14 CFR Subpart C Section 25-301

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In this post we will dig deeper into 14 CFR Subpart C Section 25-301. This is the first regulation in Subpart C, Structures.

§ 25.301 Loads.
(a) Strength requirements are specified in terms of limit loads (the maximum loads to be expected in service) and ultimate loads (limit loads multiplied by prescribed factors of safety). Unless otherwise provided, prescribed loads are limit loads.
(b) Unless otherwise provided, the specified air, ground, and water loads must be placed in equilibrium with inertia forces, considering each item of mass in the airplane. These loads must be distributed to conservatively approximate or closely represent actual conditions. Methods used to determine load intensities and distribution must be validated by flight load measurement unless the methods used for determining those loading conditions are shown to be reliable.
(c) If deflections under load would significantly change the distribution of external or internal loads, this redistribution must be taken into account.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-23, 35 FR 5672, Apr. 8, 1970]

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(a) I discuss this part in detail in this post: Limit and Ultimate Loads

14 CFR Subpart C Section 25-301
14 CFR Subpart C Section 25-301: Limit and Ultimate Loads

(b) Typical Interpretation: Inertia loads must be applied on structures. Loads and moments must be in equilibrium with reactions at the aircraft structure interface points. Acceptable load application methods (simulated and real world) are body loads, pressure loads (using load spreaders) and point loads where applicable.

For example during landing, the net inertia loads from all the internal items of mass (passengers, interiors structures, seats, crew etc.) travel through out the aircraft structure. They travel along their respective load paths and are finally balanced by the reaction loads at the landing gear and aerodynamic lift under the wings.

The entire aircraft structure acts as the load distribution path without failing. All internal loads and moments are in equilibrium at any given point in time, ultimately loading the runway or the ground at landing load levels.

Hence, the applied loading needs to resemble as closely as possible, the real landing loads when sizing is done. Conservatism in included in the form of safety factors as needed. In addition, it must also be shown that the applied loading is verified to be a reliable way of loading the structures.

In case of cabin interiors, the ultimate loads prescribed in § 25.561 is a well established reliable way to apply those loading conditions.

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(c) Typical Interpretation: The effect of large deformations must be accounted for in the applied loading. For example, in case of the airplane wings, the large deflection of the wing tip results in follower forces normal to the surface of the wing, these must be accounted for.

Similarly, in case of large deflections of full height unsupported cabin partitions or bulkheads, the effect of follower forces must be accounted for unless it is proven to be more conservative by not doing so. This consideration applies not only to the partition itself but also the downstream and upstream primary structure load path.

14 CFR Subpart C Section 25-301: Large Deformations
14 CFR Subpart C Section 25-301: Large Deformations

In other words, not considering follower forces may be more conservative for the cabin partition itself. But, if the load path beyond the partition or bulkhead is negatively impacted, then it becomes important to account for such nonlinear geometry follower force effects.

We can see in the figure above that follower forces could result in two different types of reaction loads at the aircraft interface, but the interface may not be designed to accept such loading. Again, this is just an example to explain the reason for this particular FAA regulation. Thus, it may be required to supply such data to the aircraft integrator to meet compliance requirements as deemed necessary.

I hope this post on 14 CFR Subpart C Section 25-301 was useful to you guys. It would be even more awesome if you can comment below on your own unique experiences with this regulation. For example, maybe there was a unique DER interpretation of 14 CFR Subpart C Section 25-301, that was required to be complied with and you found it interesting, share it 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.