Channel 4 Tonight…..

Comet disasters program Channel 4

The other difference between the tank failure and the airborne failures is that the latter happened at 500mph so any fuselage skin thrust outwards by the cabin pressure at failure would be ‘snatched’ by the slipstream increasing the break-up forces.
Calling the ADF apertures ‘ADF windows’ in the program was fair enough as it was a term used in the reports.
Anybody know what skin gauges they used on the redesigned Comet 4?

Thanks again chaps, great info!!

The wing flexing was to replicate the aerodynamic loads on the airframe, and that, yes, the water inside the fuselage was pressurised, to the same extent that the air would have been pressurised in flight. The net effect being to completely replicate the flight cycle. Like you, I assume, but have no direct evidence, that the water ‘outside’ was at atmospheric pressure – I can’t see why you’d need to do otherwise. What matters is the pressure differential, the speed of pressurisation and relaxation, and the other loads on the airframe (hence the wing flexing).

Great info gentlemen thanks…..In the CH4 programme it shows the wings being subjected to constant jacking up and down as you quite rightly state…….my next nieve 😮 question is, would the water INSIDE the fusilage be at a higher pressure than the water outside, which Im assuming was basically at open atmosphere pressue….

The fuselages were both filled with water and immersed in water (the latter presumably to negate the weight of the water filling. ) The wings stuck out of the tanks and I believe were subjected to jacking forces to simulate flight loads.

All pressure vessels (boilers, compressed air tanks etc) are tested by pressurised water filling as a liquid is incompressible. When a vessel failure occurs there is no stored energy in the water so the failure is only develops a small amount in relative safety – as opposed to what could be effectively an explosion if a gas or vapour were used.

Which is what happened to the Comets in flight – explosive expansion of the pressurisation air which ripped the fuselages to bits. If they’d been filled with water and not air, all that would have happened would have been the initial small tear in the corner of the window.

But of course full of water it a) wouldn’t have been able to leave the ground b) the passengers couldn’t have lit their cigarettes.

As I understand it, once the crack has developed sufficiently to allow significant air to escape, you’re well on the way to a catastrophic failure. I believe the test fuselage was both surrounded by and filled with water. This gives two benefits: Firstly that it is easier to achieve and control the required pressure differential, not least because with water outside and air inside the depth of the water around the fuselage is a significant factor – i.e. the loads would be lighter at the top of the fuselage because the water is shallower. With water inside the differential is the same and the two effects negate each other. Secondly that the water will dampen (no pun intended) any movement of skin or structure when the failure does occur, so it’s easier to analyse the results because you don’t have to retrieve this or that skin panel from the other side of the airfield.

Anyone care to add to that, or correct my own inaccuracies or naiveties?
🙂

Please excuse my nievity, but testing the fusilage in the water tank…..am i correct in thinking that whilst submerged in the tank and with the fusilage pressuriesed, any cracks/weakness would show up as escaping air bubbles etc etc….

Of course these criticisms are justified in the sense that documentary makers should always make every effort to get the small details corrct. However they are not made for knowledgable experts, they are made for a genrally interested public the majority of whom will have been educated and entertained.

Therde will be programmes we have all watched about subjects we know a little about but not enough to notice errors of detail. Experts in these fields will no doubt whinge on about then in parallel threads.

Did anyone happen to see the repeated 1996 programme about the Harrier last night?

On the other hand, while it is not always correct to extrapolate a few relatively minor gripes into a major criticism, it is difficult to name any such programme, no matter how well produced or how worthy, without several almost childish errors – which generally would appear to have been fixable quickly, easily and at little cost. It may not be a valid criticism in every case, but if someone telling a story of which the ‘meat’ is all about attention to detail can’t get the detail right, what faith should the rest of us have that they have actually understood the story they’re telling? And if they haven’t understood it, why are we trusting them to educate or entertain us with their version of events? Perhaps this is the point that really needs ramming home, particularly during negotiations for what advertising is associated with the film’s showing.

Well said. That the plans were not the right ones just shows a lack of attention to detail which was indeed just sloppy and easily fixable.

No, of course the plans on the table don’t have an effect on the actual content. I could privately agree with my wife that spotting the difference was a bit sad. Calling the radio enclosures ‘windows’ was rather more sloppy, but, even though it has many of us frothing at the mouth, even this might perhaps be excused, as structurally they are similar to the window openings.

On the other hand, while it is not always correct to extrapolate a few relatively minor gripes into a major criticism, it is difficult to name any such programme, no matter how well produced or how worthy, without several almost childish errors – which generally would appear to have been fixable quickly, easily and at little cost. It may not be a valid criticism in every case, but if someone telling a story of which the ‘meat’ is all about attention to detail can’t get the detail right, what faith should the rest of us have that they have actually understood the story they’re telling? And if they haven’t understood it, why are we trusting them to educate or entertain us with their version of events? Perhaps this is the point that really needs ramming home, particularly during negotiations for what advertising is associated with the film’s showing.

