ATSB Control Cable Report

[Old Akro]

I've just read the ATSB report on control cable terminal fittings and can't stop myself tilting at the windmill again.

The conclusion might be valid, but report is terrible. Like most ATSB reports I've read recently, it is not transparent, doesn't cite it sources, nor build a logic path to its conclusion and it fails to touch on some obvious questions.

Firstly, it labels the control cable problem as chloride stress corrosion cracking. This conclusion is pretty much pulled out if the air without any explanation. Its been a while since I studied metallurgy, so I did some reading. I might of missed something and someone else might know better, but documents like the UK based National Physical Laboratory's report " Stress Corrosion Cracking" or a paper by Howard Hilbron " Chloride Stress Corrosion Cracking - Causes & Solutions" both suggest that chloride stress corrosion cracking requires a combination of high level of stress, acidic environment and elevated temperatures. All this in the back of my hangared aeroplane? Please explain.

I can't find anything from the FAA that labels the US failures as chloride stress corrosion cracking.

Furthermore, the crack in the photo on p9 does not fit the descriptions I have read on chloride corrosion stress cracks. I read that chloride stress corrosion cracks are a) perpendicular to the stress (the crack in the photo is parallel with the stress), b) there tends to be only one crack without surrounding micro cracks (ie the crack that causes failure only) and c) the crack in the photo is in a lug area of the turnbuckle that is not stressed at all.

Whenever I have seen a failure investigation a photo of the failed surface has been included. This report does not. Ordinary visual inspection of the failed surface almost always allows some observations to be made about crack propagation and whether fatigue, ductile or brittle failure is involved. The documents I have found all talk about chloride stress corrosion cracking having a very distinctive appearance with a triangular or lightning shape. Its hard to see, but this does not appear to be evident in the photos shown in the ATSB report.

From what I can see of the failed surface, it looks dull and smooth-ish which would suggest that the crack had been developing for many years. If this is the case, shouldn't there be some discussion about maintenance diligence?

Assuming the turnbuckle shank is 1/4 inch diameter, the 303 stainless steel shank should have a tensile strength of about 1.8 tonne. The tension on a Piper elevator control cable is 40 lb +/- 5 lb or about 20 kg. The stainless steel turnbuckle is in no way stressed. How could the ATSB label stress being a factor without any sort of basic analysis? Fatigue cracking through vibration from loose cables might be more conceivable, yet consideration of any other failure mode is conspicuous by its absence.

Secondly, the report acknowledges that there are 3 styles of turnbuckle, but it appears from the photos that one style is forged and the other two machined. A first year engineering student will tell you that turning marks act as a stress concentrator and an initiation point for cracking. But the report does not mention this.

Most of the focus and sterobinocular microscope study is of corrosion pitting on the shank, but since a) that is not the site of the cracking and b) there are huge margins of strength in this area, it doesn't seem to be at all relevant.

Thirdly, it would be pretty nice to do some sort of investigation to see if there is a correlation to operating condition and the failure, ie were all the failed turnbuckles from aircraft tied down outside near the sea?

There is an unacceptable failure rate of these turnbuckles (ie more than 1), but I have no confidence that this report as got to the bottom of it. My guess would be that the turnbuckle actually fails when it gets down to about 10% of its cross section area remaining. It ought to be relatively easy to see at this juncture. Could it / should it have been picked up in 100 hourly inspections?

Should cleaning with scotchbrite (as recommended by Piper) and / or coating it with a light oil like LPS #1 be recommended? Wouldn't some discussion of prevention be warranted?

It seems to me that replacing the control cables once every 15 years is a reasonable thing to do. I'm not arguing against the conclusion at all. I just think we deserve to get more intellectually robust reports for the money we are paying - especially when its conclusion affects a large number of people and aircraft.

[Kharon]

Nicely done Akro – "The conclusion might be valid, but report is terrible."

This depends on your point of view. It appears, once again we have to look overseas for explanations and expertise. What we see, again is the ATSB report being used to justify the CASA predetermined outcome. Long live the MOU.

The presented report has little to do with 'operational rationale' and everything to do with making it appear that the CASA decision is correct and fully supported by 'technical expertise'; the 'strictly no liability' policy shines through. Just get used to it, the trough dwellers have developed an appetite for such things, there will be more.

People attending the GA/AMROBA meeting got a taste of what's on offer from the leadership. With Mc Comic once again making friends and influencing people whilst laying down the law on a micro managed prosecution data base, just for you.

