Composite Structures certification requirements 1

[Tads]

Hi All,

I have a quick question in regards to the certification of composite aircraft structures. I'm trying to find more info on certification requirements for structure.

FAR 25.603 seems to be a little ambiguous on material selection, and the fatigue section tends to focus more on alloys.

Can anyone help?

 

[abuhanifah]

Please refer to FAA Advisory Ciucular (AC 20-107A).

 

[douchaa]

Check out the FAA memorandums and DOT/FAA/AR-... documents

eg.,
Memorandum 21-Dec-2001: "Static Strength Substantation of Composite Airplane Structure"

etc.

 

[ChrisSpedding]

...and for composites material data go to MIL-HDBK-17.

 

[JeanAlesi]

Design/certification Considerations in Composite Aircraft Structure
Joseph R. Soderquist
National Resource Specialist, AIR-103

 

[Tads]

Hi All,

Thanks for all the great responses, very useful information. Have looked at FAA Advisory Ciucular (AC 20-107A), and to MIL-HDBK-17. Both have good information for general test program requirements.

Jean Alesi - had a search for National Resource Specialist, AIR-103, but couldn't find it. Do you know if it's available online?

The impression I am getting, is that there is a heavy reliance on tailoring to suit each OEM. I have read through the history of the Eurocopter Squirrel composite certification to FAR 27, which was a good insight as well.

Has anyone got any other examples of a case study of a composite certification case study? I'm just trying to get my head around the various approaches.

Joe

 

[SWComposites]

Joe,

Do you have a specific application that you want to certify? Is there a specific topic that you need help with? I'm quite familiar with the process and requirements, but its a very big topic.

Steve

 

[Tads]

Steve,

The question was initially a general one, to try and understand what a reasonable testing program is for primary structure - such as a rotor blade. I have seen a few test programs, and from the guidance in AC 20-107A relies fairly heavily on the experience of the manufacturer.

Specific questions, in relation to primary structure are:
1. Do ultimate and limit load conditions normally need to be demonstrated by component static test? Or can coupon testing be relied on?
2. What level of damage is normally assumed to be in the structure? I have seen varied approaches in the civil area from tooling marks, to intended delamination.
3. For fatigue, what is a typical cycle goal to justify safe life? 2 times expected service life is something which I commonly see, however I have also seen fatigue issues being discounted when laminate stresses are below 40% of ultimate (like an equivalent alloy endurance limit).

If anyone could provide examples from their experiences in this area, it would be greatly appreciated! I am quite comfortable with material, and process certification requirements, it's the actual strength issues I'm confused with!

Regards,

Joe

 

[SWComposites]

Joe,

1. Limit and ultimate load conditions normally need to be demonstrated by test unless the manufacturer can produce test data on sufficiently similar structural components. The catch here is in figuring out what is similar. As a very general rule of thumb, if two structures have the same or very similar materials, the same basic structural configuration, same type and magnitude of loading, same approximate size and has the same critical failure modes, then they are similar, otherwise they probably are not.

2. For ultimate load tests, the structure should contain damage at the critical locations that is the worst case of either 1) defects that would be acceptable per the applicable process specfication or 2) accidental damages (considering any potential growth) that will not be reliably detected by the aircraft structural inspection program. Generally fatigue testing for 2 "lifetimes" is used to demonstrate that these levels of damage do not grow in size (in composites, if the damages do grow then you will be stuck with a very nasty analysis, testing and inspection program).

3. Limit load tests for structure that must meet damage tolerance requirements should contain damage sizes and shapes that can be reasonable expected to occur but will not be immediately obvious to the pilot. Ofter these damages must be fatigue cycled for at least the loadings expected to occur for over two inspection intervals.

4. The only case that I have heard of where someone certified a composite structure for "safe life" (where damages grow under fatigue loading) is for some rotorblades. And I think that they used more of a damage tolerance approach (damage growth prediction linked to an inspection program; the safe-life approach is missing the inspection part). They had the potential for delaminations to occur and grow.

Two lifetimes (usually with a small load enhancement factor) is usually acceptable to the FAA to demonstrate "no-growth" of damages. If there is growth, then the number of cycles is going to have to be linked to the frequency and reliability of the planned inspection program, and will have to be negotiated with the FAA ACO.

You might look at AC29-2C, especially the Misc Guidance section 8 on Substantiation of Composite Rotorcraft Structure. I think that a major update and expansion of MG8 is coming out, but the current version is on the FAA web site.

You can also look at the paper on the 777 Empennage Certification Approach on my web site at:

http://www.swcomposites.com/Papers.htm

Also there is a large chapter on Damage Resistance, Durability and Damage Tolerance in Mil-Hnbk-17, Volume 3, Rev F.

Hope this helps,

Steve

 

[Tads]

Thanks very much for the advice Steve, just what I was after. I'll have a good read of your paper, very helpful stuff.

Joe

 

[pipsqueek]

We are just about to get Transport Canada certification, and then FAA certification on an all composite ballistic cockpit installation.
We are not a manufacturer but used an AMO'd composites manufacturer. Because the composites we used were not covered by a standard off the shelf spec, ie Mil-spec we had to write our own materials specs including test specifications, this can be very costly. You cannot use other OEM specs for composite materials if you are not the controlling authority on that specification.
Another aspect is making sure all other tier suppliers are coverd and audited by your prime manufacturers QA department, thus it is necessary to use an AMO'd prime.
We also had to physically test all our components with the certification authority as witness.
When dealing with certification authorities always double the time you think it will take

 

[Sparweb]

Tads,

You mentioned that you'd seen a document about the composite certification of the AS350 - Ecureuil. I would be interested in seeing that, if you don't mind. It might shed some like on better approaches to some mods we've done to the belly panels - and mods we may do in the future.

In what form is the document? Paper or electronic?


STF

 

[Tads]

Hi,

The article I read was in the journal: "American Helicopter Society Proceedings", and was titled something like: "Certification overview of the AS350 Ecureuil project" - you should be able to find it electronically through most libraries. Let me know if you can't find it...

JOe