Marianne Guffanti, U.S. Geological Survey, 926A National Center, Reston, Virginia 22101, USA, email@example.com
Captain Edward K. Miller (Retired), Air Line Pilots Association, 535 Herndon
Parkway, Herndon, Virginia 20172, USA, firstname.lastname@example.org
Volcanic eruptions pose a serious threat to aviation, but one that can be mitigated through the combined efforts of scientific specialists, the aviation industry, and air-traffic control centers. Eruptions threaten aviation safety when finely pulverized, glassy, abrasive rock debris ("ash") is explosively erupted to disperse as airborne clouds over long distances at cruise altitudes. Numerous instances of aircraft flying into volcanic ash clouds have resulted in hundreds of millions of dollars of aircraft damage and seven cases involving engine failure. The best safety strategy is to avoid ash encounters, which requires that pilots, dispatchers, and air-traffic controllers quickly learn of occurrences of explosive eruptions and the whereabouts of airborne ash clouds globally.
In an explosive volcanic eruption, gas-charged magma abruptly depressurizes as it nears the Earth's surface, violently exploding out of a constrained vent. The ejected magma quickly cools in the air, fragmenting into glassy shards and sharp-edged bits of minerals. The erupted materialalong with sulfur dioxide and other gases released from the decompressing magmais entrained upward in a convecting, columnar mass. Eruption columns rise quickly from their source vents at velocities of 15 to >600 ft/sec and can be energetic enough to reach cruise altitudes of jet aircraft and beyond, to 150,000 ft (Self and Walker, 1994). The larger pieces of volcanic debris fall out of the column within minutes to hours and are deposited around the volcano, while the smaller particles (termed ash) can remain suspended in the atmosphere for days.
Once ejected into the stratosphere, the ash and gas droplets are spread by winds aloft as a diffuse cloud of small particles having insufficient reflectance for detection by weather radars onboard aircraft. Ash clouds can extend over large areas and travel great distances from the source volcano after eruptive activity has stopped. For example, satellite images showed that the 1992 eruption of Mount Spurr, Alaska (USA), produced an ash cloud that could be detected for three days up to 3100 miles downwind of the volcano over Canada and Great Lakes region of the United States (Schneider and others, 1995).
Numerous instances of jet aircraft flying into volcanic ash clouds have demonstrated the serious damage that can be sustained. Ash particles are angular fragments having the hardness of a pocket-knife blade and, upon impact with aircraft traveling at speeds of several hundred knots, cause abrasion damage to forward-facing surfaces, including windscreens, fuselage surfaces, and compressor fan blades. Moreover, the melting temperature of the glassy silicate rock material that comprises an ash cloud is lower than the operating temperatures of modern jet engines; consequently, ingested ash particles can melt and 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. Experimental tests (Dunn and Wade, 1994) determined the following mechanisms that can affect aircraft performance due to exposure to a volcanic ash cloud:
(a) deposition of material on hot-section components,
(b) erosion of compressor blades and rotor-path components,
(c) blockage of fuel nozzles and cooling passages,
(d) contamination of the oil system and bleed-air supply,
(e) opacity of windscreen and landing lights,
(f) contamination of electronics,
(g) erosion of antenna surfaces, and
(h) plugging of the pitot-static system which indicates the airspeed of the aircraft.
An ash cloud eventually dissipates in the atmosphere, and ash concentrations drop. However, the threshold concentration at which ash poses no harm to aircraft is not known, and indeed, may never fully be characterized for all situations involving aircraft. It is usually assumed that ash identifiable on satellite images continues to present a hazard to aircraft (ICAO, 2001). Accordingly, the consensus of the aviation community is that if an ash cloud can be discerned, it should be avoided.
Ash that falls out of a volcanic plume and settles to the ground is also a hazard to aviation by impairing operations at airports in the vicinity and downwind of erupting volcanoes. Between 1971 and 2001, ash falls have forced closures or have severely impacted operations at more than 40 airports in 15 countries, viz., USA, Japan, Mexico, New Zealand, Guatemala, Colombia, Indonesia, Papua New Guinea, Philippines, Argentina, Falkland Islands, Puerto Rico, Ecuador, Italy, and British West Indies (data modified from Casadevall, 1993). Problems at airports include:
(a) difficult landing conditions due to reduced runway friction coefficient, especially when the ash is wet,
(b) loss of local visibility when ash on the ground is disturbed by engine exhausts during take off and landing,
(c) deposition of ash on hangars and parked aircraft, with structural loading considerably worsened if weight is added by precipitation absorbed by ash, and
(d) contaminated ground-support systems.
Guidelines for dealing with ash at airports have been published (ICAO, 2001; Casadevall, 1993) and include recommended ground-operating procedures for aircrafte.g., limiting reverse thrust during landings and using a rolling take-off procedure.
