April 20, 2010

further reaction to Icelandic volcano

Experts react to the ongoing volcanic ash cloud story, with comments on risk and uncertainty, and a Q&A with Channel 4 viewers.

Probability and precaution

 Prof Nick Pidgeon, School of Psychology, Cardiff University said:

“In the debate about the rights and wrongs of taking action to suspend European airspace because of volcanic ash, it is important to recognise that risk and uncertainty are a normal part of many of the decisions we all take every day.  We should also not lightly forget the intense public scrutiny that follows any form of major aviation disaster, particularly where the possibility exists that the risks of such an event could have been avoided in advance.

“As individuals and a society we seem particularly sensitive to death when it comes in such a catastrophic fashion, a reaction quite unlike, for example, our response to the annual human cost of death and injury on our roads. Once the existence cloud was known just one incident of major engine failure, let alone an accident caused by the ash, would have left the airlines and aviation regulators with a heavy responsibility as well as a legacy of distrust amongst the public possibly for years to come. Under such circumstances it is hardly surprising that a precautionary approach to risk has been adopted.”

Q&A between Channel 4 viewers and some of our experts:

• What is the scientific model used for calculating volcanic ash in the air? Is there a formula? Does the computer model have a name? Is it only the Met Office that use it?

Dr. Grant Allen, Centre for Atmospheric Science, University of Manchester:

“The UK model is called NAME (Nuclear Accident ModEl) and is a pollution dispersion model which has been developed by the Met Office ever since the Chernobyl accident in 1986. It was developed to model dispersion of nuclear fallout, but treats ash and dust in exactly the same way.  It is a theoretical dispersion model which uses equations of fluid motion applied to the atmosphere. To work well, the model needs several things, the most important of which are: a) a good knowledge of the wind structure of the atmosphere; and b) a good knowledge of the emissions from the site (volcano) itself.

“Although the model is based on sound physics and has been tried and tested many times before and found to work well, it is the knowledge above that is used to drive the model that is not perfect.  We simply don’t know the wind structure of the atmosphere with perfect accuracy, nor do we know the evolving nature of the plume itself. Other countries use similar models to NAME for this purpose and all suffer from the same problem.  This is NOT to say that they aren’t fit for purpose as they have been shown to work well for broad decision making, but they will not be able to perfectly tell you where the plume is to the nearest metre!”

• Critics of the model claim it is “completely based on computer calculations” with no scientific evidence behind the model – is this true?

Dr. Grant Allen, Centre for Atmospheric Science, University of Manchester:

“See answer to question above.  Dispersion models are based on sound physics and have been well validated in the past.  They are not perfect as they rely on the less than accurate knowledge of the state of the atmosphere as inputs to the model.  They are, however, fit for this purpose and have performed well so far when compared to observations of the plume from laser ranging stations for example.  Furthermore, the airlines are forgetting that measurements and observations are being made for decision making purposes; no one is relying solely on computer models for decision making.  Observations and measurements will trump computer models every time but both modelling and observations are giving the same result in this incidence.”

• Results of 40 or so European test flights over the weekend, including a British Airways flight on Sunday, suggests the risks to aviation are not as high as computer models predict – who is right?

Dr. Grant Allen, Centre for Atmospheric Science, University of Manchester:

“The concentrations of ash which are dangerous to jet engines is simply unknown.  No one knows for sure what density of ash plume can bring a plane down.  The test flights so far could well have flown through a patchy part of the plume. Engineering tests in the ground need to be performed in future so that we know exactly what concentrations are dangerous and which are not.  This is the problem, not the computer models or observations of the plume.”

Chris Yates, Aviation Consultant:

“Whilst it is true to say that multiple test flights by some of the major carriers in Europe have found clean air in the skies above, it is also very evident that other test flights (most notably by the Met Office) have found that the dispersal of ash is not uniform and pockets of high concentration exist at multiple flight levels throughout the ash cloud presently drifting over us.  Since forward looking weather radar and other sensor systems aboard modern aircraft cannot see this stuff, the very real possibility exists that that a passenger jet could fly into one of these high concentration pockets and suffer serious damage.

“Tests show that there is no evidence of volcanic ash in the upper flight levels above 35,000 ft and it appears the CAA is minded to allow overflight of the ash cloud.  However, it must be borne in mind that there are risks inherent in doing so.  If an airliner were to suddenly depressurise at altitude, standard operating procedure calls for pilots to make an emergency descent to 10,000ft where breathable air exists.  Given that the ash cloud, according to reports has at base at 8,000ft and rising upward, this emergency procedure would put an already stricken airliner in the midst of an arguably more dangerous situation.  Both airframe and aero engine manufacturer operating procedures call for volcanic ash cloud to be avoided at all costs.

“There is a fair argument to make that much more detailed assessment of the composition of volcanic ash cloud is need to inform decision making to avoid such widespread airspace closures in the future.”

• Are the criticism of “health and safety gone mad” justified?

Chris Yates, Aviation Consultant:

“Airspace closure and consequent grounding of flights is most decidedly not ‘health and safety gone mad’ as some have begun to characterise it.  Safety must always be the overriding principle guiding decision making and controlling operations in the aviation world.  Where significant risk to those of us who fly regularly exists, it is right and proper that safety agencies step in and ground flights for the greater good.  Sufficient evidence is available from previous incidents to show clearly the sort of damage that can be done to an airliner caught in a high concentration of ash at altitude and this is well documented.  Financial consequences notwithstanding, no airline would be foolish enough to operate in conditions where the safety of crew and passengers is put in jeopardy.”

• Do planes get to fly because they fly over the dust?  Or are there gaps in the atmosphere where ash is absent?

Dr. Grant Allen, Centre for Atmospheric Science, University of Manchester:

“Both are possible; the latter is more relevant today.”

• Do planes stay under the ash or do they go through some dust but not enough to cause engine failure?

Dr. Grant Allen, Centre for Atmospheric Science, University of Manchester:

“The latter is true, but no one knows how much dust constitutes a definite hazard.”