AW139 lost tail taxying DOH

[9Aplus]

for last one, hope they will be smart enough.....

[outhouse]

Having been away from PP for some time I have how an opportunity to catch up on my reading. I found this thread fascinating and the technical stuff by blakmax an educational gem.
Many thanks
outhouse.

[Encyclo]

Good analysis Blakmax,

The issue you bring up in the defect not being detectable through NDI/NDT is the reason OEM actually do DT (destructive testing) on a portion of the parts they build, specifically to identify production/quality issues as you describe.

I would hope AW being the professionnals they are (not being sarcastic) would have identified adhesive flow issues after the panels are cured in the autoclave.

[amicus]

Based upon all that I have read in this thread, I would insist upon proof loading all existing articles and new booms and all field repairs until AW can prove conclusively that their in-process works on a repeatable and controlled basis. Even then, I would continue to require 100% proof load their booms for several years before reducing the proof test requirement to an acceptable statistical basis.
We can debate all the nuances of hydration and fillets forever, but safety demands a short term and provable system and that is exactly what 100% proof loading provides. Clearly AW has not had and does not have control of in-process bonding procedures and NDI is not capable of detecting these incipient production defects. I have no issue whatsoever with correcting and improving In-process at AW, but we still have a bunch of highly questionable booms in service and inadequate repairs, so proof loading to limit load is the only short term sane and workable procedure from a safety of flight standpoint. Maybe Blakmax can get his yearned for consulting gig and ultimately fix things at AW, but that will take much time and meanwhile we have existing booms and a lousy cold bond SRM repair schemes from AW. This is a safety issue and needs addressing immediately.

[slowrotor]

Here is another method that anyone can use to inspect for disbonds:

Sometimes in the evening or morning, dew will condense on the aircraft surface and the substructure can be seen in the pattern of the dew. A spar or bulkhead disbond might be seen as an area without dew. The dew clearly shows the exact edge of a sound bond. Or at least a bond that has thermal contact.

I have observed this on my aircraft, a Grob G109 many times. Not sure if it will work on honeycomb, but it sure works for skin to spar bonds.

Just another way to visually inspect when conditions permit and the humidity is just right.

[spinwing]

Mmmmm ....

Unfortunately .... this method will not help those who operate in the "Middle East" ......

[Eng AW139]

Lots of morning dew over here..in the Gulf

[spinwing]

Mmmmm....

Yeah trouble is its usually all over the Heli-decks making them lethal slippery ....

D'ya reckon you'd be able to find a de-bond using that method?

Good luck with that!

[Heli-phile]

Update please AW139

Where are AW139's still operating and where have they been "withdrawn from service"??.

Not being involved with this particular type I am not up to speed with what inspections have/are being conducted resulting from the tail boom "re-deployment"

Blakmax and other contributors - great posts, keep up the pragmatic and informative posts

[blakmax]

It gets worse

Sorry to say folks, but in Post # 235 I requested information on the surface preparation method used for bonding the repair to fix the disbonds. I tongue in cheek made the comment "Don't tell me is just a solvent wipe". Sadly, I have been advised that the procedure stipulated was a hand abrade and solvent wipe. Now if you have followed my explanation for surface preparation to involve a step to provide resistance to hydration, then you will realise that a hand abrasion and solvent wipe on an aluminium surface will never produce any resistance to hydration whatsoever. I am certain that these repairs will disbond, probably in the shorter term. If you have such a repair, have a look BF to see if it is lifting.

Hand abrade and solvent degrease may produce adequate short term strength if tested directly after the specimen is made. But if you leave the specimen in a warm, humid environment for a few months, the specimen may not even stay together long enough to be tested.

Now I am not familiar with the EASA regulatory system, but FAR 29.605 with reference to processes states " The methods of fabrication used must produce consistently sound structures." For a repair to restore the certification basis for an aircraft, the repair processes must meet the same certification requirements as the original construction. Is there any evidence anywhere that scuff sand and solvent wipe actually produces a "consistently sound structure"? I can show hundreds of examples where it has not.

