What's minor wear?

[KirbyWan]

Howdy all,

I've struggled with (and asked a couple related questions about) this for my entire time designing repairs for aircraft. There is an allowable damage limit for one area of a part. But for everywhere else it says "No damage". But at some point minor, but measurable, damage becomes non-relevant. There are occasionally references for sheet metal parts that permit 10% or even 20% wear and give instructions for blending. In the attached figure the wear limits indicated are permitting the removal of ~.015" from the original V-blade profile (this is on a V2500 TR) which is much less than 10%, but this is a structurally more significant part than most sheet metal details.

However if I have .002" wear on a surface away from the "repairable zone" am I stuck? How do I justify accepting this wear as is and moving on. At what point can I call it negligible damage and move on without further analysis?

I was looking at justifying it by noting the part looks to have an anodize coating and since this is for a wear surface I would expect it hard anodized similar to MIL-A-8625 which could have a coating thickness of up to .0045. I also thought I could justify it by noting the wear damage was less than any expected measurement tolerance the part is being held to. I know that's an assumption since I don't have the drawing, but I would be surprised to find an extrusion with a thickness tolerance tighter than ±.010". I'm also assuming the part wasn't fabricated at the minimum material condition and my additional .002" might push it beyond that.

Is there an FAA or SRM (Goodrich or Airbus A320 in this case) reference for "below this call it negligible." that I'm missing?
Are there other, better justifications for blending minor wear that I could use?
Are the justifications I've listed reasonable or bogus?
What have you done when faced with similar minor issues you need to put your engineering stamp of approval on?

Thanks for expanding my knowledge when I'm in a dilemma and stumped.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[KirbyWan]

More information about this:

The damage is actually in the repairable zone. The part meets the minimum material limits as indicated in the drawing above.

The reason we are beating our heads against the wall may be just a manual interpretation problem. I have included the relevant page of the manual below.

This is an overhaul for a lease return. So no temporary repairs are acceptable. The allowable damage limits has two checks. First is the "Remaining Material" check. If the remaining material meets the requirements indicated in the figure, we're done? No inspection intervals, so a permanent repair, right? Then there is the damage check for "Wear, Nicks, Gouges, Scratches and Abrasion". Here it says to blend it, has a length limitation for wear, which is not an issue. Then it has an inspection interval, so is this a temporary repair? Is this a permanent repair with a required re-inspection or repetitive inspection? I think the leasing company might be arguing this a temporary repair and it has to be replaced to be considered a permanent repair. Of course the returner wants the lowest cost solution and they're mad at us because we assumed we were good the first time through. (Not positive about these opinions from my location here in the engineering dungeon.)

How would you interpret this check section?
This may be more about meeting customer and lease return inspector expectations than about a specific engineering issue, but I would still like your thoughts on this and my desire for a negligible damage baseline from my first post.

Thanks all!

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[Sparweb]

It sounds like you are looking for an action that "terminates" maintenance on the part once the wear has been addressed. I don't see such a thing either, in the data you've posted.
For this part, no matter how much wear is detected, and no matter what is done to address the wear, an inspection (assume visual?) becomes necessary.
Not much point of giving you an allowable wear diagram, then is there?

I can see the trivial scenario, where a 0.0001" deep scratch is detected, and blended out 20:1, still provoking a 5000 flight inspection interval on the part.
I doubt this should be the case, but hang on, let's ask a few more questions:
Is this part safety-of-flight critical?
What contacts this surface to cause the wear?
How is the part loaded; is it highly stressed?
How frequent are inspections on the part anyway?
Is the new inspection visual or NDT?
How accessible is the part for inspection?

You may not have the whole story yet, but you're definitely making progress to finding it.


STF

 

[KirbyWan]

This V-blade is the forward frame of the thrust reverser structure that presses against a rest on (I think) the fan case and is clamped in tension with the opposite hand reverser when stowed keeping them in place along with the hinges and latches on the upper and lower beam of the TR and the inner V-blade that sets closer to the engine CL. Considering the section thickness and the closing mechanism is a simple over-center style clamp that a human engages I don't think the stresses mostly hoop are significant and there are many load paths if it fails (has a frame member like this ever failed?) This is a part that would get a cursory look at every time the reverser half is opened, since it's right there where you unclamp the reverser halves, but this inspection is not normally done until a c-check (though I'm really only familiar with CMMs not AMMs.) The inspection is only visual as far as I can tell.

The outer V-blade is listed in the A320 SRM as Secondary structure per the Primary and Secondary Structure - General Details figure, and in table 122 it clearly labels the V-blad as PSE (Principle Structural Element) and that it is not FCS (Flight Critical Structure) nor is it Safe Life.

Now if you will allow me to vent for a moment, Airbus has the most confusing mish-mash of terms for what it thinks of as important but can't seem to agree with itself how important. In the schematic breakdown everything on the nacelle is listed as secondary structure (51-11-12-001 "Primary and Secondary Structure - General Details - Sheet 1"). They have a handy Venn diagram (51-11-12 "Structure Breakdown - Schematic - Sheet 1") which has a 45° cross-hatch for Metallic Principal Structural Elements (and confusingly also uses that 45° cross-hatch to indicates flight critical structure) which is contained wholly on the primary structure side of the line dividing the structure between primary and secondary structure. I have no damn idea what Airbus means. If anyone would like to enlighten me I would greatly appreciate it.

