Composites in SRMs? 1

[madvlad]

Ok I'm just curious and throwing this out there to express my curiosity...

That 787 is going to be flying pretty soon, and they've spent the last five years hyping up how easy to inspect it will be, ie that if you can't detect any damage by tap-test, it's ok. That got me thinking...

That CAN'T be realistic, that's a fabrication by the PR department. So...what's the 787 SRM gonna look like? I mean...if you get a scratch in aluminum, you blend it out, within the limits of the SRM. Or make a doubler as shown in such and such drawing.

But none of that applies to composites. Even the structural identification sections, are they going to identify every single lamina and resin used on the whole plane? They usually do for fairings and nosecones etc...

I've seen composite fairings where the allowable repair is to cut the damaged facing, fill the core, and patch it up...and that's fine, but that's a lot different from primary structure.

So I'm just curious as to what an SRM looks like on a FAR 25 composite aircraft...are they going to trust joe mechanic, AME, to mix the epoxy in the right proportions, at the right temperature, at such and such humidity, on primary structure, on an airplane that carries 300 people?

What does the military do on composite aircraft? Are field personnel allowed to make composite repairs? Or do OEM contractors do all the repairs? I have no access to military AMM/SRMs obviously, I'd love to read one and see how they approach the issue. Has anyone got one they can share? lol..

 

[FieldTeam]

The USAF uses T.O. 1-1-690 "General Advanced Composite Repair Processes Manual". This is a general Technical Manual that supplements the specific SRM and encompasses repairs to graphite, buron and aramid/epoxy advanced composite materials.
Personnel qualifications and certifications are addressed in this manual and allow for field level repairs.

 

[bf109g]

I recall Airbus saying that they would use typical aluminum doubler repairs installed with rivets/hi-loks for the GLARE skin used on the A380 fuselage.

I would think that repairs to the 787 fuselage skin would necessitate pre-cured repair doublers that would be bonded onto the airplane. It follows that you would have to have the correct pre-cured repair doubler for the damaged area, say around a door. What happens if you have damage in an area where there is no pre-cured doubler available? I guess you'd have to make your own doubler in accordance with certain ply charts, prepreg or wet layup over a mold having the correct curvature for that area. And do it per processes shown in the SRM or internal Boeing processes.

Sounds to me like an out of service event for fuselage skin damage on the 787 could last quite a long time. Anybody else have ideas on this?

 

[MNLiaison]

I've heard they have provisioned for blind bolted Titanium doublers for strength 'restoring' repairs on the fuselage solid laminate structure. Bolted repairs seem to make the most sense for AOG type situations with bonded repairs possibly being performed at checks.

Bonded repairs pose a problem as there currently is no way to check the strength of the bond line. So how do you provide disbond arrestment features to meet damage tolerance requirements? You have to analyze the structure with the repair missing to see if it can sustain limit load or some other derivative.

Airbus provides specific size limits for bonded repairs to primary structure based upon the analysis of the repair missing and still sustaining limit load.

Here is a link some presentations given at the FAA Composite Damage Tolerance Workshop in 2006.

https://www.niar.wichita.edu/niarwo...WorkshopJuly2006Chicago/tabid/99/Default.aspx

 

[SWComposites]

"ie that if you can't detect any damage by tap-test, it's ok." WRONG! DON'T EVEN START TO THINK THAT. READ THE SRM WHEN IT COMES OUT.

"what's the 787 SRM gonna look like?" ITS GOING TO BE BIG AND COMPLICATED.
IT WILL INCLUDE PREPREG AND WET LAYUP BONDED REPAIRS, AND BOLTED REPAIRS

"Even the structural identification sections, are they going to identify every single lamina and resin used on the whole plane?" YEP

I've seen composite fairings where the allowable repair is to cut the damaged facing, fill the core, and patch it up...and that's fine, but that's a lot different from primary structure" YEP, THAT'S CORRECT IT IS

"So I'm just curious as to what an SRM looks like on a FAR 25 composite aircraft..." GO GET A B777 OR A340 OR A380 SRM AND HAVE A LOOK

are they going to trust joe mechanic, AME, to mix the epoxy in the right proportions, at the right temperature, at such and such humidity, on primary structure, on an airplane that carries 300 people?" THIS IS ALREADY ALLOWED ON BOEING AND AIRBUS AIRCRAFT FOR PRIMARY STRUCTURE (EMPENNAGE, FLAPS, ETC).

"What does the military do on composite aircraft? Are field personnel allowed to make composite repairs?" YES; REPAIRS ARE DONE IN THE FIELD AND AT REPAIR DEPOTS.

Or do OEM contractors do all the repairs? NO

 

[bf109g]

Titanium bolt thru repairs...I wonder what the pull-up limit will be for those doublers. Titanium is tricky to roll, even with the correct equipment. Especially if your maintenance people aren't used to using titanium.

 

[madvlad]

MNLiason some of those presentations are pretty cool, thanks for the link. The UAL one is great, and I would have really liked to have seen the Boeing presentation. It's very interesting that there hasn't been any damage reported yet to any 777 CFRP floor beams.

SWComposites: getting these things is difficult sometimes. If you can tell me where to find a 380 SRM (with enough torrent seeds to actually finish the download to burn it) I'd be all ears! But yeah I'll be looking at the 777 one.