The worry is that this was a repair to fix a disbond and it has replaced that disbond with a repair which almost certainly will result in another disbond. The only reason these repairs do not fall off is that they installed fasteners to restrain the patch. The question is, has the repair design provided adequate strength to carry the load through only the fasteners? Next question: Even if the fasteners can carry the loads, is there enough stiffness to prevent crippling failure once the insert disbonds from the skin?

I agree with Amicus. This is a safety of flight issue, and in the short term it is relevant to the AW139 and the focus is on Agusta and something must be done.

However, there are just as many other platforms out there using the same repair methods and built with similar deficiencies in their construction methods. It really comes down to defining what is a "consistently sound structure". The only long term solution I believe is to amend the airworthiness regulations to mandate demonstration that production processes not only meet static strength and fatigue requirements, but also demonstrate long-term bond durability.

I think it is time that the regulators ackowledge that the current regulations do not adequately address a possible (common?) failure mechanism which can result in loss of aircraft. I am aware that the FAA is trying to manage this by amending an Advisory Circular AC-20-107, but what value is advice? Surely a possible (probable?) cause of structural failure requires mandatory regulation, not optional advice? How important do we feel this matter is? If even one regulator or manufacturer would organise a conference or meeting on the subject and cover our out of pocket costs, my company would be prepared to waive professional fees to present at that conference.

Regards

Blakmax

[amicus]

Thank you, blakmax, your agreement is most sincerely appreciated, and clearly the advice and suggestions from agencies in instances such as the AW 319 boom and similar risky and questionable instances worldwide are currently totally insufficient and inadequate.
From my perspective, I agree with your suggestion re a conference and for a substantial improvement of bonding and repair procedures and support it as an excellent step forward. There are both short term and longer term fixes, obviously, but we do need far better answers from the relevant agencies as their current posture is unacceptable to aviation engineers and repair stations. My primary focus is on the safety of flight issues, as an immediate and critical area, but there are many longer term improvements to be made.
I remember giving a composites paper at an AIAA conference on East Coast some years ago and met and interacted with an excellent Fokker bonding expert who had spent his whole career dunking bonded specimens into the North Sea off of Holland and his slides of resultant bond line degradation were both scary and telling. Incidentally, blakmax, he concluded that, based upon his research, from an environmental resistance perspective, that phenolics were far superior to epoxies as adhesives. And I did not mind at all being scared by his data set, as I think engineers work best when scared.
Equally clearly, I think we can agree that engineering in general and bonding in particular remains an art rather than a science and obviously this holds true for both NDI and repairs.
Please continue to beat any drum within range and know you are on the side of the angels, as I hope that I am also.
All best regards,
amicus

[airsail]

Has any work been done in regards to the use of silane in the surface preperation process when bonding alloy/nomex core to alclad sheet?
I know it is used in sheet to sheet bonding but the process used for this probably wouldn't work for honeycomb.

[blakmax]

silane on nomex

Sorry airsail, silane is effective on metallic sufaces when correctly applied, but I am unaware of its usefullness for nomex. I'd be surprised if it did anything because it needs a chemically active surface to bond to and I don't see how that could be achieved on core.

Regards

Blakmax

[spinwing]

Mmmmm ....

Gosh this is getting scary ...... so the big question is .... "When as aircrew do we refuse to fly the aircraft ?"

It is a bit difficult for aircrew to argue .... that the aircraft is not fit to fly when either there has been a new "boom" fitted or an "approved repair" has been made .... and now as some of us have been informed that nearly any repair made to Agusta specification will fail .... we (the Royal we!) now cannot argue that we didn't know it may fail (quickly) ....

Could be some crews are going to be blamed for any incident that may occur to them for just accepting the repaired aircraft????

And of course ...its not just the tail boom that is delaminating .... as pointed out previously ... this is just the part which has so far been stressed to failure ...

I'm starting to feel very uncomfortable ....

[RVDT]

Bed time reading

[Um... lifting...]

RVDT-
The Anglophones are going to point out that if "Aluminium" is spelled wrong on the cover, how good can the rest of it be?

