Concorde, Ten Years On, Part 2

The Concorde accident to F-BTSC on 25 July 2000 is about as well understood as to causes as any accident can be. There is also, unusually, a more or less linear connection of causes from an exceptionally rare event: the deposition of a particularly hard and sharp strip of metal, which shouldn’t have been mounted in the first place exactly because of such possibilities, on exactly the part of the runway at which Concorde’s tires bear the greatest load – and the aircraft indeed running over it, and it’s not a big strip. The Concorde’s ground run goes up to just about 200 kts at rotation, I understand, compared with that of a Boeing 747 at about 160 kts. Furthermore, the delta wing generates some negative load, putting even more weight on the tires, at rotation, before it changes to positive and the aircraft lifts off. The sequence of events that then ensued was, as far as I know, not anticipated by anyone in the development or certification or analysis of the aircraft. To my mind, it is hard to see how it could have been. To me, this is a freak accident, the «not expected to occur during the operational lifetime of the aircraft», which is the strictest category of likelihood contemplated in civil aeronautical certification.

But some differ, for example Tom Ferrell in this note to the York Safety-Critical Systems Mailing List. Tom thinks the accident had precursors, which showed, in advance of the accident, that

Regardless of causal agent, the Concorde was susceptible to severe damage from a relatively common occurrence.

He means there had been tire burst incidents, which indicated problems with the design. So is this just a matter of personal taste, say, like wine? Ladkin tastes “freak” and Ferrell tastes “foreseeable” in the same glass, and that’s it? Or is there, as I would prefer to believe, an objective way of evaluating the views, such that one can be shown to be right (or more accurate) and the other wrong (or misleading) in some way?

I think it is partly a matter of what you lump together, and what you don’t. Do you lump together all tire bursts, including this one, and all damage, including this damage, or don’t you? Is this lumping arbitrary, a matter of individual perception? I don’t think so. I think there are objective principles, on which so far I have only an intuitive handle.

How to indicate these principles? I try to show them here by means of a hypothetical cross-examination of Ferrell’s claim. Here goes.

M’lud, regardless of causal agent, the Concorde aircraft was susceptible to severe damage from a relatively common occurrence.

I see, thank you, counsel. What was that common occurrence?

A burst tire, m’lud.

Thank you, counsel. And what was that severe damage?

A 32cm square hole in the lower wing skin, m’lud, which also served as the fuel tank skin.

I see. Had that ever happened before in the history of the airplane?

No, m’lud.

You say “susceptible”. Had damage ever occurred to the lower wing skin, except in this case?

Six times, m’lud.

And how many times was that due to your “common occurrence“, a tire burst?

The lower wing skin was punctured on five occasions when a tire burst, m’lud.

But that is not what I asked you, counsel. I asked you in which of these events the damage to the lower wing skin was due to the tire burst.

It is supposed, three times, m’lud.

You say “supposed“, counsel. Why so?

As far as we know, in those cases, m’lud, the damage sequence was causally initiated by a tire burst. It is conceivable, although very unlikely, that a contemporary but independent damaging event caused the lower-wing-skin penetration, but there was no evidence for that.

I see. Thank you for your care in phrasing this, counsel. And what were the two other events?

In one, on 29 January 1988, the tie bolts holding the two wheel halves together sheared, and in the resulting sequence one of the bolts penetrated the Number 7 tank, leaving a half-inch hole. In the other, on 15 July 1993, there was a braking-system jam, and the Number 8 tank was punctured as a result of the damage sequence.

So, if I understand you, counsel, you tell me that, before the fatal accident at Gonesse, three times it had occurred that the lower wing skin was punctured due to your “common occurrence“, a tire burst.

Yes, m’lud.

And how many years did the Concorde fly in service before the Gonesse accident?

Just over 24 years, m’lud. The first revenue flight was 24 May, 1976.

And how many flight cycles?

About 84,000, m’lud.

I see. That is quite a long time. And, to me, quite a large number of flights, although of course by no means so large as with most aircraft in commercial use nowadays. So are those three occasions a lot or a little, counsel?

