AW139 lost tail taxying DOH

[shak'n]

Here's a thought:

In my limited experience with AW139, we too had debonding i.e. bubbling of the tail boom skin only on the left hand side directly under the #1 engine (LHS) exhaust stack. This also coincided being directly under the tail boom coanda strake.

The organization I was with had this bubbling/debonding occur on at least 2 out of the 3 aircraft it owned and all with less than 300hrs. One with only around 150hrs when the debonding was observed. The role of the aircraft was SAR/EMS requiring a lot of normal cruise flight regimes but also periodical slow speed searches, hoisting etc requiring extended hovering.

We observed one bubble appeared quite large immediately after shutdown but seemed to reduce in size after cooling down. At the base I was at, teletemp decals were fitted only on the left side (unfortunately) and the tail boom monitored. During normal flights, T/Os, cruise and Ldgs, no portion of telitemps discoloured. We then had to do a slow speed search (<20KIAS) for approx 1 hour duration which included a little downwind maneuvering . On shutdown from this sortie the first couple of segments had discoloured. We then did periods of hoisting and hotter segments became discoloured. I do not remember what the hottest temp we finally recorded but it was within the hottest one or two. Unfortunatley we did not put a teletemp on the RHS of the boom in the a similar position in order to get a comparison of temps on either side of the boom to see if both sides were seeing similar temps.

It is clear though that the bubbling/debonding is linked to the high temps being recorded there on the LHS. When looked at in conjunction with the Coanda strake, the airflow or vortex which it sets up beneath it particularly in the hover, must trap the exhaust gas from #1 engine immediately below it. It in effect becomes a Weber-like BBQ recirculating and increasing the localized heat against the skin. See the link below which shows the visualised airflow around these strakes in the hover admirably:

http://www.nwlink.com/~blrweb/pdfs/H...20analysis.pdf

(Hope this link works out)

I believe the laminar flow on the RHS (while in the hover and throughout the flight regime) prevents localised heat build up and the ensuing softening of epoxy and bubble formation. As I said, regrettably we did not fit teletemps on the RHS of the boom to compare the heat build up of each side against the flight envelopes. I am now no longer in a position to access a AW139 but recommend to anyone who does, try it and see what the teletemps on each side of the boom do with different flight regimes - in particular hovering, slow speed maneuvering, or extended downwind operation (on-ground, taxiing, hovering, slow speed).

A very easy quick fix: If the above is true, the simplest quick fix would be to amputate the first 2 feet of Coanda Strake to allow laminar flow of the rotor downwash to remove localised heat build up against the skin on the LHS of the tail boom i.e remove the hot exhaust gas of #1 engine with the laminar flow in the hover or at slow speed maneuvering . The remaining length of strake should still provide adequate Tail rotor thrust offset in the hover as originally designed to do but prevent localised "weber" effect of trapped, oven-like temps beneath the strake immediately under the exhaust stack of #1 engine.......

I'm not suggesting this as a permanent fix as I think a better tail boom design/product should be designed by those so-called design engineers who get paid all the big bucks and have limited responsibility for their end product need to do their job properly as we as aircrew are expected to do every day.

But, food for thought as a short term solution don't you think???

[blakmax]

The role of heat in sandwich panel failure

The suggestion that this problem is driven by heat has some initial plausibility, but realistically the only way that adhesion fillet bond failure can be driven by heat is by the water content of the cells causing steam pressurisation during the operational heat cycle, combined with weak bonds between the adhesive and the core, and as I already suggested this is probably due to inadequate drying of the nomex prior to bonding and a lack of environment control in the bonding facility at the time of bonding.

Service heat can never cause interfacial failure between the skin and adhesive.

This is not operator caused. The PM reports I have is that these defects occur in incredibly short TIS after fitment (a few hours???) Now I don't care if the operator had a dozen naked maidens dancing on the boom, that would not cause interfacial defects. They are processing problems and that is a manufacturer issue which should continue to drive their warranty claims through the roof.

In defence of Agusta, I have seen exactly the same deficiencies in bonded structures from other rotary manufacturers as well as fixed wing manufacturers. THIS IS NOT ISOLATED TO ONE MANUFACTURER. It reflects a deficiency in the regulations in respect to adhesive bonded structures, combined with deficient engineering training in adhesive bonding. This is not a reflection on any manufacturer including Agusta, because to loosely quote Dick Chaney, you may know what you do know but you can not know what you don't know.

The route to success is to ask those who actually do know more than you to help you to sort out your adhesive bonding problems. The alternative is to take decisions with substant risks to company profits as well as human life. There are experts who are out there who can help and it is no discredit to a company to recognise that their knowledge base is limited to the requirements (and deficiencies) of current regulations. The choice is do you produce a product which barely complies with the requirements of the regulations or do you produce a product which meets the requirements of the regulations and has a justifiable reputation for excellence and never requires warranty replacement? While I clearly state my capabilities in this area, I acknowledge that there may be other experts out there. I simply urge manufacturers such as Agusta, EC, RHC and others to seek help to establish standards which actually produce quality products with sound structural capabilities and not just to pretend they have divinely acquired knowledge which is often demonstrably not backed up by service performance.

