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.

 

[KirbyWan]

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/8110.103B.pdf

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[LiftDivergence]

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!

 

[KirbyWan]

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.

 

[rb1957]

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 ?

 

[KirbyWan]

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.

 

[rb1957]

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 ?

 

[KirbyWan]

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.

 

[rb1957]

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

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

 

[VN1981]

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

 

[KirbyWan]

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.

 

[SWComposites]

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?

 

[rb1957]

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

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

 

[KirbyWan]

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.

 

[rb1957]

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 ?