Tuesday, 20 April 2010

Volcanic Ash: Assessing Risk

How does one assess the degree of risk involved in flying when there is Volcanic ash?

That there is damage possible to jets is proven by the images released by the Finnish air force which show the effects of volcanic dust ingestion from inside the engines of a Boeing F-18Hornet fighter:

"Five of the air force's Hornets were involved in a training exercise on the morning of 15 April, just hours before the imposition of airspace restrictions due to the ash cloud spreading from a major volcanic eruption in Iceland. One aircraft's engines have been inspected so far using a boroscope, with melted ash clearly visible on its inside surface. The air force decided to release the images to show the potentially damaging effects of current flight activities, says chief information officer Joni Malkamýÿki. "The images show that short-term flying can cause substantial damage to an aircraft engine," the air force says. Continued operation could lead to overheating and potentially pose a threat to the aircraft and its pilot, it adds."(1)

Now this was released on the 15th April, so there was clearly sufficient ash up in the jetstream to cause appreciable damage at that point, and according to reports it was already over Northen Finland (2). Yet this seems further afield than had currently been reported by the weather maps which give a visual distribution of the ash cloud's dispersal.

Part of the problem is that the area of dispersal is derived from a mathematical model. As the Daily Telegraph reports:

"The decision was based on a computer model operated by the Meteorological Office's Volcanic Ash Advisory Centre, which suggested there was a cloud of ash covering northern Europe. This prompted a warning from the Met Office, which triggered the wider European ban, via Eurocontrol, the Brussels-based air traffic control centre. However, the model is no more than that - a mathematical model. There was no empirical evidence to back up its findings. Yesterday, the European Commission suggested that the American method of dealing with such episodes, whereby airlines decide whether to fly based on facts and supported by risk assessment, might offer a better approach."(3)

So let's turn the clock back a little to 1989, and see one of the incidents that made the current worst-case scenario so great:

"Despite a new warning system designed to prevent such encounters, several planes last week made potentially disastrous trips through ash from an erupting Alaskan volcano. The incidents leave many wondering what went wrong.I n the most serious event, a Boeing 747 operated by KLM Royal Dutch Airlines lost power in all four engines on Dec. 15 when it flew through an ash cloud at 37,000 feet about 75 miles northwest of Anchorage. The plane plunged more than 13,000 feet before pilots restarted its engines, says Ivy Moore of the Federal Aviation Administration (FAA) in Anchorage."(4)

Notice - as with the other notable case in Indonesia, that the plane suffering the power loss was extremely close to the volcano. Iceland is over 1,000 miles from Britain, and the models being used to track the spread go back to 1995. Yet it should be noted that eruption of Mount Pinatubo in 1991 caused damage but not engine failure to 16 aircraft, some of which were around 650 miles away. The safety margin may be larger than we might expect.

The mathematic model seems to have been developed around 1995 by M. Roth an others, in a paper entitled "Visualization of volcanic ash clouds":

"Ash clouds from a volcanic eruption-invisible to radar and nearly indistinguishable from weather clouds-pose a serious hazard to aviation safety. This article describes a system developed by the Alaska Volcano Observatory and the Arctic Region Supercomputing Center for predicting and visualizing the movement of ash clouds. Using meteorological and geophysical data from volcanic eruptions, a supercomputer model provides predictions of ash cloud movements for up to 72 hours. The AVS (Ash-cloud Visualization System) controls the execution of the ash cloud model and displays the model output in 3D form, showing the location of the ash cloud over a digital terrain model"(5)

Notice that this model is good for up to 72 hours, while the Iceland eruption has been causing a model to be used in considerably excess of that period. I cannot find any details of any extensive work in journals on improving the model, which is only as good as the sampling it can make. Over a small area, this is likely to be fairly accurate, but as the dispersal of the ash cloud increases, and some falls to ground, it is not clear exactly how many points of reference are available.

