Dromader Mk II.

[Kharon]

I find the previous thread on the M18 accident was closed by the Mods; can we please try to keep this version alive, there is much to consider from the latest M18 accident.

Bear with me, there is a point to the preamble. The reason 'why' the wing and fuselage parted company has and still does drive me nuts; I keep putting it aside only to have it sneak up on me. So, the books got hit; did a fair amount of research, ending with a very good ATSB report from 2008 on a similar event. Still couldn't scratch the itch - more midnight oil and no further forward. There was something missing, a piece of puzzle which escaped my clutches. Anyway – from the deep dark corners, some half learned, almost forgotten fragments of aerodynamic law crept out – Negative G damage rate incurred during taxi. I checked back over my notes and it was not mentioned and yet this is a very real, powerful destructive force.

I have no doubt that the design and certification engineers would have considered this and I dare say the ATSB (2008) would be aware of this sinister factor, but there was no official accounting for it. The aircraft are 'hard used' particularly during fire bombing operations; fast turn around; fast taxi in and out over rough ground, maximum loads over a full days operation in rough conditions. Potentially going on for weeks.

It was only a notion, but it seemed to fill in some of the questions. The 'in-fight' data, loads, stresses and management are well understood by the pilots; it's a pretty fair bet that the 'boys' are as careful as can be (under the circumstances) with the aircraft; the engineering can be considered as 'up to scratch'; all in, the only element which I can find not accounted for is damage done - 'on the ground'.

Now I am not saying – at all – that this is the answer; but it is a concept worth chewing over. I know a bloke, who knows a bloke, who is a wizard on the 'engineering' side. This is a dark art and not a subject much discussed. I spent a very valuable hour on the telephone with a very fine gentleman, who allowed his dinner to go cold while attempting to explain a small part of the mystery to me. I owe him a beer or two and many thanks. There are in front of me three pages of scrawl which I will attempt to place in order, for consideration; all fascinating stuff.

For instance did you know the 'fatigue bible' (AFS 120/73) started life as a response to (type uncertain) bomber wings falling off – design life of 27 hours. NACA tested several to destruction in the 40's but the real research was Australian (DCA), done on P51 aircraft wings (50's) and set the benchmarks for the bible. The rest is history, as they say. Anyway the rate of damage used for ground operations is estimated at 9.73 x 10 to minus six or 10.25%, which is slightly higher than that use for calculating gust and 'manoeuvre' rate of damage. Metal as we all know, has a memory.

So, for what it's worth; is there a possibility that constant taxiing over rough ground at high weights could have contributed to the M18 accident? – discussion – with moderator forbearance.

[currawong]

Look again at where the failures occurred versus where the ground loads are transmitted.

[Old Akro]

Negative g would mean that the pilots lunch & thermos will rose off the seat then fall on the floor. Or, more seriously maps & pencils would jump around.

Primarily, taxy loads will be positive g bumps followed by zero g or close to zero g as it "falls" on the other side of the bump.

I've done some certification work on vehicles above GVM (armoured vehicles, etc). The problem is that you never really know which component is the limiting factor that determines GVM. I assume that its the same with aircraft. There are possibly a dozen points which might be the limiting factor, but its hard if not impossible to determine the critical one. We change wheels, tyres, springs, shock absorbers, suspension bushes and place severe speed restrictions on these vehicles to deal with loads above the homologated axle loads.

For vehicles (and I'll bet aircraft too) the GVM (MTOW) is set by a regulatory or marketing decision and the vehicle built, then tested to that weight. So, the designing engineers know that that weight is OK, but anything above that and you are on your own!

As soon as you increase the mass of a land vehicle (read taxying aircraft) you need to increase the spring rate. If this is not done, the chance of using all the suspension travel and "bottoming out" increases dramatically. Depending on design, bottoming out can nearly be considered as having no suspension at all and a solid metal to metal load path from the wheel to structure allowing shock loads to be transmitted directly to the structure.