ATSB reports should be consigned to the same place as the safety digest – but; the culture of fear prevails and the majority will meekly accept this as part of the thin gruel provided to industry, shrug and get on with life. Pavlov – eat your heart out.

[Sunfish]

This report? FAIL! Amateurish bull****. I did better around 1973 as a Second year engineering student when I spent Nine months on the metallurgy of 30mm DEFA cannon cartridge casings at Footscray after the Australian made ammo was determined to have inferior accuracy compared to the French stuff - due to bullet pullout force variations we thought.


Reading the report, it is apparent that CASA simply said to ATSB "Find me some examples of stress corrosion cracking to justify replacing old control cables".

So some poor work experience student at ATSB got the job.......and dutifully looked....and found nothing.

The late and great Ian McRitchie, pilot and metallurgist, would turn in his grave at this.

The findings:

›
Findings
From the evidence available, the following findings are made with respect to the examination of the flight control cable terminals submitted by the Civil Aviation Safety Authority and should not be read as apportioning blame or liability to any particular organisation or individual.

Key findings

• The flight control cable terminals of part number MS21260 and AN669 had been manufactured from a material (SAE - AISI 303 Se austenitic stainless steel) that
is particularly susceptible to chloride stress corrosion cracking.

• Many of the terminal samples originally submitted for examination revealed corrosion pitting and surface discolouration, however none exhibited evidence of stress corrosion cracking.

• Failure of two flight control cable terminals during the course of the investigation were
confirmed to be a result of a chloride stress corrosion cracking mechanism.

• Visual inspection alone has been shown to be an incomplete defence against stress corrosion cracking related failures.

• Failure of control cable terminals due to stress corrosion cracking is an ageing aircraft issue with a potential for safety impact that will increase over time (how the F*** do you justify that statement?)
.


So here is what my year Two lecturer would have said:

Page 1:

"Stress corrosion cracking occurs when a component made from a susceptible material is placed under a sustained tensile stress and subjected to a corrosive environment."

Stress level, crack propagation behaviour, time frame? References?

Please also differentiate between fatigue crack behaviour and SCC?

Page 2:

"At the time of the release of Revision 2 of the AWB, CASA began an examination of a large population of control cable terminals to identify if any of the selected terminals had sustained SCC, and to assess the potential risk to the Australian fleet.

A number of cable assemblies 3 were sourced from a maintenance organisation in north - western Victoria and were believed to have been in service for greater than 30 years. The cables and terminals had been removed during routine maintenance from a number of different types of aircraft, and from different flight control systems. "


I'm sorry but the methodology here seems flawed. Were these turnbuckle components representative of the general population of control cables in the australian fleet? It sounds to me as if these assemblies were items that had already been scrapped by the maintnenance organisation, a such they don't represent the fleet do they?

Wouldn't a better method been to have chosen a sample of aircraft of known ages and inspected their cable assemblies? Did you pass statistics 101? What did they teach you about experiment design and hypothesis testing?

"The AWB was updated as Revision 3 on 5 June 2012, with an added description of a terminal that had cracked from the internal surface of the swaged section, at the interface with the wire cable. "

Yes, but was this a case of stress corrosion cracking ???????????????



Page 3:

"Slight differences were observed in the design of the terminals received, but they were essentially the same component. The primary difference was in the centre section that included a hole for safety wire installation; one population exhibited a hexagonal form (Figure 1 bottom) while the other exhibited a rounded mid - section
with wrench flats (Figure 1 top). The terminals also exhibited three different manufacturing markings which included ‘bell’, 'F’ or ‘OO’ (Figure 2).

The terminals marked with an ‘F’ were all of the type with the hexagonal centre section
, while the ‘bell’ and ‘OO’ samples were round. Additionally, all the ‘bell’ and ‘OO’
items were identified as part number AN669, while approximately half the ‘F’ termin
als were part number AN669 with the other half being part number MS212604. The two part numbers were consistent with those identified in the CASA AWB."

How do you know the differences were "slight"? Microstructure examination? Are all components roll formed? Stress concentrators? Did you attempt to trace the manufacturers by the marks?

What are the specifications for AN669 and MS212604 terminals? What are the tensile limitations?????

Page 9:

While your photographs of pitting corrosion are good, I fail to understand how the single photo of the failed terminal demonstrates anything.


