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AW139 lost tail taxying DOH

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AW139 lost tail taxying DOH

Old 28th Aug 2009, 10:03
  #81 (permalink)  
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Crab, agree, but I have no idea what the inspection criteria would be re the 139 tail boom, however whatever they may have been they did not detect a severity that required replacement it seems. As you know impact damage on these structures has to be severe to cause visible deformation of the structure, it is more lightly that any visible indications were chipping of the external paint surface. Possibly the use of an Ultra Sonic detection system called FLAUS that has I understand shown success in detecting hidden de-bonding and internal voids may have help, I donít know if this technology is in use or approved for use.

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Old 28th Aug 2009, 11:52
  #82 (permalink)  
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Are we all agreed this incident would be far less likely to happen in the air?
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Old 28th Aug 2009, 12:44
  #83 (permalink)  
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Well, we hope.
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Old 28th Aug 2009, 19:40
  #84 (permalink)  
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chopjock, do you mean in forward flight or in the air full stop (e.g. incl. hover)?

Last edited by birrddog; 28th Aug 2009 at 20:06.
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Old 28th Aug 2009, 19:55
  #85 (permalink)  
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I would have thought this `occurrence` should be classified as an accident,as the aircraft was taxiing,rotors running,etc, period of operation of the aircraft..
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Old 28th Aug 2009, 20:42
  #86 (permalink)  
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Could those who fly the AW139 answer this question.

Obviously there was substantial stress on the tailboom when they were doing their specific taxy procedure and thats when the boom gave way.

Now lets say they had become airborne without the separation happening, what other stages of the flight would have had that same large amount of stress on the boom that it could have given way.

Not being an aerodynamicist I am unsure of what parts of a flight regime have the highest stress loadings.

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Old 28th Aug 2009, 21:33
  #87 (permalink)  
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I would have thought that when airborne, even at MTOW the tail boom would be easier to pull around than when on the ground. Therefore had it been air taxi ing, this accident may not have happened.

A bit like pushing a boat when afloat is much easier than pushing a boat aground.(or on wheels).
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Old 28th Aug 2009, 21:39
  #88 (permalink)  
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If this "incident" had occurred in the air it would be an "accdient" for sure.

It is an interesting question you pose about the forces....but I would imagine hovering cross wind in a very strong wind at max weight would also produce quite a bit of force. Whether making tight turns on the ground....or forgetting the nose wheel lock and trying to make a turn would exceed the airborne forces is for the design engineers to answer to.

No matter.....tail booms are not supposed to fail this way for any reason shy of a massive overloading such as a tail boom strike or crash. (IMHO anyway!)
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Old 28th Aug 2009, 22:39
  #89 (permalink)  
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It's bin a few years but...

The only sure way of finding disbonding or voids within and GRP/CFRP structures is by using Ultrasound Techniques (a La - The Midwife's Pics of your loverly babies in mummy's tummy)

The 2p test (it used to be 10p in my day!) is okay for finding voids or disbonding near to the surface but is not reliable for deeper structures. TAP tests on Blades are looking for disbonding at the surface or the skin joint to the Spar - but not in the Spar itself.

X-Rays are also not particularly accurate and are open to "interpretation" issues.

Voids are prone to 'speading' under stress so all small knocks should be registered ASAP and assessed 'as soon as practicable' (I love that phrase!) and mapped for future periodic monitoring.

Also, to repair CFRP is a really specialist job - you cant use a normal drill, for instance, as it won't cut the Carbon Fibres without dragging them through the resin, and any repair scheme would need to be drawn up (though not for this particular Tail Boom!) by more specialists, and conducted by yet more specialists. (I remember a RAF Harrier GR7 once waited 18 months for an intake "Bird-Strike" Plate repair scheme that took two weeks to complete)

Although it is a very useful material - I personally don't like CFRP for structural uses!
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Old 29th Aug 2009, 01:51
  #90 (permalink)  
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I distinctly recall Nick Lappos telling us that the stresses on the tailboom are far greater in flight than while ground taxying. So, it's probaby wrong to even discuss whether it would have failed in flight - it definitely would have done!
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Old 29th Aug 2009, 02:32
  #91 (permalink)  
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So, it's probaby wrong to even discuss whether it would have failed in flight - it definitely would have done!
Could not agree more!!

