I have some experience in this specific issue based on my involvement as a consultant on a particular crash investigation. The relevant aviation authority has so far not released the final report. (Why???)
I note the comment from Kev
OK, so Robinson get the design wrong
. That is not entirely true. The DESIGN is not the cause of this problem. I know that this is an adhesive bond durability issue which is almost certainly related to the surface preparation processes used during production. It has very little if anything to do with erosion of the paint layer. I have personally seen only one example of erosion, but I have seen many more examples of bond failure where there was negligible evidence of erosion.
The tape is not the answer, and anything other than a clear transparent tape makes the issue of inspection even worse.
The real issue here is: Did RHC do everything required by the regulators to produce a reliable structure? The answer is probably YES. The FARs require static strength and fatigue testing. They would not have recieved certification unless they did those tests.
FAR 29.603 requires that critical bonded structures demonstrate strength by one of three methods, one of which is by reliable NDI, and another is by static testing. I know that RHC inspects every blade (I am not that confident with tap testing) and undertakes a series of QA tests on companion samples cured with the blades. I know RHC undertake a static strength test on 1/100 blades, and I expect that these results will show that the blades are "sound".
FAR 29.605 requires the use of processes "known to produce a sound structure". How do you define a "sound" structure? If it passes NDI and QA testing, is it a sound structure? In the
short term YES. In the
long term, the same structure may not be "sound".
I therefore believe that RHC has done everything they could to comply with the FARs.
So where does the problem originate? The essence of this problem (and a number of other similar bond failure examples) is that
there is no requirement in the FARs to demonstrate long term resistance to degradation of adhesive bonds. This requirement is referred to indirectly in AC20-107B, but its lack of prominence does not reflect the dire consequences of failure to address adhesive bond durability before any certification tests are undertaken.
The mechanism of bond degradation for most metallic surfaces is hydration of the surface of the metal and this takes some time in service to occur. The classic example is for aluminium surfaces where Al
2O
3 oxides hydrate to form Al
2O
3.2H
2O. In the process, the chemical bonds between the oxides and the adhesive dissociate so the hydrate can form, resulting in disbonding. I have no conclusive evidence that this occurs with stainless steels such as the current RHC blades, but I would be very surprised if there was not a similar hydration mechanism involved in these failures.
The hydration process takes time and is exacerbated by operation in areas of high temperature and humidity. Hence, short-term static strength and fatigue tests will not prevent these failures unless they are conducted after a considerable period of environmental exposure, and the FARs do not require such exposure tests.
There are in existence short term tests which will provide evidence of bond durability and these tests are supported by between 15 and twenty years of military experience with negligible bond failures for bonded repairs performed using compliant processes.
The answer to this problem is to mandate these tests, not just by RHC but by all manufacturers of all adhesive bonded structures.
Regards
Blakmax