Page 14:

"While no cracking was identified on the large batch of terminal samples submitted by CASA, evidence of pitting was observed on several of the terminals, with pitting attack being a known precursor to SCC.

It was noted that the terminals were all collected from an operator in north - western Victoria, and although the full operational life of the terminals was not confirmed, the
environmental conditions in north-western Victoria were considered to be relatively benign for SCC, due to the relatively low average humidity and the low potential for environmental chloride exposure."


Yes, pitting is a known precursor to SCC, it is also a precursor to fatigue cracking. WHy didn't you differentiate between the Two earlier? Why didn't you also address the loading conditions on the turnbuckle as well as the absolute prohibition of bending loads?

But most importantly, why didn't you conduct ultimate tensile tests on some of your samples to determine if any of them no longer met the performance specifications referenced in the standards you should have looked up and referenced?

Yes, North Western Victoria appears benign, but did all of your samples spend their lifetimes solely iin NV Victoria??????


There are many other assumptions in this report that are too numerous to mention.

Yes, there most probably is an argument to replace cables after Fifteen years, but your report does not demonstrate this conclusively. For example, how many failures have there been in cable assemblies that have otherwise passed a severe "on condition" inspection after Fifteen years requiring replacement if there is the slightest sign of corrosion of any sort?

D +, see me next week about resubmitting this.

[Sarcs]

The 'causal' chain...remarkable isn't it that in the space of three posts the credibility and veracity of a bureau report can be so completely decimated and ridiculed! :ugh:

There has been many threads on the apparent falling standards of bureau investigations and reports over the last 12 months or more...perhaps the collective 'we' should be batting them back at the ATSBeaker with the kind of summarised questions that Sunny and OA ask in a REPCON or two..just a thought??:E

https://www.atsb.gov.au/voluntary/repcon_aviation.aspx

[my oleo is extended]

Ok, I spent a few hours yesterday methodically digesting the ATSBeakers (I mean CAsA influenced) investigation report, and........
And this crap has surely got to be the straw that breaks the trough dwellers back? Put it to you this way, I have a mate who is a metallurgist with a an Engineering company in South Africa. I flicked a few key points from the investigation to him for a guernsey and he was, based on what I gave him, totally appalled as well.
He also agrees that the report is 'light weight', not fluid, and in my words full of mi mi mi mi-ing once again. The curse of the Beaker strikes again.
Does 'he who does not know what a mid-air is' even understand what the field of metallurgy is?
My question is this. If the ATSBeaker don't have a qualified metallurgist on the payroll (I can only assume this based on the final reports content and shallow pithy commentary), why don't they bring in a 'technical expert' to assist? That provision exists within the TSI, not a problem??

Perhaps we should email copies of this pony pooh to ICAO and the NTSB for a 'peer review', comment and feedback? Or maybe get some third world Category 2 state who doesn't even have a safety investigative body do the work?
Or do we use the report as toilet paper or bring it out at Xmas time for a good ol drunken laugh among friends?

I understand how the Beaker wouldn't have a clue which way North and South are, but surely Jules Verne, Beakers right hand prophet, could or should have advised him of the quality of this report? Or is the report just a copy or cut and paste of something the CAsA ALOO provided the ATSBeaker with?

[T28D]

Perhaps we should email copies of this pony pooh to ICAO and the NTSB for a 'peer review', comment and feedback?

Thisareally good idea, whohas an Email address of a Senior ICAO staffer.

[Sunfish]

For the avoidance of doubt, I have no objection to replacing control cables at Fifteen years or sooner at the direction/ suggestion of the regulator or any other authority.

However the ATSB report literally reminds me at my own earliest failed attempts at investigation and the root cause was the same - failure of experimental or research design.

The hypothesis to be tested is "the increased failure probability of control cable components beyond fifteen years old poses an unacceptable risk".

If this is true then you either replace them - hard time method of reducing risk, or, if it can be demonstrated (another hypothesis) that a detailed inspection and test can spot problems and is cheaper, then place them "on condition" at designated intervals.


Then of course there is the little problem of infant mortality with the replacement cables.

[owen meaney]

It should be stressed that there are both internal and external stresses, plus residual stress and accumulative stresses.

[Up-into-the-air]

That's it meany, now how about an in-depth exposure of how the report should have been designed.

