Fabrication Methods

In this post, we will explore the next regulation, 14 CFR Subpart C Section 25.605 – Fabrication Methods. Manufacturing process specifications and quality control is the focus of this post.

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§ 25.605 - Fabrication Methods

(a) The methods of fabrication used must produce a consistently sound structure. If a fabrication process (such as gluing, spot welding, or heat treating) requires close control to reach this objective, the process must be performed under an approved process specification.

(b) Each new aircraft fabrication method must be substantiated by a test program.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-46, 43 FR 50595, Oct. 30, 1978]


In the previous posts, we looked at:

In the last post on materials, we discussed the importance of the quality of materials. Such materials are used to manufacture parts and products. But how can we make sure that every time a product is manufactured, especially the same part is used on multiple aircraft, the part is essentially the same in terms of structural integrity? This is where the fabrication methods regulation kicks in. It is not enough to simply use quality materials, the fabrication methods used also are of critical importance.

(a) Fabrication Methods: Consistency

Consider our good old sandwich panel, it is typically manufactured in large quantities, meaning mass produced.

In part (a) of this regulation, the FAA is basically saying that the "manufacturing process specifications" of these panels, or the fabrication methods, must be nailed down to be consistent and certified. In such situations, it is acceptable to prove to the FAA that the panels produced meet the requirement of consistent quality and strength requirements, with an approved testing schedule of samples of those panels. The emphasis of this regulation also shines light on the quality control of the fabrication methods.

This way, every single panel does not need to be tested for compliance with the regulations. WHY? Because you already have a certified "process specification" with coupon testing and documentation procedures in place to convince the FAA that you don't need to test every panel. This also ties into developing A-Basis and B-Basis allowable values.

Joints are critical for the health of the load path of any assembly structure. Not only is this regulation applicable to individual parts, but also to assemblies that use various types of joints including fastened and bonded joints. For bonded joints, the fabrication methods relate to mixing of the structural adhesive, and testing samples of that adhesive using lap shear test coupons to ensure consistent results. Same applies to potted insert fabrication methods in sandwich panels.

(b) Fabrication Methods - New Methods and Testing

As Boeing, Airbus and other OEMs did with radically new composite airframe structures for example, if some day you come up with brand new fabrication methods to manufacture a jet engine out of carbon nano tubes, or may be you figured out how to make the wings from a special kind of spider web material (I am kidding of course, but you get the idea, and don't let me stop your imagination from flying high..), it must be proven to the FAA that the same structure is in compliance with all the regulations via an extensive testing program.

These new radical fabrication methods must also have certified process specifications to ensure consistent quality of the part or end product that can use the results of the testing program for compliance.

Alright, I think that pretty much sums it up. Cheers! Oh and don't forget to comment and share this post, use the sharing methods below.

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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.