Understand how FAA AC 21-16G guides the use of RTCA DO-160 qualification evidence in aviation certification, including recognised revisions, compliance limitations, fire-testing considerations, and FAA acceptance pathways.
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FAA AC 21-16G is an Advisory Circular issued by the Federal Aviation Administration on 22 June 2011.
Its full subject is:
RTCA Document DO-160 Versions D, E, F and G, Environmental Conditions and Test Procedures for Airborne Equipment
The Advisory Circular identifies RTCA DO-160 versions D, E, F and G as containing acceptable environmental qualifications for showing compliance with certain airworthiness requirements. It strongly encourages the use of DO-160G for new articles.
However, AC 21-16G is not itself a regulation. It states that it is not mandatory and describes one acceptable means, but not the only means, of complying with applicable airworthiness requirements.
This distinction is fundamental:
RTCA DO-160 provides environmental conditions, categories and test procedures.
AC 21-16G explains how the FAA may accept and use the resulting qualification evidence.
The applicable airworthiness requirement remains the compliance objective.
The approved certification plan defines how that objective will be demonstrated.
AC 21-16G therefore does not replace RTCA DO-160, Part 25, Part 33 or the applicable certification basis.
It connects them.
Environmental qualification evidence can remain in use for many years.
Aircraft equipment may be:
Installed on several platforms
Modified during its service life
Requalified after design changes
Supported by evidence generated to an earlier DO-160 revision
Used under a Technical Standard Order
Reassessed for a new installation
Accepted through similarity or delta analysis
This creates practical certification questions:
Which DO-160 revisions are acceptable?
Can qualification data from an earlier revision still be used?
When is similarity sufficient?
When is requalification required?
Do the declared categories represent the intended installation?
Which environmental sections support particular certification findings?
Where does DO-160 evidence stop being sufficient?
AC 21-16G provides an FAA position on these questions.
It does not eliminate programme-specific engineering judgement. It provides a framework for using the evidence more consistently.
RTCA DO-160 defines standard environmental test conditions and procedures for airborne equipment.
The tests provide a laboratory means of demonstrating equipment performance under environmental conditions that may be encountered during aircraft operation.
AC 21-16G also makes an important limitation clear:
DO-160 testing is not intended to be a measure of the equipment’s service life.
A successful environmental qualification programme can demonstrate performance under declared conditions. It does not automatically prove:
Unlimited operational life
Suitability for every aircraft installation
Compliance with every airworthiness requirement
Acceptance after uncontrolled design changes
Complete aircraft or engine certification
Adequacy of categories that do not bound the intended environment
Qualification evidence must remain connected to the equipment configuration, installation and certification objective.
AC 21-16G identifies DO-160 versions D, E, F and G as containing acceptable environmental qualifications for certain airworthiness requirements.
It also provides a revision-comparison appendix covering changes from:
DO-160C to DO-160D
DO-160D to DO-160E
DO-160E to DO-160F
DO-160F to DO-160G
For Technical Standard Order activity, the Advisory Circular states that versions D, E, F and G provide an equivalent level of safety when the applicable version is identified in the Environmental Qualification Form.
That statement must not be overstretched.
Equivalent level of safety within the Advisory Circular’s stated context does not mean that:
Every test section is technically identical.
Every revision can be substituted without review.
Every category remains suitable for a new installation.
Every TSO permits unrestricted use of an earlier revision.
Later equipment changes can be ignored.
The relevant revision, categories, configuration and qualification basis should be declared and assessed.
Qualification engineers often concentrate on:
Test methods
Instrumentation
Fixtures
Vibration profiles
Temperature cycles
Electromagnetic compatibility
Functional monitoring
Test reports
These activities are essential. However, qualification does not exist in isolation.
Testing becomes valuable when its evidence supports an engineering, procurement, acceptance or certification decision.
AC 21-16G sits at the interface between:
Environmental qualification
and
FAA compliance activity
Qualification planning should therefore consider:
The applicable certification basis
The selected DO-160 revision
Environmental categories
Installation location
Equipment configuration
Required functions during exposure
Declared acceptance criteria
Environmental Qualification Form content
Similarity or delta assessments
Additional compliance methods
Authority agreement
A technically successful test can still provide inadequate certification evidence if it addresses the wrong requirement, category, configuration or installation.
AC 21-16G applies to manufacturers and installers using RTCA DO-160 for environmental qualification of airborne equipment.
The reference to installers is important. Environmental categories often depend on where and how the equipment will be installed.
Relevant questions include:
Is the equipment installed inside or outside a pressurised area?
Is it located in a temperature-controlled zone?
Is it exposed to external airflow?
Is it mounted near an engine or propeller?
Is it within a designated fire zone?
What structural vibration reaches the mounting?
What fluids may contact it?
What wiring, cooling or hydraulic connections are present?
Must the equipment operate during the environmental exposure?
Does the original qualification configuration represent the new installation?
A manufacturer may qualify equipment against declared categories.
