R22 & R44 blade delamination

[61 Lafite]

Surely you can't be serious. Have you had a look at the pictures a few posts above yours? 400hrs, paint well within limits - unservicable. How can that possibly be called a solution?

I can absolutely be serious, and look at the paint that you say is 'well within limits' - it's all flaking off the end of the blade - how can you possibly call that 'paint well within limits'?

There is no way that just happened in one flight before the problem was noticed.

The SB/AD info from Robinson is very clear: if there's a problem with the paint, it needs to be sorted out, otherwise delamination may happen.

Here you see a problem with the paint, and delamination has occurred.

When you buy your aircraft, you get 2 years warranty. After that, all you know is that a lot of research and development has gone into making the machine as durable as possible, but while many, in fact most, parts on all the aircraft will go all the way to TBO, some won't. It happens on engines, pumps and, under some circumstances yes, paint and even blades. Check the statistics - not much normally goes wrong. If Robinson had it their way, they'd be using a better primer, but legislation won't allow them.

Go to the safety course, and the unofficial advice? if your blade has paint problems, for god's sake don't buy primer from us - use the old stuff we're not allowed to use!

If you want a machine with a 12 year warranty on all parts irrespective of operating environment, then not only shouldn't you buy a Robinson, I'm not sure there's any aircraft out there you should buy. Yes, it sucks, but hey - these are aircraft and ownership can suck big time. Ditto boats, and (if you remember the rhyme) women !

I'm not unsympathetic to the costs, and neither are Robinson, as I believe some assistance does sometimes appear to people who get these problems at low hours. Virtually everyting manufactured on the planet has a small percentage of issues. People need to learn to live with that uncertainty, and manage it with the manufacturer's assistance as best they can.

Lafite.

[topendtorque]

these are aircraft and ownership can suck big time. Ditto boats, and (if you remember the rhyme) women !

we're all ears there, Lafite ol' son

[Sam Rutherford]

"performance penalty normally associated with blade tape."

Tell me more about this - I thought the negative effects were minimum to the point of undetectable in flight.

We're looking at the pros/cons of blade tape for our desert operations.

Thanks, Sam.

[Nubian]

Sam,

Our technicians used a bladetape from 3M, which worked wonders reducing leading-edge-erosion on our blades.
Now, operating in the desert of Libya, you should be prepared to change it within 25 hours or so, but the clue is to not use one long tape, rather cut it into 1 meter bits or so, and change the appropriate bit as it starts to bubble/loosen. Easily detected when preflighting/walkarounds between flights, and you might hear it during flight.
It will save you a lot of hassle/work and lots of $$$ in less wear on your blades.

As for performance, I think that must be practical next to nothing when(in the practical world), but I'm sure in the theoretical one will give you slightly less performance. Haven't ever seen any difference at least.

Good luck with the planning and trip!

cheers

[500e]

3M tapes
Erosion Protection

HONTEK appear to make a product that is the last word in erosion protection!
read the blurb, then try and get product, ( have been trying for 18 months) was told no way to do in field, send blades to Karmen Helicopters it is not a field product, yet they are saying repair re treat blades as required.
HONTEK CORPORATION - Sand and Erosion Protection - Welcome

HONTEK CORPORATION - Publications Downloads

[helisphere]

So when an aircraft goes down from a delamination/debonding blade, can this be difficult to tell whether the delamination was actually the cause? I just wonder about some of these accidents that are caused by a divergent main rotor for no apparent reason. Is it at all likely that a delamination could be the cause of the divergent main rotor in these accidents?

[Arnie Madsen]

Can anyone name another manufacturer of bonded metal blades that has a similar history of ongoing problems .? I am not aware of another. Thanks.

[blakmax]

Blade disbonds

can this be difficult to tell whether the delamination was actually the cause?

Helisphere

Thank you for taking the time to read the paper.

It is possible by visual inspection to determine the type of bond failures present, and there are essentially two types: Cohesion, where the adhesive is fractured and residue remains on both surfaces, and adhesion, where the adhesive separates from one surface. Most structural film adhesives contain a "carrier cloth" which is a supporting cloth added during initial adhesive manufacture to aid handling. Cohesion failures usually fail through the plane of the carrier cloth because this is the weakest plane in the bond, provided the interface is strong. Cohesion failures require higher loads than adhesion failures, and in extreme cases adhesion failures can occur with very low loads.

