Passengers queue up at the international terminal at Bali's Ngurah Rai airport in Denpasar for information of flight delays due to volcanic ash near Indonesia's resort island on July 10, 2015. SONNY TUMBELAKA/AFP/Getty Images The ongoing eruption of Raung* (see below) in Indonesia is beginning to become a travel headache across the region surrounding the volcano. Ash from the explosions have now caused multiple airports across Indonesia to close, including in some of the popular tourist area of Bali. The eruptions so far at Raung have been fairly small, so these closures must be especially troubling to tourist destinations as this sort of activity can be prolonged at volcanoes like those found in Indonesia---just look at the years of explosions at Sinabung. However, when it comes to modern air travel and volcanic ash, it is best to play it safe to prevent a potential disaster. The most famous air travel disruption from volcanic ash is the 2010 eruption of Eyjafjallajökull in Iceland, where a strong eruption in Iceland closed airspace over Europe over almost a week. This was an unprecedented closure of airspace due to ash and a major reason for this closure was that the European Aviation Safety Agency (EASA) had little idea of the concentration of the ash across much of Europe at different flight levels. However, it was clear that ash was in the atmosphere and even in low concentrations (too low to seen by pilots), extensive damage can happen to jet engines as it sucks in the ash. It was the right decision based on the threat that the ash posed to aircraft, although many airlines were infuriated by EASA's response.
The dark brown ash plume (center) from Raung in Indonesia, seen by Terra on July 10, 2015. NASA Earth Observatory This damage is due to the nature of volcanic ash. It is really just tiny, airborne glass shards that melt at temperatures found in modern jet engines. This means when the ash cools in the engines, it coats and blocks important parts of the engines, causing the engines to shut down. We know this thanks to incidents where jets have actually flown through ash plumes unknowingly and experienced just that. Luckily, they were able to recover so they could land safely, although the aircraft sustained extensive damage to their engines and glass. Just last month, the EASA issued new rules on how it would approach a new volcanic ash crisis over Europe. Unlike the 2010 events, it looks like a total closure of airspace of Europe isn't in the cards. Instead, the decision to fly would be up to the individual operators based on data collected at different flight levels. So, in theory, airlines could choose to fly above or below the ash (if possible), as long as they carefully assess any damage that might be found on their aircraft due to the ash. Some airlines are even testing realtime volcanic ash monitoring from their aircraft, although the technology is still in its infancy. Right now, where there is a threat of volcanic ash to aircraft, pilots turn to Volcanic Ash Advisory Centers (VAAC). They monitor ash to issue statements and maps about potential ash hazards to aircraft, dividing the world up into different zones that are watched by a series of VAAC offices. For example, the current eruption at Ruang is under the domain of the VAAC based in Darwin. Across North America, an eruption from the Cascades would fall under the Washington VAAC while the Aleutians are covered by the Anchorage VAAC. The VAACs are needed because volcanic ash doesn't appear in most air traffic control radar systems, so aircraft likely have no idea they are entering volcanic ash unless the plume is thick enough to be noticed visually. The typical VAAC statement (see below) contains information about the flight level that ash is likely to be found, the overall location of the plume and some forecasts about how it might move in elevation and location due to weather. They also usually include information about the sources of the information about the plume, such satellite images, pilot reports or ground reports. These reports help airlines decide how to reroute or cancel their flights if a volcanic eruption is underway.
Annotated VAAC statement for the activity at Raung on July 7, 2015. Click on the image to see a larger version. Darwin VAAC, annotated by Erik Klemetti So far, the United States has been lucky when it comes to avoiding these volcanic air travel crises. We had some taste during the 2010 Eyjafjallajökull eruption or when eruptions in Alaska and Kamchatka cause disruptions in travel to Asia. However, if a sufficiently large and ash-laden eruption were to occur to one of the Cascade volcanoes, like Rainier, Hood, Shasta, Lassen or others, a large swath of airspace across North America could be affected. What exactly are the threats caused by volcanic ash to aircraft? Beyond the threat of engine stall, pilots are told that ash and "smoke" (actually sulfur dioxide gas) could get into the cockpit, St. Elmo's Fire could appear, fire warnings may occur in forward cargo holds and pitot tubes could be blocked, causing incorrect flight speed information. The ash itself is highly abrasive, so flying through ash will damage the leading edge and windshield of the aircraft's cockpit. Landing at airports coated in ash can cause runways to become slippery as well, so caution needs to be taken. The biggest problem is that a lot of the ways to avoid the hazards of volcanic ash are very different than how pilots avoid severe weather hazards. Due to this difference, the FAA offers some suggestions of how to deal with volcanic ash. Ground operations are told to tell aircraft to try to avoid the ash along with warning that if there is ash on the ground, aircraft will kick up a lot of the ash. In flight training manuals, pilots are told that "it is recommended that pilots encountering an ash cloud should immediately reduce thrust to idle (altitude permitting), and reverse course in order to escape from the cloud." This is based on the experience of other aircraft that have encountered ash. Beyond this, the International Civil Aviation Organization released a report called "Flight Safety and Volcanic Ash" after the 2010 Eyjafjallajökull eruption that offers advice on risk management during a volcanic event. That being said, there is very little regulation in the United States for how the FAA will respond to a volcanic ash crisis. The assumption is that, much like after 9/11, the FAA can close airspace to all civilian air traffic. However, much like what happened in Europe, a prolonged closure will bring the wrath of airlines and passengers. Although the Cascades are quiet right now, it is inevitable that another explosive eruption will occur, so planning for how we will respond to an eruption is vital, especially in a country where air travel is so important. *Don't confuse Raung with another Indonesian volcano called Ruang (I know I did).