The installer must still establish that those categories and assumptions are appropriate for the actual aircraft installation.
A DO-160 qualification label is not a substitute for installation assessment.
One of the most persistent misunderstandings in environmental qualification is:
The equipment passed DO-160,……. therefore it is certified. That is not an accurate conclusion.
RTCA DO-160 provides controlled environmental conditions and test methods.
AC 21-16G describes how the FAA may accept evidence generated using those methods.
The applicable regulation defines what must be shown.
The certification programme determines how the showing will be completed.
In practical terms:
| Document or activity | Primary role |
|---|---|
| Applicable airworthiness requirement | Defines the compliance objective |
| RTCA DO-160 | Provides environmental categories and test procedures |
| FAA AC 21-16G | Provides FAA guidance on using DO-160 qualification evidence |
| Certification plan | Defines the agreed means of compliance |
| Environmental Qualification Form | Records the applicable tests and categories |
| Test report | Records configuration, method, achieved conditions and results |
| Installation assessment | Confirms relevance to the intended aircraft installation |
| Authority finding | Determines regulatory acceptance |
Testing generates evidence…..It does not independently make the regulatory finding.
AC 21-16G recognises that using the newest version of DO-160 may not always be practical.
Existing qualification evidence may remain useful where a detailed assessment shows that changes do not adversely affect the environmental qualification.
That assessment should consider:
Hardware changes
Material changes
Software changes affecting environmental behaviour
Connectors and wiring
Enclosure changes
Mounting changes
Cooling arrangements
Power inputs
Internal component changes
Environmental categories
Installation location
Declared performance criteria
Reuse by similarity should not be treated as an administrative shortcut.
The engineering case must show why the previous evidence remains applicable.
Where the original categories do not adequately represent the intended installation, additional testing or analysis may be required.
For many fire-test and qualification programmes, paragraph 7(h) of AC 21-16G contains the most consequential statement in the document.
It distinguishes between:
14 CFR Part 25
14 CFR Part 33
This distinction matters because equipment qualification evidence can be mistaken for complete engine certification evidence.
It is not always sufficient.
A common claim is:
We passed DO-160 Section 26, so all fire compliance requirements are satisfied.
That statement is too broad.
DO-160 Section 26 addresses fire and flammability testing for airborne equipment.
Its evidence may be highly relevant.
However, the compliance value depends on:
The applicable DO-160 revision
The selected test category
The equipment configuration
The installation
The relevant rule
The required equipment function
The product certification boundary
The agreed means of compliance
AC 21-16G makes this particularly clear by treating DO-160F and DO-160G Section 26 differently.
Paragraph 7(h) states that DO-160G Section 26 is sufficient for Part 25.
This is an important FAA position for transport-category aeroplane programmes.
However, it should not be rewritten as:
Every DO-160G Section 26 test automatically closes every Part 25 fire requirement.
The programme must still determine:
Which Part 25 requirement applies
Whether Section 26 is relevant to that requirement
Which test category is appropriate
Whether the tested configuration represents the installation
Whether mounting and interfaces are representative
Whether required functions were monitored
Whether additional installation evidence is needed
Whether the proposed compliance method has been agreed
“Sufficient for Part 25” must be read within the Advisory Circular’s scope and the specific certification programme.
The position changes for aircraft engines.
Paragraph 7(h) states that DO-160G Section 26 is not sufficient for Part 33.
For Part 33 fire and flammability guidance, the Advisory Circular directs users to AC 33.17-1.
This distinction exists because engine certification can involve requirements beyond generic airborne-equipment fire qualification.
Depending on the requirement and product boundary, the compliance showing may need to address:
Engine-specific functions
Fire-zone installation
Fuel and oil systems
Fluid pressure and flow
Shut-off and isolation
Safe engine shutdown
Representative vibration
Cooling and ventilation
Shielding
Connections and interfaces
Type-design conformity
Continued safe behaviour during or after fire exposure
A component may survive a defined Section 26 test and still require further evidence for a Part 33 finding.
The wording of paragraph 7(h) is more precise than many summaries suggest.
The FAA position described in AC 21-16G is:
DO-160F Section 26 is insufficient for Parts 25 and 33.
DO-160G Section 26 is sufficient for Part 25.
DO-160G Section 26 is not sufficient for Part 33.
Part 33 fire guidance is directed to AC 33.17-1.
This is why the qualification revision matters.
It is not enough to state that equipment was tested to “DO-160 Section 26”.
The revision must be identified.
The Environmental Qualification Form, test plan and report should clearly record the applicable version and category.
Two customers may request:
DO-160 Section 26 testing
Their certification objectives may be very different.
| Programme | Possible objective |
|---|---|
| Aircraft equipment programme | Support an equipment or installation finding under Part 25 |
| Engine programme | Support an engine type-design finding under Part 33 |
| TSO programme | Demonstrate environmental qualification required by the applicable TSO |
| Supplier validation programme | Generate development or customer-acceptance evidence |
| Modification programme | Support a changed installation or supplemental approval |
The laboratory method may look similar.