Adhesion failure is caused by time-dependent degradation of the chemical bonds at the interface between the adhesive and the substrate (in this case the metal). The bond may start off with a high strength but over time the strength decreases, and usually the mechanism is that the surface of the metal forms a hydrated oxide and in the process the chemical bonds between the metal and the adhesive layer dissociate to enable hydration to occur and disbonding results. Failure naturally occurs at the weak interface, so all the adhesive remains on one surface and none on the other.

There is a third possibility; that failure occurs before the interface is fully hydrated, and this results in a mixture of cohesion failure and adhesiuon failure, commonly termed "mixed-mode" failure. The proportion of adhesion failure will increase with time as the interface hydrates, so if the structure is highly loaded at an early stage, the failure load will be higher and the proportion of cohesion failure will be higher. The converse is also true, such that the longer the bond is in service, the weaker it becomes, so failure will occur at a lower load and there will be a higher proportion of adhesion failure, and the locus of failure will migrate from the plane of the carrier cloth towards the interface.

The rate of strength loss depends upon a number of factors, the most important of which is the process used to prepare the surface during the factory bonding process. If the process produces chemical bonds which resist hydration. then the bond will last a long time. If not, then a shorter life will result. Other factors include higher operating temperatures and humidites, both of which will accelerate the hydration process and hence shorten the service life of the component.

Now, of all the tip disbonding examples I have seen there is a high percentage of adhesion failure. Erosion by itself can not cause adhesion or mixed mode failure in otherwise sound adhesive bonds. Hence, the entire discussion about which tape to apply is meaningless unless the bond failures are confirmed as pure cohesion failures.

I have also observed adhesion failure on one crashed blade at a number of locations well inboard from the tip where erosion is not evident at all. (Release of the report from a certain Pacific country is being delayed internally.)

The real problem is that it is remarkably difficult to categorically point to a disbond and say that this was without doubt the absolute primary cause of the accident. This is because even though the bonds exhibit failure modes which are characteristic of low bond strength, the loads which caused that specific bond to fail may well have been associated with an event which occurred as a consequence of another causal event, but equally they may be the causal event. There is rarely any marker such as fatigue striations to give a definitive identification. Occasionally, there is discolouration of the adhesive due to the presence of liquid water, and this would positively indicate a pre-crash defect.

Hence, it is necessary to eliminate or ameliorate all other probable causes and to look for circumstantial evidence to support the hypothesis that bond failure caused the crash. For example, if the pilot had thousands of hours with that type, had a perfect medical record and post-mortem assessment confirmed the absence of alcohol or drugs, then it is reasonable to suggest that the probability of inappropriate pilot control input is extremely low.

Naturally, weather conditions would be taken into account, as would mission profile. If the aircraft was flying level on a set and commonly used transit route on a bright sunny day with no cloud and low winds, then it is reasonable to infer that weather and flight mission were not a factor.

Extraneous causes such as bird impact or impact with items departing the aircraft would also need to be eliminated.

Next you would examine other causes which may result in a loss of blade tracking. Most of these leave tell-tale evidence, so if that evidence is absent, then it is possible to suggest that the probability of those causes is low.

Next you would examine all bond failure surfaces. All cohesion failures can usually be discarded because the bond would have exhibited an adequate strength. Adhesion failures are important because they often indicate a defect which existed before the crash event. For mixed-mode failures, the proportion of adhesion failure AND the proximity of the failure surface to the interface are both strong indicators of weak bonds. (Often safety investigators use a scanning electron microscope to find the presence of ANY adhesive and incorrectly identify that as a cohesion failure.)

Next, you would look for any history of bond failures in the specific component, and in the case of R22/R44 there are a number of examples where aircraft have landed or crew have survived crashes with tip disbonds. If then, the component in question exhibits characteristics of bond degradation and hence a loss of bond strength, then the probability of bond failure causing the event increases. If the extent and distribution of bond degradation is high, then so is the probability that bond failure was the cause of the event.

