Substantiation by Similarity

What Is Being Demonstrated?

Substantiation by similarity demonstrates that the evidence supporting a baseline configuration also supports a new configuration, because the two are structurally equivalent in every way that matters for the requirement being substantiated. Similarity is not a shortcut — it is a delta assessment. The engineer does not claim that the new design is "just like" the old one. The engineer systematically compares the new configuration against the baseline, identifies every difference, assesses the structural significance of each difference, and determines what additional evidence is required to close the gap. Where the differences are structurally insignificant, the baseline evidence applies. Where they are significant, new evidence is required. The argument is only as strong as the comparison is thorough.

SIMILAR APPEARANCE IS NOT STRUCTURAL SIMILARITY.

Why Similarity Matters

In any engineering programme, designs evolve. A fitting is modified to improve assembly access. A skin thickness is increased to address a load case. A fastener is changed to a different standard. A material is substituted because of supply chain constraints. Each change raises the question: does the existing substantiation still apply, or must the new configuration be independently substantiated? Re-performing a full analysis and test programme for every minor change would be prohibitively expensive and schedule-destroying. Conversely, assuming that every change is "just like" the baseline and requires no assessment is dangerous. Substantiation by similarity provides the disciplined middle ground: a structured comparison that determines which changes are covered by existing evidence and which require new work.

• Similarity avoids unnecessary re-analysis and re-test when changes are structurally insignificant.
• Similarity identifies the changes that do require new evidence, preventing gaps in the substantiation.
• Similarity leverages certified heritage — the investment in the baseline evidence is preserved.
• Similarity is a delta assessment, not a blanket claim — each difference is examined individually.

The Similarity Comparison

The core of a similarity assessment is a systematic comparison between the baseline configuration and the new design. The comparison must cover every characteristic that could affect structural behaviour: geometry, material, loads, mass, interfaces, manufacturing, environment and failure modes. For each characteristic, the engineer records the baseline value, the new design value, the difference and an assessment of the structural significance. Where the difference is significant, the engineer identifies the additional evidence required. This comparison is not a checklist to be ticked — it is an engineering assessment that requires structural understanding and judgement.

The similarity comparison is the heart of the assessment. A comparison that skips characteristics, dismisses differences without analysis, or fails to identify required evidence is not a similarity assessment — it is an assumption.

What Is the Requirement?

Similarity is used to demonstrate that a specific requirement is satisfied for the new configuration. The requirement must be stated explicitly, just as in any other substantiation. "This fitting is similar to the certified one" is not a substantiation. "This fitting satisfies the limit load requirement, as demonstrated by similarity to fitting ABC-001 whose substantiation is recorded in report STR-001, with the differences assessed in delta report DEL-001" is a substantiation. The requirement, the baseline evidence and the delta assessment must all be referenced and traceable.

• State the requirement being substantiated for the new configuration.
• Identify the baseline configuration and its substantiation evidence.
• Reference the delta assessment that compares the two configurations.
• State the conclusion: the baseline evidence applies, or additional evidence is required.

When Similarity Is Appropriate

Similarity is most appropriate when the changes from the baseline are minor and well-understood. A thickness increase, a fastener upgrade, a minor geometry change in a non-critical region — these are candidates for similarity. Similarity is less appropriate when the changes are fundamental: a different material, a different manufacturing process, a significantly different load path or a different failure mode. In these cases, the baseline evidence may be irrelevant, and a full or partial re-substantiation is required. The engineer must be honest about where the boundary lies. Claiming similarity for a change that is fundamentally different is not engineering — it is wishful thinking.

The Delta Assessment

The delta assessment is the engineering work that bridges the baseline evidence and the new configuration. It is not a statement — it is an analysis. The delta assessment examines each difference identified in the similarity comparison and determines whether the baseline evidence still applies, whether the baseline evidence is conservative for the new configuration, or whether new evidence is required. A difference that increases capability and does not change the failure mode may be covered by the existing margin. A difference that increases demand or changes the failure mode requires new analysis or test. The delta assessment must be documented, reviewed and referenced in the substantiation argument.

Delta assessment logic:

  For each difference identified:
         │
         ▼
  Does the difference increase demand?
    ├── YES ► New analysis required at the increased demand
    │
    └── NO
         │
         ▼
    Does the difference decrease capability?
      ├── YES ► New analysis with the reduced capability
      │
      └── NO
           │
           ▼
      Does the difference change the failure mode?
        ├── YES ► New evidence required for the new failure mode
        │
        └── NO
             │
             ▼
        The difference is covered by the baseline evidence
        (document the reasoning)

Heritage and Precedent

Heritage — evidence from a previously certified or qualified configuration — is a powerful form of baseline evidence. It represents an investment in analysis and test that has already been accepted by an authority or a customer. Using heritage through similarity preserves that investment. However, heritage must be used carefully. The baseline evidence must be applicable: the certification basis must be compatible, the loads must be comparable, and the configuration must be genuinely close. Heritage from a different certification basis, a different load environment or a fundamentally different configuration is not heritage — it is a false analogy. The engineer must verify that the heritage is relevant before relying on it.

Heritage is evidence only when the baseline certification basis, loads and configuration are genuinely compatible with the new application.

Documentation of a Similarity Assessment

A similarity assessment must be documented as a controlled report, not as an email or a spreadsheet. The report should state the requirement, the baseline configuration, the baseline evidence, the comparison, the delta assessment, the additional evidence (if any) and the conclusion. The report must be reviewed independently, just as any substantiation report is reviewed. A similarity assessment that is not documented is not substantiation — it is an opinion.

• State the requirement being substantiated — Be specific — load level, failure mode, acceptance criterion.
• Identify the baseline configuration and evidence — Reference the baseline report, revision and certification basis.
• Present the similarity comparison table — Cover geometry, material, loads, mass, interfaces, manufacturing, environment, failure modes.
• Present the delta assessment — For each difference, state the structural significance and whether new evidence is required.
• Reference any new analysis or test performed — The new evidence must be traceable and reviewed.
• State the conclusion — The baseline evidence applies, or the new evidence demonstrates compliance, or both.
• Obtain independent review — The similarity assessment must be checked by an engineer who did not perform it.

Common Errors in Similarity Claims

Similarity is one of the most abused concepts in structural substantiation. The following errors recur with sufficient frequency that every engineer should be alert to them.

• Claiming similarity without a documented comparison — "it looks the same" is not an engineering assessment.
• Dismissing a material change as "just a different grade" — different grades have different strength, toughness and fatigue behaviour.
• Assuming a thicker part is always conservative — a thicker part may be worse for fatigue, for residual stress or for mass-driven loads.
• Ignoring load changes — the configuration may be similar, but if the loads have increased, the margins may no longer hold.
• Assuming the failure mode is the same without checking — a geometry or material change can shift the governing failure mode.
• Using heritage from an incompatible certification basis — the baseline evidence may not satisfy the current requirements.
• Failing to document the delta assessment — the comparison is done in someone's head and is lost when they leave.

Key Takeaways

• Similarity is a delta assessment, not a blanket claim — each difference is examined for structural significance.
• The similarity comparison must cover geometry, material, loads, mass, interfaces, manufacturing, environment and failure modes.
• Similarity is appropriate for minor, well-understood changes; fundamental changes require re-substantiation.
• Heritage is evidence only when the baseline certification basis, loads and configuration are genuinely compatible.
• The delta assessment must be documented as a controlled report and independently reviewed.
• Similar appearance is not structural similarity — the engineering assessment, not the visual impression, governs.