The required evidence package may not be.
Differences may include:
Test article conformity
Witnessing
Installation simulation
Fluids and flow
Pressure
Operational state
Failure criteria
Post-test inspection
Report ownership
Authority involvement
Certification-plan traceability
The correct question is not simply:
Did the equipment pass Section 26?
The correct question is:
Does this evidence support the specific certification finding being sought?
AC 33.17-1 provides FAA guidance relating to engine fire protection requirements under Part 33.
It should be consulted where the certification objective concerns an aircraft engine rather than only airborne equipment qualification.
The applicable revision should be confirmed at programme launch. Internal Resonate research identifies AC 33.17-1B as the later engine fire-protection reference.
The relationship can be summarised as follows:
| Reference | Role |
|---|---|
| RTCA DO-160 Section 26 | Equipment-level fire and flammability test methods |
| FAA AC 21-16G | Guidance on the FAA’s use of DO-160 environmental qualification evidence |
| FAA AC 33.17-1 | Guidance for engine fire-protection compliance |
| 14 CFR Part 25 | Airworthiness standards for transport-category aeroplanes |
| 14 CFR Part 33 | Airworthiness standards for aircraft engines |
These references should be mapped to the actual product, installation and certification basis.
A component should not be labelled casually as a “Part 25 component” or a “Part 33 component”.
The more accurate question is:
Which approved product type design contains the item, which function does it support and which applicant owns the compliance showing?
A component may contribute to:
The engine type design
The aircraft installation
Both engine and aircraft compliance findings
A TSO article approval
A supplemental modification
A supplier qualification programme
Nacelles, pylons, engine externals, wiring, controls, sensors and fire-protection equipment can cross organisational and certification boundaries.
The compliance matrix should identify:
Product
Applicant
Authority
Certification-basis paragraph
Equipment function
Required function during fire
Test method
Installation conditions
Conformity requirements
Witness requirements
Evidence owner
Final acceptance route
This prevents a test method from being mistaken for a complete compliance strategy.
AC 21-16G remains the current active public FAA document in this series, although enterprise research identifies AC 21-16H as an in-work deliverable without a stated issue date.
That status should be checked at the start of every new certification programme because advisory material can change.
The questions addressed by AC 21-16G remain highly relevant:
Which qualification evidence is acceptable?
Which DO-160 revision applies?
Can legacy evidence be reused?
Do the categories represent the installation?
Which limitations apply?
Is additional analysis or testing required?
Does the evidence support the actual certification finding?
Has the proposed route been agreed with the appropriate authority?
These are not administrative questions…..They determine whether a programme generates useful evidence or expensive paperwork.
A disciplined programme should follow a controlled sequence.
Identify the applicable product, authority, certification basis and compliance paragraph.
Determine whether the item supports aircraft, engine, TSO, modification or combined findings.
Define the DO-160 revision, sections, categories and installation assumptions.
Review AC 21-16G and any requirement-specific advisory material, including AC 33.17-1 where applicable.
Record the proposed method in the certification plan or compliance matrix.
Define hardware, software, mounting, interfaces, fluids, pressures, electrical loads and operating states.
Capture achieved conditions, observations, deviations and functional results.
Produce the Environmental Qualification Form, test report, conformity evidence and supporting assessments.
Present the evidence against the actual certification objective rather than relying on a generic claim of DO-160 compliance.
The statement is incomplete. Revision, sections and categories matter.
A laboratory report records evidence. It does not issue aircraft or engine approval.
Equipment qualification may not represent a new mounting, thermal environment, vibration input or fire zone.
Design and installation changes can invalidate previous assumptions.
Aircraft and engine certification objectives differ.
DO-160G Section 26 is not sufficient for Part 33 under AC 21-16G.
AC 21-16G is guidance describing an acceptable means, not the only means, of compliance.
The deeper lesson from AC 21-16G is straightforward:
Qualification evidence and certification evidence are related, but they are not automatically interchangeable.
Environmental testing generates controlled engineering data.
The certification basis defines what must be demonstrated.
The applicant proposes the means of compliance.
The authority determines whether the evidence supports the required finding.
Experienced qualification engineers understand all four layers.
AC 21-16G is often treated as a supporting document to RTCA DO-160.
That understates its importance.
The Advisory Circular provides essential context for how the FAA may use environmental qualification evidence generated under DO-160 versions D, E, F and G.
It helps manufacturers, installers, certification teams and qualification engineers understand:
Which revisions are recognised
How evidence may be applied
How legacy evidence may be assessed
Why installation categories matter
Where limitations exist
When additional compliance activity is required
Nowhere is this clearer than the distinction between Part 25 and Part 33 fire compliance.
DO-160G Section 26 may support the FAA position stated for Part 25.
It is not sufficient for Part 33.
For qualification engineers, that distinction is not a regulatory footnote.
It can determine the entire test and compliance strategy.
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