The Dromader has undercarriage mounted to the wings (read wing spar) at maybe 1m outboard of the fuselage. The original certified Dromader had a MTOW of 4200kg. The Australian CASA issued STC allows an increase to 6600 kg. That is an additional 2400kg of mass that is carried on the main gear legs. I would think that (especially given the aircraft's service on unimproved strips) that an upgrade of the undercarriage spring rates (ie oleo pressure) should be required.

With no suspension upgrade the suspension will almost certainly "bottom out" on some bumps. This would transmit large shock loads to the main spar which in turn will put the inboard bottom wing mount in tension (similar to negative g in flight).

It seems conceivable to me that taxy loads from taxying on rough strips at increased MTOW could have a significant fatigue effect on the failed wing spar joint.

The failed wing spar joint is a pinned joint, not a bolted joint. This means that there is no clamping force between the various joint components. Thus there is no shear force damping effect and all of the shock vibration from taxying loads would be borne by the failed wing attach bracket.

I don't think you could discount fatigue caused by taxy loads as a contributing factor.

[TBM-Legend]

For instance did you know the 'fatigue bible' (AFS 120/73) started life as a response to B17 bomber wings falling off – design life of 27 hours.


Please provide the evidence for this. My research shows no such occurrence on the Boeing B-17..the wing design like that of the DC-2/3 are shown of classic strength in their day...

As a matter of interest a number of T-28 and B-26 aircraft shed wings in the Vietnam War due to fatigue. The cause was put down to taxying with heavy underwing ordnance on rough taxyways and operating off sub-standard natural or PSP runways at high weights. This is effectively a military version of an ag operation and Old Akro has hit the nail on the head..

[currawong]

When in the air heavy, I see the wings bend. So they are loaded up. Add to this turbulence.

When on the ground, they do not bend. They have only their own weight. Add to this bumps.

Several tons difference by my reckoning.

Also, the lower spar attach is subject to compression, not tension in the situation you describe.

[Hempy]

Currawong, I don't think anyone is suggesting that the load factors on the ground are anywhere near the load factors experienced in flight, more that ground ops are a factor to be considered in the 'accumulation' of fatigue over time. And possibly that the Dromader in question was subject to rougher than average handling whilst taxiing thus accumulating fatigue faster than other a/c operated differently. Surely then it's worth a consideration.

[yr right]

There are no additional wing loadings at the wing fitting from standard on the ground. It has a centre section that carrys all the weight

[Sunfish]

Currawong, in flight the wing load is spread across the wigspan. On the groundits taken at Two points (forgetting tailwheel) this is a very differnet loading system and needs to be analysed separately from flight loads.

[currawong]

Sunfish,

Refer posts 2 & 7

Put simply, the wing attach point that failed most recently was outboard of the undercarriage.

If it was the centre section that was failing I would say you might have a valid point. But its not.

[dubbleyew eight]

what category of certification was the dromadier done to? normal??

[Neville Nobody]

This accident was a Turbine Drom, not a standard one. The airframe was designed to carry 1600 KG. Basically they stick a bigger engine on without any airframe mods.They are approved/certified by STC. They carry twice the load faster and some times cruise very close to VNE. If you read the ATSB report they cover the factoring of time with high weight loads and the history of this particular aircraft. At a wild guess I'd say most other turbine Droms have a similar history.

[Kharon]

TBM – Sorry, it never occurred to me check that part of my notes. Being a complete ignoramus in the subject, struggling to frame a question and to understand the answer, I just accepted that statement – verbatim. The information did come from an impeccable source – simply as a bit of background to the subject. I will contact the source and check; probably down to my appalling handwriting, but I have jotted WWII/ 40's research/(looks like) B17/ wings off/ Italy/ ferry flight/ time ex 27 hrs/ - but I will check.

"Old Akro has hit the nail on the head"... Indeed, and it was most helpful. Remember the 'Kermode' series: Mechanics of flight and Flight without formula: and how helpful they were, when trying to 'nut out' a problem. I wish there was similar for this subject. There are some hefty tomes available, but I can't make anything of them; which is why, I suppose, I asked the questions. There is little in the way of 'pilot education' on the subject, let me hasten to add there is no need for it, not at that level anyway.