Remember this: No matter what design methodology is used, the shear strength of an ineffectively manufactured bond will eventually reach zero, despite the best intentions of the manufacturer. What varies is the time for the bond to achieve zero strength, or the intervention of a load which exceeds the degraded bond strength and results in premature failure. Unfortunately refusal to acknowledge gaps in technological capabilites have a high potential to cost lives, and those costs exceed warranty claims any day and subsequent legal costs may exceed the cost of specialist support by a considerable margin.

Regards

blakmax

[JimL]

This thread is a tribute to Blakmax and the other practitioners who have volunteered knowledge and advice.

Well done - keep the information flowing.

Jim

[212man]

Hear hear!

Blakmax, do you have any friends Who are metallurgists, gas turbine experts or AFCS specialists? If so, I think we have pprune sewn up!

Thanks alot for your contribution - it's been a joy to read.

[SASless]

I have a rule.....I must learn something new every day!

This week I have more than caught up with my sleep having read Blakmax's posts.....after reading each one of the posts I accomplished my daily goal and thus could fall back into the hammock with a clear conscience!

Definitely enjoy his posts!

[griffothefog]

SAS,

What if he's a spook from Eurocopter? Me paranoid???

[heliski22]

And hear, hear again!

I've been checking this thread several times a day just to see what new gems of wisdom have been brought forward. Supplemented by the observations of others with some hands on experience, blakmax's posts have been fascinating!

In the Unversity of Life, every day's a schoolday!!

Griffo, just cos you're paranoid doesn't mean they're not out to get you!!!

blakmax, if I read your last post correctly, is it probable that the longer the aircraft is in service without defects showing up, the less likely it may be that there will be problems at all?

22

[Hilife]

I’m not a lover of anything AW, but there are a lot of 139’s out there flying in all manner of operations and this is the first I’ve heard of a tailboom failure and supposedly on a low time machine.

Appreciate manufacturing flaws are possible, but post #24 suggests that this machine had history of a tail strike and after an inspection the machine was released to service.

I’m not privy to the circumstances and severity of the tail strike – if indeed this ever happened, but if true, it could be the root cause of this latest incident and not down to poor build, manufacturing processes, excessive localised heat or bonding design flaws.

Just a thought.

[Shell Management]

Hilife

A very good point - and there was indeed a previous incident. This fact has not been lost on the oil companies. That is why there have been no groundings of our AW139 fleets.

We hope the full picture will emerge over the next few days.

[blakmax]

Forget the tail strike theory

Firstly, thanks guys for the encouaging comments, and no, I am not a spook for any company. I am a private consultant having recently retired after 37 years involvement in aircraft science and maintenance on military aircraft. I am happy to share my specialist knowledge on adhesive bond forensics in the hope of improving safety.

Now to deal with the tail strike theory. If the adhesive bonds were damaged by impact or overload of any description, then the failure would be by cohesion, with the adhesive fractured. There would be residual adhesive on both surfaces. The descriptions from people who have taken sections from disbonds clearly indicate interfacial failure, and a well bonded structure should never fail interfacially even under impact conditions.

And Yes, 212, I do have a friend who is a metallurgist and he is a damned good failure analyst.

Still waiting to hear from Agusta....

[Thridle Op Des]

I'm wondering if the tailbooms are subcontracted to Alenia Aeronautica in Naples. On the FW side they are rapidly progressing towards pressurised structures in bonded composite, albeit with slightly different structural approaches between Boeing and AB so the science of adhesives is becoming much more interesting.

[airwave45]

Given the known softening of epoxy at temperature,
The static temps on the booms during the day being 70-80 Deg C (ambient temps are mid 40's add in direct sunlight in the sandpit, 70 Deg C is easy)

The guys fly the 139's in an unusual profile offshore in the Qatari gulf (where A7 GHC failed), the cruise and approach are same as everywhere else, the lift to get back in the cruise is unusual.

The guys will lift off the deck, wait (I assume for egt, but I'm just SLF) then 9 times out of 10 will lift backwards up about 100' (Top of the legs of the jack ups are about that high, so it's easy for SLF to guage height)

the heli's come up at about 75 to 85 deg off the deck (90 being straight up) I'm used to helicopters exiting the decks in forward flight and notice the exit procedure as it's pretty close to the bow leg and hacks me off as I know neither driver is looking backwards to guage clearance from the leg.

This exit will surely have the boom / fus passing through the Exhaust gas, heating an already hot structure.

I assume this is done as the 139 is such a tail heavy beastie, but don't know.

