14 CFR Part 25.609 – Protection of Aircraft Structure Components

In this post we will explore the next regulation, 14 CFR Subpart C Section 25.609 – Protection of Structure.

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§ 25.609 Protection of Structure

 Each part of the structure must -

(a) Be suitably protected against deterioration or loss of strength in service due to any cause, including -

(1) Weathering;

(2) Corrosion; and

(3) Abrasion; and

(b) Have provisions for ventilation and drainage where necessary for protection.

In the previous posts, we looked at:

Protection of structure is done in many ways, such as:

  • Special coatings
  • Surface chemical treatments
  • Heat treatments
  • Special design features
  • Protection Tapes etc.

(a) Structure Component Protection: Treatments

Let us discuss one such important treatment that affects aluminum parts, which is "Anodization".

Benefits of Anodization:

So what exactly is this process? Its a critical process to achieve protection against corrosion for components made from aluminum alloys such as AL 2024, 7075 etc. There are three main types of this process:

  1. Type 1 (phosphoric acid thin layer)
  2. Type 2 (sulfuric acid thin layer) and
  3. Type 3 (thicker layer hard anodized)

For more details on this topic, refer to this link: Anodization

Check out this cool video:

So the benefits are obvious, it provides corrosion protection in areas of moisture and acidic environments. But from a stress engineering standpoint, what are the downsides?

Drawbacks of Anodization:

The main mechanism of anodization is the creation of microscopic pores on the metal surface during anodization. This process therefore has a detrimental influence on the fatigue or endurance capability of these parts. Watch the video above to learn more about the process.

Stress Impact:

The way this detrimental impact on the fatigue allowable stress is accounted for is by:

  • Either knocking down the fatigue allowable stress using a particular factor (depending on a company's internal guidelines). This factor is between 0.0 < K < 1.0 (the thicker the protection layer, the higher the anodization knockdown factor K due to deeper pores)
  • Or, assuming a stress concentration effect, thus dividing the applied tensile stress on the component with the anodization factor K

The final stress value including the factor effect is used in the margin of safety calculation.

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Other Damage Mechanisms:

In addition, there are other mechanisms such as 'abrasion' caused by mechanical vibrations and the like which also damage components and surfaces. Protection against such damage can be achieved using special sealant tapes, check this link for an example: Abrasion

Alright, now that we have covered that part, let us look at part (b).

(b) Structure Protection: Design Features

This part is a bit interesting because depending on its interpretation, the term 'ventilation' can tie into one of the previous regulations I covered, 14 CFR Subpart C Section 25-365: Pressurized Compartment Loads.

Note that here, we are talking about protection of structure against rapid decompression loading using design features such as ventilation or drop down panels. This is covered in detail in part (g) in the link above, so check it out. Vent areas are included throughout the cabin and other areas to prevent excessive pressure differentials as discussed in the link above.

Now, in the context of corrosion due to humidity or acidic conditions, ventilation could be as simple as providing a path or conduit for humidity to escape. Similarly, in areas that are expected to get wet, such as toilet floors or door entry areas, there may be special drain hole design features that prevent water accumulation, thus reducing or preventing corrosion.


Aerospace components made from Aluminum alloys are typically protected against corrosion using thin layer anodization. So as a stress engineer, you must account for the fatigue capability knock down effect when writing fatigue margins. It is also important to note that fatigue allowable stress is usually a low number compared to ultimate or yield strength. So any knockdown could really hurt your fatigue margins.

So there you have it, you have once again wasted a perfectly good hour of your life reading th....... just kidding. I really hope you enjoyed this post.


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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.