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Is an AMOC needed for a double drilled hole?
2

Is an AMOC needed for a double drilled hole?

Is an AMOC needed for a double drilled hole?

(OP)
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.

RE: Is an AMOC needed for a double drilled hole?

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 ?

RE: Is an AMOC needed for a double drilled hole?

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!

RE: Is an AMOC needed for a double drilled hole?

(OP)
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.

RE: Is an AMOC needed for a double drilled hole?

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.

RE: Is an AMOC needed for a double drilled hole?

(OP)
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.

RE: Is an AMOC needed for a double drilled hole?

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.

RE: Is an AMOC needed for a double drilled hole?

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

RE: Is an AMOC needed for a double drilled hole?

(OP)
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.

RE: Is an AMOC needed for a double drilled hole?

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

PK

-----
Nert

RE: Is an AMOC needed for a double drilled hole?

(OP)
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.

RE: Is an AMOC needed for a double drilled hole?

let us know the outcome, please.

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

RE: Is an AMOC needed for a double drilled hole?

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

RE: Is an AMOC needed for a double drilled hole?

(OP)
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.

RE: Is an AMOC needed for a double drilled hole?

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.

RE: Is an AMOC needed for a double drilled hole?

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

RE: Is an AMOC needed for a double drilled hole?

(OP)
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.

RE: Is an AMOC needed for a double drilled hole?

how can someone other than the OEM do the analysis ?

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

RE: Is an AMOC needed for a double drilled hole?

(OP)
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.

RE: Is an AMOC needed for a double drilled hole?

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 ?

RE: Is an AMOC needed for a double drilled hole?

(OP)
One of the documents the FAA response referenced was FAA order 8110.103. I'm going through it now and it seems to have a lot of info and examples of how to accomplish getting an AMOC.

https://www.faa.gov/documentLibrary/media/Order/81...

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

RE: Is an AMOC needed for a double drilled hole?

Kirby,

This is a common issue, where the OEM wants an arm and a leg, which is why companies like the one I work for exist. To substantiate your AMOC you need FAA approved data, which an FAA DER can provide.

You do not necessarily need a ton of analysis. However the bottom line is you need very solid reasoning why the repairing action you are doing meets the minimum level of safety put forth by the FAA.

First, determine if you need a static AMOC, and DT AMOC (is this even FCBS for the 777?), or both. I looked at the AD on the FAA RGL and it looks to me per paragraph (g) that the inspections do not need to start until some threshold per the SB (which I do not have to look at). There are other follow on inspection cases, but all after the initial SB except for (l) which has a hard time of 48 months. Remember, you do not need an AMOC until you will be out of compliance, which is the inspection threshold.

It looks like you are trying to take advantage of paragraph (p) to terminate the need for inspections, but your repair has gone awry.

In this case, all you should need to do is show that your design is at least as good as the SB repair statically (and in terms of DT if it is FCBS). And then present that to the FAA saying that you have removed the unsafe condition, except using an 8110-3 repair design similar to the SB / SRM. An additional comparative analysis may be your friend.

Here is what I would do:
1. Design the repair using your engineering judgement. Consult a reliable DER / other engineers who have TR sleeve experience / FAA as required
2. Statically substantiate it like any other repair is required to be
3. In your substantiation document, make an AD compliance section (this should be in there anyway). Be very detailed. Go over each paragraph of the AD and list how it is affected and why, and also describe the normal requirement and how you have affected it. In this case you would say, these inspections are required at some time in the future. They could be terminated using XXXXX but we have deviated. The substantiation herein shows the deviation meets the minimum level of safety etc.
4. Write an AMOC request letter outlining everything.
5. Send it to the FAA ACO handling post production support for the 777 in a package with the repair plan, 8110-3, and substantiation.
6. Be ready to answer questions.

AMOC requests can be daunting but they're really not that bad. I don't have expertise working this type of structure and not much experience with the 777 (I mostly work older aircraft support). But this is the type of thing certain companies specialize in.

Just make sure everything is clear, well presented, and has sound engineering behind it. The FAA will work with you as long as you have your ducks in a row so to speak.

Also, don't just take my word for it either - it is the ACO's JOB to help you through this.

Keep em' Flying
//Fight Corrosion!

RE: Is an AMOC needed for a double drilled hole?

(OP)
So Here is the response from the FAA to my questions about this (posted below). I asked him if I could post his response and he allowed it.

...

Hello Kirby

In general, anytime you are doing work at an AD affected area/location and or are deviating from the AD requirements then you will need an AMOC. This includes the service information and its applicable instructions required by the paragraphs of the AD. CFR 39, specifically 39.15, 39.17 and 39.19 provide the general requirements for when an AMOC is needed; including paragraph (r) of the AD. Also, you can find more detail information in FAA Order 8110.103

From what you provided, in the course of replacing the inner wall panel, as required by the SB work instructions (which is mandated by the AD), you needed to deviate and make minor repair to the panel. I would assume this is a deviation to the SB instruction of installing a new inner wall panel and also for the installation of the panel, or repairing the panel to make it equivalent to a new non-damaged panel. Thus, you would need to request an AMOC.

