In this post, we will dig into the next regulation, 14 CFR Subpart C Section 25-603 – Materials. Its all about quality, material quality is critical to safe and long lasting aerospace structures.

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§ 25.603 - Materials

The suitability and durability of materials used for parts, the failure of which could adversely affect safety, must—
(a) Be established on the basis of experience or tests;
(b) Conform to approved specifications (such as industry or military specifications, or Technical Standard Orders) that ensure their having the strength and other properties assumed in the design data; and
(c) Take into account the effects of environmental conditions, such as temperature and humidity, expected in service.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-38, 41 FR 55466, Dec. 20 1976; Amdt. 25-46, 43 FR 50595, Oct. 30, 1978]

In the previous posts, we looked at:

So much rides on this regulation. It is probably fair to say that the entire path to certification hinges on compliance with this one regulation.

Let me explain what I mean...

(a) Materials - Statistical Basis & Test Data

Aerospace material applications are heavy on process specifications approved by the FAA, and strength values developed based on extensive FAA approved testing, as well as actual fleet flight experience. One of the most commonly used documents in this regard is MMPDS (Metallic Materials Properties Development and Standardization). We discussed in detail about the tables in this critical document, learn more about A-Basis and B-Basis allowable values.

Typical MMPDS Materials Mechanical Properties Table
Figure 1: Typical MMPDS Mechanical Properties Table, 2024 Plate

If a part is made from a material listed in MMPDS, and the various material strength, temperature dependency and fatigue data found in this document is strictly followed in sizing the components, then the path to certification is that much more simple with the FAA.

The point here is that materials used in aerospace applications must have very well established strength, durability, and process specifications, with supporting test data for a clear path to certification and use in aerospace applications. WHY? SAFETY...... that is the cardinal rule.

Let us say you are an adventurist, and you are about to embark on an expedition that you have never been on before. How do you really feel safe about it? You guessed it, with the knowledge of people who have already been there done that and documented everything about that adventure.

You researched it, understand it and make sure to implement every rule of it, and that is how the FAA feels confident about products that come to its doors for certification as well. A manufacturer than can consistently and diligently demonstrate this has a lot to gain and maintain.

(b) Materials - Approved Specifications

Aerospace grade materials must comply with stringent manufacturing process requirements. What the heck does that mean?

In simpler terms, any aerospace part must be made from a material that complies with strict manufacturing process standards acceptable for use in aerospace applications.

NAS Materials
Figure 2: NAS Materials

Per 14CFR 25.603(b), all structural materials and fasteners must conform to approved specifications. For example, aluminum materials must be specified using Aerospace Material Specifications (AMS) maintained by the Society of Automotive Engineers. Fasteners must be specified using Army-Navy (AN), Military Specification (MS), and National Aerospace Standard (NAS) specifications. If a material specification or fastener specification is used which is not published by one of the organizations listed above (example: Hi-Loks) then that specification must be approved by the respective ODA as a part of each project on which it is used.

Aerospace materials specifications include composition, statistical test data requirements, temper data, tested strength data, among other important information. Any supplier is required to comply with these requirements in order to be a certified supplier acceptable to the procurement departments of aerospace manufacturers.

WHY? It is all about safety of you and me.

Humans without wings flying at 30,000 to 40,0000 feet above sea level, sitting in a seat inside a pressurized barrel is unnatural and no joke.

It is serious business and extremely risky. And without quality materials helping us do that, it is simply impossible.

(c) Materials - Environmental Effects

Is it enough to follow the specifications? Of course not.

Temperature effects, humidity effects, corrosion treatment (anodization) effects, static electricity, sonic fatigue, pressure, mechanical vibration fatigue and many other considerations are required for certification with the aerospace certifying authorities (example FAA, EASA etc.).

Let us consider the engine nacelle for example. If there is one part of the aircraft where Armageddon shines, that is the engine nacelle and the engine itself. It is probably where the worst of the worst conditions come together in perfect synergy, and where the first line of safe flight defense begins.

Environmental effects such as the impact of high temperature on the fatigue and durability of a component, corrosion resistant anodization treatment of Aluminum parts etc. must all be accounted for in the sizing of these parts.

The key to the critical path to certification (CPTC) for these conditions is the use of a material that complies with all of these important regulations.

Learn more about how this regulation ties into the Proof of Structure regulation. Learn more about A-Basis and B-Basis allowable values.

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Surya Batchu
Surya Batchu

Surya Batchu is the founder of Stress Ebook LLC. A senior stress engineer specializing in aerospace stress analysis and finite element analysis, Surya has close to a decade and a half of real world industry experience. He shares his expertise with you on this blog and the website via paid courses, so you can benefit from it and get ahead in your own career.