14 CFR Subpart C Section 25-305

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In this post we will get into the next regulation, 14 CFR Subpart C Section 25-305.

§ 25.305 - Strength and Deformation

(a) The structure must be able to support limit loads without detrimental permanent deformation. At any load up to limit loads, the deformation may not interfere with safe operation.
(b) The structure must be able to support ultimate loads without failure for at least 3 seconds. However, when proof of strength is shown by dynamic tests simulating actual load conditions, the 3-second limit does not apply. Static tests conducted to ultimate load must include the ultimate deflections and ultimate deformation induced by the loading. When analytical methods are used to show compliance with the ultimate load strength requirements, it must be shown that—
(1) The effects of deformation are not significant;
(2) The deformations involved are fully accounted for in the analysis; or
(3) The methods and assumptions used are sufficient to cover the effects of these deformations.
(c) Where structural flexibility is such that any rate of load application likely to occur in the operating conditions might produce transient stresses appreciably higher than those corresponding to static loads, the effects of this rate of application must be considered.
(d) [Reserved]
(e) The airplane must be designed to withstand any vibration and buffeting that might occur in any likely operating condition up to VD/MD, including stall and probable inadvertent excursions beyond the boundaries of the buffet onset envelope. This must be shown by analysis, flight tests, or other tests found necessary by the Administrator.
(f) Unless shown to be extremely improbable, the airplane must be designed to withstand any forced structural vibration resulting from any failure, malfunction or adverse condition in the flight control system. These must be considered limit loads and must be investigated at airspeeds up to VC/MC.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-23, 35 FR 5672, Apr. 8, 1970; Amdt. 25-54, 45 FR 60172, Sept. 11, 1980; Amdt. 25-77, 57 FR 28949, June 29, 1992; Amdt. 25-86, 61 FR 5220, Feb. 9, 1996]

In the previous posts, we looked at 14 CFR Subpart C Section 25-301, and 14 CFR Subpart C Section 25-303.

The 14 CFR Subpart C Section 25-305 regulation is related to both. Now we have some more specifics on the strength and deformation requirements under limit and ultimate loads. OK lets break it down.

14 CFR Subpart C Section 25-305

(a) Limit Load Interference

We already know that no permanent detrimental deformation is allowed up to limit loading. meaning there are may not be any permanent deformation impacts that could compromise safety under limit loading.

In other words, even if there is plastic deformation under limit loading, the structure's deformation cannot interfere with safe operation of the aircraft.

Example, under limit loading there could be impact or contact with the adjacent structural members that may not have been designed for any load sharing.

Or, maybe a protrusion/interference situation under limit loading could cause safety issues for the occupants in the area.

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Other instances maybe where flight limit or ultimate loads are induced into the interior structure from aircraft structure deflections.

If an interior structure is designed in such a way that these deflections are constrained, for example vertical deflections of floor beams, it could be severely detrimental to both the aircraft structure and the interior structures. So care needs to be taken in addressing these issues for compliance.

14 CFR Subpart C Section 25-305 Deflections, FAA Policy Statement Example

Figure 1: 14 CFR Subpart C Section 25-305 Deflections, FAA Policy Statement

The figure above shows an example scenario of the effects of deflection. If you would like to know more, there is a policy statement from the FAA with additional explanation on compliance for cabin interiors, click here.

14 CFR Subpart C Section 25-305

(b) Ultimate Load Pass Condition

Under ultimate load the structure must be capable of holding the load for a minimum of 3 seconds. For example, in any ultimate load static test, the load is held for 3 seconds in front of the witnessing DER before the test is considered a success.

How do we interpret this? One way to think about it is that 1 second may be too short to conclude the structure is truly sustaining the applied loading without failure. Another way is to cover the possibility that the ultimate load in a crash situation may last up to three seconds. Parts of the aircraft may deform under loading and sustain the load on other components for up to three seconds. These are just my thoughts though.

The exception to this condition is when aircraft seats are tested to 16.0G dynamic loading. In that test, the seat certification does not require a 3 second hold, the load is considered as an instantaneous G load amplification. The seat is accelerated on a track and the acceleration is continuously measured. At the end of the track are a bunch of wires that stop the seat. They are adjusted to make sure the peak acceleration of the seat reaches is at least 16.0G.

When actual static tests are conducted, its more straight forward to show compliance, the proof is in the test results.

However, more often than not, analysis is used for interiors certification. Therefore it is important to account for the deflections and prove that they are not detrimental as described in the FAA policy statement link above. In doing so, well established SDC (structural design criteria) guidelines must be used to demonstrate compliance.

14 CFR Subpart C Section 25-305

(c) Stiffness Dependent Load Amplification

This is not so much of a concern for interiors, with the exception of Sustained Engine Imbalance Analysis, the analysis of the effect on interior items close to an engine that is out of balance.

The regulation is more applicable to components that are relatively flexible, that tend to resonate and oscillate. This amplifies the deflection and thus the induced load at resonance within the dynamic range of engine vibration.

On engine nacelles for example, an equivalent static load factor may be used to size components subject to vibration and resonance within the engine frequency range.

14 CFR Subpart C Section 25-305 (e) and (f) are out of the scope for interiors so I will stop here. Hope you learnt something new today about 14 CFR Subpart C Section 25-305.

Make sure to comment below and let me know what you think.

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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.