With respect, m’lud, I offer no opinion on that question.

So there are these “common occurrences“, which had occurred – how many times, counsel?

Aviation Safety Network has a record of 55 occasions after service introduction in which tires burst, m’lud.

Common enough, I suppose. And these common occurrences had caused damage other than to the tire on – how many occasions, counsel?

Aviation Safety Network has a record of 28 occasions on which other damage occurred, m’lud.

Does that include the two above in which the damage was not initiated by a tire burst, counsel?

Yes, m’lud.

So there were 26 occasions on which, as far as we know, a tire burst initiated damage to other parts of the aircraft?

Yes, m’lud.

So I think you have established, counsel, that a common occurrence, a tire burst, could cause damage, and thus that the aircraft was susceptible to damage from this common occurrence. But you want to establish more than that, don’t you, counsel. You wish to say that the aircraft was susceptible to severe damage.

Yes, m’lud.

Is “severe damage” a technical term used in aviation, counsel?

No, m’lud.

So it is your term, counsel. What do you mean by it?

I mean that the safety of the flight is affected by the damage, m’lud.

Thank you, counsel. Is there any similar term used in aviation?

The U.S. National Transportation Safety Board Part 830 defines an “incident” to be an occurrence other than an accident, associated with the operation of an aircraft which affects or could affect the safety of operations. The same regulation defines an “accident” to be an occurrence [associated with the operation of an aircraft] in which any person suffers death or serious injury, or in which the aircraft receives substantial damage.

I see. Is there a definition of “substantial damage“, counsel?

Yes, m’lud. “…..damage or failure which adversely affects the structural strength, performance, or flight characteristics of the aircraft, and which would normally require major repair or replacement of the affected component. Engine failure or damage limited to an engine if only one engine fails or is damaged, bent fairings or cowling, dented skin, small punctured holes in the skin or fabric, ground damage to rotor or propeller blades, and damage to landing gear, wheels, tires, flaps, engine accessories, brakes, or wingtips are not considered ‘substantial damage’ for the purpose of this part.” This definition is similar to other definitions of significant damage, used in definitions of accidents and incidents in, say, the International Civil Aviation Organisation Annex 13, which defines reporting requirements for its member states.

Thank you, counsel. And in which of those 26 tire-burst incidents you enumerated above was “substantial damage“, according to this definition, incurred?

In the incident at Washington Dulles airport on 14 June 1979, m’lud. The performance of the aircraft was affected in that fuel was lost through the debris penetrations of the tank at a rate of up to 4 kg per second. It was unable to continue its flight to London. The aircraft lost 7 tonnes of fuel before it landed again at Washington Dulles.

And in others, counsel?

In no others, according to the definition, m’lud.

I see. Are there incidents in which a fuel tank was penetrated, in which the performance of the aircraft, its structural strength, or its flight characteristics were not substantially affected?

Yes, m’lud. On 29 January 1988, the incident in which the wheel-half tie-bolts broke and a bolt punctured the tank on take-off from London, the flight continued to its destination, New York.

I see. How large was this puncture?

The hole was half an inch, so about 1.3 cm, in diameter, m’lud.

So it appears that a puncture in a fuel tank, even a fairly large hole, does not necessarily count as “substantial damage“?

No, m’lud, it does not necessarily count so.

Are there any other common technical meanings of “severe damage” or “significant damage” which we might want to consider, counsel?

I think so, m’lud. For example, damage which could affect the safety of flight, the definition I suggested.

Could affect“, counsel, or “does affect“? For example, during the 29 January 1988, was the safety of the flight affected?

Apparently no, m’lud.

Was the safety of flight affected in any of the other tank-penetration incidents besides the 14 June 1979 incident at Washington Dulles?

I don’t believe so, m’lud.

Could it have been?

I believe so, m’lud.

How?

Maybe fuel streaming from a hole can catch fire when it meets engine exhaust, m’lud.

I see. Does it commonly do so, counsel? Do you know of any other incident in commercial aviation when fuel streaming from a smallish hole, such as this, caught fire?

