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14 CFR Subpart C Section 25-305

14 CFR Subpart C Section 25-305

RE: 14 CFR Subpart C Section 25-305


Having been a DER now for over 18 years, the FAA requires us to show compliance to the rule, not interpret it. However, obviously, the working level engineer needs to understand what his substantiation needs to comply with. As a recommendation, try looking into FAA Advisory Circular AC 25-21 "Certification of Transport Airplane Structure". It attempts (weakly albeit) to provide some background and intent of the rule. The other place to look is into the earlier CAR 4b equivalent regulations and predecessors. These where written at a time when we did not have soo much assistance from the legal folks and actually prescribes regulations in laymen terms.

Good luck

RE: 14 CFR Subpart C Section 25-305

Cool, thanks so much crackman for that link to the circular.

I definitely agree the terminology of "interpret" is mainly to understand, but yes it is required to be complied with.

Are you on linkedin by any chance? Would love to connect with you there.

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Stressing Stresslessly!

RE: 14 CFR Subpart C Section 25-305

crackman- As a DER, while you may be employed by a private company, you are ultimately responsible to the FAA, right? The engineering documentation submitted for your review/approval must demonstrate full compliance with both the letter and intent of any associated FAR. You described the situation like this, "the FAA requires us to show compliance to the rule, not interpret it.". But the responsibility for demonstrating compliance with the rules lies entirely with the vendor. As a DER you must decide if the documentation they submit is satisfactory or not. And this might involve some "interpretation" on your part. But since you are the person accepting responsibility when you sign off on the design, you have every right to be picky.

RE: 14 CFR Subpart C Section 25-305

Yes, the 8130 is the DER or UM's sign off, and this is the part that varies widely from DER to DER and this is where the interpretations could differ, ideally the interpretations are all the same.

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RE: 14 CFR Subpart C Section 25-305

tbuelna and stressbookllc

I will try to address both of your comments. During a review of data to demonstrate compliance, many aspects are looked at. The FAA does provide a lot of guidance however. Particularly, you can find guidance in the FAA Advisory Circulars which present acceptable methods of compliance but of course they are not the only way to show compliance. Additionally, thru DER recurrent training, the FAA provides training material which also provides guidance. In fact, one of the recent training courses provides a very thorough discussion of the use of finite element models and the expected levels of validation for demonstrating compliance. All of this material is available thru the FAA website.

With regards to the older regulations, the couple of links you presented stressbookllc are indeed some of the documents I mentioned. However, the following link a the transportation department (which can be reached thru the faa website) presents full copies of all of the older regulations:


In particular, if you look thru the table of contents, locate the CAM 4 manual and it contains numerous appendices with guidance material. Also, if you look thru the older regulations you will find numerous technical approaches to the regulations.

good luck

RE: 14 CFR Subpart C Section 25-305

crackman and other DER's are the experts here, but I think we should understand how crackman was using the word "interpret". (crackman correct me if I err). The DER's I work with (and especially the one training me as an ODA UM) have explained to me that when they use the word "interpret" in the manner crackman did, they mean that the FAA reserves to itself the authority to "interpret" regulations that are not appropriately detailed. That is one of the primary functions of an Advisory Circular. The FAA recognizes that the regulation can have a number of different methods of compliance. The AC is the FAA "interpreting" the regulation to say that one (or more) acceptable method of compliance is detailed in the AC.

I think crackman would say that when performing the compliance review to compare the design data to the regulations, if a regulation requires "interpretation" to determine exactly what has to be complied with, then only the FAA can make that call. I expect to be corrected if I am off base.

In the case where tbuelna used the word "interpret" I think I would use the words "compare and evaluate" instead. The idea is that the regulation should give the standard, and the DER either finds compliance with the standard or not. If the standard itself is fuzzy (meaning none of the sources crackman identified such as an AC, seminar, policy letter, white paper, order, etc exist to clarify the standard) then only the FAA can clear up the fuzziness of the standard.

RE: 14 CFR Subpart C Section 25-305

I agree with debodine. As both a DER and DAR, I don't interpret the regulations, I read them, and evaluate the data presented by the applicant. If my evaluation shows that the applicant has shown compliance to the regulation, then and only then can I sign off on an 8110-3. If there is some ambiguity within the regulation, further evaluation is necessary with consultation of Advisory Circulars, FAA Orders, Letters of Interpretation, or consultation with my FAA advisor. The words have meaning, don't read more, or less into what is written. For newer regulations, don't forget that the Federal Register will contain a lot of information regarding what the FAA thinks the proposed and final rules mean. As stated, the older CARs and related CAMs can provide insight. Just be careful that the intent of the regulation didn't change form CAR to 14 CFR. Most were simply recodified, but several were changed, or have been changed since ~1964 when Part 25 was codified.

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