Structural Repair

[SkyPunk]

Recently exposed to a repair (if one would want to call it that) involving filling/fairing a metal structure with adhesive EA934/Magnabond6398. Then drilling through said filled holes to mount a flight control bellcrank. Two of the four holes received this pasty repair. Dare I ask? Has anyone ever seen such a thing in a production/mod line?


Skypunk

 

[rb1957]

maybe in Timbucktoo,

and a flight control bellcrank too !!

 

[berkshire]

SkyPunk (Aerospace)
I think you may be looking at what the Fed's call a "Clandestine repair". Take lots of pictures before you start working on it.
In a case like this, most often there is no logbook entry.
If there is and you report it, the Feds may take certificate action against the person who did it.
B.E.

 

[rb1957]

following up on berkshire, i think you have to report it else you'll get dragged down with it. advise the owner first, he might be able to suggest who/when it was done; plus i'm sure he'd rather know first (before the local FAA).

 

[rerig]

I have seen EA934 used as a liquid shim at fitting locations. What you see might not be a repair.
This application is normally no thicker than 0.030".
Most use peal shims, but it could be a liquid shim.

 

[MNLiaison]

Skypunk,

If the original structure was a sandwich panel with honeycomb core then this type of "repair/mod" does not sound at all that crazy. Due to the low compression capability of most honeycomb cores, phenolic aramid or metal, when installing additional holes after original panel manufacture it is typical to locally pot the core around the bolt location with paste adhesive and allow to cure prior to fastener installation. The paste adhesive has better compression capabilities then the original core therefore when torquing down the fastener the paste adhesive will not crush like the original core would have. On Boeing aircraft, 747, there are SRM allowed repairs where this process is used to bolt a repair plate to a composite honeycomb structure to repair face sheet damage. I can't say for certain that this is the type of repair that you witnessed but my two cents.

In fact I was just involved with a repair to a 757 nose inlet cowl acoustic panel which is a sandwich honeycomb structure where the SRM calls out to drill fastener holes, remove core around the holes for 1.0 inch dia., then pot with paste adhesive, and install doubler with thru bolts. Unfortunately the mechanics didn't allow the potting compound to fully cure so when tightening the fasteners they crushed the core.

Regards,
MnLiaison

 

[berkshire]

SkyPunk (Aerospace)
I think you have to clarify what you have there.
To me filling/Fairing means the filler/putty is on the outside of the skin. To me bolting or riveting a fitting to that is a no-no.
Potting a Metal honeycomb structure to get a hard point for attaching a fitting is acceptable, if, it is done in accordance with the SRM for that aircraft, or according to figure 2.43 chapter 2 paragraph 115 of AC.43-1a, or fig 3-10 page 3-10 of AC.43-1B.
Tell us is this a single skin or a honeycomb?
B.E.

 

[SkyPunk]

Would like to thank you guys for helping me with my sanity check. You all have reflected exactly my thoughts. No honeycomb, no doubler, no sandwich at all other than that provided by fitting, and nas1149 washer. My thoughts are that it is an excercise absurdity. Had to rub my eyes, take a few laps around the facility, and chug about 10 cups of coffee to ensure that I was not dreaming. It gets better, cannot elaborate at this time.

 

[whalesA3D]

Skypunk,
Safety of flight begins with you. A bad repair is just that, a bad repair. At least report it to your supervisor or a competent inspector.

 

[berkshire]

SkyPunk (Aerospace)
Having confirmed that you have at least one substandard repair, you now need to go over the aircraft with a fine tooth comb. You have already alluded to the fact that there are more. You may be more right than you think you are.
Have you or your supervisor talked to the owner yet?
Above all do not sign anything off until you are 100% sure you have found everything.
B.E.

 

[Stache]

As aviation professionals we have a duty to warn others of potential problems. If we find something and know its wrong and don’t report it we could be held legally responsible if something should happen even years later.

The FAA works on safety related stuff and the mechanic, engineering’s and other are the eyes and ears of the system. Under FAR part 43.11 you have a duty to inspect and provide the owner or lessee with a list of discrepancies. This would meet the rule for discrepancy. Once you notify the owner in writing it is there responsibility to have it repaired correctly.

Good catch, its people like yourself that makes the system work.

Stache

 

[planedr]

SkyPunk (Aerospace)
Having confirmed that you have at least one substandard repair, you now need to go over the aircraft with a fine tooth comb. You have already alluded to the fact that there are more. You may be more right than you think you are.
Have you or your supervisor talked to the owner yet?
Above all do not sign anything off until you are 100% sure you have found everything.
B.E.

With everything there is a caveat. What category aircraft? How old? Military or civil? What country registration? What work are you contracted to perform?

That asked, be careful following this advice. It is true but.... You are not responsible for recertifying every repair on the aircraft. You are responsible for certifying the work you have accomplished. Yes there may be issues on the aircraft. If you "find" issues unrelated to the maintenance or work you are contracted to perform you are exposing yourself and your employer to alot of legal liability.

Unless you have every SRM and every document for the aircraft history, which I doubt, you cannot recertify every repair. Maybe a DER signed off that area and repair 10 years ago. Good for him. Maybe not, have you read the original Non routine or sign off? If I need to rework it, it gets brought up to date. You cannot go over the airplane looking for issues, you will be unemployed and your employer bankrupt very quickly.

