AW139 lost tail taxying DOH

[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