[Possum1]

I've found it strange that Piper would allow the turnbuckles to remain if they clean up with scotchbrite considering the expense of complying with the AD. Our 1974 Piper Cherokee Cruiser would have been one of the first aircraft in the country to be subject to the FAA mandated AD as it went into the workshop for its 100 hourly the day after(March 5th).

Not surprisingly the 40-year old control cables were found to be corroded. I asked about the turnbuckles which were serviceable but at $20 each for new ones, it was an easy decision to agree with the workshop to fit new turnbuckles despite Piper not requiring it, considering the expense of the new cables and the labour involved in removing and then refitting the entire interior of the aircraft from the front seats back.

I also took the opportunity of having a lot of corrosion in the rear fuselage tunnel removed and treated. The AD and the corrosion removal/treatment all added about $2000 to the bill but I think it will be well worth it in the long run.

[Kharon]

"The response disappointed me. All the cables volunteered for testing are gathering dust."

This post highlights one of the industry failings; I can well understand and empathise with the frustration expressed, but really, the thread was originally started to discuss the quality of the ATSB report. For me the issue is, had I ordered and paid for an analysis, the same as Clinton proposed and been presented with a report like the ATSB effort, would I feel I had got value for money, or a haircut?. Caveat emptor and all that.

[Old Akro]

Clinton

I'm trying to critically review the ATSB reports. I'm being very careful to criticise the reports and not the ATSB. It's a bit of a Quixotic quest to demonstrate that the ATSB fail us. I hope to get more people critically reading their reports. I hope that some of those people might be from the media or government and that it might help them realise that our peak safety investigation body is delivering sub standard reports that are not transparent, do not facilitate peer review, do not table sources, fail to canvass all relevant issues and there is some evidence in some reports that they change or mis-report things.

Regarding the cable issues, I both agree and disagree with you. The cable replacement issue was done in a rushed ham fisted way. The fact that no-one else around the world has followed CASA's lead helps demonstrate this. However, for the at least the Pipers I fly its not a big expense and seems reasonable practice to replace them every 15 years. My old cars get new wheelbearings & ball joints every few years whether they need the or not. Its the same principle. I feel happier driving my 1972 Alf with new steering rod ends. I feel happier flying my 1979 Seneca with new control cables - especially after seeing some of the NASA work on flutter

If we test the cables (which I could do for you - or set up for you to do), it won't get over the "but if it only saves one life...." safety motherhood argument. You can only test a small population of cables and there will be the argument that they came from the more engaged / diligent owners / maintainers.

The better way of attacking this flawed decision is to attack the process. I THINK this could be done effectively by using the FAA published documents, plus what ever the US AOPA submitted. I think there is also useful information on Beech Talk & CSO Beech. The US is not taking the same approach as Australia and their processes are more robust, transparent and better supported by analysis. Without having looked, I'm sure there will be many aspects where the US contradict our own regulators.

[Sunfish]

I agree with old Akro, just follow the FAA advice and be done with it! Why does CASA have to reinvent the wheel - and then get ATSB to justify it?

[Jack Ranga]

Clinton, just say the tests were done. You don't think CASA would dismiss the test results out of hand? That or completely ignore them?

I kind of agree with you on the 'don't whinge if you are not prepared to do anything' line but it seems to me that a lot of good people have been worn down by these two organisations, know they haven't got the financial resources to fight them & couldn't give a rats anymore?

[Sarcs]

Who needs ATSB reports anyway?
Watch to learn about cable failures

Everyone concerned about wear and tear on aircraft control cables should take a few minutes to view a video on the important subject. CASA has produced the video to clearly explain the dangers posed by unidentified cable wear and the steps that need to be taken to avoid problems. This follows an increasing number of reports of cable failures in recent years, particularly control cable terminal failures.

Senior CASA engineer Roger Alder features in the video, setting out why cable problems can develop, how to inspect cables and where problems are most likely to develop. Roger Alder says aircraft control cable failures should not happen if inspections are carried out properly. Many aircraft now have cables inspected in situ, which can mean emerging problems are not identified at an early stage, risking cable failure. It is recommended cables are removed from aircraft and inspected on the workshop bench, so they can be examined under magnification and opened up. Likely areas of wear, such as where cables run over pulleys, should be marked on the cables before they are removed.

View the cable video on CASA's You Tube channel now.

[YOUTUBE]cable video
Maybe it's all a grand plot to make the ATSB redundant, well in aviation at least, let mimimiBeaker get back to his train sets and sailing down on the pond and leave aviation to the experts!!:E