Do the plans have any real effect on the actual content of the film – a lot of what you saw was set dressing and whilst a lot was obviously not Comet in the bigger scheme of things its probably only of annoyance to aviation enthusiasts. Of note is the picture of the Elba fuselage section which shows apart from cracks some of the build standard employed on the aircraft.

There are too few programmes on TV these days about aviation and sadly those that do get commissioned are often spoilt by silly errors.

This was no exception.

As has been said, the plans they were pouring over were NOT of Comets, or anything de Havilland for that matter. The ‘roof windows’, shown as glazed windows (what did they think they were? Sunroofs??), were actually ADF aerial hatches (where ‘YP started to break up, not the side windows where ‘YU failed in the tank) and (IIRC – it’s a long time since I read up on it) the bodies of those found in the water were found to be exhibiting burns caused by exposure to the Mediterranean sunshine, not from burning fuel on the surface.

Agreed, the red-haired ‘expert’ was very annoying, but good to see Roger Topp being interviewed and Brian Rivas (his book, with Annie Bullen, on John Derry is an excellent read).

And good to see the green paint I applied to the interior of F-BGNX at Salisbury Hall in 1990 still there! Happy days, eh Bruce?

Lee

A side note; the overhead fuselage section shown in the programme, is on display in the London Science Museum as part of their exhibit on Alan Turing and the uses of early computing.

DAI

I enjoyed the programme too.

Nothing new in there, of course, but some interesting perspectives, and despite some flaws a reasonably rounded presentation of the facts. I have to admit I ground my teeth at the description of the radio aerial enclosures as ‘roof top windows’. And my wife told me I was very sad indeed for spotting that the plans on the table in front of the ‘investigation team’ were of a Consolidated CV-880, or maybe a DC-8, but certainly not a Comet. 😀

A point that BOAC took from the investigation, even if no-one else did, and which I haven’t seen in any televised account, was the role of the fuselage top paint played. Had they left the top unpainted, as was the original intention, it seemed likely that someone might have spotted the initial cracks around the aerial enclosures while performing one of the frequent aerial changes. Following trials in hot climates a white top was felt necessary, perhaps not least because the weight penalty was less than extra air conditioning. Apparently the white paint used was flexible enough that it didn’t show the crack in the skin below it.

Well fool you and the rest of you who have dismissed this with out even watching it. You are missing a damn good programme about the investigation into the lost Comets by those who did the investigation into the crashes.

Yes, I enjoyed it too, and expected it to receive its share of “expert” criticism in the forum. My only gripe, as with so many modern documentaries, is the dramatisation element. Why producers feel the need to explain by dramatised scenes instead of intelligent talking heads and contemporaneous photographic footage I do not understand.

And I’ll echo an earlier post – four Comet IV trips in the late sixties – sublime.

To clarify my point – the way that fatigue cracks work is they grow from microscopic flaws that are below the threshold that can be detected using standard methods (dye penetrant, etc.). These have to be assumed to be present, whether from manufacturing flaws, flaws in the material, or whatever. The 2 times pressure test (that’s my understanding, based on this: http://www.oocities.org/capecanaveral/lab/8803/fcogalyp.htm#court) was a spike overload which led to the crack tip plastic zones, and significantly retarded crack growth on the experimental section, and with the understanding of fracture mechanics at that time, it wasn’t foreseen that the crack growth rate would be retarded in that way. It’s not an intuitive mechanism.

Fracture mechanics is still a relatively new science, and was in its infancy at the time of the Comet.

Fatigue crack starters are assumed to be present in everything at a microscopic level, and the fatigue cracks that become dangerous are the result of these microscopic cracks growing.

It’s not cold working strengthening the material (in general, it’s expected that crack growth rates go up with increased tensile strength).

It’s a more subtle behaviour than that.

It’s about plasticity and crack closure. Essentially the initial flaw gets opened up significantly by the spike overload, and there’s a plastically deformed zone around the crack tip. It then closes back up, and subsequent loads are not sufficient to open the flaw up again to its previous size, thereby temporarily retarding the crack growth rate. These mechanisms are still being investigated.

Eddie – the 2 x ultimate test was done as the first test on the specimen so there would have been no crack tips present. My understanding is that the window rads were subjected to a straight cold work stretch. Many more modern airliners have nominally square windows with corner rads similar to those of the Comet. The real trick is to understand material properties and duty cycle and prove your assumptions with a realistic test.

Anyway a good number of other aircraft manufactures both in the UK and US managed to produce very successful pressured fuselage designs without the problems experienced by the Comet.

The ginger woman was bloody annoying and ruined it for me. Where do they find em? Had to switch off.

They preformed 12000 pressurisation cycles on the Fuselage but only after a 3 x Ultimate pressure test……..which cold worked the window corners and strengthen them.

It’s not cold working strengthening the material (in general, it’s expected that crack growth rates go up with increased tensile strength).

It’s a more subtle behaviour than that.

It’s about plasticity and crack closure. Essentially the initial flaw gets opened up significantly by the spike overload, and there’s a plastically deformed zone around the crack tip. It then closes back up, and subsequent loads are not sufficient to open the flaw up again to its previous size, thereby temporarily retarding the crack growth rate. These mechanisms are still being investigated.