Materials, Processes and Controls for Composite Repairs 2

[CompositeGeek]

Greetings All,
I am looking for feedback regarding field and in situ repair of aircraft composite components from an engineer's point of view. Here are some of my MANY Questions.

1. At what point do you say? - Give up do not bother repairing it, just replace the part. What are the factors you weigh?
2. What happens when the materials used in the original component are not readily available? (Your in Sri Lanka, there is no local outlet mall for prepreg). Will you substitute? If so what are your considerations?
3. Do you have preferred processing methods for repairs? Heat Lamps, Bonders, Ovens, Autoclaves, etc?
4. Do you strictly limit repairs to the practices in the SRM? What controls do you apply to manage to your policy?
5. How do you factor in the variation of individual skill in performing repairs? Materials? Processing equipment? What do you do to control these factors?
6. Do your answers to 1- 5 change between primary, secondary and interior structure components? Do they change between General, Commercial and Military Aviation? Should they?
7. What will change when 70% of the aircraft is composite re the Boeing 787, Airbus A350, Lancair Columbia 300, Stealth, etc? Can we continue to conduct field repair the way it is today given the significant increase in composite materials without sacrificing performance, safety, or economics? If you were King of the world what would you do?

I now work in this arena after 20 years in the OEM side of the house, daily I see variation in approaches for all of these topics. I am constantly surprised at the range of opinions on these subjects. And the lack of forums for discussing them. I sure am curious to hear from any and all.

Composites and Airplanes - what was I thinking?

There are gremlins in the autoclave!

 

[berkshire]

Hi Composite Geek,

I will try to give you opinions on the questions you pose:
The answers I am giving you are based on experience running a repair station
Doing advanced composite repairs on type certified class fiber and carbon fiber
Sailplanes, Experimental aircraft such as the Glassair and Lancair types and radomes
I also used to work on things like cruise missiles
I know I am going to get a howl from people working the heavier industries saying yes but these are lightly loaded structures
However I think the same basic principles apply.

Question 1
When the cost of the repair exceeds the value of the part or when you cannot guarantee your repair.
Question 2
If the aircraft is type certified
When you wish to substitute materials, it is best to go back to the manufacturer for a written statement.
I.e. “ Rutapox xxxx can be used in place of Epicote xxx if the physical and electrical properties meet or exceed those for
Epicote xxx or 4 layers of xxxxx 300gram glass may be used in place of 2 layers of xxxxx glass.
If you cannot do this then you are moving away from approved data and would need to do an engineering evaluation.
3/4
The methods for the repair are dependent upon the original method of manufacture i.e. was it open mold, Vacuum
Bagged or autoclaved. It is good practice for the repair station to use the manufacturers SRM where practical.
I personally feel you should ask the manufacture first before you do an engineered solution.
5
Skill levels of individuals rate highly, hand skills are important in this type of work. Particularly on primary structure.

6-non structural interior such as cabin wall panels is not as demanding but is no excuse for sloppy working either.

7
The methods and techniques have not changed much over the years, what has changed is the scope and application of the material, particularly the use of carbon fiber and aramid fibers

 

[RPstress]

I think berkshire's replies are good. I might differ slightly on his answer to 3/4 "..good practice...to use the manufacturer's SRM where practical." If the damage is not adequately covered by the SRM, I would have thought it mandatory to go back to the manufacturer unless you have a licence to design repairs. That's basically what their In-service Support departments are for.

With regard to primary/secondary structure, generally these days we would not do a bonded repair to primary structure.

The issue of bonded repairs to important bits is tricky. Personally I would not like to see a bonded field repair to PS except maybe for a gentle fly-to-repair station where it would be replaced. I think that in the future bonded repairs to PS would be ok in a properly licensed facility with all the correct training, equipment, working practices, etc., in place. However, I would like to see a stricter set of requirements from FAA, etc., on such a facility. Bond integrity probably comes down to process consistency and control, and regulation has a big part to play in ensuring that.

Historically, composite repair materials have been badly handled by manufacturers, with far too many being required. This has begun to be properly addressed, but still doesn't appear to be central to their thinking. If you can't find any of the repair materials specified then as berkshire says, you must go back to the manufacturer to check that the material you want to use is acceptable.

 

[rerig]

Yes, except most of the time the OEM won't help or the assistance is cost prohibitive.

The OEM wants to sell a new part to the operator. That is why we have DERs.

If I sound bitter...I am. Having worked at 5 different Repair Stations and on more different aircraft types than I can count on 2 hands. And having Operators insist we fix the parts because "Do you realize how much a new one costs!".
And having a sales staff that won't take "No" for an answer, because they promised we could repair anything with nothing forever!.

RE: Airbus Engineering: "Very nice repair and substantiation. No we aren't going to approve it. The Elevator skin must be replaced in it's original assembly fixture." 5" damaged area to upper skin surface. SRM 3" max.

Operator wants it fixed cheap. Sales says we fix Boeing stuff with this size damage all of the time. Fix this one. Get a DER Approval or something.