I have never used full pedal while ground taxing, however have "hit the stops" many times in flight, with many different types of a/c.......cross winds, hot and high, etc.

This would have been a catastrophe if the a/c would have gotten airborne........
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Old 29th Aug 2009, 05:43
  #92 (permalink)  
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You would use more pedal hot and high because the tail rotor is less effective in these conditions. So yes you would use more pedal but not necessarily apply more force to the boom.

In a strong croswind a larger percentage of tail rotor thrust would be used to counter the effect of the wind to the vertical fin and boom itself so would not be transferred as an increased tortional force to the boom.
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Old 29th Aug 2009, 06:13
  #93 (permalink)  
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NDI of disbonds

Guys, the problem with NDI of bonding problems is not just limited to composites. Adhesive bonds depend on chemical reactions at the interface between materials and in some cases those chemical bonds are susceptible to degradation in service, usually by hydration at the interface. This occurs on composite and metal bond surfaces. NDI can ONLY detect defects which cause an air gap, and hence interupt transmission of sound waves. NDI can NOT detect interfaces which are degrading but have not yet separated. In other words, NDI can tell you if you have a disbond, but it can not tell you that you are about to have a disbond.
I did ask what the repair method was (and go no response) because if it involves injecting fresh adhesive, that will only fill the air gap so that NDI will pass the repair. It will NOT and can never restore bond strength because the surfaces are not chemically active and hence no chemical bonds can be formed.
My next question would be "what was the production method used to prepare the composite bond surface?" because if it relied solely on sacrificial peel plies (removed to produce a "clean" surface) then there is a potential for further problems. The surface must not only be clean, it must be chemically active, and removal of peel plies does not produce a chemically active surface. Worse yet, there are some peel plies which transfer release agent onto the bonding surface, hence causing contamination.
The real problem is that the FARs for certification of aircraft structures require testing for strength and fatigue, neither of which are the cause of bond degradation in service. It is actually possible to fully certify a structure which has a high potential for in service failure through bond degradation. And NDI will not find the disbond until it has occurred. The trick is to find the disbond before it reaches a critical size.
Now, can anyone tell me A: what is the repair method? and B: do they use peel plies during construction?
This is not an attack on this particular manufacturer. There are many well intentioned constructors out there who make the same mistakes.
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Old 29th Aug 2009, 10:34
  #94 (permalink)  
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Surely the tail boom structural failure force on any helicopter is well in excess of any load that could possibly be applied through max pedal, whether on the ground on in the air?
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Old 29th Aug 2009, 11:58
  #95 (permalink)  
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Mmmm ...

blakmax .....

We who have been flying these machines in the middle east have for some time been concerned by statements presumed to have come from Agusta saying .... "these de-bonding issues are only prevalent on machines operating in the middle east" .... would the adhesive breakdowns you mention be more problematic with the heat issues we suffer ie a/c parked in high ambient (+47C) temps, differential expansion/contraction of the structure ... also the area at issue is prone to exhaust heating action from rotor downwash.

All of which we now find very worrying ....
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Old 29th Aug 2009, 12:25
  #96 (permalink)  
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Yaw Loads

FAR 29.351 describes the design yawing loads. I'd assume the JAR's are similar.