Actually, m’lud, I don’t.

Are there any other ways in which safety of the flight could be affected by such a leak?

When the aircraft lands, m’lud, the brakes heat up, and leaking fuel could fall onto hot brakes and catch fire.

Has this happened, counsel?

Yes, m’lud.

Are there ways to prevent it happening?

Yes, m’lud. If a crew knows they have a leak – and if the leak is substantial you can usually see the stream behind the wing from the rear passenger seats during flight – then they can have fire services meet the plane on landing and cover the brakes and ground under the leak with fire-suppresant foam. This mostly suffices.

Thank you, counsel. So igniting this fuel is a event for which there exist known and effective countermeasures.

Yes, m’lud.

So although such an event “could affect” the safety of flight, it mostly doesn’t do so.

It appears not, m’lud.

So it appears that penetrations of the fuel tank in themselves do not count as “substantial damage“, and they do not necessarily count as damage which affects the safety of the flight. But they might count as events which could affect the safety of flight if we are sufficiently imaginative in devising scenarios.

It seems so, m’lud.

Let us see how imaginative I may be. As far as I understand quantum mechanics, atomic particles may engage in random motion, that is, displacement of position without apparent cause.

As far as I also understand quantum mechanics, m’lud, that is so.

So it could be, counsel, that all the atomic particles in a Concorde translate 4 meters to the left all at the same time, leaving the passengers sitting, well, somewhere in space outside the fuselage.

I suppose it could be, m’lud.

And those passengers would probably fall to the ground and injure themselves or die.

I suppose so, m’lud.

So it could be, counsel, that the Concorde, indeed any aircraft, suddenly leaves its passengers sitting outside the airframe, leading to serious injury or death.

I suppose so, m’lud.

I am, counsel, as you see, sufficiently imaginative in devising scenarios. You have presented me with two partially overlapping definitions of significant damage, of which the second is indeterminate between “could be” and “is“. I don’t find the “could be” interpretation very helpful, as you see, because I am, as you also see, sufficiently imaginative. And I don’t think any objective safety property of a commercial airplane should depend so heavily on my sufficient imagination. So I am going to interpret “severe damage” as meaning damage which is either substantial in the sense of NTSB rule 830 or which does (not “could” but “does“) affect the safety of flight.

Yes, m’lud.

On which occasions, then, did your “common occurrence“, a tire burst, initiate a causal sequence in which severe damage resulted?

On 14 July 1979 at Washington Dulles, m’lud, and on 25 July 2000 resulting in the crash in Gonesse.

The damage which resulted in the Gonesse crash was then, by definition, substantial, as well as severe, wasn’t it, counsel.

Yes, m’lud.

So, since this severe damage actually happened on that occasion, we can say that, even before this occurred, the aircraft was susceptible to exactly this severe damage, in the sense that, since it did happen, it follows that the aircraft was susceptible to its happening, simply through the usual meaning of the word “susceptible“.

Yes, m’lud, that is what I claim.

Let’s look a little closer at this word “susceptible“. There are some people who claim that human beings spontaneously ignite. Not often, but occasionally. All that is left is ashes. If that is true, and I believe that this is a very, very big “if“, then human beings are “susceptible to spontaneous combustion” aren’t they, counsel?

Yes, m’lud. But I share your scepticism of the phenomenon.

The point, counsel, is this. We know whether or not human beings are susceptible to spontaneous combustion only in so far as we know actual examples of human beings spontaneously combusting.

It seems so, m’lud.

And, further, let us suppose that there are certain circumstances C in which human beings spontaneously combust, and if those circumstances do not obtain, then they don’t. Then, surely, we are obliged, by virtue of not wishing to mislead our fellow men and women, to say that human beings are susceptible to spontaneous combustion in circumstances C and to indicate that, if circumstances C do not obtain, there is nothing to worry about.

That seems to me reasonable, m’lud.

And I take it that you do not wish to mislead me, counsel!

Certainly not, m’lud!