The scope of the ash cloud may be accurate, but elsewhere in the world, a different judgement is being taken on its concentration:

"Environment Canada expected no domestic problems from the cloud, which has shut European airports. 'The ash cloud is very diffuse, moving slowly and should not affect Canadian airports,' said spokesman Laura Cummings."(6)

How well the forecasting agrees with a actual data is also unclear, but the European Commissioner of Transport, Mr. Reute, shows how problematic the situation is, and how the tests currently taking place do not show the level of risk given by the model:

""In a case where, we do not have the data it is a tremendous and terrible responsibility for the authorities to say, 'oh well go on up'. That is why test flights are so important to have some kind of empirical evidence to help us move on from the mathematical model," he said. Mr. Reute also said that so far there have been 40 test flights that have been operated across Europe and none of them had found evidence of volacanic ash in their engines, windows or lubrication systems."(7)

A mathematical model for weather patterns, or in this case, ash dispersal, can be seen as roughly as a grid, like that used in graph paper, except this is 3-dimensional. The greater the number of points of data on the grid, the more accurate the forecast will be. Of course, weather data on winds and cloud cover is not only taken by weather measurements on the ground, but weather balloons and weather satellites. After the ash reaches a critical dispersal, it no longer shows on radar, and only special planes can test it. This reduces the amount of data being fed into the model considerably, and hence its accuracy.

To change the analogy, it is like measuring the depth of the ocean beneath a boat by sonar, or by sight and dropping plumb lines. Hazardous rocks in a random pattern are more dangerous, because they cannot be measured properly. If one was sailing in thick fog, the boat would have to drop anchor or move extremely slowly. Boats can do that; planes, of course, cannot.

The solution is better measuring techniques. The Swiss have unveiled their use of a much more sophisticated system:

"Laser weather technology, originally devised for 3D humidity maps, is the ideal solution for monitoring the volcanic ash cloud, says a top MeteoSwiss official. Bertrand Calpini and his team at the MeteoSwiss Aerological Station in Payerne have been using the innovative Light Detection and Ranging (Lidar) weather measurement system to map the ash cloud over Switzerland. The Federal Civil Aviation Office said on Monday morning that the cloud, which arose following the eruption of a volcano in Iceland on April 14, remained over the country and another ash cloud was expected soon. Large parts of Europe enforced no-fly rulings for a fifth day on Monday because of the cloud, causing the worst air travel chaos since the September 11 attacks. The high-tech instrument, launched in 2008, provides continuous data on the vertical distribution of humidity in the atmosphere up to an altitude of 10km. It can also detect fine particles, including pollen, and can build up 3D temperature."

Calpini says this system provides better intelligence on the ash cloud: "We have been able to do excellent monitoring of the spatial extension of the cloud using satellite images, and in particular computer modelling by Britain's Met Office. We have a very good 2D view of the situation in Europe. But for knowing where the higher concentrations are, whether above the central Swiss Plateau region or over Munich, Lidar is THE ideal tool."

The irony is that the technology has been around for 10 years, and yet no one has thought to make a serious investment in it. The Swiss, however, are able to accurately gaige what is going on. As Calpini notes:

It arrived over Switzerland on Friday at midnight at 6,500 metres. On Saturday morning it then descended to 3,500m and tripled in intensity into a thinner layer of less than 500m. On Saturday afternoon it descended to 3,000m and increased in intensity. Then overnight the cloud diluted, descended to 2,500m and entered the so-called "mixed layer" - the air we breathe at ground level. The last interpreted observation at Sunday lunchtime shows a strong easing and dilution effect with traces around 3,500m.

