FAR 25.613 and A/B Basis Applicability 2

[skiaddict]

Hello everyone,

I'm a bit new with dealing with FAR 25.613 but I have a question, or a request, that many of you may be able to help with.

I work for an aircraft component supplier (mainly landing gear hydraulic controls, actuator). The airframer is attempting to flow down some requirements regarding applicability of FAR 25.613 and A/B basis materials to our components. My understanding is that FAR 25.613 relates mostly to critical structural components of the aircraft and therefore, would not be a firm requirement for the types of parts we make. The reason this poses a problem for us is that we're using 17-4 CRES at H1025 in our assemblies and only an S-basis property exists in MMPDS. The sampling and testing program for us would be prohibitively expensive.

Does any one have any insight on documentation that relates to the intent of FAR 25.613 and the regulations regarding A/B basis material? It seems to be generally vague with the exception of some white papers which speak about A basis for critical aircraft structure and B basis for redundant critical structure.

 

[skiaddict]

After doing some more research, I supposed the better question is:

Does an S-Basis material from MMPDS satisfy Section A of FAR 25.613

FAR 25.613

a.) Material strength properties must be based on enough tests of material meeting approved specifications to establish design values on a statistical basis.

 

[SWComposites]

"Does an S-Basis material from MMPDS satisfy Section A of FAR 25.613"
> no, it does not. The FAA specifically removed the inclusion of S-basis values in 25.613 some years ago. S-basis values do not have a statistical basis.

> you need to prove that either a) your component is not flight safety critical (hence not needing a stress analysis and allowables), or b) you conduct sufficient system level tests at high enough loads to certify be test.

 

[skiaddict]

Ok, fair enough. If that's true, then I guess I'm having a hard time understanding why the S-Basis exists.

In MMPDS,
Section 1.4.1.2 "Statistically Calculated Values"-
Statistically calculated values are S (since 1975), T99 and T90. S, the minimum properties guaranteed in the material specification, are calculated using the same requirements and procedure as AMS and is explained in Chapter 9.

The way that the FAR is written, it appears that Section A is to ensure that materials with design properties that are established on a statistical basis are being used. Section B then goes on to speak about the requirement for A or B basis materials for critical structure. If A or B basis were required for all components, why have Section A at all?

Also,
> you need to prove that either a) your component is not flight safety critical (hence not needing a stress analysis and allowables), or b) you conduct sufficient system level tests at high enough loads to certify be test.<

Where is this written? Or is this more of a tribal knowledge thing?


All of our components will be proof, burst, impulse and endurance tested to FAR 25.1435 for qualification. For serial production, each unit will then be functionally tested and proof pressure tested before shipment. We do not have any flight critical components.

 

[RPstress]

Long ago (about '97/'98) on 767-300 Boeing stopped using S-basis for cert calcs. It was recognised that S-basis was a safe minimum but it is without statistical validity. One of their tasks was to track down where S-basis was used (inherited from 767-200 calcs done in the '80s) and specify a mini test programme to get a B-basis value with a minmum of tests for an adequately high value. S-basis is still quoted in MMPDS, but it should only be used for non-cert calcs or by material suppliers, for whom it still holds some relevance (not entirely sure why, but I've seen them quote it in the odd discussion).

 

[skiaddict]

RPstress,

Thank you. I think I have a good understanding now...or at least I think I do.

 

[SWComposites]

There is a whole huge pile of tribal knowledge about aircraft certification that isn't written down anywhere, particularly not in any FAA documents. And to makes matters even more confusing, the different FAA ACOs have different interpretations of various regulations and means of compliance, not to mentions the different interpretations by the various regulators in other countries.

"S-basis" allowables exist for historical reasons in MMPDS. And to (somewhat) understand why the FARs are written as they are, you need to dig up and read all of the previous versions back to the 1950's.

 

[irq]

skiaddict: you can read more about s-basis in the following thread: thread2-283628

It has been pointed out that in absence of A/B allowable instead of developing of expensive statistical allowable we can use e.g. s-basis values and validate the analysis by a test. I have seen this type of approach e.g for metallic brackets, where analysis was done based on s-basis or other material data sheets. Finally 3-5 parts have been tested to include some statistics of material properties variation. Would be a single test assuming high MS not enough to satisfy FAA regulations and show that the part can carry more load than enough? From where can come the statistical requirement on a part level? FAR 25.613 implies the statistic for material properties only.

 

[WKTaylor]

skiaddict, et all...

A fairly good discussion of metallic materials S, A and B basis properties is given in AC25.613-1 MATERIAL STRENGTH PROPERTIES AND MATERIAL DESIGN VALUES

http://www.airweb.faa.gov/Regulator...6D9B0057C55D?OpenDocument&Highlight=materials

Similar discussion for composites is available in various ACs.

Many/most FAA Advisory Circulars are available for free download at:

http://www.airweb.faa.gov/Regulator...rgAdvisoryCircular.nsf/MainFrame?OpenFrameSet

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[WKTaylor]

Note: some organizations have a rational basis for using "S" values as follows...

S(a)cceptable = S / SF

S = S basis mechanical property

SF = safety factor ~= +15% = 1.15 [or whatever their rational SF is)

in this example... S(a) ~= S * 0.87

NOTE: I have seen this SF applied to materials and fasteners, not [A or B basis] listed in corporate or MIL/MMPDS design documents

Mechanical properties often took a significant "hit"... but could be used none-the-less under this rationale.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[MNLiaison]

Linked is a great paper on the MIL-HDBK-5 conversion to MMPDS. On page 8 they specifically talk about the S basis history, how it was linked to A-basis and what its use will be going forward. It should shed a lot of light on the subject.

http://projects.battelle.org/mmpds/Aging Aircraft 2003 MMPDS Paper-final1.pdf

Regards