Then when you claim that the Concorde is “susceptible to severe damage [resulting from] a common occurrence“, which we have more or less agreed is a phrase which may be able to describe the Concorde aircraft, I now want to know if there are any circumstances C which you should be telling me about, under which severe damage resulting from your common occurrence, a burst tire, may be realised. Please note the condition: you are to tell me about circumstances C in which, if they obtain, the accident sequence results, and for which, if circumstances C do not obtain, the accident sequence does not result.

Yes, m’lud. The accident sequence is as follows. A titanium strip lay edge-on on the runway at or near the rotation point of the Concorde. It cut sharply into a tire, causing a tire burst resulting in at least two chunks of tire of size approximately 4.5 kg. It is presumed that one of these expelled chunks impacted the lower wing skin, the skin of one of the fuel tanks, causing a shock wave which blew out the fuel tank skin from inside, near the impact point of the tire chunk , resulting in a hole of size about 32 cm square being formed in the fuel tank, and of course fuel streaming out. The fuel ignited, and burned from very near the fuel tank hole, causing varying loss of thrust in two engines, as a result of which the aircraft was unable to attain positive-rate-of-climb flying speed, and was also subject to thermal damage from the fire under the left wing. As the damage progressed, control was lost and the aircraft crashed in Gonesse.

Thank you, counsel. How do we know that the loss of thrust rendered the aircraft unable to attain the appropriate flying speed?

That is elementary aerodynamics of Concorde, m’lud, and is not disputed.

Thank you. How do we know that the fire caused loss of thrust?

Calculations show that, if air is ingested into the engine intakes at a temperature approximating that of the burning fuel, thrust is lost at more or less the observed and recorded rate.

Thank you. How do we know the fire was present at exactly the unfortunate point to be ingested?

Photographs of the accident, m’lud.

Thank you.

How did the fire attain the state in which it was photographed?

We don’t know, m’lud. We would expect a fuel fire to start when it has been ignited by hot gases from the turbine engines, behind the engines, which of course were in reheat at the time. As far as anyone knows, the front of such a flame cannot travel relatively forward at the speed at which the aircraft was travelling.

So we would expect the fire to remain behind the wing structure, behind the engine exhaust?

Indeed so, m’lud.

But this fire didn’t. Its front came forward underneath the wing, and you have indicated we do not know why.

That is correct, m’lud. There is speculation that it might have been ignited by an electrical spark from some wiring in the undercarriage bay.

Do we know that, counsel?

No, we do not know, m’lud. It is speculation, because we cannot otherwise understand how the flame front came forward under the wing.

Thank you. So we basically do not know the causal sequence between fuel released from the tank and the the engines consequently operating at reduced power.

It seems so, m’lud. We do not know why the flame front moved forward.

But of course there would have been no flame front had fuel not been streaming out of a hole.

Indeed so, m’lud.

How big was the hole?

About 32 cm square, m’lud.

That is a big hole! Did such holes occur during any other of your “common events“, counsel?

No, m’lud. The largest was 1 inch x 1.5 inches, caused by metal debris on 15 November 1985. The second largest was the hole of size 0.5 inch diameter on 20 January 1988, which event we have already mentioned.

So this hole, in the Gonesse accident, was 160 times larger than the largest hole which had previously been caused, and 790 times larger than the second-largest hole which had previously been caused. That is an enormous difference, counsel! Why is that?

The hole in the Gonesse accident, m’lud, was not caused through tank penetration by debris, but through shock-wave convergence punching a hole through the tank skin from inside.

That is, if I understand you, counsel, a much larger hole, two to maybe three orders of magnitude larger than any that had previously occurred, made by a completely different mechanism.

That appears to be so, m’lud.

And are such kinds of events, reminding me of your phrasing “common occurrence“, counsel, common in commercial aviation?

No, m’lud. This occurrence of the phenomenon is unique in the history of civil aviation as far as we know.

Thank you for your frank answer, counsel. But people knew about this phenomenon, did they?

Military engineers knew of the phenomenon from battle-damage studies, m’lud. It is not clear if any engineer in civil aviation, if anyone involved with civil aviation, knew of this phenomenon before the Gonesse accident. After the accident, military engineers informed the accident investigators of what they knew.

