14 CFR Subpart C Section 25-307
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In this post we will get into the next regulation, 14 CFR Subpart C Section 25-307.
(a) Compliance with the strength and deformation requirements of this subpart must be shown for each critical loading condition. Structural analysis may be used only if the structure conforms to that for which experience has shown this method to be reliable. The Administrator may require ultimate load tests in cases where limit load tests may be inadequate.
(b) -(c) [Reserved]
(d) When static or dynamic tests are used to show compliance with the requirements of § 25.305(b) for flight structures, appropriate material correction factors must be applied to the test results, unless the structure, or part thereof, being tested has features such that a number of elements contribute to the total strength of the structure and the failure of one element results in the redistribution of the load through alternate load paths.
[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. 25-72, 55 FR 29775, July 20, 1990]
In the previous posts, we looked at:
- 14 CFR Subpart C Section 25-301: Loads
- 14 CFR Subpart C Section 25-303: Factor of Safety
- 14 CFR Subpart C Section 25-305: Strength and Deformation
The 14 CFR Subpart C Section 25-307 regulation provides guidelines on proof of structure and more detail on compliance requirements of the 14 CFR Subpart C Section 25-305 Strength and Deformation regulation. OK let us break it down.
14 CFR Subpart C Section 25-307
(a) Substantiaton by Analysis or Test
The certification process involving compliance with 14 CFR Subpart C Section 25-305 Strength and Deformation regulation can be achieved either by analysis, by test, or a combination of both. It is important to note however that certification entirely by stress analysis is generally not acceptable. There will be some level of testing, required to produce acceptable allowable loads and margins of safety.
First, it is not sufficient to show compliance only by analysis of one critical load case. For example, 9.0G forward is generally a critical load case.
Compliance must be shown for every critical load case. For instance, 6.5G DOWN, or Rapid Decompression, or any other load case that is required to be evaluated including any load sharing and contact due to deflections, as shown in the previous post, 14 CFR Subpart C Section 25-305 Strength and Deformation.
In case of cabin interiors, stress analysis methods and procedures are well established. Industry standard classical hand calculation methods, and margin of safety calculations are employed. In addition, finite element analysis and modeling is used for analysis. Many critical safety margin calculations utilize tested allowable loads.
Examples:
- Panel Flexure and Core Shear (click to read more about this) allowable stresses
- Panel Joints allowable loads and moments testing
- Sandwich panel insert testing
Almost all testing conducted in cabin interiors structures is to ultimate loading. The combination approach above is mainly intended to minimize testing effort required to show compliance. However, care needs to be taken in selecting efficient and accurate analysis methods and acceptable allowable data to ensure cost savings.
14 CFR Subpart C Section 25-307: Compliance Balancing Act
In case of a full scale static test, clearly the opportunity for debates and arguments is vastly minimized and so is the need for various uncertainty factors that may otherwise be required. But a full scale static test could be a lot more expensive. So it is a delicate balancing act.
14 CFR Subpart C Section 25-307
(d) Redundant Load Path
The second part of this 14 CFR Subpart C Section 25-307 regulation provides guidance on redundant load paths.
It is required to determine A and B basis allowable values during testing of components such as inserts, panels etc. The regulation provides some guidance in the selection of these values. Depending on the ODA unit Member or DER's disgression, B-Basis allowable values may be acceptable for components along a load path that has other redundant load paths in case of the failure of one of the members. A good example for this would be panel pins that join various panels.
14 CFR Subpart C Section 25-307: Redundant Load Path
Panel pins are typically placed at every 2 to 4 inches along the panel joint. This type of a joint inherently has redundancy built into the joint. The same is true for joints with multiple inserts.
However, B-Basis is typically not allowed at critical locations such as floor fitting inserts or top attachment inserts. Here A-Basis is generally required. More details are included in this post on A-Basis and B-Basis values.
The other part of this 14 CFR Subpart C Section 25-307 part (d) guidance is related to the use of correction factors or fitting factors. During static testing, it is common practice to load the structure up to the limit load (ultimate load/1.5), pause for three seconds and then back down to zero load. Then deflection measurements are taken to ensure there is little or no permanent deflection at critical points on the structure.
Following that, the structure is loaded again all the way up to ultimate load, which usually includes a 1.15 fitting factor. So this is an example to ensure compliance with part (d) of 14 CFR Subpart C Section 25-307 regulation. During stress analysis as well, we use fitting factors in margin of safety calculations. A lot of the regulations overlap in terms of interpretation, we will cover the fitting factors and other material factors in more detail in another future post.
So there you have it, make sure you comment below.
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