.
A substitute should not be selected merely because it is convenient.
For an aerospace fluid fitting, failure is not limited to complete rupture.
Potential failure criteria include:
visible leakage beyond the permitted limit
fluid spray
sustained burning of leaked fluid
rupture
separation
loss of pressure
inability to maintain required flow
loss of commanded function
seal extrusion
fitting loosening
firesleeve displacement
flame propagation
aggravation of the fire hazard
The exact pass or fail criteria must be agreed before the test.
The laboratory should not be expected to invent certification acceptance criteria after exposure.
A strong test plan defines:
permitted leakage
prohibited leakage
detection method
pressure limits
flow limits
required function
test duration
abort criteria
observation method
post-test checks
reporting requirements
“Survived the fire” is not a measurable acceptance criterion.
Leakage can be difficult to assess during a severe kerosene-burner test.
The flame, airflow, smoke and glare can obscure small but important releases.
Detection may therefore include:
direct visual observation
pressure monitoring
flow monitoring
reservoir-level monitoring
downstream optical detection
video from several angles
defined leakage collection
post-test pressure or leak checks
The chosen method must be appropriate to the fluid and failure mode.
A small gaseous leak may need a different detection approach from a hydraulic-fluid spray or a fuel leak that immediately ignites.
The report should distinguish:
seepage
weepage
droplets
continuous leakage
spray
rupture
ignited leakage
These terms should be defined in the test procedure where they affect acceptance.
A technically correct burner exposure can still produce poor evidence if the fitting is installed unrealistically.
The test arrangement may need to reproduce:
production fitting material
production joining process
correct torque
adjoining hose or tube
firesleeve and termination
bend radius
mounting restraint
orientation
adjacent heat sinks
firewall interface
pressure
flow
fluid temperature
vibration
mechanical load
shutdown conditions
The correct test article is not always the most convenient laboratory specimen.
A fitting installed in a short, highly restrained engine line may behave differently from the same fitting attached to a long, flexible hose.
A bulkhead fitting may need its surrounding structure represented.
A fitting passing through a firewall may need to be assessed both as a fluid connection and as part of the fire barrier.
A fitting fire test should begin with a signed, revision-controlled Qualification Test Procedure or equivalent approved test plan.
The document should identify:
| Area | Required information |
|---|---|
| Test article | Part number, serial number, revision, materials and production status |
| Requirement | Standard, revision, regulation, customer specification and classification |
| Configuration | Hose, tube, fittings, firesleeve, joints, mounting and orientation |
| Fire exposure | Burner type, fuel, calibration, target point and duration |
| Fluid system | Fluid type, temperature, pressure, flow and reservoir arrangement |
| Mechanical input | Vibration, loading, direction, control and tolerances |
| Monitoring | Temperatures, pressures, flows, leakage, function and video |
| Acceptance | Defined pass, fail and abort criteria |
| Quality controls | Conformity, calibration, witnesses, hold points and deviations |
| Evidence | Raw data, photographs, video references, observations and final report |
This review must happen before hardware and test time are committed.
A poorly defined test can be executed perfectly and still generate unusable evidence.
AC 20-135 Change 1, Engineering Report 3A, AS1055 and AIR1377 remain important historical and technical references.
However, current FAA practice has moved forward.
FAA AC 20-135A recognises SAE AS6826 as an acceptable means of compliance for applicable powerplant fire-protection requirements.
AS6826 introduces a more prescriptive approach to:
burner requirements
temperature calibration
heat-transfer calibration
test planning
representative hardware
boundary conditions
test fluids
operating conditions
fire-intended functions
component-specific pass or fail criteria
post-test validation
reporting
AS6826 also identifies AS1055, AIR1377 and Engineering Report 3A as important related material.
This does not make legacy AS1055 evidence invalid.
It does mean that new programmes should confirm:
the controlling requirement
the accepted standard revision
whether AS6826 applies
whether previous evidence remains technically applicable
whether the certification authority accepts the proposed method
Legacy precedent should be reviewed, not assumed.
This may remove the mechanism most likely to create hazardous leakage.
This can test the adjoining hose while missing the joint.
This may provide unrepresentative internal cooling.
Different fluids can change heat transfer, leakage and ignition behaviour.
Correct temperature does not prove the required heat-transfer performance.
Thermally degraded joints and seals may fail only when mechanically disturbed.
The termination can be the most vulnerable part of the assembly.
This creates ambiguity and weakens certification evidence.
The report provides evidence. The responsible applicant and authority determine compliance.
Provide the following where available:
applicable standard and revision
aircraft or engine regulation
component description
fitting part number
production drawing
material specification
adjoining hose or tube details
firesleeve details
intended installation
fire-zone location
required classification
exposure duration
fluid type
fluid temperature
working pressure
flow rate
vibration requirement
mounting and orientation
burner target location
leakage criterion
required operating function
specimen quantity
witnessing requirements
conformity requirements
reporting expectations
required accreditation status
If these details are not yet complete, the first task should be a requirements review.
Quoting a fitting fire test from the standard number alone is rarely sufficient.
Resonate Testing provides UKAS-accredited aircraft powerplant component fire testing under QS00118 for the scope listed in the current UKAS schedule.
The listed scope includes aircraft powerplant components, hoses, hose assemblies and fluid assemblies, with references including:
FAA AC 20-135
FAA Engineering Report 3A
ISO 2685
FAA Fire Test Handbook Chapters 11 and 12
SAE AS1055
SAE AIR/AS1377
SAE AS4273
applicable listed CS and FAR provisions
relevant listed Technical Standard Orders
RTCA DO-160G Section 26
Each enquiry remains subject to technical, safety, feasibility and accreditation review.
A proposed programme should confirm:
the exact standard revision
component type
test configuration
operating conditions
fluid hazards
vibration requirement
acceptance criteria
certification route
required evidence package
Our role is not simply to place a fitting in a flame.
It is to help customers define a credible test, reproduce the relevant conditions and generate evidence that can withstand engineering and certification review.
Control the flame. Represent the fitting. Apply the operating conditions. Measure the leakage. Protect the evidence.
Control the flame. Represent the fitting. Apply the operating conditions. Measure the leakage. Protect the evidence.
Use the contact route to share the requirement, test profile, drawings or specification for review.
Whether you’re looking to contact us for the first time or have another testing requirement, we’d love to hear from you.
