AW139 lost tail taxying DOH

[[email protected]]

sox6 - blakmax has done an excellent job here in highlighting shortcomings in people's knowledge about the construction of the aircraft they fly and operate.

Unless you can come up with a really good reason why a tailboom that had been repaired and declared airworthy should suddenly fail in such a spectacular fashion I suggest you let blakmax educate you as he doing to the rest of us.

We have all taken composite technology for granted and assumed since it replaced/enhanced aluminium construction that it must be as reliable and failure proof - after all, all the manufacturers are using it so they surely wouldn't compromise safety for a few more kilos useable AUM would they?

Blakmax has clearly detailed how, unless the composite is put together properly in the first place, it is doomed to failure at some point and environmental factors are simply the trigger and accelerant.

[DOUBLE BOGEY]

CRAB

Thanks for putting us all in our place, however, there is no information released that supports any of the hypothesis that has been put forward so shall we keep all the other SWAGs open as well.... like did the bump on the helideck weaken it.

If it is a heat/debonding type issues then it is clear that none of us have learnt anything at all since the first SMS began.

A Greek SMS Reported some while back that a de-bonding issue caused by excessive heat had caused a catastrophic mainplane failure and a fatality as a result. The Pilot I think was called Icarus!!!

DB

[sox6]

I am always open to being educated. I simply challenge the level of confidence of some statements being made. The rest us may have a low level of knowledge ourselves but automatically assuming someone with more must have all the answers is just being naive. Lets not confuse knowledge about composites with knowledge of this failure.

If a psychology professor was posting here about a recent accident and discussing the pilot's errors with that level of confidence would that be automatically accepted just because of a few academic qualifications?

[Brian Abraham]

I am always open to being educated. I simply challenge the level of confidence of some statements being made. The rest us may have a low level of knowledge ourselves but automatically assuming someone with more must have all the answers is just being naive.

sox6, if you want to do yourself a favour, go back through the thread and you'll find blakmax lays his credentials on the table. Impressive they are, an expert in composites. He is not alluding to what caused the failure, but giving an education in the problems associated with the use of composites.

[sox6]

Brian If you do yourself a favour and look on this very page you will find I have specifically challenged some apparently very explicit statements above that do more than simply allude to this failure.

[JimL]

It is not clear whether we should be posting on this thread or the other one - started to discuss the take-off procedure.

AW have developed a take-off procedure for the AW139 which is in conformance with the PC2e philosophy; further, they have had it certificated as a CAT A helideck procedure. It is analogous to the North Sea HAPS procedures but, when flown into a clear sector (the 180 degree obstacle clear sector), it provides engine failure accountability up to its new MCTOM of 6.8t.

In fact it is good enough to provide deck-edge clearance (and of course drop down) even when taken off from the TD/PM of a helideck of any size in the North Sea and elsewhere (212man, remember the discussion we had some time ago). With a take-off mass of 6.4t, the drop-down is minimal up to about ISA + 20 with zero wind.

As the greatest danger to the aircraft in a helideck departure/arrival is collision with obstacles, heliport procedures should not be used. There is less danger with a PC2-with-exposure procedure.

Having said all of that, we are all in the dark as to the procedures used, and reasons for using them, unless one of them is prepared to join this discussion.

Jim

PS - FSTD TGL # 14 (new)–- Guidance on the qualification of Electrical Motion Systems for FSTDs

[TheOldTimer]

Dear me what a shame, a very educational and fascinating series of posts and a thread that was in my humble view, one of the best I have seen recently, seems to be going in a direction that really adds little to the original development of the discussion.

[Capt JB]

AW 139 tail lost..!!

Ok, interested to know what the happened there. I have A7-GHC as registration, from web records.
If anyone has the serial number and operator of this bird it will be very much appreciated to keep up with my database

[Sandy Toad]

BT 139-193 and 139-194 Issued .Both Mandatory Precautionary Inspections for signs of de-bonding. First one for Helicopters that have experienced "Tail Scrapes, Tail Strikes or Tail Bumps". Latter for rest of fleet.

