Tangmere

I don't think that you can expect the report to make such a specific technical recommendation. There may be circumstances (even if we cannot here and now conceive of them) where a horizontal cotter would be the correct engineering solution. Such a specific recommendation as you suggest would carry very significant weight coming from the RAIB and have the effect perhaps of "regulation". This would be highly undesirable not least from the RAIB's point of view as they would then be in the position of having specified a design standard which is neither their purpose nor their competence.

The most important thing to come from the report is a reminder not to go making design changes without fully and properly considering the engineering ramifications. That they have also conveyed the meaning that a vertical cotter is better than a horizontal one, (which is no more than a reminder of the lessons of the past), suggests to me that this is in fact a very well written report.

I also wouldn't expect the RAIB to provide operators with guidance on what to do if they identify a deficiency. Not their role. The ORR (HMRI) have the role of enforcement and will no doubt be issuing their own instructions if they feel that operators can't, won't or don't pay attention to the findings of the report.

 

That's a fair point but, as you say, the RAIB only make recommendations and these do not have to be followed, especially if there is good reason for not doing. In essence, it is no different from deviating from an Approved Code of Practice. If you can justify the deviation, it doesn't make it illegal.

 

I think that first statement puts the rest of your post into context.

 

I am not going to single out any individual or individuals however I think it becomes obvious who is an Engineer and/or has Engineering knowledge on this thread and I feel that it is time for those who cannot really contribute to the pool of knowledge to desist.

 

You said 'seriously questioning' . If that is serious then I stand corrected. The suggestions have also been dismissed by others. Thinking outside the box is one of the reasons those in the RAIB are generally not professional railwaymen or have in-depth knowledge of the subject that they are investigating.

Are you saying that we should not discuss it or that the discussion should be confined to those who can show that they have appropriate qualifications to do so?

 

Whilst this may be a valid point in some scenarios there is also the view that some people can't - or won't - see the wood for the trees and it sometimes needs an external view to clarify matters; even the RAIB admits its limitations and looks to outside guidance but whether it has been given the right answers in this case is one of matters that this thread is currently considering. Surely that is one of the rationales for a "discussion" thread ?

 

Without singling out individuals, its also clear that there are posters who lack the skill and experience to properly understand this kind of bureaucratic document.

 

Two notes: first, the nut (and probably the cotter pin too, although I didn't see anything about the original material specification for the cotter pin) was also made of the same material, and the nut has not been ruled out as the cause of the failure; and second, I think people are questioning a number of things, not just the material choice: the cotter pin orientation, and the fit of the gudgeon pin into the crosshead have also been discussed as potential causal factors.

Noel

 

Gudgeon pin nut-original spec BR102A-6 (EN8)
Cotter pin (SL/DN/K-569) -original spec 24/17 (EN3A)

 

good isn't it... RAIB used a engineer with a speciality in bolted joints as a consultant, and of course he/she got to examine all the parts, see all the drawings, hear all the witness evidence etc, but actually it seems you get a better analysis from a bunch of people on a Web forum...

 

History might be instructive here. The first technical enquiry into the Comet losses (the Abell Committee) completely missed the crack propogation at the high-stress areas in the window corners (the true cause), and after considering a very long list of potential causes, decided that the likely cause of the loss was fire. And that enquiry, looking as it did into an incident which included a large loss of life, had an even larger galaxy of technical talent available to it. Accident Investigation Boards aren't god-like, they are human; they can miss things.

Noel

 

But if I remember rightly, it was engineers who found the cause.

 

Indeed. Built a huge pressure tank to take a complete Comet fuselage.

 

Yes, after a second crash (which happened after remedial action for all the possible causes identified by the Abell Committee had been taken) showed that they must have missed something.

Noel

 

One of the benefits of using a specialist consultant is that having examined all the components a clear description of the condition is given together with photographs which those with suficient exppertise can follow. The expert did see all the drawings and a good number have been made available in the report so we can see them too. References are made to witness evidence and it seems fair to assume the important and pertinent information is in the report. One reason for writing a technical report is so others can understand the iissues and gauge the pertinence of the information without having had to gather the evidence themselves.

