rimary Structure, Secondary Structure & their Loading.

[Bazzo]

Two questions really,
No. 1, CS-25 (yeah I’m in Euro Land) makes no mention of “Secondary Structure” but does refer (but not define)to “Primary Structure” Can anybody quote me the definition and origin of Primary and Secondary Structure which doesn’t come from the SRM? FAR regs would be acceptable.
And No. 2
As secondary structure is not ruled out in CS-25 Subpart 3, my logic says that CS-25.305 (which is “Strength & Deformation” and equates to FAR 25.305) must also be applied to secondary structure. Can anybody prove different?
Thanks for all comments

 

[rb1957]

as dumb as it sounds ... primary structure reacts primary loads ... flight loads, pressure, landing; secondary structure reacts secondary loads ... shrouds, fairings, cabin interiors.

as for no. 2 ... a resounding yes.

as for a formal resference for this, i'd look into the FAR ACs

 

[rb1957]

from AC23.13 ...
"7. What are the definitions for the common terms used in this AC?
...
b. Primary Structure. Primary structure is that structure which carries flight, ground, or pressurization loads, and whose failure would reduce the structural integrity of the airplane."

 

[SWComposites]

There is no definition of either in the FAA FARs. There are some relatively vague definitions in some of the ACs, mostly defining "primary" structure, such as given by rb above. "Secondary" structure is everything else, more or less. And yes, 25.305 applies to all structure; however, the means of compliance is often different between the classes of structure.

Why do you ask?

 

[Bazzo]

@RB Thanks, there’s never a dumb answer only a dumb question. There’s a definition in the (Airbus) SRM (I haven’t bothered to look in Boeing’s) which more or less agrees with what you said, but as CS 25 references “Primary Structure” I wanted an “Authority” reference. I never though to look in the ACs though. Your “resounding yes” to 2 means you agree or you can prove different? I hope the former

@SWC Thanks too for your reply, why do I ask? Easy, someone asked me the question and going by CS-25.305 I was fairly sure that there was no special dispensation for secondary structure (not like damaged structure) but thought I’d throw in out in the forum anyway.

 

[rb1957]

"resounding yes" means "yes, 305 applies to both primary and secondary structure"; as SWC.