Please feel free to contact me with any further questions or comments.

FAA Specialist

...

Original email that I sent:

...

Mr. FAA Specialist,

During the installation of a Trent 800 thrust reverser duct inner wall acoustic panel per Boeing SB 777-78A0094 a hole was misdrilled. I would like to create a minor repair for the damage however this SB is associated with AD 2016-11-16. The repair does not affect the intent of the SB which is to replace the panel as the terminating action for a repeated inspections in earlier SBs. Is an AMOC needed in this instance? In discussing this with other engineers, the point was made that because this is controlled by an AD any future repairs to this panel would likewise require an AMOC. Would you agree with that position? I have included a copy of the Engineering order for you to see the specific damage.

Thank you for your assistance.

-Kirby

...

Kirby Wilkerson

Remember, first define the problem, then solve it.

RE: Is an AMOC needed for a double drilled hole?

another wrinkle to your situation ... the AD replaces the liner on OEM B/P provisions (I assume). what if those provisions have repairs incorporated into them (like repairing your misdrilled hole) ? what if repairs are needed in the future (does the SRM still apply) ?

If the AD applies to repaired provisions, could you repair the misdrilled hole, reinstall the original liner, sign the plane out, then sign it back in to do the AD ?

The point to the AD is to replace the liner, yes? incidental repairs are, well, incidental.

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

RE: Is an AMOC needed for a double drilled hole?

(OP)
RB1957,

What's B/P? Bondment Panel? It's not a liner. It's the inner duct sidewall. The main structural component of the thrust reverser. We have asked the question about performing SRM repairs to the post AD reverser on the inner duct sidewall requiring an AMOC. Have not had a response on that yet. This repair is not covered by the SRM though. If it was just oversizing a fastener that would be covered (by the AD/SB even), but it's beyond that, with two holes too far apart to make one larger hole or too close together to be within the allowable damage. (okay at 2.5D, we're at 1.5D) So the only option is a doubler, which is easy enough to substantiate as providing an acceptable level of safety to restore one hole.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

RE: Is an AMOC needed for a double drilled hole?

Blue Print ... ie original design.

liner, duct ... it's all the same to a non-nacelle guy !

so there's no SRM repair for your condition. But earlier discussion seemed to say it wouldn't matter if you were doing an AD incorporation ... no deviation from the AD is permitted.

I was posing the thought, what if there was a pre-existing repair ? would you have to go back to the OEM to get a specific AD incorporation ? what about future repairs ?

If you're worried but the fatigue of two holes close together, then the doubler repair will be messy. is it possible to do an interference rework (like FTI) ? If static concern, is there one ? (but proving there isn't one is the rub !)

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

RE: Is an AMOC needed for a double drilled hole?

(OP)
Since the fastener is holding on a fitting that is on the other side of the panel, and the double drilled hole only affects this one side, my primary concern is pull through and a doubler will restore that. And with a couple added fasteners, there should be no issues with transferring any shear loading. It feels like a straight forward repair other than for regulatory issues.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

RE: Is an AMOC needed for a double drilled hole?

"a straight forward repair other than for regulatory issues." ... aye, there's the rub !

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

RE: Is an AMOC needed for a double drilled hole?

Very interesting topic. Kriby, what tools do you folks use to statically substantiate a composite repair doubler? FEM or Hypersizer?

RE: Is an AMOC needed for a double drilled hole?

(OP)
I use an Abbaris spreadsheet. Over the scale of the fastener location the panel is effectively a flat plate. FEM would be a sledgehammer to crack walnuts.

Kirby Wilkerson

Remember, first define the problem, then solve it.

RE: Is an AMOC needed for a double drilled hole?

Kirby - where do you get the material properties for the parent and repair materials to put into the abaris spreadsheet? And how do you know they are valid for the parent material?

RE: Is an AMOC needed for a double drilled hole?

this s/sheet is "just" a compliance model, yes?

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

RE: Is an AMOC needed for a double drilled hole?

(OP)
I don't know what you mean by that. It has the stress strain behavior of selected materials and you build up the OEM ply layup to define the stiffness and load capability of the laminate as designed. You could then build up a repair laminate to compare stiffness and calculate MS. In this case I used the load capability of the OEM laminate and compared it with a titanium doubler.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

RE: Is an AMOC needed for a double drilled hole?

ok "equivalent strength" ... I thought you might be looking into the fastener forces (into the dblr) ... this is the key fatigue issue with adding doublers to fuselages.

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

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