I have had several PMs and direct questions about my theory with R44 crashes and I have been waiting for the report to be released. For all of those who use the mantra "Wait for the report" what do you do if the report is not released? I am not totally aware of the legal implications of releasing information before the report is released, but given the time which has elapsed, I see no alternative. However, I shall couch my comments in the terms I gave to the IIC and the comments already released by the NTSB. I will NOT preempt the IIC's conclusions.
Firstly Aaarj, bits DO fall of helicopters if there is a structural failure. This was the case here.
I do need to do a bit of posterial tin plate protection so I here quote the NTSB assessment at A08_25_29_Recommendation in relation to DQ-IHE.
The fracture faces in the remaining portions of the adhesive bond joints showed mixed cohesive and adhesive failure fracture features with a large percentage of adhesive failure, indicating that the bond strength deteriorated after the blade was manufactured.
Further
The Safety Board has determined that the adhesive fractures in the main rotor blade from the Fiji accident helicopter propagated from the blade tip and leading edges and cannot rule out the possibility that the in-flight breakup was initiated by a bond failure at the tip of the blade.
Adhesive failure fracture features result from a dissociation of the chemical bonds at the interface due usually to hydration of the oxide layers on metallic adherends. All epoxy (any polar molecule) will absorb atmospheric moisture and that provides the water necessary for hydration. Hydration resistance is controlled by surface preparation at the time of manufacture. In every case, adhesive (more correctly adhesion) failures result in very weak bonds and in the extreme can result in negligible bond strength. They are time dependent, with strength decaying exponentially from the time of manufacture.
I then refer you to my paper:
http://www.adhesionassociates.com/pa...0Explained.pdf
Note that some of the pictures are of a specific failed R44 blade.
This paper explains the significance of mixed-mode failures. These result when the interface has commenced degradation, but at some time a sufficiently high load is applied which results in joint failure. If such joints had not experienced that higher load, then they would maintain apparent integrity. The longer they are in service, the lower the failure load needs to be. Mixed-mode failures are in fact clear evidence that the bond strength has degraded below that at the time of manufacture.
Now, currently blade integrity is managed by an AD which requires frequent inspection. What did the NTSB say about tap testing?
The Safety Board is concerned that tap testing is not adequate for detecting bond defects in critical bond joints of the main rotor blade, such as areas between the skin and spar at the tip of the blade and between the skin and tip cap. The intact main rotor blade from the Fiji helicopter was tap tested on site and at the Safety Board’s Materials Laboratory. Neither tap test detected the 0.5-inch debond area that was visually detected at the tip of the intact Fiji blade.
The Safety Board believes that the actions required by AD 2007-26-12 may result in the detection of some defects but will not detect hidden bond flaws at the spar-to-skin bond joint and the skin-to-tip cap bond joint. The investigator-in-charge of the Fiji accident reported that the main rotor blade from the helicopter involved in the event in Australia was tap tested prior to flight, but a bond defect at the skin-to-spar bond joint was not detected. This failure, as well as the Safety Board’s tap testing of the intact Fiji blade, which did not detect the debonding, demonstrates that tap testing does not detect bond flaws in the main rotor blade consistently.
Even if the tap test was an effective method for disbond detection, is that an adequate measure to assure structural integrity? I again refer you to a recent paper:
http://www.adhesionassociates.com/pa...d%20Joints.doc
This paper clearly shows that for joints susceptible to mixed-mode and adhesion failures, the usual methods for establishing tolerable defect sizes are inappropriate. Often, the size of tolerable defects is established by testing joints with teflon inserts to simulate defects, so if the structure survives loading with that size defect, all such defects should be safe. That assumption is simply not true for mixed-mode and adhesion susceptible surfaces. It is also not true for micro-voiding (porosity). Why? Because the test conditions result in adhesive bonds surrounding the defect which are not degraded. In practical disbonds which occur due to adhesion, mixed-mode degradation or porosity, there is no way of ascertaining that the surrounding adhesive has not been degraded. In fact it often is very degraded. It may still maintain sufficient contact to pass NDI, yet the interface or the adhesive is so degraded that the bond strength is well below that required to provide sufficient bond strength.
The AD also made reference to erosion of the leading edge paint leading to the exposure of the adhesive bond and suggests that this leads to the failure. I have seen evidence of such erosion, but I assert that if this was the cause of failure, the failure in a joint with an appropriate level of strength, there would be cohesion failure (fracture through the adhesive at high bond strength). The evidence is that there have been a number of cases of at best mixed-mode failure. It is my contention that the only role erosion could possibly play is if the bond interface was already degraded, then erosion may possibly (?) contribute to an earlier failure than would have occurred in the absence of erosion. In other words, failure would occur without erosion. So paint integrity or the use of leading edge tapes would not prevent bond degradation.
In summary, the NTSB agreed that there was evidence of adhesion and mixed mode failures in DQ-IHE blades. My papers explain the significance of mixed-mode failures and adhesion failures with regard to strength. The papers also explain that even if effective NDI methods existed, they are inappropriate for surfaces susceptible to adhesion or mixed-mode failure, and porosity.
It is my opinion that the only way to assure flight safety is to withdraw a number of blades from service and test them to destruction. The NTSB report noted failures in low-life blades
The blade with the earliest debonding had accumulated only 331.2 hours TIS.
Start there and follow through to the designated service life.
I would also add that TIS is not the correct parameter. Time since manufacture (TSM) may also be just as important. The ATSB reported on one blade which exhibited disbonding at the root fitting after ZERO flight hours.
I feel I have a duty of care to make my observations public, even if CAAFI and others do not wish to do so. Even if the French accident is unrelated to this cause, something must be done to address the integrity of blades. And for those Robbie floggers, there is also evidence of problems on other types. The root cause is that the current regulations do not require demonstration of bond durability. They only require demonstration of static strength and fatigue resistance. The requirements for adequate bond durability are buried in advisory circulars and policy documents, and are often not understood by certifiers. But even if that changed tomorrow, it would not change the fact that there is a risk of lives being lost because of inadequate bonded structures in current service.
Regards
Blakmax