About 60 of the world's ~1500 young volcanoes typically erupt each year (Simkin, 1994). The size and intensity of these eruptions varies considerably, with smaller eruptions (e.g., the ongoing activity at Soufriere Hills in the British West Indies) being more common than larger eruptions (e.g., at Pinatubo, Philippines, in 1991). Typically, an eruption involves episodes of activity that are separated by non-eruptive intervals of hours to months. The duration of a single episode usually ranges from a few minutes to tens of hours. The entire eruptive period of a volcano can last for months to years. As a result, a volcano observatory that monitors such activity may need to change alert levels numerous times over the course of an eruption from a single volcano.
The ash hazard to aviation is not a rare possibility on a worldwide scale, given that many major air routes overlie the world's volcanically active regions. It is estimated (Miller and Casadevall, 2000) that volcanic ash can be expected to be in air routes at altitudes greater than 30,000 ft for roughly 20 days per year worldwide.
From 1973 through 2000, about 100 encounters of aircraft with airborne volcanic ash have been documented (Figure 1). That number can be considered a minimum value, because not all encounter incidents are publicly reported. Locations of 32 source volcanoes whose eruptions impacted airports or produced ash clouds encountered by aircraft are shown in Figure 2. Aircraft have been damaged by eruptions ranging from small, recurring episodes (e.g., Etna, Italy, 2000) to very large, infrequent events (e.g., Pinatubo, Philippines, 1991). Severity of the encounters has ranged from minor (acrid odor in the cabin and electrostatic discharge on the windshield) to very grave (engine failure requiring in-flight restart of engines). Engine failures have occurred 150 to 600 miles from the volcanic sources. Fortunately, engine failure leading to crash has not occurred.
Figure 1. Plot of number of reported aircraft encounters with volcanic ash clouds from 1973-2000. Data modified from (ICAO, 2001).
A high of 25 encounters is documented for 1991, primarily related to the eruption of Pinatubo volcano, which was the second largest eruption in the 20th century. Over the past decade since 1991, an average of two aircraft/ash encounters per year (21 total) have been reported. Again, this is a minimum value.
Figure 2. Locations of volcanoes responsible for ash/aircraft encounters, 1973-2000 (circles) and airport closures, 1971-2001 (squares). Active and potentially active young volcanoes shown with triangles.
The potential for a disastrous outcome of an ash-cloud encounter has been illustrated by three dramatic incidents (Miller and Casadevall, 2000). The first occurred in 1982 when a Boeing 747 flying at night over water with 240 passengers flew into an ash cloud about 100 miles from Galunggung volcano, Indonesia. The aircraft lost power in all four engines and descended 25,000 ft from an altitude of 37,000 ft above sea level. After 16 minutes of powerless descent, the crew was able to restart three engines and make a safe landing in Jakarta. A few weeks later, a second Boeing 747 with 230 passengers encountered an ash cloud from another eruption of the same volcano. The aircraft lost power to three engines and descended nearly 8000 ft before restarting one engine and making a nighttime emergency landing on two engines. In both cases, the aircraft suffered extensive damage, and large death tolls were barely averted. A third nearly tragic incident occurred in 1989 related to an eruptive event at Redoubt volcano in Alaska, USA. A Boeing 747 with 231 passengers was nearing Anchorage International Airport and flew into what looked like a thin layer of altocumulus but was actually an ash cloud. The aircraft lost power to all four engines and descended for four minutes over mountainous terrain. Within only one to two minutes of impact, the engines were restarted, and the aircraft was safely landed in Anchorage. Damage was estimated at more than US $80 million.
Some more recent documented encounters in August 2000 did not involve engine failure but were nevertheless very dangerous (Rossier, 2002). A Boeing 737-800 nearing Japan's Narita Airport flew into an ash cloud from an eruption that had occurred about an hour earlier at Mijake-jima volcano, located about 100 miles from the airport. The aircraft's engines functioned, but the flight management computer and electronic engine controls failed. Handicapped further by severe loss of visibility due to abrasion of all but a small part of the windscreen, the crew managed a safe landing. Soon thereafter on the same day, a 747 had a similar experience. For each aircraft, replacement of engines and windscreen cost US $5 million.
