Flying in Volcanic Ash

The biggest political problem of the week seems to be that airlines have stopped flying in Europe, because of the ash cloud from the volcano Eyjafjallajökull. I must say that in Bielefeld it is wonderful to see the sky without the usual 15 or so condensation trails and the ensuing cirrus, but my wine/tea/coffee merchant and his son are stuck in Namibia at the end of a hunting holiday and desperately need to get back to work, so I understand well the economic side of this also.

Those who don’t understand what volcanic ash can do to gas turbine engines might want to check out this 2003 NASA report concerning damage to the engines of an aircraft which flew through an ash cloud on its way to Europe some years ago. The cloud was not visible to the pilots, and visual inspection of the engines on landing revealed no damage. But the engines were severely damaged. Many thanks to Robert Dorsett for finding this reference.

I have been reading a lot of half-thought-out commentary, but little that enumerates the issues. So here goes.

1. Volcanic ash contains a high proportion of silica. This particular eruption sequence has shown concentrations from just under one-half to about two-thirds, depending on the type of eruption (an eruption sequence is not necessarily uniform in type or composition), if some unnamed geologist cited by an anonymous poster on a forum is to be believed. (For those who wish to troll through the 90 pages of chatter on this on PPRuNe, I recommend in particular the contributions of the gentleman or lady name of “Sunfish”, who appears to be an Australian engineer, for example this one.)

2. The ash is very fine stuff.

3. The silica melts in some parts of the turbine, and gives other parts a nice glass coating as a consequence.

4. There are almost no data points for the behavior of engines under exposure to volcanic ash. There are just the occasional damage reports, as above. It is known that higher concentrations will cause flame out and seizing, but I doubt that the effect on engines of lower concentrations has been determined by anything much in the way of testing. For example, behavior on exposure to volcanic ash is not part of the certification requirements for engines. It looks like if you fly through it for a couple of hours then everything is OK on a visual inspection (thank you BA), but I doubt anyone knows what might happen if you fly through it for a week (an order-of-magnitude increase in exposure).

5. Suppose some engine, somewhere, has a problem. Then standard safety regulatory action would be to take the engine type out of service until it has been determined what the problem is. In this case, until one can rule out that flying numbers of hours through an ash cloud was not a causal factor. If it was a causal factor, then the fleet is grounded until all the engines can be rebuilt. That could take rather a long time – months, not weeks. And if the engine happens to be an intercontinental one, flying under ETOPS, then what do you do about ETOPS approval for that type, for those engines exposed to ash? ETOPS is predicated on independent failures, not on common-cause failures such as flying through ash.

6. Airlines dependent on transatlantic traffic to generate revenue, such as BA, are going to be hurting. But it would hurt a lot more to have ETOPS rescinded on the airline’s entire 777 fleet pending rebuild/overhaul of the engines.

7. The likelihood that one engine, somewhere on one wing, in Europe, will have a problem in the next couple of weeks, is, just on general experience, not small. For the consequences of that, see point 5 above.

It is a hard problem. The problem arises from (a) the environment – the fact that the ash cloud is there; (b) long established procedures for regulating aviation safety, which requires that a fleet be grounded upon evidence of a problem; (c) the unknown but tangible likelihood that some problem will occur; (d) the severe consequences of such a problem, given the established procedures for regulating aviation safety; (e) the severe economic consequences of closing down airline travel in such a busy part of the world.

I have no solutions. And I very much doubt that anyone else has any, either. As a safety person, I favor keeping aircraft out of this stuff until it goes away.


1. Thomas Netter pointed out to me a broadcast on France Culture today by Olivier Duhamel (available today, Tuesday 20 April, from the France Culture daily programming site, see time 07:55, and I take it later from the archives), who, Thomas said, pointed out that risks were evaluated with respect to aircraft, rather than taking a systems approach to aircraft travel and evaluating the general social cost of grounding. So let’s do it, superficially. Let the general cost of grounding for everyone be X per week. We have so far suffered X. If one engine shows up with ash damage, that will cost 2-4X, right there, since regs will require the fleets be town down and inspected, and I doubt that can be done in less than, say, a month. If we then ignore the regs, and have an aircraft lose both engines mid-Atlantic, that’s €300m – €1 billion out of insurers’ pockets (for which all air travellers have to pay, even though they might think it is only one airline). Not to speak of the political consequences for those who decide to let aircraft fly, when one is then lost. So those are the severities (some of them). Unless you can evaluate the likelihood of (a) discovering damage to one engine somewhere, and (b) having an ETOPS aircraft lose two, sometime in the future, due to ash damage, you cannot evaluate the social risk (usually taken as the multiplication of likelihood with severity for all hazards). I don’t hold much truck with saying that something isn’t being done, when no one can do it.

2. John Rushby just pointed out a thread in PPRuNe TechLog, which contains this interesting comment on what happens to gas turbines in ash clouds, by MFgeo.

3. The International Herald Tribune aka New York Times has this story today dealing inter alia with the politics. Apparently, [begin quote]The region is grappling with a new blow to its ability to act decisively during an emergency. ……… Most noisily, the head of the International Air Transport Association said before the announcement to partially lift the aviation ban that “the decision Europe has made is with no risk assessment, no consultation, no coordination, no leadership.” The industry group’s director general and chief executive, Giovanni Bisignani, went farther, saying that the crisis is a “European embarrassment” and “a European mess.”[end quote]

I think, in contrast to these suggestions, that the individual countries in the EU, which have legal responsibility for their airspace, have acted decisively, with “risk assessment” and “leadership” and what have you: the airspace is more or less closed; some flights with minimal possible exposure are taking place. You can’t get much more decisive than that. People who disagree with these measures could make their divergent risk assessments public. How about it, IATA?

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