[blakmax]

aluminum vs aluminium

I have never worked out why it is aluminum yet it is not titanum or uranum?

[Um... lifting...]

Would appear that Sir Humphry Davy felt it should be 'num'. Since he had apparently isolated (and inhaled sadly, leading ultimately to his death) various 'ums' and 'iums', in this writer's view he was entitled to call it any old thing that he wished.
Everyone was perfectly content and then some anonymous crank with a classical education felt qualified to object to the spelling in a political-literary journal of all things. I ask you, personal feelings on spelling aside, does this seem just?
This would seem to be the 19th century equivalent of the PPRuNe pedant.

Etymology


Nomenclature history

The earliest citation given in the Oxford English Dictionary for any word used as a name for this element is alumium, which British chemist and inventor Humphry Davy employed in 1808 for the metal he was trying to isolate electrolytically from the mineral alumina. The citation is from his journal Philosophical Transactions: "Had I been so fortunate as..to have procured the metallic substances I was in search of, I should have proposed for them the names of silicium, alumium, zirconium, and glucium."[42]
By 1812, Davy had settled on aluminum. He wrote in the journal Chemical Philosophy: "As yet Aluminum has not been obtained in a perfectly free state."[43] But the same year, an anonymous contributor to the Quarterly Review, a British political-literary journal, objected to aluminum and proposed the name aluminium, "for so we shall take the liberty of writing the word, in preference to aluminum, which has a less classical sound."[44]
The -ium suffix had the advantage of conforming to the precedent set in other newly discovered elements of the time: potassium, sodium, magnesium, calcium, and strontium (all of which Davy had isolated himself). Nevertheless, -um spellings for elements were not unknown at the time, as for example platinum, known to Europeans since the sixteenth century, molybdenum, discovered in 1778, and tantalum, discovered in 1802.
The -um suffix on the other hand, has the advantage of being more consistent with the universal spelling alumina for the oxide, as lanthana is the oxide of lanthanum, and magnesia, ceria, and thoria are the oxides of magnesium, cerium, and thorium respectively.
The spelling used throughout the 19th century by most U.S. chemists ended in -ium, but common usage is less clear.[45] The -um spelling is used in the Webster's Dictionary of 1828, as it was in 1892 when Charles Martin Hall published an advertising handbill for his new electrolytic method of producing the metal, despite his constant use of the -ium spelling in all the patents[39] he filed between 1886 and 1903.[46] It has consequently been suggested that the spelling reflects an easier to pronounce word with one fewer syllable, or that the spelling on the flier was a mistake. Hall's domination of production of the metal ensured that the spelling aluminum became the standard in North America; the Webster Unabridged Dictionary of 1913, though, continued to use the -ium version.
In 1926, the American Chemical Society officially decided to use aluminum in its publications; American dictionaries typically label the spelling aluminium as a British variant.

Present-day spelling

Most countries spell aluminium with an i before -um. In the United States, the spelling aluminium is largely unknown, and the spelling aluminum predominates.[47][48] The Canadian Oxford Dictionary prefers aluminum, whereas the Australian Macquarie Dictionary prefers aluminium.
The International Union of Pure and Applied Chemistry (IUPAC) adopted aluminium as the standard international name for the element in 1990, but three years later recognized aluminum as an acceptable variant. Hence their periodic table includes both.[49] IUPAC officially prefers the use of aluminium in its internal publications, although several IUPAC publications use the spelling aluminum.[50]

[[email protected]]

Are North Americans not in fact anglophones? and if so why would they object to the spelling?

[Um... lifting...]

Now, now... you Britons can't have it both ways. Out of one side of your mouths saying we don't speak English and out of the other claiming we do. Perhaps 'Self-appointed Guardian twits of British linguistics' would have been more appropriate usage.
Such as this twit with the arched eyebrow and superior sneer, who may or may not be correct, depending upon whether your reference is literature or the traffic code (G.B. Shaw came down on the side of the traffic code).
'Punctuation hero' branded a vandal for inserting apostrophes on street signs | Mail Online