Pressure, flow, viscosity and temperature describe important aspects of hydraulic operation.
Aerospace hydraulic fire-test integrity
A substitute hydraulic fluid may reproduce specified pressure, flow or viscosity. That does not establish that the fire test represents the aircraft system being qualified.In aircraft fire testing, the fluid is one part of an interconnected system. The test article, seals, gaskets, hoses, fittings, coatings, insulation, contamination state, operating conditions and installation configuration can all influence the result.
The correct engineering question is not simply:
Does the substitute produce similar hydraulic conditions?
It is:
Does the complete test configuration represent the approved Skydrol-equipped system and the hazard being assessed?
That is a much higher standard of evidence.
Hydraulic similarity is not qualification equivalence
Pressure, flow, viscosity and temperature describe important aspects of hydraulic operation. They do not, on their own, establish chemical, material or installation equivalence.
A surrogate may move through a circuit at the required pressure and flow. It may provide similar cooling under selected conditions. However, that does not demonstrate that it will interact with the test article in the same way as the specified phosphate ester fluid.
Nor does it demonstrate that the test article represents the production design.
A defensible qualification test must connect the tested configuration to:
• The applicable certification or qualification objective.
• The approved or intended production configuration.
• The specified fluid and material interfaces.
• The critical operating and boundary conditions.
• The required fire exposure.
• The agreed acceptance criteria.
• The applicable conformity and traceability arrangements.
The purpose is not merely to expose hardware to flame. It is to generate evidence about how the relevant aircraft component or installation performs under the defined fire hazard.
Current Resonate technical material treats test-article configuration, representative boundary conditions, fluid pressure, fluid temperature, flow, airflow, vibration, functional operation, instrumentation and acceptance criteria as interconnected parts of a defensible fire-test programme.
Skydrol is part of a fluid-material system
Skydrol is a family of phosphate ester aviation hydraulic fluids. Its function in a fire test cannot be reduced to pressure transmission alone.
The fluid may contact or operate alongside:
• Seals and O-rings.
• Gaskets.
• Hoses and hose liners.
• Fittings and joints.
• Coatings and finishes.
• Adhesives and sealants.
• Cable insulation.
• Protective sleeving.
• Metallic interfaces.
• Surrounding structural materials.
The response of the assembly depends on the compatibility and interaction of these elements.
Where the approved design specifies EPDM seals, EPDM gaskets or another phosphate ester-compatible material, the qualification article should represent that specification unless an alternative has been formally assessed and accepted.
This does not mean every Skydrol application universally requires EPDM. The controlling requirements are the approved design definition, component specification and applicable material approval.The important point is that seal material cannot be separated from the fluid when evaluating system behaviour.
A different fluid may produce different swelling, softening, shrinkage or degradation behaviour. A different seal material may alter leakage initiation or retention. The resulting leak may then change how much fluid enters the fire zone, where it enters and whether it forms a drip, stream, mist or pressurised spray.
The relevant qualification unit is therefore not simply the fluid.
It is the fluid-material-installation system.
Resonate’s current Skydrol technical position identifies seal response, hose response, coatings, insulation, decomposition, residues, leakage development and post-fire condition as potential differences that a hydraulically similar substitute may not reproduce.
Correct fluid does not compensate for incorrect hardware
Using the specified Skydrol is important where actual-fluid chemistry forms part of the hazard or evidence objective. However, using the correct fluid does not make an otherwise unrepresentative test valid.
A test may still be weakened by:
• Incorrect or unverified seals.
• Substitute gasket materials.
• Non-production hose construction.
• Unapproved coatings.
• Altered fittings or joints.
• Incorrect assembly processes.
• Unrepresentative mounting.
• Missing insulation.
• Different component modification status.
• Uncontrolled contamination.
Correct chemistry applied to a non-conforming assembly can still produce questionable qualification evidence.
The more complete engineering principle is:
Where qualification concerns a Skydrol-equipped aircraft component or installation, the test should represent the specified fluid, conforming or accepted representative hardware, approved material interfaces and critical operating conditions.
Any departure should be visible, justified and accepted through the appropriate programme controls.
Conformity protects the meaning of the result
A fire-test result has limited qualification value if the tested article cannot be connected to the design being assessed.
Conformity provides that connection.
Depending on the programme, conformity and configuration evidence may include:
• Test-article part number.
• Serial number.
• Drawing issue.
• Modification status.
• Bill of materials.
• Manufacturing route.
• Material certificates.
• Seal and gasket identification.
• Hose and fitting specification.
• Coating and finish records.
• Assembly and tightening requirements.