[[email protected]]

DB, I would be very surprised if the tail strike hadn't weakened the structure - surely that is why it had an AW approved repair carried out. The point we keep coming back to is that the industry may not be quite as clever as it thinks when manufacturing or repairing composite structures.

Sox6 - if you read blakmax's post that prompted your reply in context, he is referring to the fact the environmental issues such as OAT and exhaust gases can precipitate failure but only if the bond is not perfectly manufactured - I read this as a generic statement regarding compositis and not one specifically dealing with the tail boom collapse.

He has been critical of the aerospace industry as a whole as well as the regulators, he just happened to mention AW in that post.

[500e]

DB
".Like did the bump on the heli deck weaken it."
Now there is a scary thought, I understood it was passed as serviceable after repairs by AG.
sox6
blakmax is questioning the methods\ quality of build and giving his suggestion for failures of this type, he would appear to be able to back up the talk with results in the field, he has laid out types of failure with suggestions as to the reasons and a rough guide as how to examine same, the thing I find worrying is there appears to be no way to check for incipient flaws?.
So the only way to prevent failure is in build quality which is what he appears to be saying, as Crab said,
"Unless you can come up with a really good reason why a tail boom that had been repaired and declared airworthy should suddenly fail in such a spectacular fashion I suggest you let blakmax educate you as he doing to the rest of us".

Thanks for the information you have imparted both here & in PMs direct BM . We have been doing in nearly correctly, without understanding the reason! will reduce vac to around 5lb rather than 8\9 we were using. This is on a boat hull not aircraft, relax everyone.
I have no knowledge of bm other than he gave me some Free pointers regarding bonding, which as this is his living I found most helpful.

[blakmax]

response from blakmax

To quote a famous Australian "Crikey!" Things were quiet last night and when I awoke this morning all hell has broken loose!

For Sox6. You do have a point, and we will eventually have to wait until the report is out before we can confirm my theories. However, my discussions were not restricted to just this incident. If you go back through the thread you will see that several posts relate to bonding issues in the tail boom on a number of other aircraft. Indeed there are examples of failures after very short TIS. The desriptions of the disbonds in all of the other cases are consistent with interfacial failure between the adhesive and the skin or the adhesive and the core. These types of failures are indicative of processing problems. There are very few actions which an operator or maintainer can do that will result in interfacial failures.

The next issue is; Will the report actually be undertaken by people who can recognise bond defects? I mean no reflection on investigators, who really do a good job, but adhesive disbonding probably constitutes a very small proportion of the investigations so there is only a limited opportunity to develop the knowledge base necessary to differentiate between the different types of failures and to identify the probable cause. I am unaware of any formal training available in adhesive bond forensics. Therefore much of the knowledge base is derived from hand-me-down knowledge which is only as good as the original data.

I have also seen a number of glaring errors in formal reports on accidents involving another helicopter type. In one case, the investigator found what he beleived was fatigue markings in an adhesive bond. Unfortunately these features were in the middle of the adhesive layer ahead of the failure surface and the actual failure was at the interface. There is no way any fatigue in the middle of the layer can be related to a failure which occurs at the interface.

In another case, the investigator showed an example of a good bond from an area away from the failure site. In fact that "good" bond exhibited about 50% microvoiding which was not even commented on.

From my side, this subject has been my primary responsibility on a day to day basis for over 37 years. I have had the luxury of having the time to develop expertise in the field, the opportunity to access a wide range of bond failures and to identify processing issues which cause specific failures. These skills enabled me to write an engineering standard on composite and adhesive bonded repairs, write two handbooks on repair design and repair bonding processes, develop three training courses and participate in the development of two other courses. Since the implmentation of these measures, the bond failure rate in repairs fell from 43% in 1992 to only three cases since then and in each case the cause could be easily attributed to poor practices.

The reason I have been getting involved with this thread is to increase people's understanding of composite and adhesive bonding issues.

You will also note that I have offered my services to Agusta on investigating any bonding issues for this specific case as well as those reported by others in this thread. I acknowledge that until something more definite is available, this still remains a WAG.