I have lectured at the Institute of Mechanical Engineers, the subject being why the grid operator flexes generating plant. The symposium being on does flexing plant lead directly to faiilures due to boiler stress cracking. The debate was interesting and it should be noted that not all the assembeled experts agreed wiith each other all of the time. The diferences were marginal but seriously debated, more robustly than has occured in this forum.

Getting back to Tangmere the major discussion points are in the report and the expert makes at least references in the text which is not emphasised furthur but is a discussion point in this forum. in 46 the robustness of the taper gudgeon pin is emphasised including the reference to difficulties removing a well fitted pin. The mechanism for the nut to come loose is clear but not the mechanism for the gudgeon pin to work loose although there is a reference to a bad fitting pin behaving diferently in para 47. The discussion apears to mainly be not that thre report is deficient but more to the weight of relevance of certain issues which is fair technical discussion.

 

At the time of the Comet losses jet travel in pressurised aircraft was in its infancy and investigators had little prior data to work with. Not a good parallel to draw with 34067 IMO.

 

First, it's not clear if the investigator in the Tangmere case had previously looked into case(s) of secured nuts coming loose in a rail application, so one could equally well make the same point here. However, I don't think this point is significant in either case.


Second, that incident was merely the first the came to my mind in which a group of skilled investigators looked into an accident and missed the cause.

You could try looking at the Boeing 737 rudder reversal failures, where over a period of quite a few years, Boeing 737's experienced numerous un-commanded rudder reversals - including one fatal crash. The actual cause was not accurately identified after any of these - and the fatal crash generated a full-blown NTSB investigation. It was only after another fatal crash (in Pennsylvania) that the cause (a hydraulic valve mis-operation mode) was finally identified. At that point (1994), powered hydraulic systems had been used in aircraft for about 50 years.

I seem to remember others, too, but I don't recall the details off the top of my head, and would have to go look them up. (They involve rocket failures.) The message from all of them is the same - sometimes accident investigators miss things. This is not to say that they aren't thoroughly competent individuals doing their best - just that people are imperfect.


Now, can we please return to discussing the technical details of this particular incident, rather than making remarks about the people discussing it? Thanks.

Noel

 

Engineering as a profession is practically built on: "There but for the grace of *insert chosen agent of good fortune* go I." Engineers talk things over after failures because they don't want to make the same mistake, not to delight in the failures of others. No, most of us don't design steam locomotives, or even work with them, but it's still useful information about things that break; the question of how best to keep bolted assemblies together in a vibrating environment is practically universal.

I happen to think the RAIB report is good (and I'm sure they're so glad of my positive opinion), but they've actually got pretty limited data to work on (through no fault of their own). If you were setting up an experiment to research gudgeon pin assembly failures, then this incident would be followed by a request to do better next time and stop wasting money. Imagine the conversation:

"So what happened?"

"Well, the cotter broke, the nut came loose, and the pin came out."

"That was the point of the exercise, yes. In what order?"

"Well, the pin came out last."

"Very funny. All right, did the cotter break, or did the nut break it?"

"Probably failed on its own. Not very well made."

"Probably? What about the nut, we're sure that didn't fail?"

"Well, we don't really have any data there..."

"When I told you last week I was going to be interested to see the footage, you should maybe have taken that as a hint. Ok, how many pieces is it in?"

".... ...."

"Are you telling me you ******* lost it?!"

Enough silly stuff. What is known is that Tangmere's gudgeon pin assembly differed in certain ways from the BR spec, and from other active Bulleids. It failed. Nobody has come up with any comparable incident on another Bulleid in SR, BR or preserved service. The RAIB report includes the line: "The reasons for this could not be established with certainty because some components could not be found after the accident."

The RAIB therefore made some recommendations that roughly translate as: "Make sure you give an appropriate level of scrutiny to any design changes. And don't make cotters like that." It's not unreasonable advice.