• Fluid grade and batch.
• Cleanliness or contamination status.
• Pre-test conditioning.
• Approved deviations.
• Photographs.
• Inspection records.
• Witnessing or conformity statements.
Not every development test requires formal certification conformity. Development testing may use representative or deliberately simplified articles to investigate a design question.
The distinction must remain clear.
A development article can provide useful engineering information. It should not automatically be presented as a conforming qualification article.
Current Resonate AS6826 material requires the test article to be identified by material, dimensions and configuration, supported by a representativeness statement matching production intent. It also places configuration, boundary conditions, instrumentation, acceptance criteria, procedure and reporting within the same controlled test plan.
Build integrity and test integrity are connected
Aerospace fire-test integrity depends on more than the burner.
A test can use a calibrated fire exposure and still answer the wrong engineering question if the build is not representative.
Likewise, a conforming test article can produce weak evidence if the operating conditions, instrumentation or test procedure do not represent the applicable hazard.
A defensible programme therefore needs both build integrity and test integrity.
Build integrity
The article must represent the relevant production design or an agreed test configuration.
This includes the correct:
• Materials.
• Interfaces.
• Assembly details.
• Modification state.
• Wetted components.
• Coatings and finishes.
• Mounting and installation features.
Test integrity
The test must apply the required exposure, operating conditions, instrumentation, sequence and acceptance criteria.
Depending on the applicable requirement, these conditions may include:
• Hydraulic pressure.
• Fluid temperature.
• Flow rate.
• Leakage condition.
• Actuator operation.
• Cooling airflow.
• Differential pressure.
• Electrical loading.
• Mechanical loading.
• Vibration.
• Test duration.
• Burner orientation.
• Post-test inspection.
The exact conditions depend on the applicable regulation, standard, installation and agreed means of compliance.
Resonate’s current fire-test material treats production-representative hardware, calibrated fire exposure and justified operating boundary conditions as separate but connected requirements.
What a substitute may fail to reproduce
A substitute fluid should not be rejected merely because it is a substitute. It should be assessed against the evidence the test is required to produce.Where chemistry and material interaction matter, substitution may affect several mechanisms.
Seal and gasket response
The fluid and elastomer combination can influence dimensional change, material integrity, sealing performance and leakage development.
If hazardous leakage is an acceptance criterion, changing that combination may alter the failure mechanism being assessed.
Coating and finish degradation
Fluid exposure can affect incompatible paints, coatings and finishes. Heat and fire may intensify or accelerate that damage.
A substitute may not reproduce the same pre-fire or during-fire material condition.
Insulation and wiring effects
Fluid contact can influence insulation, sleeving, adhesives and nearby electrical protection.
The resulting damage may affect electrical continuity, short-circuit behaviour or post-fire inspection.
Hot-fluid decomposition
A fluid exposed to severe heat may decompose differently from a surrogate.
The test may therefore produce different vapours, smoke, residues or contamination.
Any claim about specific decomposition products or toxicological consequences must be supported by the current safety data sheet and suitable primary technical evidence.
Interaction with metallic and non-metallic materials
Material response depends on the exact fluid, material, temperature, exposure duration and condition.
Broad compatibility claims should not replace component-specific material approval.
Leakage behaviour
Leakage is not controlled by pressure alone.
Seal condition, viscosity at the point of release, joint behaviour, hose damage and thermal distortion can affect whether the release forms a drip, stream or spray.
That distinction can materially change the local fire hazard.
Residual contamination
Post-fire deposits may affect inspection, cleaning, electrical condition, continued operation or failure analysis.A substitute may leave a different residue or no comparable residue.
Post-fire condition
Qualification may require more than survival during exposure.
The component may need to:
• Retain pressure.
• Remain contained.
• Perform a safe shutdown.
• Avoid hazardous leakage.
• Maintain a defined fire-intended function.
• Support a specified post-test inspection.
Different fluid-material interactions can change that final condition.
These mechanisms support the current Resonate technical position that viscosity, density, pressure, flow and fluid temperature do not establish chemical equivalence.
When might a substitute be acceptable?
The conservative approach is not an absolute prohibition on substitutes.
An alternative fluid or configuration may be acceptable where:
• The test objective is deliberately limited to hydraulic operation.
• Fluid chemistry is outside the hazard being assessed.
• Material compatibility is not part of the evidence question.
• The applicable procedure permits substitution.
• The relevant properties have been compared across the required operating range.
• Material, thermal and leakage differences have been assessed.
• The design organisation accepts the justification.
• Customer or certification-authority agreement is obtained where required.
