R22 blade debonding
 

Taken from 1st Quarter 2015 Service Difficulty reports. First time I had seen this.. sorry if this old news.

http://www.flightsafetyaustralia.com...0871-1_adj.jpg

http://www.flightsafetyaustralia.com...0871-1_adj.jpg

Robinson R22 Beta Main rotor blades—main rotor blade unserviceable. SDR 510020871 (photo above)
Initial visual inspection and subsequent tap test revealed both main rotor blades had debonding at the skin/spar bond.
Failure due to glue joint failure. P/No: A0164. TSN: 1624 hours/96 months

Yes unfortunately this is not the first and won't be the last.


Its a shame to see the blade in such poor general condition with regards cleanliness etc.

Duct tape will be the solution.....

HuH Bellrider - 'Duct Tape', your being to kind & hey duct tape would be way over engineering......that'd be by far the strongest part of a F.Robinson built machine.

Is it any surprise.:mad:

advantage - bruno g.

Most likely flown many more hours than the logged time states and look at the general condition! Robinsons are the most abused helicopter in general service today!

That's what I was thinking... who would fly a machine with that degree of abrasion on the blades?

I would and do happily fly helicopters with abrasion on the blades - paint worn by dust particles - no big deal - it's not an issue to a well built helicopter - I just won't fly a Robinson! Why? I prefer to live. All helicopters types have histories of fatal accidents. I prefer to fly the types that are well built and clock up fatals mostly as a result of pilot error. One day I might come a cropper and it will be my fault. Why fly a type that can just fall apart in a gentle cruise without pilot provocation? Too much like Russian roulette to me.

OOW

As OOW says, why not? Unless it's a Robbo blade there are many of us who have seen that degree of erosion as quite normal, and worse. Plenty of outback red dust there which is an absolute bugger to remove from planes, trains and automobiles so you learn to live with it.

I didn't word my post very well... let me try again...

My understanding is that Robbies are not to be flown with abraded blades... is this true?

If so, then that machine shouldn't have been flown... setting aside any prejudices about Robbies.

The blade is not eroded, a close look at the metal on the leading edge will show that. It does seem to have been flown in heavy rain earlier in its life with fresh paint which is why the paint is flaked off as it is. That is quite often seen.

Also very often seen is bug damage like this, even with just one day at Low Level it can be like this and a real pain to clean.

There are plenty of hypothesis around regarding dry air - moist seaside air etc etc that suggest the commencements of de bonding. In this photograph I believe there is a lot more to the story.

Have a look at the damage to the skin above the rear spar area and what looks like warping of the skin forward of that area. The so called bug marks are consistent enough in my book with having been in contact with a quite resistant type of bush such as wattle or similar light and reasonably soft branched brush. Hence the wide smudge and usual red colour from such material.

Before I would go jumping up and down about the product I would get the pilot in a corner with some hard questions.

Blade condition
 

I note the discussion about the condition of the blade. The principle objective of the AD to manage paint quality was to prevent undercutting of the adhesive by particle impact. If the paint condition was maintained, then there would be no undercutting of the adhesive, which it had been suggested caused the bond failures experienced prior to the AD.

My personal opinion as an adhesive bond failure forensics specialist is that the basis of the AD is bovine excrement and this photo of the blade proves it. While the paint on the leading edge has been eroded, the paint adjacent to the skin-to-spar bond is not degraded at all. If particle undercutting is the primary cause of these failures, the the paint adjacent to the bond would be absent and it is not. Hence the basis of the AD is to put it mildly CRAP! This blade has disbonded yet the paint adjacent to the bond line is intact, so there is absolutely no evidence of undercutting.

Next, let us examine what we can see of the bond failure. If you look carefully at the image you can see a hatched pattern on the surface of the bond failure. That is the "carrier cloth" a knitted material embedded in the film adhesive when it is manufactured. It is there to provide an ability to handle the adhesive film without the adhesive fragmenting as it is handled.

In terms of failure forensics this carrier cloth is important. There are three failure modes for adhesives. Cohesion failure is the strongest form, and the failure exhibits evidence of the carrier cloth because in the absence of other failure modes that is the plane of weakness because of the stress concentrations associated with the carrier cloth and that this is the plane with the least adhesive material.

The next failure mode is adhesion failure which occurs at the interface between the adhesive and the metal. This is the weakest form of failure and in extreme cases, failure can occur without any load at all. Failure is characterised by the adhesive being on one surface only at any location. Importantly there is no failure through the carrier cloth.

The last form of failure is by mixed-mode, which is a combination of adhesion and cohesion failure, and the strength of the bond depends on the proportion of strong cohesion failure and weak adhesion failure. Importantly, as the bond strength weakens, the locus of failure migrates from the carrier cloth towards the adhesive to metal interface.

Now the above discussions relate to shear failure. If the failure is by peel, then there will be an apparent failure which is a mixture of failure through the carrier cloth but also there will be evidence of failure at the interface but the failure will be within each cell of the carrier cloth. The failure is characterised by evidence of each cell of adhesive being peeled away from the surface, thus presenting the apparent adhesion failure.

Now, what the heck has this to do with this case? I see apparent evidence of the carrier cloth, so the failure of the skin to spar bond is not by adhesion or mixed-mode failure which I have seen in other failures of R44 blades. But if this is true cohesion failure which would expose the carrier cloth, then the bond should have been strong enough to sustain flight loads and (sorry TET) even moderate impact loads such as from contact with flora.

I believe that a closer examination would show peel failure characteristics. So what would cause peel failure? I strongly suspect that there has been weak adhesion or at least mixed-mode failure of the bond to the tip cap retainer fitting aft of the spar and that has caused the skin to peel because of the lack of support for torsional loads in the skin-to spar bond. I would then suspect that the process used to treat the fitting is deficient in providing longer term bond durability.

Now I would suggest that this is a theory and closer examination of the blade would be needed to confirm (or reject) my theory. The key is to examine the skin to tip fitting bond. Is there evidence of adhesion failure?

It is a real pity that the level of understanding of adhesive bond failure forensics appears to be so low within regulators and investigators and appears to be totally absent in manufacturers of bonded structures.

Regards

Blakmax

PS
 

If you want to really understand why the current tap test for such disbonds is useless, I suggest you read this:

http://www.adhesionassociates.com/pa...ES%20final.pdf

It actually shows that for short bond lengths where there is a history of adhesion or mixed-mode failure, then NDI may not actually prevent bond failure because the bond strength may decay to unacceptable levels before any detectable disbond can be found. In fact there may not even be a disbond at the time of failure.

Draw your own conclusions about the validity of currently approved methods for assurance of structural integrity of principal structural elements with short overlap bonds.

Regards

Blakmax