There are countless aircraft, of all makes and marks which operate off rough strips every day within their design limitations, that's what kicked off the thought bubble. There are some pretty fancy theories floating about on the 'why' of this accident and some ugly accusations. The ATSB (http://www.atsb.gov.au/media/3532975/ao2008084.pdf)2008 report is very good as was the CASA response; some cherry picked paragraphs –HERE (http://www.pprune.org/australia-new-zealand-pacific/429828-merged-senate-inquiry-97.html#post8478410) –. I liked the way no one got 'shut down', sacked or prosecuted. ATSB made sense of the accident, CASA fixed up the 'over-time', misunderstandings and limitations then everyone went back to work. Lets hope it's repeat story for this iteration.

OA & TBM thanks for the good words – and food for thought. (more headscratchin).

[currawong]

If you have not already I would read this -

www.atsb.gov.au/media/4111849/ai-2011-150_final.pdf

No opinion from myself is expressed or implied.

[Old Akro]

Put simply, the wing attach point that failed most recently was outboard of the undercarriage.


Yep! I missed that. So, the increased mass of the fuselage will have no direct effect on the wing attach on the ground.

Although, there may still be a shockload issue from "bottomed out" suspension.

At the time of the accident, I did read the inspection requirements for the increased MTOW aircraft and my memory is fading. But I do wonder about the bottom pinned joint. The condition of the bolt, tapered bush and mounting hole diameter all become quite critical. Any slop in the mount (I'm talking 1/10,000 th of an inch) and the joint will start to flog itself to death.

The taxi issue may not be the bumps as much as wing rocking caused by taxying on rough ground where the outboard wing section will experience negative g as it oscillates up and down. Couple this with the suspension bottoming out and the forces might become quite high.

[Lumps]

Oleos are easily adjusted for increased weight and operators account for this simply adding more nitrogen (there is something like 800psi of nitrogen in the oleos already)

[Kharon]

Currawong – Thanks for the link, duly downloaded and saved. It's a good example of an ATSB report, which cheers me up.

Are G meters fitted as routine, I understand that speed and bank angle control preclude the need (not that thick) but, when it is as critical as it appears to be in this case – is that not a bad idea (to be sure)? For example, the aircraft is running in – fully loaded then releases (dumps) the hopper load, assuming wings level, is there a negative effect then or additional stress (for want of better) created during that brief period?

This bit has me puzzled, any ideas:-
P6 (16 pdf). -The aircraft manufacturer advised that the bank angle limitation in PZL AFM Supplement No. 16 was a result of flight testing. CASA advised that, during the development and approval of STC SVA521, the STC owner/developer was unable to discover the reason for that limitation so it elected to include the limitation for the STC with CASA approval.

The rest is interesting.

P7 (17 pdf). 4.10. Level Flight
The aircraft shows dynamic longitudinal instability with free control stick /after about 20 seconds and two vibration cycles, the aircraft shows tendency to reaching the stall speed or exceeding the allowable maximum flight speed/. There was no parallel notification of a longitudinal instability issue in any of the other AFM supplements. The certification flight test report in support of STC SVA521 (see STC flight tests on page 9) stated that the test aircraft’s longitudinal stability was ‘adequate at all times’.

The following is most informative; but, on first read through it leaves me wondering about 'damage' done before the manufacturer, CASA and ATSB got things under some semblance of control? I will re read it, but I wonder was there any 'retrospective' initiative?

P9 (19 pdf). At least one maintenance organisation applied the time in service factoring in an alternative manner. The method used was to subtract the additional flight time derived by the application of the weight-dependent factor from the total permissible airframe life. The maintenance organisation reported that this method was used to prevent maintenance from being conducted more frequently than necessary on items that were not affected by the aircraft’s increased weight, such as the aircraft’s engine and flight instruments. The extent to which other organisations may also have performed the service life adjustments this way was not determined.