John Dixson
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Old 29th Aug 2009, 13:47
  #97 (permalink)  
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The real issue with disbonds

Hi spinwing

You say the manufacturer says
these de-bonding issues are only prevalent on machines operating in the middle east
Regretably these issues have been reported elsewhere. Reference NTSB A08-25_29 dated 09 Jun 2008. Interfacial degradation is exacerbated by hot and wet environments. Hence, a number of OEMs have incorrectly blamed tropical environments for similar failures. I am also aware of manufacturers blaming wash fluids, bird crap, whatever. The real reason is a failure of resistance to environmental degradation at the interface and this relates directly back to the failure of the FARs (or other regulations such as JARs, DEF STAN 00 970, MIL STD 1530 etc.) to require demonstration of the durability of the interfacial chemical bonds in the operating environment.

The current FARs require that bonds use "processes known to produce a sound structure". But what constitutes a sound structure? If it passes NDI is it a sound structure? Not necessarily because NDI can not interrogate bad interfaces. If it passes bond strength tests, is it a sound structure? Not necessarily because if the test is conducted before the interface is degraded, then there will be a false-positive result. If it passes fatigue testing is it a sound structure? Not necessarily because if the test is conducted before the interface is degraded, then there will be a false-positive result. None of these requirements address the time-dependent degradation of the chemical bonds at the interface. Hence, the FARs and other reg's which require strength and fatigue tests and the use of processes that produce a sound structure do not prevent a common (dare I say the most dominant) failure mechanism in adhesive bonded structures.

I stress again that it is NOT the manufacturers who are liable, it is the regulators who permit certification of structures which are susceptible to a known deficiency. Eventually some smart lawyer will stop sueing the manufacturers and realise that it really is the regulations themselves which permit structures to be certified such that they fail to prevent a known, common failure mechanism. I published a paper on this in 1996 and I am aware that the regulators have seen this paper ( see my web site at adhesionassociates.com ).

I have been trying since 1996 to have this deficiency rectified. If it takes a smart lawyer to defend a manufacturer or another even smarter lawyer to seek damages for defcient regulations, I would be available for discussions. My objective is not to make money from other's demise, but to prevent further needless loss of life. In the case of this specific aircraft, the on-ground failure avoided loss of life. It does not mean that the structure which in all probability met certification requirments was actually airworthy. I am well aware of other cases where "rotorheads" have not been so fortunate. The problem is not just limited to rotary wing aircraft. There are examples from fixed wing structures as well.

And can I stress again, if ANYONE has ANY testing evidence that injection repairs for disbonds actually restore ANY strength, I'd be very interested. The difference between removing the NDI signature and actually restoring strength is critical. Similarly, if an adhesive disbond exhibits interfacial failure (metal or composite) this is a processing and certification issue, not a loads issue.

On a related issue, I have data (admittedly old data) which shows that injection repair for delaminations in composites actually REDUCES fatigue life compared to doing absoultely nothing. I actually forced one OEM to admit that injection repairs for delaminations could not restore ultimate strength and at best slowed down delamination growth. Any comments?


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Old 29th Aug 2009, 16:40
  #98 (permalink)  
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It sounds reminiscent of when the Mosquitoe was deployed to the far East 65 years ago!
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Old 29th Aug 2009, 17:28
  #99 (permalink)  
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Some more detailed pictures, sent by a friend:

I'm glad everyone is safe

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Old 29th Aug 2009, 17:45
  #100 (permalink)  
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Thank you blakmax, for the interesting input and the background technical information. I had always felt that with the new emerging composite structures that a important part of the structure, the glue and the efficiency of its bonding would have been well covered, maybe I need to expand my reading list.
The matter of the structure being affected climatically and the approach taken after events that could damage or reduce its load carrying capability is a matter I feel needing some attention. I feel myself that these structures need a regular form of inspection using a system that will be able to assess any degrade of its integrity. Certainly after an event that may have cause a weakness that could remain hidden for a number of years a more involved inspection should be required for the life of the structure.
The fact that after repair defects could spread under repeated loading and not be visible or seemingly detectable in the early stages using present methods is interesting. I am sure that JAA offer some advisory on this whole subject ?
I have just seen the snaps from Aser, I apologies to Crab, indeed the statement WTF seems to be well justified.

Thanks for your indulgence.
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