And what of the tire pieces that caused this phenomenon?

It was shown by experiment, m’lud, that a piece of rubber weighing about 4.8 kg and travelling at a relative speed of about 120 m/s, that is something over 300 mph, which could in theory occur due to a Concorde tire bursting at the point in the take-off sequence at which it did, could trigger the shock-wave phenomenon with a proportionate loss of tank skin.

And you have said that two chunks of tire of about that size were found amongst the runway debris.

That is correct, m’lud.

Could any other phenomenon of which we know, say a tank penetration by debris consequent to a “common” tire burst (I use your phrasing), cause the release of a 32 cm square piece of the fuel tank wall?

Not that we know of, m’lud, no.

So we have only one explanation to hand of the known size of the hole?

That is correct, m’lud.

And this explanation, this phenomenon, is otherwise unknown in the history of civil aviation.

That seems to be so, m’lud.

Thank you. If I understand you, this phenomenon was triggered by the impact of a chunk of tire of about 4.5 kg or so?

As far as we can tell, m’lud.

And chunks of this size, of tire pieces or indeed of other material, are frequent, or usual, during your “common occurrences“, tire bursts?

Actually no, m’lud, they are not.

I see. Have they otherwise occurred in any of the tire-burst events, counsel?

Actually, m’lud, they have not.

That is, they are unique to the Gonesse accident?

It appears so, m’lud.

How did they occur?

The tire was apparently cut by a titanium strip lying on the runway near the rotation point of the aircraft, m’lud.

I see. Pieces of metal left lying on the runway cut Concorde tires into 4.5 kg chunks, apparently?

Not any pieces of metal, m’lud, according to experiments undertaken after the accident. Titanium. Titanium is unusually hard. Other metals just crush when the tires run over them.

I see. But titanium strips are to be found lying on runways every so often, I take it?

Actually no, m’lud. This is the only recorded instance ever of a sharp titanium foreign object lying on a runway with commercial operations. Of course we don’t know about the military, since they do not share their records.

Why would that be, counsel, that this is the only instance?

One reason, m’lud, might be poor record-keeping. Another reason might be that titanium is not used on aircraft in places in which it might fall off on a runway.

Oh! So why did it happen here, counsel?

A mistake, m’lud.

I imagine a very, very rare mistake, counsel?

Yes, m’lud. As I mentioned, this is the only recorded instance of such debris lying on a runway at a commercial airport.

So, if I understand you, counsel, the shock-wave phenomenon can only happen via large chunks of debris, and the only way in which large chunks of debris from a burst tire have been known to occur is in this very accident, through cutting by a titanium strip, debris of which there has not been another recorded instance, in part because the use of titanium in a way in which it might separate from the aircraft during take off or landing is proscribed?

That seems to be so, m’lud.

So the circumstances C in which your “common occurrence” can lead to “severe damage” are, as far as we can tell:
(a) a flame front in the streaming fuel from an unusually large hole “moving forward” through an unknown mechanism to burn under the wing, in front of the engine intakes;
(b) a fluid shock wave punching out the unusually large portion of the fuel tank wall to create the hole;
(c) an unusually large chunk of debris creating the shock-wave sufficient to punch out the unusually large hole;
(d) this unusually large chunk indeed impacting the fuel tank wall, rather then being ejected in another direction;
(e) a titanium metal strip lying on the runway near enough to the rotation point to cut a tire which happens to run over it into suitably, unusually large chunks.

That seems to be so, m’lud.

I conclude, counsel, in the words of your claim, that the Concorde aircraft is susceptible to severe damage resulting from a common occurrence (tire burst) under the circumstances (a)-(e) just elaborated. And that, in order not to mislead us, your claim should include the supplementary phrase “under the circumstances (a)-(e) just elaborated“. If you agree to include that wording, counsel, I shall grant your claim. If not, I shall reject it. Accordingly, do you wish to remain with your original wording, or to amend it?

Counsel’s reply is not recorded because the recorder had used up its batteries.

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