However, I get the impression from other SRM's I've read, that what you can do in the field on those components is pretty minimal (compared to their metal equivalents), and you have a small number of very specific repairs, else "contact OEM". The other thing of course is that all hot-cured processes are out. I'm interested in how that will translate to an entire composite airplane.

Sure, I'll read the 787 SRM when it comes out, but that'll be another 2 or 3 years at least.

About the tap test and the notion "if you can't see the damage, it's ok": I agree with you, that's sketchy... But go look at the Boeing presentation in the link from MN, that's their story and they're sticking to it! Not even tap test...visual inspection only!

I haven't worked with composites (on a professional level) yet, I do boring old equipment racks and interiors stuff, because there's no structural composites work at my current workplace. It's just something I'm really curious about, because my experience so far is probably only 10% applicable to the 787 and A350.

 

[thruthefence]

"joe mechanic, AME, to mix the epoxy in the right proportions, at the right temperature, at such and such humidity, on primary structure, on an airplane that carries 300 people"

Do you trust "Joe Mechanic" to do any of the thousands of other mission critical tasks that are preformed 24-7 by this batch of troglodytes?

Please understand that the 787 will likely NOT be maintained by "Homer & Jethro's Flying service" out at the crop duster strip, where Homer has to clean out his snuff spit bucket, in order to mix the epoxy.

 

[madvlad]

Do I trust a mechanic to do a repair? Well...I trust a mechanic to bolt two parts together. I have a really hard time trusting him to glue two parts together.

The problem is that A) bonded joints are more sensitive and they are MUCH MORE DIFFICULT to inspect for flaws or imperfect bond than conventional fasteners, and B) they occur more frequently when you can't do the repair on a bench in a controlled environment (like you can with a flap or spoiler). It's not an insurmountable problem; I'm saying that I'm interested in how Boeing et al will go about it.

p.s.
In those presentations MN linked to... There's a hydrogen fuel tank which failed catastrophically because somebody accidentally left a piece of tape in between the core and the facing during layup, and it delaminated. That happened to NASA, not Homer and Jethro.

 

[thruthefence]

And who is pretty much running NASA? Like you say, Not your run-of-the-mill aircraft mechanics.

 

[KirbyWan]

Most aircraft repair stations have a bonding shop with a controlled contamination area with a team lead (and possibly a whole team) who has years of experience doing bonding operations. We control materials that are time and temperature sensative using a reporting method to insure we do not use expired adhesives. I have seen rolls of 350°F cure adhesive tossed in the garbage because we did not use them up in time. Yes, if you ask a mechanic who may be an expert at driving rivets and shaping metal to do a bonding operation he's never done before, he may not make an effective bond, but if a shop is asking unqualifed people to be bonders, then that shop has a management problem. That same mechanic (assuming he has the craftsmanship skills that make him good at shaping metal and driving rivets) under the direction of a master bonder will probably be able to make bonds that meet the repair requirements. Craftsmanship (and craftswomanship, some of our top bonders are women) is critical no matter what repair methods are being used.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[bf109g]

You also need to consider the organization. If you are strictly a line and light check operation, you may or may not have the ability to accomplish bonded repairs. Having worked at one of these, I can tell you that I've had a few repairs fall off in my hands when I touched them.

Line ops will always resist removing the component from the airplane if the weather is decent or it can be rolled into the barn.

What makes it doubly fun is when you throw a union into the mix. The airline may purposely limit the capabilities of the hub to avoid union classifications, i.e. sheetmetal, composites, systems, avionics, etc. Then they may agree in the contract that all Mx that can be accomplished before a heavy C or D check has to be given to the union. So you end up having generic personnel with little or no experience accomplishing repairs with little or no facilities in which to accomplish the repairs. Hence composite repairs lying on the ramp.

 

[berkshire]

madvlad
I ran a repair station for many years specialising in fiberglass then later advanced composite repairs to aircraft.

One of the paragraphs of page one of the manual for the station says it all:- " It will not maintain or alter any item or article for which it is rated if technical data, equipment, materials,or TRAINED PERSONNEL are not available".

You just, do not, throw a rivet smasher onto a bonded repair unless that person has been well trained in cleaning and bonding a repair.
And yes epoxy is mixed in the shop using calibrated gramme scales by trained personnel. Some manufacturers not trusting their mechanics to even do that right, make prefabricated sealed epoxy kits where the hardener is in a sealed compartment in the kit and activated at the time of mixing.
Then of course there is prepreg if a manufacturer cannot trust anybody to mix resin right.This is a hot process and has been done in the field for years.
Bottom line there are stations out there that can do the job right. They are just not the kind of stations BF 109 refers to.They are stations with people like SW Composites and Kirby Wan present.
B.E.

 

[thruthefence]

see:

14 CFR 65.81 You can't do the work without the training.

see also:

14 CFR 43.13 Can't do the work without the tools & tech data

The devil is in the details, Education & tooling are the first things to be scrimped upon in a bad economy. Don't blame the guys on the floor.

As a non practicing Engineer, and an Aircraft mechanic & business owner for 35 years, I am frankly afraid of the trend to composite construction for the very reasons sited. It is unforgiving of error. And as stated, the errors can be introduce on alot of levels.