Then it comes down to a balance of probability that bond failure was the cause and not the result.

Regards

Blakmax

[61 Lafite]

A very interesting summary, blakmax.

The FAA notice on the blade AD in 2008 quoted on about page 8 of this thread notes:

This amendment is prompted by 11 reports of blade debond, some occurring in flight causing the pilot to feel excessive vibrations and land, and some found during routine maintenance.

I didn't see anything in it stating that there was any evidence of it being a factor in any fatal (or non-fatal) accidents. The Israeli report stated

There is real probability that some of the similar accidents in recent years that involved that type of helicopter were caused by a similar failure process.

(I note also that it mentions erosion as evident on the disbonded blade). I find it hard to reconcile these two: I have no doubt the Israeli investigators have done a thorough job, as have the US investigators on all the Robinson accidents in the US.

However if there really were a 'real probability' that similar disbonding had caused fatal accidents, why has it not been recorded as a possible factor far more frequently than we have been hearing about? A 'real possibility' maybe, but one missed by all other accident investigators.
For me, this is very pertinent to Blakmax's last sentence:

Then it comes down to a balance of probability that bond failure was the cause and not the result

Can any significant number of other accident investigators all have been mistaken in not even quoting this as a possible factor, and if we assume thay all did the highly professional job we know they all do, I find it hard to accept the 'real probability' the Israeli report suggests.


Lafite

[Shawn Coyle]

There was an accident in the Caribbean that was most definitely blade debonding - only the spar remainded on one blade.

As for debonding not being mentioned in other reports - accident investigators are human, and sometimes go off on tangents and miss the real cause of the accident.

[Hughes500]

Blades tape does make a difference to performance. In an OH6 the manual states that if blade tape is fitted ( doesnt say how much) that is equivent to an extra 70lbs on auw !
On my 500's if you polish the blades having cleaned them really well the machine will go about 5 to 6 kts faster. If you dont believe ask a glider pilot about unpolished wings or dirty wings on a glider ! was scepticle about it til I tried it !

[500e]

Hi BM Another interesting read.
I do not think the tape enquiry was really to do with the dis-bond, rather that Frank keeps saying dis-bonding is due to paint erosion on leading edge joint, this would at least reduce this argument from the factory & make it harder for them to reject warranty support.
If the Hontek product works as advertised, it would be a cheap cure rather than paint every X hours & low blade life.
Hi H500how are the Xmass lights

[blakmax]

why has it not been recorded as a possible factor far more frequently than we have been hearing about?

There are a number of reasons why this type of failure has not been reported in other events. Firstly, many safety investigators are ex-pilots and tend to focus on what they are familiar with.

Next, most investigators would not encounter a sufficient number of bond failure causal events to establish an adequate level of expertise in identifying the specific characteristic of a bond failure.

Thirdly, to my knowledge there is no formal training available in adhesive bond failure forensics, and much of the knowledge base is grandfathered within the investigative organisations and that embeds and propagates false knowledge (which is why some investigators use SEM to find the minutest trace of adhesive and then classify the failure as cohesion).

Fourthly as I have said in other postings, there is a deficiency in the FARs and JARs in that there is no mandatory requirement to demonstrate that an adhesive bonding process produces hydration resistance, and what guidance there is in AC 20-107 is weak and only came into existence a few years ago. Hence, there is no regulatory and minimal advisory guidance to the investigator to assist in focussing on that type of bond failure.

These comments are not intended to reflect on the integrity of safety investigators. They simply have not had training and exposure to bond forensics. I am sure that a similar lack of knowledge will come to light when we start to see failure of fibre-composite structures.

Regards

Blakmax

[delta3]

Hydration

Blakmax

is there a "easy" explanation why hydration weakens the mechanical bond? Is it for instance the mechanical strength of Al2O3 versus AL(OH)3 or is it changes in the bonding bridges?

d3

[Hughes500]

500e

Mrs 500 wants to know why I want to purchase 200 m of Xmas lights, the expression " dont even think about putting them on the house" was the polite reply !!!!!!!

