Under the aegis of the International Civil Aviation Organization (ICAO)—the United Nations Technical Agency charged with regulating international air navigation—a global framework is in place to mitigate risks to aviation from volcanic ash clouds. Motivated by severely damaging encounters of passenger aircraft with volcanic-ash clouds in the early 1980's, ICAO established globally coordinated procedures to:

  • Report eruptions that are imminent or underway.
  • Detect ash clouds in the atmosphere and forecast their expected movement hours into the future.
  • Issue special warning messages to aviation meteorological offices, airline operation centers, and air-traffic management centers.
  • Together, these actions are aimed at assisting aircraft avoid potentially hazardous airspace.

    Eruption reporting

    Timely reporting of volcanic activity, beginning with information about the precursory build-up phase and carrying through to eruption cessation, can help airlines and air-traffic managers to optimize flight planning and carry out safe and efficient en-route flight diversions.

    ICAO recommends that Volcano Observatories use an alert-level system developed specifically to quickly communicate key information about activity at a restless or erupting volcano to dispatchers, air-traffic managers, and aviation meteorologists. The system uses a simple, color-based ranking that focuses on conditions at a volcano (not on the ash cloud as it moves downwind) and works for both increasing and decreasing activity. As of 2013, the color code is used by Volcano Observatories in the United States, Russian Federation, New Zealand, Iceland, and by the Darwin Volcanic Ash Advisory Center in Australia as part of its Ash Advisories for the Southwest Pacific region.

    GREEN  Volcano is in normal, non-eruptive state.
    or, after a change from a higher level:
    Volcanic activity considered to have ceased, and volcano reverted to its normal, non-eruptive state.
    YELLOW  Volcano is exhibiting signs of elevated unrest above known background level.,
    or, after a change from a higher level:
    Volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.
    ORANGE  Volcano is exhibiting heightened unrest with increased likelihood of eruption. timeframe uncertain,
    Volcanic eruption is underway with no or minor volcanic ash emissions.
    RED  Eruption is forecast to be imminent with significant emission of volcanic ash into the atmosphere likely,
    Eruption is underway with significant emission of volcanic ash into the atmosphere.

    To convey the color-code rankings and other operationally critical information, a specialized message template—called a Volcano Observatory Notice for Aviation (VONA)—has been developed by ICAO for use by Volcano Observatories. The VONA template and guidance on its dissemination are given in ICAO's Doc. 9766, Handbook on the International Airways Volcano Watch.

    Ash Cloud Detection and Forecasts

    Through the efforts of ICAO, a worldwide system of nine Volcanic Ash Advisory Centers (VAACs) has been established to determine the locations and movement of ash clouds in the atmosphere. VAACs rely primarily on data from civilian meteorological satellites, Volcano Observatory notifications, and pilot reports to detect and track ash clouds. The VAACs run atmospheric dispersion models to forecast cloud movement hours into the future. VAAC Advisories are issued in text and graphical form, and these are the basis for the generation of aviation hazard warnings called SIGMETs (for SIGnificant METeteorological information). SIGMETs are issued by the worldwide system of aviation Meteorological Watch Offices to inform airline dispatchers, pilots, and air-traffic managers of hazardous weather-related hazards including ash clouds

    Communicating the Hazard

    VONAs, VAAC Advisories, and SIGMETs are sent to the parties who make or support operational decisions about pre-flight route changes, flight cancellations, and in-flight diversions—i.e., airline dispatchers and pilots, air-traffic managers, and aviation meteorologists. The messages are updated and re-issued over time periods of minutes to hours. Most of the messages are disseminated via the global telecommunication networks organized for air-traffic services under the authority of ICAO, and some are also posted on agency web sites. For the United States, roles of the involved government agencies and how they are coordinated are outlined in the National Volcanic Ash Operations Plan for Aviation.

    Source Message Type
    En-Route Flight Crew  Special Air Report (AIREP): Report of observations of volcanic activity made by flight crew during flight. Also known as pilot report (PIREP).
    Volcanic Activity Report (VAR): ICAO form for reporting observed volcanic activity or an aircraft encounter with an ash cloud.
    National Civil Aviation Authorities  Notice to Airmen (ASHTAM or Ash NOTAM): Message to alert pilots of any ash hazards en route or at a specific location (such as an airport).
    Volcanic Ash Advisory Centers  Volcanic Ash Advisories (VAA): Text and graphical description (VAG) of the location of detected ash cloud with forecast of its expected movement hours into the future.
    Meteorological Watch Offices  SIGnificant METeorological Information (SIGMET): Warning message about meteorological hazards to flights, including volcanic ash clouds.
    Volcano Observatories  Volcano Observatory Notice for Aviation (VONA): Succinct description of activity at a volcano with focus on ash emission.