P9 (19 pdf). STC SVA521 and the associated documentation included a list of required equipment and modifications, such as the turbine engine and increased capacity hopper. It did not require the installation of vortex generators or M18B standard elevators. CASA advised the ATSB that its intent for the approval of the STC ‘was always that aircraft fitted with STC SVA521 be in the same configuration as the aircraft used for testing’.

P14 (24 pdf). The result was that an operator or pilot could, under STC SVA521 and depending on the supplements that were incorporated in an aircraft’s AFM, apply a variety or no operational limitations to agricultural operations in M18 and M18A Dromader aircraft at weights between 4,200 kg and 5,300 kg (Table 5). This lack of clear and unambiguous operational limitations increased the risk of their inconsistent application by operators and/or pilot’s, increasing the likelihood of the unknowing erosion of engineering safety margins and aircraft life.

P21 (31 pdf) Appendix A.

The 'Currawong (http://www.atsb.gov.au/media/4111849/ai-2011-150_final.pdf)' report is well worth a careful read; but drawing the 'right' inferences is tough going; for my ol' wooden head..

[Old Akro]

Oleos are easily adjusted for increased weight and operators account for this simply adding more nitrogen (there is something like 800psi of nitrogen in the oleos already)

Yes, but..

1. The pressure needs to be increased by 64%. If they are 800 psi as you suggest, this means they now need to have over 1300 psi. This is the "at rest" pressure. Under full compression this will be many times higher. Can the standard seals handle this?
2. Does the STC require increased pressure in the oleos?
3. Is it actually done in the field? Does the STC supply a revised maintenance schedule that reflects this?

[currawong]

Put simply, the airframe is/was operated beyond design life, at beyond design weights, at beyond design airspeeds.(ATSB)

The effect of ground operations on the fatigue life of the airframe is a very very distant fourth consideration, in this case, in my humble opinion.

[Old Akro]

Put simply, the airframe is/was operated beyond design life, at beyond design weights, at beyond design airspeeds.(ATSB)

And inspected to a different regime than sanctioned by the manufacturer

I don't disagree. I was responding to Kharon's pondering about ground loads. However, my experience with car GVM increases is that frequently its something outside the main load path that is overlooked which causes grief.

[yr right]

grd loads are nulled. it has nothing to do with ground loads at all.


cheers

[Kharon]

No disagreement from me either: as stated, I was only asking the question – out loud – due to a lack of knowledge and fishing for answers. I did find a reference book though -Fundamentals of Structural Integrity - A.F. Grandt (http://books.google.com.au/books?id=y76sdpYYhU4C&pg=PA4&lpg=PA4&dq=B+17+structural+failure&source=bl&ots=grpeA9GR16&sig=fqHAyipJ10FsA0XXF1Z6RytnjGk&hl=en&sa=X&ei=Brt3U_-3EcXTkAXd14GYBA&ved=0CC4Q6AEwAQ#v=onepage&q=B%2017%20structural%20failure&f=false)- which, for me at least, is a helpful guide. The ground damage itch is scratched and I learnt a bit – so colour me happy.

Seems that damage due 'ground loads' was not a 'significant' factor; thought there may be something in it, but Lumps trumped the oleo card which pretty much put a hat on it for me.

Hard to get past the Currawong statement though, a second careful read of the ATSB report takes you, inevitably to the conclusions drawn.

Put simply, the airframe is/was operated beyond design life, at beyond design weights, at beyond design airspeeds.(ATSB).

The effect of ground operations on the fatigue life of the airframe is a very very distant fourth consideration, in this case, in my humble opinion.

It is still a complex issue though, even the ATSB don't leave it cut and dried. Going back to the 2008 report; it seems as though there was a 'joint', no blame effort made to effect a cure, which I applaud. I will reserve my opinion on the time scale and 'official' response until we see a final outcome of this last event.

And anyway – it's all YR's fault – he got me started. (Thumbs up and big smile).