Just one problem... the Aircraft is leased from Airbus and must be returned with Airbus Approved Repairs only.
Answer: You Engineering guys are worthless.

So fire me!

Rant Over.

Rerig.

 

[berkshire]

Hi guys,
Rerig must be working on heavy aircraft from the howl that went up.
Rp stress is correct. It really is mandatory to go back to the manufacturer if your repair is outside the scope of their service manual. I personaly have not had a problem getting repairs approved.But then again most of my repairs are on lighter aircraft by German manufacturers.
Not on things like MD 11s or 747s or Airbuses.
With regard to the remark by Rp stress on bonded repairs, if it is done in a repair station environment, that is already covered by 145.201 and 145.205. I do not think additional regulations are required.
If you are doing it under part 43 of the FARs you had better be using the manufacturers repair manual or a repair schedule from them, because AC 43 13 is really not much help. It just covers the simplest and most basic repairs.
Then again I do not think that most of you are working outside of a repair station or manufacturing environment.
B.E.

 

[CompositeGeek]

Howdy Gents/Ma'ams (as applicable),

I appreciate the feedback regarding my questions. My company supports the repair facilities around the world - there is less control over the issues you describe than you apparently believe.

One customer was attempting to repair a disbond over 60% of the exterior surface of an 6' by 6' cowl panel with a substitue material because they COULD NOT BUY the material in the US, even though the material manufacturer was a US company! Crazy but true. The engineer I was working with just had to fix it (see above from rerig sme kinda deal).

I am amazed at how little the buyers, engineers, and mechanics really know about the materials, how they are used, what restrictions apply, etc. Almost daily we have requests from referencing only advertising property sheets on their requests for quotes and they want me to tell them which specifications the materials meet! Many of these folks actually ask ME for copies of material specs and SRM pages when I ask them for clarification of a requirement.

Many repairs performed in the field are to secondary structures and control surfaces (Probably incorrecty - I think of all control surfaces as primary, I am not an aircraft designer- but I figure if it steers the plane it must be important!). To me these folks are ALREADY DOING primary structure repair.

There are portable Phosphoric Acid Anodize systems used for prepping metalbond repairs. In any other setting the person operating a PAA line for an OEM or process line is certified, and specially qualified - but in a repair scenario I believe all you need is your A&P tag punched. Ask the next tech you talk to how he applies the bonding adhesive - BET HE USES A BRUSH! with thickness tolerances of .0005 to .0012" thickness show me a brush that does that! Then they complain that the adhesive bond line doesn't work right!

Now we are seeing vast increases in usage of composite materials and bonded structures in the industry. In less than three years there will be 2 major commercial aircraft with over 70% composite bodies and wings carrying 200 plus passengers. What happens when you poke a hole into a 787 fuselage with a forklift? Replace the barrel?

We need a seed change in composite repair issues in the aerospace industry. Far more scrutiny in the highly specialized world of composites and more stringent qualifications of people working in this area. Are you as nervous as I am?

The change will only take place when the OEMs and material houses get their houses in order and stop looking for the quick buck that only benefits themselves. And we cannot count on them for the original thinking in this subject.

My experience in an OEM setting was that up to 30% of repair attempts failed to restore the part to the intended designed repair requirements. And that was with all the original tooling and process equipment at my beck and call. How many NDT failures of composite repairs have you encountered? nuff said!

Again thanks for the feedback and allowing me the rant. I really do appreciate your view points but I honestly think rerig has a more realistic view of what is happening than you realize.


Composites and Airplanes - what was I thinking?

There are gremlins in the autoclave!

 

[RPstress]

Quite often you find that the attachment areas of flaps and ailerons are considered primary, but the body of the part is secondary. The usual reason is that having the part actually depart the wing would be very dangerous, whereas having a chunk of it fall off or be ineffective structurally is just dangerous. Hence you could possibly have bonded repairs to a composite flap skin away from the track fitting attachment areas.

With regard to doing a bonded repair to PS, I'd feel unhappy about having it done in even the best sort of environment where secondary structure is repaired today, even in an OEM. On the other hand, I've never actually worked in a non-OEM repair facility, so I confess a degree of ignorance there. I seem to remember Ric Abbot saying that the Beech Starship biz prop-job had repair patches for parts of the wing, where the patches were made by Beech and were a properly controlled part with a bunch of instructions for bonding them on. They did quite a bit of dev to try and make repairs as easy as metallic (unlike Airbus and Boeing).

Re Rerig's rant, I think the SRM damage size limit on some A340 forward wing panels (very thin glass, from memory) was all of a half inch square! Unbelievably impractical.

 

[berkshire]

Quick comment on the starship.

When Beech engineers were looking for ways to do maintainability on the aircraft
One of these Guys was a customer of mine.
So he says to the other engineers, “There are guys out there already doing this, why don’t you ask them.” They were incredulous.
There were people out there, doing this type of repair. As a result he picked my brain
for several weeks and fed the info to the other engineers who largely ignored it as NIH but some of it stuck.