The nine Volcanic Ash Advisory Centers (VAAC) located around the world are responsible for coordinating and disseminating information on volcanic ash that may endanger aircraft that fly through the ash cloud formed from a volcanic eruption. They were set up by the International Civil Aviation Organisation (ICAO), an agency of the United Nations, as part of the International Airways Volcano Watch (IAVW). The individual VAAC are run as part of national weather forecasting organisations of the country that they are based, e.g. the NOAA or the Met Office.

The centers were set up in the 1990’s to improve forecasts of the locations of ash clouds from volcanic eruptions following incidents where commercial aircraft had flown through volcanic ash resulting in the loss of engine power. British Airways Flight 9 a Boeing 747 lost power to all four engines in 1982 over Indonesia after an eruption of Mount Galunggung and KLM Flight 867 another Boeing 747 again lost power to all four engines in 1989 over Alaska after an eruption of Mount Redoubt. It was recognised following these and other incidents that volcanic ash was a danger to commercial aviation and that the only way to ensure that there would be no loss of an aircraft was to alert pilots in a timely manner to divert their flight around the cloud.

[edit] Danger of volcanic ash to aviation

Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions,^[1] less than 2 millimetres in diameter. The ash enters the atmosphere from the force of the eruption and convection currents from the heated air and is then carried away from the volcano by winds. The ash with the smallest size can remain in the atmosphere for a considerable period of time and be carried well away from the eruption point. The ash cloud can be dangerous to aviation if it reaches the heights and enters the paths used by aircraft.

Volcanic ash has a melting point of approximately 1100 degrees Celsius, which is below the operating temperature of modern commercial jet engines, about 1400 degrees Celsius. If ash is ingested into the engines by an aircraft flying though the ash cloud then the ash melts in the engine on fuel nozzles and turbine blades rather than passing through the engine.^[2] Sulfur dioxide, another product of volcanoes which is also carried within the ash clouds following an eruption is corrosive to aircraft that fly through it.^[3]

When flying at night ash clouds are not visible to pilots, in addition they do not show up on radar, the small size of the ash particles do not return an echo to the onboard weather radars of commercial airliners. Even when flying in daylight the ash cloud is visible it may be interpreted as a normal cloud formed by water vapour and not seen as a danger, especially if it has travelled well away from the eruption location.^[3][4] In the image from the volcano Chaitén volcano the ash cloud has spread thousands of kilometers from the eruption site crossing the width South America from the Pacific coast and over the Atlantic.

Ash cloud from the 2008 eruption of Chaitén volcano stretching across Patagonia from the Pacific to the Atlantic Ocean

It was recognised that there was an issue following the incident in 1982 with the British Airways Flight 9 and therefore the ICAO established the Volcanic Ash Warning Study Group in 1982. Due to the difficulty in forecasting accurate information out to 12 hours and beyond the VAAC were set up by the ICAO as part of IAVW.^[3][5]

[edit] Purpose

The purpose of the VAAC is to gather information from volcano observatories, satellite images, pilot reports on ash clouds released by volcanoes, these are then analysed and forecasts made on future movements. This alert and forecast is then issued to interested parties in the aviation industry including Meteorological Watch Offices and Area Control Centers for aircraft and adjacent VAAC where an ash cloud may move from one area to another.^[6]

To be able to perform this function each VAAC develop models on the dispersion of volcanic ash. Using the reports available from satellites, volcanic observatories and pilot reports they identify ash clouds and then use the models to forecast where they likely to be a danger to aviation.^[7]

[edit] Locations

There are nine VAAC locations each with a defined area to monitor. The centers coordinate with adjacent VAAC, flight control centers within and adjacent to their area as well as meteorological offices within and adjacent to their area of operation.

Coverage of the nine VAAC around the world

The areas covered by the VAAC are set either by coordinates or by Flight Information Regions (FIR) that are internationally agreed as part of the IAVW program of the ICAO and the areas that each has responsibility is set out in the IAVW handbook ^[5]

The areas of responsibility for VAAC cover most of the world, with the exception of part of northern Russia and the Arctic; part of the South Pacific and the Antarctic, the areas that are not covered are shown by the hatched areas on the map.

[edit] Detection

Satellite data is the main source of information on ash clouds; it can provide the timeliest information. The remote nature of many volcanoes and that only a few have permanent monitoring from volcanic observatories mean that onsite reports will often not arrive quickly. Though they are remote in terms of population, aircraft may still fly over them regularly. For example the Aleutian Islands are on the Great Circle route between Asia and North America has a number of volcanoes. Over 50,000 aircraft fly through the area each year which has 40 volcanoes that have been active in historic time.^[7] Pilot reports if present will only be available for flights during daylight hours as they cannot observe ash clouds during the night.

Airline great circle track at top, jet stream track at bottom

Even satellites will have issues as cloud cover of the volcano can limit the detection capability. Volcanoes in high latitudes are not covered as well by weather satellites as those in lower latitudes due to the curvature of the Earth from the position of a geosynchronous orbit^[8]. The details of the ash erupted, in terms of concentration, size of particles are often not available to the forecasters, with this data a better forecast can be made.^[3]

[edit] Alerts

When an ash cloud is detected the VAAC will gather all the available information on this and then make use their computer models to predict the path of the ash cloud at different flight levels used by aircraft. They will then issue an alert to aviation and meteorological offices as stated within the IAVW procedure manual ^[5]. This alert will be in the format of an alert stating that it is a Volcanic Ash Advisory.

Within the alert the following information will be provided The name of the volcano, The country, location and crater elevation of the volcano The source of the information, e.g. satellite or pilot observation, Details of the eruption including time of day in UTC and date of the eruption Details of the ash cloud including the flight level and size Detail on the current movement of the ash cloud Forecast movement for 6, 12 and 18 hours ahead following the time of the advisory Next update time

[edit] References

[edit] External links

• Anchorage VAAC http://aawu.arh.noaa.gov/vaac.php
• Buenos Aires VAAC http://www.meteofa.mil.ar/vaac/vaac.htm
• Darwin VAAC http://www.bom.gov.au/info/vaac/
• London VAAC http://www.metoffice.gov.uk/aviation/vaac/
• Montreal VAAC http://meteo.gc.ca/eer/vaac/index_e.html
• Tokyo VAAC http://ds.data.jma.go.jp/svd/vaac/data/index.html
• Toulouse VAAC http://www.meteo.fr/vaac/
• Washington VAAC http://www.ssd.noaa.gov/VAAC/washington.html
• Wellington VAAC http://vaac.metservice.com/vaac/