If the helicopter is flown backwards and up through it's own exhaust gas 5 to 10 times a day, this might not help the already hot and stressed boom bonding product.

a little bird told me that 2 other booms have been found out of spec on the ramp at GH, that would make 3 on that ramp with issues.

Shell / Conoco _have_ grounded 139's here. (we are back to 412's) others are still flying 139's

[spinwing]

Mmmmm ...

Airwave ..... you are making comments about pilot technique which are really not necessary to this thread .... if you are concerned about how they arrive or depart your platforms you should take one of the pilots aside and ask him to explain to you why they are using their to your eyes unusual procedures.... they most likely will be quite happy to explain. A lot is to do with decks placed out of wind or with NO WIND with high ambient OATs and High humidity often all at the same time ..... makes for different techniques than used in the North Sea.

With regard heat being blown down on the booms during takeoff .... I don't think that is a problem ... helicopter by design have a bloody big fan that tends to blow hot air across the booms all the time ... whether on the ground on takeoff or in flight .... were stuck with that!

Cheers

[Um... lifting...]

The pause is a momentary check of the power index (measure of first power limit in the 139) to determine the limit to which to pull during the takeoff.
The backward takeoff permits a safe landing back to the deck if at any point during the procedure an engine fails. Standard in the 139 for certain PC1 operations, though I can't see why it would be relevant to this thread.

[chopjock]

I think Airwave has very valid points and are relevant to this thread. Surely anything the pilot can do to minimize the risk of exhaust heat to the tail boom is good? (Even though he shouldn't have to).

For example, in the cruise at 155kts there is virtually no downwash on the tailboom and max cooling air flow.
The more time spent in the hover and climbing backwards will obviously add risk to the tailboom getting hotter for longer.

Imagine this, a pilot carries out a twin engine type lift from a platform, backing away and climbing until safe to fly away on one engine. That's all very well and good so long as you still have a tail boom left.

[Heli-phile]

AW 139 tailboom failure

I don't think I have ever seen a more disturbing failure on an aircraft structure.
My thoughts go to the "lucky" crew on this imminent flight, ...and the previous flights!!

[ShyTorque]

The quoted "unusual", rearwards climbing, departure is likely to be the normal Class A takeoff profile for a raised helideck, as published in the Agusta Flight Manual. The crew are required to fly the aircraft in the published manner to satisfy the performance criteria for public transport operations.

[Shell Management]

Yes, ShyTorque, very likely the FM has been followed. What is unusual is that other people who don't know what this is are judging it!

airwave45
Gulf Helicopters withdrew its other AW 139 from service pending further understanding of the cause of this incident. Shell are not currently placing any restrictions on the use of small number of AW 139 in Shell service.

[blakmax]

The role of heat in sandwich panel failure

Ok, irrespective of the trajectory of the aircraft at take off and/or the presence of dancing girls on the boom, there is really a high thermal input to bonded sandwich panels in service. I am aware of some measurements on composite structures sitting on the tarmac in northern Australia where the temperature exceeded 80C (180F for those in the US) just through solar heating. Exhaust gasses may or may not add to this depending on the previously mentioned big cooling fan just above.

Irrespective of the temperature, what is the mechanism which causes the failure? In reality it is the presence of the afore mentioned moisture absorbed by nomex (about 5%) and epoxy (about 2%). When heated, this leads to pressure in the cells both due to the expansion of air but also the release of water vapour which adds to the pressure. It is this pressure which results in the failure of the bond to the core. Even worse, if this panel is heated to above 100C (212F) in service or for hot-bond repair, then the pressure greatly increases.

Now there is another aspect which must be considered. If the nomex core has not been adequately dried before bonding, then the moisture evolving during the adhesive cure cycle will inhibit chemical reactions between the core and the adhesive, thus weakening the bond which forms. In service, the pressure developed in the cells as described above, when combined with the weak fillet bonds due to poor chemical reaction results in excessive pressure which disbonds the skin at the fillet bonds and results in the skin distortion described elsewhere in this thread as the skin disbonds.

So, the upshot of all of this is that the heat cycle in service facilitates the failure; it does not actually cause it.

It is still a processing problem.

A further aspect to this. A number of people have suggested that there is a softening temperature for adhesives at around 80C (180F). That is true for most 120C (250F) curing adhesives, but for 177C (350F) curing adhesives, that temeprature is closer to 140C (about 285F). That temperature is the glass transition temperature Tg above which the adhesive transforms from a glassy rigid material to a soft rubbery material. However, exceeding Tg does not cause interfacial failures, so if the failures in this case are interfacial adhesion failures, then Tg is not a factor.

Regards

blakmax

PS OK SASless, you can go and have another snoo. (Snoo is the singular of snooze.) Now I think it is time for one more boo.

[IntheTin]

blakmax,

With all this discussion about moisture, would washing the aircraft down after the last flight of the day have some bearing on this problem?