That said if the flap is falling off and you have a general visual within 15 feet write it up. Why do you think SRM's are full of obsoleted repairs? Unless AD'd there is no compulsion to remove any once legal repair that may be obsolete now.

If it's that bad your employer should close up and sign off the work accomplished push it outside and call the local regulators. Again very bad legal implications.

 

[berkshire]

planedr (Aerospace)
The main issue here is the return to service. If the aircraft is getting an annual or progressive inspection, whoever does it is accepting responsibility for the aircraft until the next guy does it.
If I am signing that aircraft off, I want to make darned sure there is nothing in there that will bite somebody.
Because if it does it's my ass and certificate on the line. Finding an item like a puttied up bell crank is enough to make me stop work and want to talk to the owner of the aircraft before I go on.
B.E.

 

[MJFellow]

Referring to Manufacture's Description
Hysol EA 934NA is a two-component thixotropic paste adhesive, which cures at room temperature and
possesses superior strength to 300°F/149°C and higher. Its thixotropic nature and good compressive
strength make it ideal for potting, filling and fairing, as well as for shim applications. Hysol EA 934NA is
qualified to MMM-A-132 Type 1, Class 3 with a room temperature cure.
I think, It should be Okay for short period.
Good luck

 

[planedr]

berkshire

Back to the caveats. It is different as a GA, Restricted or a transport cat. Which is it?

I agree with your position. However it is not usually your responsibility to recertify every repair on the aircraft. Nor is it your responsibility to check anything outside the maintenance program. If so every check would be a "D" check. A similiar example I use is if the paperwork calls for a detailed visual (10X) and the inspector uses UT. If something is found you fix it. But there is no justification for doing anything beyond what is required by the approved maintenance program.

The argument here surrounds "approved" maintenance. If you accomplish the approved maintenance in the approved manner. You release the aircraft back to service. It has no bearing your certificate period.

Maybe the bearing surface of the underlying material is good and the surface needed fairing for the bell crank mount? Not saying I would necesarily do it but....

 

[berkshire]

planedr (Aerospace)
This is part of the problem here we do not have all of the picture.
I would not have a problem with fairing putty placed over a depressed area where the bell crank was attached to the surface. but that was not how I read Sky punks original post. He mentioned drilling through the filled holes to attach the fitting. and you are right about not checking things outside a maintenance program. I guess that's OK in a progressive maintenance environment. However I work in General aviation where most maintenance is 100 hr or annual inspection. And as far as I am concerned, any time I find a substandard repair, the whole rest of the aircraft is suspect until proved otherwise.
B.E.

 

[planedr]

Berkshire

I understand the GA world completely and you are totally correct. The first time you question a customers aircraft unwisely, transport category or restricted even, you will get your anatomy handed back to you (if you are lucky).

Sorry if I came across too strong but I seem to spend an excessive amount of time on this very subject. No offense but some A&P's feel they are the be all and end all of airworthiness.

I can, even after producing the original NR and SRM etc have a fight with an A&P about a repairs legality.

Safety is first, but we all need to remember the legal context in which we work, sad as it is.

 

[SkyPunk]

Appologize for the vagueness. Repair is fill and fair 3/8 hole {full thickness} then drill 1/4 hole through and mount. Absurd??? Pathetic??? How about Engineer with a degree.

 

[rb1957]

you'd wonder why they didn't bush the hole ?

btw, as described it probably isn't too bad, the potting would be reasonably rigid (sort of like the core). I'd want to see that the potting was captured on both sides (say with the fitting on one side and a large washer on the other).

 

[RPstress]

My understanding of the repair is thus:

Edge of
oversize
hole -----|
| |--- Edge of 1/4 hole required.
________v_v _______
________|_| |_|_____
Metal ^
|
Adhesive

For a systems support bracket you might get away with it, provided the fastener and bracket attach foot clamped through good metal around the hole.

For a flight control bellcrank, which would affect safety of flight if it moves slightly, let alone if it comes loose or comes off, this is unacceptable, even though there are other good attachments.

Boeing have a freezeplug method where they shrink a metallic plug into the oversize hole and then drill that. However, it's still generally mandatory to have the attaching part clamping up through good parent material around the oversize hole. (Since your oversize is 3/8 and the fastener is 1/4, a suitable washer should extend over good material.)

For a flight control I would hesitate to use a freezeplug unless the maximum loading was "low", and with a bellcrank it probably isn't. You might be ok shrinking in material like this if the structure you're attaching to is very thick, say equal to the 3/8" hole dia.

If the structure is really thick, then it's just possible that in practise the adhesive repair is adequate. However, it really shouldn't be allowed.

If the structure with the bum holes is thin, especially if it's sheet, then the adhesive repair is almost certainly a potentially lethal danger.

Even if the fastener and bellcrank attach foot are clamped through good metal around the oversize hole, the shear forces for max loading will overcome friction due to torque up (especially if the torque mostly disappears over the years), and the adhesive will be in bearing on the fastner shank. For a 1/16" thickness of adhesive, that's not acceptable. The adhesive is quite likely to crush at around about two ksi nominal bearing stress, probably less if the environment is hot and/or moist. Once it's crushed the bellcrank fitting is essentially loose, significantly affecting the control run stiffness and exposing the attachment area to unaccounted-for prying forces, which could cause static failure at high loading or start fatigue damage for every-day loads.

-RP.