Yours in befuddlement,

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[LiftDivergence]

Hmm...I have dealt with similar things for aircraft being inspected at transfer of lease where the new operator wants a minimum of inspection requirements for the future.

But the bottom line is you've found damage that needs to be addressed, and you can address it in accordance with the SRM. Just because the repairing action has follow on inspections doesn't mean that it is temporary. Look in the airbus SRM for their definitions. It should say something like "A category B repair is a permanent repair for which follow on inspections are required." or something like that.

If the blend repair does not call it Cat C or the equivalent than it is permanent. The inspections just repeat indefinitely (until LOV, I guess).

Sounds like your problem may be the demands of the new operator or owner looking to lease again. They must understand, even if the structure is not FCS, there must be some type of BZI for this part. I've seen Airbus MPDs & ALIs - virtually every part of the aircraft has some recurring inspection, even if its just a GVI on walkaround.

Plus, 5,000 FC or two years? That's a pretty nice interval. What's the C-check timeline for the A/C? I find it hard to believe that if you told the MRO to repair IAW the SRM, and then told the operator, "yeah, you have to inspect this evey C-check" that there would be any problem.

And don't worry, anybody who has worked repairs for Airbus A/C will tell you their SRM structural classification is clear as mud...

Keep em' Flying
//Fight Corrosion!

 

[SWComposites]

Kirby - have you queried the OEM for a clarification of the SRM and requirements.

 

[KirbyWan]

LiftDivergence,

Thanks for the validation of Airbus' confusing structural classification. I figure someone would have sorted it by now. There are a few of your TLIs that I didn't get.

LOV -
BZI -
MPD -
ALI -

SWComposites,

We have not had good experiences trying to get questions answered from Airbus. We are an MRO for nacelle systems that Airbus doesn't make themselves and we generally serve second line operators.

Thanks for everyone's responses.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[rb1957]

how about asking the people who make your nacelles ? I can see that Airbus would say "I don't know".

LOV = Limit of Validity, a new DT term that is similar in practice to "safe life" (although derived differently).
ALI = Airworthiness Limitation Item, a mandatory limitation (an inspection or some such) to maintain airworthiness of the structure.

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

 

[LiftDivergence]

Sorry,

BZI = baseline zonal inspection
MPD = Maintenance planning data/document

Basically what I was saying is that if you look at the airbus continued airworthiness documentation (or in this case it may be the engine manufacturer), specifically the MPD where the airworthiness limitations and certification maintenance requirements are listed, there is an outline of hundreds of different inspections. This tells the MRO what do do at different check levels. Basically every part of the airplane, probably including your damaged item, has some baseline inspection requirement on it, straight from production.

You said the client was worried about terminating new inspection requirements due to the repair action. My point is that the repeat your SRM calls up is good enough that it probably doesn't represent anything worse than the baseline inspection. If they are looking for a way to never inspect this part again, that is unrealistic.

Keep em' Flying
//Fight Corrosion!

 

[SAITAETGrad]

Your repair is permanent. You record the need for the inspection in a supplement to your maintenance program document sometimes called a Structural Deviation, Inspection and Repair (SDIR) program.

The questions SparWeb raises are interesting. When the SRM says "no damage allowed" this really means "no damage analyzed". So, if you know what you're doing you could do your own repair approval.

But I would caution playing in the margins of drawing or sheet specification tolerances. The OEM analysis may be based on nominal values and you could get outside the margins if you try to game the system. A derivation of loads is something you should do.

I heard a story - not sure exactly how factual - that an ALCAN process engineer tightened up their internal alloying tolerances on shipments to Boeing. They still hit all the specification strength and elongation requirements.

I'm sure this guy was a hero for a while at the plant. Some time later Boeing found out. It apparently left a lot of question marks with those requirements not in the base specification such as fatigue life.

Again, not sure of the truth of that but the point is that there are underlying assumptions on the many factors that design relies on because engineering is a flawed human driven process.

 

[MNLiaison]

The structure you refer to mates up with the Fan Case and as you described rests in a "V" groove. It is my understanding this is the main load path for any axial loads (mostly aft) from the reverser to the engine, which then gets transferred to the pylon. Critical load case for this joint is from "inadvertent reverser deployment during flight" but would be loaded during any reverse thrust action. The load transfer is by contact with the V groove, that is why I believe it's labelled as "no damage zone no repair" on the aft face. Since the part extends approx. 180 degrees around a reverser I assume that wear in local areas on the aft face would cause re-distribution to the adjacent nominal section due to the gap created to the V groove. Attached is a Figure of a very similar TR V blade and Groove joint with mark ups.

The CFM56-5 on the A320 has a similar design except that the V blade was made removable, as shown in the attachment, as they anticipated wear and understood it would warrant replacement. If the V2500 nacelle was designed by ROHR / Goodrich / UTAS, based upon my experience, they really love repetitive inspections on any sort of damage no matter how small.