This is the kind of detailed picture that is not available from the UK Met Office. The report also notes that the ground level on Switzerland has less impact than pollen. There are Lidar points across Europe, but they are very spread out, and as with the other systems of measuring, it is giving only so much data:

"Albert Ansmann of the Leipnitz Institute for Tropospheric Research in Leipzig said it was a "baseless impertinence" to claim that "measurement isn't taking place in Europe. From the Netherlands to Romania, we know where and how thick the ash cloud is." With a Europe-wide network of laser instruments set up at the start of the decade, "we have been measuring like crazy since Thursday," Ansmann told SPIEGEL ONLINE. "We measured the cloud for the first time on Sunday night," said Volker Wulfmeyer, an atmospheric physicist at Germany's University of Hohenheim. "We see a structure at an altitude of eight kilometres, otherwise everything looks very clean." Hohenheim's measuring devices are located near the Stuttgart Airport."(9)

The other scientific advance noted in 1998 was a system being developed to operate on aircraft. Again, this uses laser technology rather than radar:

"Researchers have developed a laser system that shoots a beam of infrared light into the craft's flight path. Tiny dust particles, volcanic ash, and other natural aerosols, many less than a micrometer in diameter, reflect the laser light back to its source. If these particles happen to be entrained by turbulence, their swirling motion changes the frequency of the reflected light. Scientists tested a laser device in the mountain ridges of Colorado in late March and early April. During 15 hours of flying, light and moderate turbulence were detected 3 to 4 miles ahead of a research aircraft. "The system measured the turbulence, and then we felt the buffeting motion as we flew into it," says Bogue [Rodney Bogue of NASA's Dryden Flight Research Center]. He adds that tests of the system on commercial aircraft may begin within 3 years. Developed by Coherent Technologies of Lafayette, Colo., in conjunction with NASA, the laser system may provide adequate warnings on passenger craft in 5 to 7 years."(10)

Unfortunately, this does not seem to have been developed further for Commercial Aircraft, and it appears as a kind of "Tomorrow's World" technology. The television programme showcased all kinds of technological advances, but many of them never actually got off the ground. Either the technology was less accurate than the tests had shown, or the commercial impetus was simply not there, because we are now ten years after the date proposed for "providing adequate warnings on passenger systems".

The special planes which are being used by Meteorlogical agencies to detect ash may use them. It certainly is referenced in the 2000 paper "Airborne Coherent Lidar for Advanced In-Flight Measurements (ACLAIM) Flight Testing of the Lidar Sensor" and in the 2005 paper "Optical Air Flow Measurements for Flight Tests and Flight Testing Optical Air Flow Meters"(11)

I suspect commercial pressures dictated the matter - to introduce an expensive piece of equipment, when shareholders were the final arbiter of profits, was a major consideration. Whether governments or airlines will now consider this to be essential to develop on at least some airplanes in each fleet remains to be seen. For now, it seems very shortsighted.

(1) http://www.flightglobal.com/articles/2010/04/16/340727/pictures-finnish-f-18-engine-check-reveals-effects-of-volcanic.html
(2) http://www.barentsobserver.com/finnish-air-force-in-ash-trouble.4774519-16180.html
(3) http://www.telegraph.co.uk/comment/telegraph-view/7608299/Volcanic-ash-cloud-of-uncertainty.html
(4) http://news.google.com/newspapers
(5) http://doi.ieeecomputersociety.org/portal/web/csdl/doi/10.1109/MCG.1995.10024
(6) http://www.straitstimes.com/BreakingNews/World/Story/STIStory_516682.html
(7) http://www.telegraph.co.uk/travel/travelnews/7605794/Volcanic-ash-cloud-restrictions-embarrassing-say-airlines.html
(8) http://www.swissinfo.ch/eng/science_technology/Swiss_lasers_map_volcanic_ash_cloud.html?cid=8704734
(9) http://www.spiegel.de/international/europe/0,1518,689913,00.html
(10) http://www.sciencenews.org/sn_arc98/6_27_98/bob1.htm
(11) http://en.scientificcommons.org/8860907


Nick Palmer said...

It was largely the position of, and stability of, the anticyclone which has been dominating our weather for a while that directed the ash over Europe and the UK. Had we had a low pressure area the ash would all have gone over the Atlantic.

Anonymous said...

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