Hazardous Unreliable Design Features

We will try to understand what it means to avoid Hazardous Unreliable Design Features.

In this relatively short post, we will dig into the next regulation, 14 CFR Subpart C Section 25-601.

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§ 25.601 - Hazardous Unreliable Design Features

The airplane may not have design features or details that experience has shown to be hazardous or unreliable. The suitability of each questionable design detail and part must be established by tests.

In the previous posts, we looked at:

Hazardous Unreliable Design Features - General Interpretation

One simple blanket statement, the regulation basically envelopes the general requirement that mandates the avoidance and elimination of any Hazardous Unreliable Design Features that experience has brought to light.

So what is the major lesson here? The FAA is making sure that plane design is done in a way that learns from past mistakes, catastrophes, crashes etc. that were the result of using a hazardous unreliable design feature.

One of the more recent examples for this is the Li-Ion battery issues that came to light on the Boeing 787. Various measures and special conditions had to be implemented due to the fires from those batteries.

So we now know that those batteries and containment, if not designed and tested properly, is a hazardous unreliable design feature regulated by 14 CFR Subpart C Part 25.601. Hence compliance must be shown by test if a similar battery design and containment is used in a new plane design.

The FAA imposed special conditions to comply with this regulation on Li-Ion batteries and containment installations, such as 14 CFR 25.1353 for electrical equipment installations, and 14 CFR Part 25.863 which was originally not intended for batteries but for flammable fluid fire protection requirements.

Hazardous Unreliable Design Features - Interiors Interpretation

This regulation's impact on cabin interiors has also led to many improvements in design.

For example, if a glass partition was designed for aesthetic purposes, but let us say it has been proven to be one of the hazardous unreliable design features. The glass shatters under certain conditions thus seriously inhibiting safe egress of the occupants.

Maybe there are certain seat belt design features that are also hazardous unreliable design features that violate 14 CFR 25.601, unless they are tested to comply with the regulation, they cannot be certified.

The above are just examples, compliance with 14 CFR 25.601 ensures that such hazardous unreliable design features do not make it to production, unless certified by test and proven to be safe and reliable.

The intent of this regulation is not to regulate every little aspect of airplane design but to make sure manufacturers and OEMs learn from past bad ideas and hazardous unreliable design features, or even unusual and novel design features that do not have sufficient regulations to ensure safety of such features.


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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 two decades of real world aerospace 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.