Is an AMOC needed for a double drilled hole? 2

[KirbyWan]

Howdy all,

So we are accomplishing an SB to replace the inner wall of a thrust reverser. During accomplishment one of the fastener locations was double drilled and needs to be repaired. Because the SB is mandated by an AD, are we required to get an AMOC to repair the double drilled hole, or can we perform the repair using FAA accepted data and move on?

Thanks,

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[rb1957]

IMHO I'd repair IAW standard procedures and, as a side-bar, consider the impact of the repair on the issue creating the AD and it's impact on the mod structure.

another day in paradise, or is paradise one day closer ?

 

[LiftDivergence]

Depends on the language of the AD. Which one is it? There is no interpretation - the AD is a legal document and compliance is based on the letter of the AD.

If the AD states "replace the thrust reverser inner wall in accordance with SB XXXXX" compliance with that paragraph is based on having that exact configuration shown in the SB. If you have a deviation (even one fastener type etc.) from the SB, you no longer have the configuration deemed acceptable by the FAA. So yes, you would need an AMOC.

I suspect they want the inner wall replaced because the design is defective in some way, so as long as you put the new part in, the deviation you mention will not really impact the safety - so your AMOC substantiation will be easy.

Also, is the inner wall replacement itself mandated, or is the inner wall replacement considered a terminating action to inspections mandated by the AD?



Keep em' Flying
//Fight Corrosion!

 

[KirbyWan]

Thanks for the input all.

This is an RB Trent 800 TR inner wall replacement per Boeing Service Bulletin 777-78A0094 which is the terminating action for repeat inspections per SB's 777-0071 and 777-0082 per AD 2016-11-16.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[SWComposites]

Does the SRM contain a repair for a double drilled hole? If so, then you should be able to use that repair.

If not, do you have some sort of other "FAA approved data" to cover a repair?

If no other data, then it seems like you would need to request a repair from Boeing.

 

[KirbyWan]

I can substantiate a repair using FAA accepted data. The way the hole is double drilled doesn't lend itself to standard repairs. The hole is located correctly on one side but it was drilled at an angle so when they corrected it, it was two hole locations on the other side of the bonded panel. I'm going to pot it and then put a doubler on the surface with two holes. There is a fitting on the other side.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[dgapilot]

What LiftDivergance said. An AD is a REGULATION and compliance must be exactly as stated otherwise you have not complied with the AD. Any deviation from the "as written" AD requires an AMOC.

 

[SWComposites]

And where do you find "FAA accepted" dara for filling a hole with potting that is right next to a loaded hole?

 

[KirbyWan]

The potting is not carrying the load, the doubler is. This is a 1" thick panel with core and composite faces so potting is at least as strong as the core material. Doublers have been used to repair aircraft before. Did I miss something SWComposites?

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[inertia4u]

I recommend you call the ACO or the engineer listed on the AD to get your answer. Everything posted here is speculation.

PK

-----
Nert

 

[KirbyWan]

inertia4u,

I have just sent an email asking these questions. I wanted to get my head around the issues and hear from other more experienced engineers before I talked to the FAA. Thanks for everyone's input.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[rb1957]

let us know the outcome, please.

another day in paradise, or is paradise one day closer ?

 

[SWComposites]

The core is not the issue, rather the double hole in the composite skin significantly reduces the in-plane net section (bypass) strength. Depending on the joint, loading and doubler design, the doubler may or may not be effective in reducing the stress in the composite skin. Effective doublers for composites are particularly tricky to design.

What joint on the inner wall is this discrepant hole at?

SW

 

[KirbyWan]

Here's the damage description and repair design:

The connection is at the aft edge 6.5" from the upper edge of the main portion of the acoustic panel. There is a titanium aft cowl skin about 6" wide and there is a split where the upper section is curved to follow the contour of the bifurcation panel. At this split there is a fitting the SRM calls a stiffener to join the split and has 6 fasteners that connect it to the panel. One of the forward most holes was misdrilled at such an angle that there is a single hole on the inner surface and two separate holes on the outer surface about 1.5D apart. I can correct the hole on the inner surface by oversizing the Hi-Lok from HL11 to HL111. My repair design was to fab a .071 thick 6Al-4V doubler and add two additional fastener locations forward of the damaged hole and using EA934NA to adhesively bond it in addition to the fasteners. I did an analysis using Abaris spreadsheet and the load carrying capacity of the panel skins is around 4k lbs/in and the titanium should easily carry that and avoid a knife edge condition. I think I could have gone thicker, but I matched the thickness of the aft cowl skin. I of course chamfered the forward and aft edges for aerodynamic smoothness and it will be less thick then a hole repair they use on the compression fittings (they let it stick up in the airflow .125").

What do you think of the repair? Please add anything you think would be helpful, I haven't listed every step though. We're a small shop and I've never had a much more experience 'grey beard' to learn from so I've always been unsure if I'm missing something critical and a lot of what I know I've picked up here from you, rb1957 and many other, with special mention going to wktaylor who I think has forgotten more about engineering then I've learned about it so far.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[romeocharlie]

Well, you need an AMOC for that. I have seen Boeing issued a repair (i.e. misdrilled holes or whatever the reason) on the replaced/new inner wall with 8100-9 and FAA AMOC.

 

[Sparweb]

Hi Kirby,
At first when I saw your question I was interested to read a meaty topic, but as I read I realized that coming to us could lead you into a mistake, even though all members here have good intentions. I've done a number of AMOC's, so I was almost eager to start offering suggestions at once, but then I thought the better of it. You're in a situation where you are best to rely on people actually involved in solving the issue, from the FAA ACO, the rest of your engineering staff, and crew in the shop, maybe an ODA has become involved. You have a repair design in your hands, and people around you who can evaluate how it will work.

Your contributions to Eng-Tips are appreciated, too. But the time for homework is over. This is the final exam. No cheat sheets. Show your work.

STF

 

[KirbyWan]

So I did contact the FAA and yes an AMOC is needed. I asked if I could share his response here and if he's okay with that I'll share the full text. So my next issue is how can I get an AMOC for the least amount of money while maintaining turn time. Boeing wants more than $12000, 5+k for the repair analysis, And 7k for the 8100. Can I just get a DER to provide an 8110 and then submit it to the FAA for AMOC approval?

Thanks!

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[rb1957]

how can someone other than the OEM do the analysis ?

another day in paradise, or is paradise one day closer ?

 

[KirbyWan]

rb1957,

The play lay-up and materials are fully defined by the SRM. So even without the load data, shouldn't I be able to show that my repair meets or exceeds the strength (and stiffness) requirements of the original design?

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[rb1957]

yes, but the issue seems to be not exactly complying with the AD instructions. I suspect that the SRM repair doesn't apply within the AD mod (else they'd've written it up that way and you wouldn't have a question).

I quite get where you're coming from ... if I can do the SRM repair on any other piece of structure, why not here ? I guess the answer is because the AD doesn't permit you to, and to develop an AMOC requires doing a tonne of analysis.

This business drives you crazy ! (or do you have to be crazy to get into it in the first place ?)

another day in paradise, or is paradise one day closer ?