When engineers outside aerospace ask how strong aircraft materials are, the question usually implies a simple comparison: tensile strength, yield strength, or perhaps strength-to-weight ratio.

But in transport-category aircraft design, strength is not defined by a single number. It is defined by certification philosophy.

From a CS-25 / FAR-25 perspective, a material is not considered “strong” because of its measured properties—it is considered strong when its behaviour is statistically characterised, structurally validated, and accepted by the certification authority.

At the heart of this philosophy lies one deceptively short regulation: CS-25 / FAR-25 §25.613 — Material Strength Properties and Design Values.

Strength in Aircraft Structures: More Than a Material Property

Aircraft structures are not designed to maximise material strength. They are designed to achieve predictable structural reliability under all operational conditions.

This distinction is crucial.

A structural element in a transport aircraft must withstand:

• limit and ultimate loads
• fatigue over decades of service
• environmental degradation
• manufacturing variability
• accidental damage
• inspection uncertainties

Therefore, certification does not ask:

What is the strength of this material?

It asks:

What strength value can be safely used in design with quantified confidence?

That question is exactly what §25.613 governs.

What §25.613 Really Requires

CS-25 / FAR-25 §25.613 states that material strength properties used in design must be based on sufficient tests to establish design values on a statistical basis.

In practical engineering terms, this means:

• Coupon test results alone are insufficient
• Variability must be measured
• Confidence levels must be defined
• Design allowables must be statistically derived

The regulation transforms raw test data into certification-level strength values.

This is the origin of familiar aerospace concepts such as:

• A-basis allowables (99% population / 95% confidence)
• B-basis allowables (90% population / 95% confidence)

Thus, the “strength” of an aircraft material is not its average strength—it is its certified allowable strength.

Material Strength Within the CS-25 Structural Framework

Section 25.613 does not stand alone. It forms one layer of a broader structural certification system that includes:

• §25.303 — Factor of safety
• §25.305 — Strength and deformation
• §25.571 — Fatigue and damage tolerance
• §25.603 — Materials and workmanship
• §25.619 — Special factors

Together, these define aircraft structural capability:

Structural capability = Loads × Safety factors × Allowables × Damage tolerance

Material strength values from §25.613 feed directly into this equation. Without statistically justified allowables, structural substantiation cannot begin.

From Handbooks to Statistics: The Evolution of §25.613

The modern philosophy did not always exist.

Before 1992, aircraft designers relied heavily on military material handbooks such as MIL-HDBK-5. These provided fixed allowable values derived from historical testing. Designers selected material allowables from tables rather than deriving them statistically for each program.

The 1992 amendment to FAR-25 fundamentally changed this approach:

• §25.613 and §25.615 were merged
• Mandatory reliance on military handbooks was removed
• Statistical derivation of allowables became central

This shift marked a transition from lookup-based strength to program-specific statistical certification.

Later, MIL-HDBK-5 evolved into today’s MMPDS database, and composite guidance expanded through FAA AC 20-107 and CMH-17 methodologies.

Modern aircraft programs now routinely generate their own allowables supported by dedicated test campaigns.

Global Harmonisation: FAA and EASA Alignment

The strength philosophy embedded in §25.613 is globally harmonised.

CS-25, used by EASA, descends from the same regulatory lineage as FAR-25 through the earlier JAR-25 framework. As a result:

• Wording of §25.613 is closely aligned between FAA and EASA
• FAA Advisory Circular AC 25.613-1 and EASA AMC 25.613 provide parallel guidance
• Certification data and allowables are accepted across jurisdictions

This harmonisation enables a single material allowable database to support certification on both sides of the Atlantic—an essential feature for global aircraft programs.

What “Material Strength” Means in Certification

In aircraft structures, strength is multidimensional. It includes:

• tension and compression strength
• shear and bearing strength
• modulus and stiffness
• temperature effects
• moisture and environment effects
• anisotropy (especially in composites)
• manufacturing scatter

For composite materials in particular, strength depends not only on fibre and resin but also on:

• lay-up sequence
• cure process
• thickness
• defects and porosity
• ply orientation

Thus, certification strength is always application-specific.

A-Basis and B-Basis: Quantifying Confidence

Statistical allowables are the operational core of §25.613.

• A-basis: 99% of population exceeds value with 95% confidence
• B-basis: 90% of population exceeds value with 95% confidence

Their use depends on structural philosophy:

• Single load path → A-basis
• Redundant / fail-safe → B-basis
• Damage-tolerant structures → B-basis with inspection justification

These allowables ensure that material variability is explicitly included in structural reliability.

Metals vs Composites: Different Paths to Strength

Metallic materials typically use allowables derived from large historical datasets such as MMPDS. Variability is relatively well understood and isotropic assumptions are often valid.

Composites require a different approach:

• Allowables depend on laminate configuration
• Manufacturing process strongly affects properties
• Environmental sensitivity is higher
• Testing follows a building-block pyramid

This pyramid moves from:

coupon → element → subcomponent → full-scale structure

Section 25.613 governs the statistical basis at every level.

The Certification Workflow Behind a “Strength Value”

A certified material allowable typically emerges through:

1. Material specification definition
2. Controlled coupon testing
3. Statistical analysis
4. Allowable derivation
5. Structural analysis using allowables
6. Element and subcomponent testing
7. Full-scale validation

Only after this chain is complete can a strength value be considered certification-grade.

So—How Strong Is Aircraft Material?

From a certification perspective, a material is “strong enough” when:

• its statistical allowable is established
• safety factors are satisfied
• variability is bounded
• environmental effects are included
• structural tests confirm behaviour
• certification authorities accept the data

Thus, aircraft material strength is not defined by ultimate stress or yield stress alone.

It is defined by assured structural reliability.

Conclusion

CS-25 / FAR-25 §25.613 embodies a fundamental truth of aircraft structural design: strength is not merely a material property—it is a certification construct.

Over decades, the aerospace industry has moved from handbook values to statistically derived allowables, from isotropic metals to process-dependent composites, and from national regulations to globally harmonised standards.

Yet the core principle remains unchanged:

Aircraft structures are not designed to be as strong as possible.
They are designed to be as reliable as necessary—and demonstrably so.

In that sense, the strength of aircraft material is not measured in megapascals.
It is measured in confidence.