ASSESSING VOLCANIC HAZARDS AND RISKS TO AVIATION

Both volcanic-ash hazards to aviation and attendant risks are substantial. Volcanic ash is present in the atmosphere and of concern to the aviation sector daily on a global basis. Many encounters of aircraft with ash clouds attest to the costly, life-threatening risks involved. Anecdotal economic data indicate that a severely damaging encounter can cost an airline millions to tens of millions of dollars in direct repair costs. Even a low-severity encounter incurs costs when an aircraft is grounded until inspected and recertified as operationally safe. In addition, flight diversions to avoid ash can impose non-trivial costs owing to extra time and fuel needed for re-routing. Compared to ground communities, short-term volcanic risk to aviation is more dynamic: the vulnerable “community” (a plane carrying 100’s of passengers) rapidly changes its location relative to numerous hazard sources (volcanoes) and shifting hazard zones (drifting ash clouds).

A methodology for assessing long-term volcanic threats to aviation was developed as part of the rationale for a National Volcano Early Warning System (http://pubs.usgs.gov/of/2005/1164/). For each US volcano, an aviation threat score was calculated as the product of ratings given to 4 hazard factors (maximum Volcanic Explosivity Index (VEI) of previous eruptions, explosive eruptions of VEI>3 within past 500 yr, major explosive activity of VEI>4 within past 5000 yr, eruption recurrence intervals) and 2 exposure factors (proximity to major airports, daily passenger traffic in the region). Cascades volcanoes have the highest individual aviation threat scores because of high passenger loads along conterminous US air routes, but Alaska has higher cumulative aviation threat because of the greater number of active volcanoes in that region. The NVEWS methodology estimates unmitigated risk—i.e., does not factor in how efforts like volcano monitoring and ash-dispersion forecasts help to reduce risk by providing the basis for more effective ash avoidance. Prevented losses related to encounters that do not occur because of effective ash avoidance practices are notoriously hard to quantify, but nevertheless motivate efforts to improve volcano monitoring, ash-cloud detection, forecasts of cloud movement, and communication of warning messages.