Regards

blakmax

[spinwing]

Mmmmmm ....

..... The reason I have been getting involved with this thread is to increase people's understanding of composite and adhesive bonding issues.
.....


And if I might say ... your doing a bloody good job of that too!

Thank you

[blakmax]

axes grinding

Thanks for the comments spinwing. I did forget to address the issue of axe grinding. Up until this incident was reported on PPRuNe, I had never seen an AW139 and I have never had any contact with Agusta. I have only had one small contract with another rotary craft manufacturer and I have no ongoing relationship with that company. I have also stressed in the past that these observations are relevant to a wide range of manufacturers of rotary and fixed wing structures. Hence I have no axe to grind with any specific company and I have no interest in supporting one company against another.

On a different tack, this particular failure presents a unique opportunity to investigate the integrity of the bonds in the failure area. Usually, the samples have been subjected to crash related forces and environments which often mask the evidence or at least complicate the assessment. This example has not experienced fire, has not impacted the ground at high speed, it has not been imersed in water and all of the parts should have been recovered. It may be possible to determine the direction of disbond growth during the failure, so it may be possible to determine the locus of failure and the probable cause without the conclusions being clouded by unrelated events.

Regards

blakmax

[TheOldTimer]

Blakmax.
I would like to add to Spin, crab and others comments regarding your valuable, very informative and nicely placed humour that has given us all a valuable insight into the black art of composite s and the problems and considerations that designers and manufactures need to address. Since my first exposure to the inclusion of these structures in aircraft design, initially as non load carrying and graduating to become critical structures, I admit to having been concerned. This in part I felt to be an old farts reluctance to accept change. A responsibility of the designer and manufacture, policed by legislation to regulate and ensure that the best and safe practices were followed. Your articles on the scientific side of this has been informative in the extreme.
To finish, education and understanding is important as we all know, openness in the release of information to the industry also critical. This was not generally the case in my time.
Thanks blakmax and please continue to educate us all, let’s not return to the dark side of the past.
Thanks all for indulging an old timer well past his sell by date.

[blakmax]

Thanks

Thanks OT. I really appreciated all of the support both on and off line. Adhesive bonding is a truly amazing technology when the basic fundamentals are understood and correctly implemented. However there are problems with the certification basis and these deficiencies are combined with management that follows what worked last time (but didn't really work if they understood the basics). Iwas astounded to find that over 75% of aircraft designers base their designs on an average shar stress approach which was shown to be incorrect as far back as 1936. The only way that these designs work is that they have a high level of conservatism built in to the design allowable stress levels, and they undertake extensive testing. The stupid thing is that if they udnerstood the fundamentals of adhesive joint design developed in the late 1960's not only would their designs be just as conservative, they could save millions in certification costs.

It is actually possible to design a bond between aluminium up to about 0.15 inches thick such that the aluminium breaks, not the adhesive. If this rule was applied, then every one of the thousands of tests normally undertaken would result in only breaking the metal. So why do all of those tests? Design the joint, undertake a few tests to demonstrate that the bond is stronger than the metal and certification is a lot cheaper.

Regards

blakmax

[widgeon]

It is funny that despite all the advances in technology one of the best tests for a debond is still the tap test with a quarter ( or probably 10P in the UK ).

[500e]

The tap test only finds relatively large dis-bonds not a myriad of small ones, read post 78 of this thread, how do AG suggest the pilot checks for problem ? or does it require an engineer to examine before every flight.
The post regarding bubbles would suggest there was a considerable void to start with, presumably this boom was in service & subject to testing on on going basis.

[twinpac1958]

Fuselage AND Tailboom

For those that do not know, Fuselages AND tailbooms are manufactured in Poland and trucked to Vergiate (It) for assembly.

[spinwing]

Mmmmm .....


....For those that do not know, Fuselages AND tailbooms are manufactured in Poland ......

Vaguely remember mentioning this back on post #11 ...... but it was sooooo long ago now

Cheers ....