The justification must be relevant to the test objective.
Matching viscosity at one temperature is not enough if the system operates across a wider temperature range.
Matching pressure and flow is not enough if seal degradation or leakage behaviour is being assessed.
Matching hydraulic cooling is not enough if decomposition products or post-fire contamination matter.
A substitute is defensible only when the characteristics it does not reproduce are irrelevant to the required evidence or are addressed through an accepted equivalence case.
Substitution decision table
| Evidence objective | Potential position on substitution | Required control |
|---|---|---|
| Hydraulic rig commissioning | A substitute may be acceptable | Confirm compatibility, safe operation and limits of the resulting evidence |
| Pressure and flow verification | A substitute may be acceptable | Compare properties across the complete test temperature range |
| Leakage development | High risk of non-equivalence | Assess the actual fluid, seals, joints and release condition |
| Material compatibility | Normally use the specified fluid | Control grade, batch, exposure and material identification |
| Fire-resistant or fireproof qualification | Use the specified fluid where chemistry forms part of the hazard | Obtain formal agreement for any departure |
| Post-fire contamination or continued function | Substitution may undermine the evidence | Reproduce relevant chemistry, residues and operating state |
| Development screening | Simplification may be useful | Label the evidence as developmental and avoid qualification claims |
The conservative qualification route
Where the programme is intended to qualify a Skydrol-equipped component or installation, the lowest-assumption route is normally to:
1. Use the specified and controlled Skydrol grade.
2. Use a conforming or formally accepted representative test article.
3. Verify the specified seals, gaskets and wetted materials.
4. Reproduce the relevant pressure, flow, temperature and functional conditions.
5. Represent critical installation and boundary conditions.
6. Apply the required calibrated fire exposure.
7. Monitor the defined performance and failure criteria.
8. Record configuration details, concessions and deviations.
9. Inspect and document the post-fire condition.
10. Retain the evidence needed to reconstruct and review the test.
This route does not guarantee certification acceptance.
Acceptance remains dependent on the applicable requirements, agreed means of compliance, configuration, test validity, deviations and authority or customer review.
It does, however, reduce avoidable uncertainty about whether the test represented the system being qualified.
Define the evidence question before selecting the fluid
The correct test architecture starts with the evidence objective.
The programme should establish whether the test is intended to demonstrate:
• Hydraulic operation.
• Fire-resistant performance.
• Fireproof performance.
• Continued function.
• Controlled or graceful shutdown.
• Containment.
• Absence of hazardous leakage.
• Seal and material compatibility.
• Post-fire integrity.
• Installation-level performance.
• Conformity of a production-representative assembly.
Different objectives can justify different levels of representativity.
The mistake is not using a simplified test for development purposes.
The mistake is allowing a simplified development arrangement to be interpreted as representative qualification evidence without the necessary technical justification.
Questions to resolve before testing
Before the test configuration is approved, the programme should answer:
1. What exact evidence must the test produce?
2. Which fluid grade is specified by the approved design?
3. Does fluid chemistry form part of the assessed hazard?
4. Which seals, gaskets, hoses, coatings and insulation materials are present?
5. Is the article conforming, formally accepted as representative or developmental?
6. What pressure, flow, fluid temperature and leakage state must be reproduced?
7. Which installation conditions could influence the result?
8. What fire-resistant or fireproof requirement applies?
9. What functional behaviour must be demonstrated during and after exposure?
10. What acceptance criteria have been agreed?
11. What deviations require design-organisation, customer or authority approval?
12. What evidence must appear in the final report?
If these questions are unresolved, selecting the test fluid is premature.
Conclusion
Same pressure does not mean the same chemistry.
The same chemistry does not mean the same assembly.
The same assembly does not mean the same installation unless its configuration, materials, operating conditions and conformity are controlled.
If Skydrol chemistry forms part of the hazard, the specified Skydrol should be represented.
If the approved design depends on particular seals, gaskets, hoses, coatings, insulation or assembly processes, those elements should also be represented.
If the evidence is intended to support qualification or certification, the test article must be traceable to the configuration being assessed.
The conservative engineering approach is therefore not simply to test with Skydrol.
It is to test the conforming Skydrol-equipped system under representative operating and fire conditions.
Your Test Facilitator. Not simply a test facility.
Discuss your test requirement
Planning qualification around Skydrol?
Send Resonate Testing the applicable specification, hydraulic schematic, specified fluid grade, operating envelope, materials list, installation configuration and proposed acceptance criteria.
An early engineering review can identify substitution risks, configuration gaps and weak evidence assumptions before hardware and test time are committed.
