The AC 21-16G Section 26 Distinction

Among the most misunderstood areas of aerospace fire testing is the relationship between RTCA DO-160G Section 26 and engine fire-protection certification requirements. Many organisations encounter DO-160 fire qualification data during procurement, design verification or equipment approval programmes and assume that successful completion of Section 26 automatically demonstrates compliance with all relevant fire requirements. In reality, the regulatory position is considerably more nuanced.

The distinction becomes particularly important when the certification route crosses the boundary between aircraft requirements and engine requirements.

Understanding this distinction early can prevent costly re-testing, compliance gaps and certification delays. The deep-dive analysis identifies this issue as one of the most common causes of misunderstanding during programme planning.

What is DO-160G Section 26?

DO-160G is an environmental qualification standard developed for airborne equipment.

It provides standardised laboratory methods for evaluating equipment under a range of environmental conditions, including:

• Temperature.
• Altitude.
• Vibration.
• Humidity.
• Power input variations.
• Lightning effects.
• Fire and flammability.

 

Section 26 specifically addresses fire and flammability performance.

The standard provides controlled laboratory methods that allow equipment manufacturers to demonstrate fire-related environmental performance in a repeatable and traceable manner. 

 

Importantly, DO-160 is an equipment qualification standard.

It is not itself an airworthiness regulation.

It does not create certification requirements.

Instead, it provides one potential means of generating evidence that may be used within a certification programme. 

 

The Regulatory Hierarchy

A useful way to understand the issue is to consider the hierarchy of compliance documentation.

At the highest level sit the regulations:

• Part 25.
• Part 33.
• CS-25.
• CS-E.

 

Beneath these sit guidance documents:

• Advisory Circulars.
• AMC material.
• Guidance material.

 

Below these sit test standards such as:

• AS6826.
• ISO 2685.
• DO-160G.

 

Finally come:

• Test reports.
• Analysis reports.
• Compliance reports.

 

Problems often occur when a lower-level standard is treated as though it were the top-level certification requirement. 

The FAA Position

FAA AC 21-16G contains an important distinction regarding the use of DO-160 qualification data.

The guidance recognises DO-160 environmental qualification methods for airborne equipment.

However, AC 21-16G distinguishes between aircraft certification evidence and engine certification evidence.

The key conclusion identified in the source material is:

“DO-160G Section 26 may support Part 25 evidence, but FAA AC 21-16G does not accept it as solely sufficient for Part 33.” 

This single statement drives a significant amount of fire-testing programme planning.

Why the Distinction Exists

At first glance, the distinction may appear unusual.

If a component survives a fire test, why should the certification route matter?

The answer lies in the objective being demonstrated.

Part 25 often focuses on aircraft installation safety.

Part 33 focuses on engine-specific safety objectives.

Although both involve fire protection, the underlying certification findings may differ significantly. 

 

For example, an equipment qualification test may demonstrate that an electronic unit continues operating after exposure to a defined fire environment.

An engine certification requirement may additionally require demonstration of:

 

• Engine-specific functions.
• Fuel isolation.
• Oil containment.
• Safe shutdown.
• Fire-zone behaviour.
• Installation assumptions.

• Type-design conformity.

   

These factors extend beyond the scope of a generic equipment qualification programme. 

Equipment Qualification Versus Engine Compliance

This distinction can be summarised simply.

DO-160 demonstrates how equipment behaves.

Part 33 compliance demonstrates how the engine type design complies.

The difference is subtle but important.

Equipment qualification focuses on the tested item.

Engine certification focuses on the certified engine system.

Consequently, evidence generated under DO-160 may remain valuable while still requiring supplementary analysis or testing before supporting a Part 33 certification finding. 

 

The Role of AS6826

For engine fire-protection programmes, SAE AS6826 occupies a different position within the certification framework.

AC 20-135A recognises AS6826 as an acceptable means of compliance within its scope.

 

AS6826 was developed specifically to support powerplant fire-protection demonstrations and contains requirements that more directly address engine fire scenarios.

The standard places significant emphasis on:

• Fire severity.
• Installation realism.
• Instrumentation.
• Acceptance criteria.
• Configuration control.

Consequently, AS6826 frequently forms a more direct bridge between test evidence and engine certification findings.

 

Common Misconceptions

Several misconceptions repeatedly appear during programme planning.

These include:

• “DO-160 automatically satisfies Part 33.”
• “A DO-160 test certifies the component.”
• “A successful qualification closes every fire requirement.”
• “The test standard determines compliance.”

None of these statements is universally correct.

The applicable certification basis remains the controlling requirement. 

 

Practical Implications for Applicants

Before selecting a fire-test route, applicants should clearly identify:

• Product type.
• Approval route.
• Applicable authority.
• Certification basis.
• Required compliance finding.
• Proposed means of compliance.

Only then can the suitability of DO-160, AS6826, ISO 2685 or another method be properly assessed. 

Conclusion

DO-160G Section 26 remains an important and widely used fire-qualification method. However, it should be viewed as an equipment qualification tool rather than a universal certification solution.

The critical question is not whether a component passed a DO-160 fire test.

The critical question is whether the resulting evidence supports the specific certification finding being sought.

That distinction lies at the centre of AC 21-16G and remains one of the most important concepts in aerospace fire-testing programmes.