(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

The eruption of Iceland’s Eyjafjallajokull volcano in April created what has been called the worst disruption of European and global air transport since World War II.

For six days, as volcanic ash from the erupting volcano was blown over parts of Europe, more than 100,000 scheduled flights were grounded, 313 airports became immobilized, and about 10 million passengers were unable to travel, many stranded in another country without a way to get home until the airspace was reopened.

The early estimates of loss reported by the media and aviation professionals are staggering. Direct losses to the airlines were as high as $2.3 billion, and losses to airports, $429 million. Indirect or secondary losses around the globe to stranded people, grounded planes, and lost commerce (destruction of perishable cargo, foregone just-in-time production, and lost wages) may have exceeded $5 billion.

Even so, the eruption of Eyjafjallajokull was modest in size in comparison with other Icelandic eruptions in the past century and, certainly, in comparison with far more explosive eruptions in the world during the past 20 years. The potential for even larger eruptions and wider dispersal of ash particles into crowded air routes impels better management of air traffic.

For nearly 30 years, the explicit goal of the airline industry has been to avoid air space known or forecasted to be contaminated with volcanic ash and accompanying sulfur gas aerosols. Ash particles are angular rock and mineral fragments less than 2 mm in diameter with the hardness of a pocket-knife blade.

The impact of ash with aircraft traveling at speeds of several hundred kilometers (miles) per hour causes abrasion damage to forward-facing surfaces, including windscreens, fuselage surfaces, and compressor fan blades.

Moreover, the melting temperature of ash is lower than the operating temperatures of modern jet engines. Consequently, ingested ash particles can melt, then accumulate as re-solidified deposits in the engine.

The overall result of an aircraft’s flying into an ash cloud can be degraded engine performance (including flame-out), loss of visibility, and failure of critical navigational and operational instruments.

The Iceland eruption and the temporary but widespread closure of much of Europe’s air space has led to a shift in thinking from complete avoidance of ash, wherever it may be located, to the consideration of new procedures and practices that would allow flight into known or forecasted low concentrations of ash (for example, no more than 2 milligrams per cubic meter).

During the past 6 months, the global community — already deeply involved in aviation safety and volcanic ash — has focused renewed attention on identifying new operational procedures that could reduce severe effects during future eruptions in Iceland and elsewhere.

The more vexing issue is evaluating the safety of flying through low concentrations of ash before this becomes an internationally recognized aviation practice.

This past summer, an International Volcanic Ash Task Force was created by the International Civil Aviation Organization to systematically review and evaluate these and other issues.

As part of its work, the task force will evaluate the reliability and accuracy of computer models used to forecast the path, location, and concentration of ash downwind from an erupting volcano. This will be extremely challenging, because there are currently no robust methods for determining actual ash concentrations in real time, especially over large areas.

Also, the airworthiness and long-term effects on aircraft systems and turbine engines taking off and flying through low concentrations of ash are not well understood or known. Research will be needed to evaluate these effects and determine the maintenance procedures needed to deal with them.

In the meantime, the international best practice for air transport remains avoiding volcanic ash.

To improve reporting news of volcanic activity to the aviation community, in 2008 the U.S. Geological Survey began issuing Volcano Observatory Notices for Aviation when a significant change occurs in the status of a volcano.

These notices are succinct messages aimed at airline dispatchers, pilots, and air-traffic controllers to immediately inform them of volcanic unrest and eruptive activity that could produce ash-cloud hazards.

For more information, go to: http://volcanoes.usgs.gov/activity/vonainfo.php