[500e]

Flashing perhaps
Don't suppose I will be welcomed for tea & toast again

[blakmax]

Hydration

Delta 3

The mechanism for aluminium materials is that Al2O3 has an affinity to form bohemite Al2O3.2H2O. It takes less chemical energy to form that hydrated oxide than it does to maintain the chemical bonds to the adhesive, so hydration wins. In the case of the R22/R44 blades, the materials are stainless steels, so naturally the reactions will involve hydration of ferrous/ferric oxides.

I sympathise with 500e. My place is used as a visual beacon because of the christmas lights. I want to start the movement to stuff the Easter Bunny down Santa's throat and fix two problems at the same time!

[topendtorque]

accident investigators are human, and sometimes go off on tangents and miss the real cause of the accident.

one will go a long way to better this clinically correct statement, at least here in oz anyrate.

such demeanours are acceptable, IF the system has within it a fool proof device of peer review, as they can usually be overcome.

here we notice that when a mistake is made by ATSB they defend , it , at , great , length .

in the process of consequental actions there are then often, affected family members, workmates and personal and company budgets that are severely disjointed and who come away from the process feeling utterly devastated and with the feeling that ATSB couldn't give a f''k.

like hospitals, they specialise in burying their mistakes.


as for this,

Can anyone name another manufacturer of bonded metal blades that has a similar history of ongoing problems .? I am not aware of another. Thanks.

i think there is one example, indeed that I have seen on these pages here, but in defense of the topic, so far I only hear the periodic creaking of the rocker, not the tapping of the keys. it may have been back in the days of fabric, so not as close to the money.

[helisphere]

Blakmax
Thanks for all the info, yes, I finally read the paper.

So I wanted to clarify a couple things and just make sure I have this right.

1. We are basically talking about adhesion and mixed failures if the bond is the problem? Because if we are talking about cohesion failures then the bond remained strong and the disbond failure was most likely a result of something else loading the blade beyond the design limit?

2. If the blade passed some kind of initial structural or NDI test out of the factory then it is assumed that for a non-cohesive failure, the bond degraded over time due to hydration?

The rate of strength loss depends upon a number of factors, the most important of which is the process used to prepare the surface during the factory bonding process. If the process produces chemical bonds which resist hydration. then the bond will last a long time. If not, then a shorter life will result. Other factors include higher operating temperatures and humidites, both of which will accelerate the hydration process and hence shorten the service life of the component.

Now, of all the tip disbonding examples I have seen there is a high percentage of adhesion failure. Erosion by itself can not cause adhesion or mixed mode failure in otherwise sound adhesive bonds. Hence, the entire discussion about which tape to apply is meaningless unless the bond failures are confirmed as pure cohesion failures.

3. Hydration can be slowed down but not eliminated and you are saying that the most important way to slow this down is proper surface prep? But you are saying that the paint covering the bond line does very little to prevent hydration? And are you also saying that an exposed bond line will not appreciably accelerate hydration of a well prepared and executed bond?

Please make sure that I am thinking of all this correctly.

[blakmax]

Helisphere

Cohesion failures are not usually a problem because the bond should have passed initial certification and the nature of cohesion failures is that they would have achieved the required strength. The exception is if the bond has micro-voids as I suspect occurred in the AW139 failure at Doha, discussed eleswhere.

The problem with QA testing is that it is undertaken before the bond has had time to hydrate, so a good result usually always occurs. NDI will only detect voids in the bond. It can not interrogate the condition of the interface, so it does not provide assurance of a good bond, it only tells you if you have a bad bond. Worse yet, all of the inspection criteria for in-service disbonds are set on the basis of tests with artificial defects embedded in an otherwise good bond. The surrounding adhesive has not had time to hydrate before the test, whereas if you get the same size disbond in service, the surrounding bond may be hydrated and may be weaker than that tested during certification.

Hydration can be eliminated by use of surface preparation methods which have been validated using specific tests to demonstrate resistance to hydration. Such processes eliminate hydration and hence eliminate adhesion and mixed mode failures.

An exposed bondline will absorb moisture faster than if it is painted. However, paints and sealants only slow down, not prevent moisture diffusion. We are not talking about free liquid water, it is the microscopic moisture which is absorbed by most polar molecular structures (such as epoxies). This eventually reaches the interface and causes hydration.

Regards

Blakmax