Industry Insider
You and others have criticised Shell Management for his comments, which relate predominantly to operational management issues where I have no expertise so I will not directly join in the criticism of SM.
However, failures may also be driven by issues other than operational matters, such as structural deficiencies.
I think you should be auditing AW's manufacturing to tell them what they are doing wrong.
I have been trying to do this for ages with no response from AW or EASA. The frustrating issue here is that with correct design and certification procedures, properly validated processes and produced with appropriate control of humidity and contamination can actually produce adhesive bonds which will NEVER fail under any load case. Hence, to address SM's operational concerns it is possible to design and produce bonds which will not be the locus of failure even where operational airworthiness is violated. No matter what the pilot does, the structure should always fail elsewhere in the structure AWAY from the bond.
Now this is a radical concept, because if you can be sure that the bond will NEVER fail in certification testing or in service, then certification test costs will be reduced substantially. Also the cost of ownership will be reduced because in-service NDI will not be looking for interfacial disbonds (where it's value is limited anyway) it will be limited to secondary damage assessment such as in-service impact damage. That will significantly reduce the cost of ownership for the operator.
This is not an academic position based on theoretical bovine excrement. Applying these principles in a practical repair environment we have reduced bond failures for repairs performed in a large field repair facility from 43% in 1992 to virtually zero since then. The same principles applied to production will produce similar levels of difference in performance for bonded structures at original production.
The reason this does not happen now is that there is no regulatory requirement to demonstrate long-term bond durability during certification, and current damage tolerance analysis metodologies do not manage critical bond failure modes.
With respect to the current case, there may be two issues:
1. Micro-voiding due to moisture absorption by the adhesive during the production process. This reduces the bond strength significantly in joint peel, honeycomb peel and shear. It is NOT detectable by NDI. It is NOT managed by damage tolerance analysis (as currently used by AW). It results in disbonds. Nooby says
The tailboom is Aluminium skins (Aluminum for you Nth American types
) bonded to an Aluminium or Nomex paper core.
Micro-voiding is exacerbated by exposure of Nomex to the same humid environment. This is the problem with the tail boom disbonds and it may be the same problem for the tail rotors.
2. Again referring to Nooby's comment, the second issue is the durability of bonds to aluminium/aluminum. These are highly susceptible to environmental degradation usually by hydration of the oxide layer to which the adhesive is bonded. Hydration leads to disbonding at the interface between the adhesive and the metal. The worry is that short term strength and fatigue tests, NDI and damage tolerance analysis as specified by FAA and EASA will not prevent these failures.
These issues are addressed in a recent paper.
http://www.adhesionassociates.com/pa...d%20Joints.doc
I have also provided advice on management of micro-voiding previously on this or other AW 139 threads.
Now to short-circuit those who believe I have an anti-AW position, I assure you that I have also raised the same issues with another significant helicopter manufacturer and they dealt with me directly and resolved a number of issues at an effective charge out rate far less than a junior helicopter pilot's pay rate. My interest is in flight safety, not bashing particular manufacturers.
regards
Blakmax