Indicators in the cockpit and cabin that an aircraft has entered an ash cloud
may include: acrid or sulfurous odors, dusty haze within the airplane, static
electrical discharges (St. Elmo's fire) on the windshield or a white glow at
the engine inlets, engine surging, and decreased or fluctuating airspeed readings
(Table 1). An incident in February 2000 underscores the
possibility, however, of an undetected yet damaging encounter (Grindle and Burcham,
2002). A U.S. research DC-8 operated by NASA departed California on a polar
route for Sweden to conduct experiments on ozone loss. The crew had been informed
of an eruption the previous day of Hekla volcano in Iceland, and the flight
track went well north (200 NM) of the projected location of the drifting volcanic
cloud. Nevertheless, on-board scientific equipment indicated the aircraft flew
through the Hekla volcanic cloud at 37,000 ft; whereas, the crew noticed nothing
unusual on their instruments or by direct observation. Upon arrival in Sweden,
the engines were visually inspected, but no damage was noticed, and the aircraft
flew another 68 hours before its return to California. Subsequent borescope
analysis indicated clogged turbine blade cooling passages and blade coating
erosion; the cost for disassembling and repairing all four engines was US $3.2
million. This incident highlights the disturbing possibility that other undetected
encounters may result in shortened engine life (Grindle and Burcham, 2002).
Table 1. Indications an aircraft has inadvertently entered a volcanic-ash cloud (ICAO, 2001; Boeing, 2000).
The recommended in-flight procedures in the event of an ash-cloud encounter are summarized in Table 2. An important lesson learned by experience is not to try to climb out of an ash cloud because doing so increases engine-operating temperatures and thus can cause more buildup of molten debris on engine parts, resulting in rapid flame out of all engines.
Table 2. General in-flight procedures in event of a volcanic-ash
encounter (ICAO, 2001; Boeing, 2000). Consult aircraft operating manuals for
Clearly, much can be learned from analyses of encounters. However, even basic information about time, altitude, location, and observed phenomena is hard to find in the public domain. Volcanologists at the U.S. Geological Survey (USGS) and Smithsonian Institution are maintaining a scientific database about ash/aircraft encounters to improve understanding of the hazard. Information about encounters and airport impacts is being sought from the aviation community. (Note that airline company identification is not part of the database.) Pilots can report information about the location and nature of ash clouds and encounters to the USGS/Smithsonian database project by sending a Volcanic Activity Report (VAR) to email@example.com. The VAR was designed to be carried in the cockpit for pilot use and is included in Federal Aviation Administration's Aeronautical Information Manual (FAA, 2002).
Given the sobering record of damaging and life-threatening ash-cloud encounters, the primary mitigation strategy adopted by the aviation community is to avoid flying into volcanic ash. Avoidance requires that dispatchers, pilots, and air-traffic controllers quickly learn of occurrences of explosive eruptions globally and the whereabouts of airborne ash clouds. Accordingly, mitigation involves elements of:
(a) volcano monitoring and eruption reporting,
(b) detecting the location of the drifting cloud over time,
(c) forecasting the expected path of the cloud,
(d) communicating effectively among the diverse parties involved in responding to the hazard, and
(e) not least, training of key operational personnel such as pilots, dispatchers, and air-traffic controllers.
Someby no means allof the world's hazardous volcanoes are systematically monitored by scientific groups (volcano observatories) that integrate a variety of data streams and provide public reports about significant volcanic activity. Eruptions herald their coming over periods of weeks to years with various physical and chemical signals (called "unrest") that are related to the rise of magma from depth toward the surface of the Earth. Modern instrumentation, combined with knowledge of the eruptive history of a volcano, provides a means to monitor and interpret precursory earthquakes, ground deformation, gaseous emissions and other signs of restlessness. Volcano observatories give as much forewarning as possible of impending eruptions; however, eruption forecasting is far from an exact science. Volcanoes do not erupt in a uniform style or with consistent precursors; nor do episodes of unrest inevitably lead to eruptive activity. Once an eruption is underway, volcano observatories can provide important information to the aviation community about eruptive style (how explosive), the likely course of the eruption, and when the volcano has returned to a quiescent state.
Reports by pilots of their visual observations of unusual phenomena at volcanoes are valuable to volcano observatories, especially regarding remote and/or unmonitored volcanoes. A volcano observatory often tries to corroborate a pilot report of eruptive activity against other data, as a volcano can experience increased steaming or display unusual local cloud effects not related to actual eruptive activity.
A successful example of volcano monitoring and eruption reporting with a strong focus on the ash hazard to aviation is the Alaska Volcano Observatory (AVO). This multi-agency scientific group operates under the aegis of the USGS Volcano Hazards Program. Alaska has numerous active volcanoes that underlie increasingly busy North Pacific routes carrying heavy passenger and cargo traffic between the Unites States and Asia and Russia. As of July 2002, AVO operates seismic networks at 23 of Alaska's 41 active volcanoes and has plans to instrument additional volcanoes over the coming years. The seismic networks provide continuous, automatically processed data streams that allow AVO to track earthquake evidence of unrest and eruptions in real time. AVO also examines satellite images on at least a twice-daily basis and incorporates reports received from pilots and ground observers. Combining these monitoring techniques and information sources, AVO typically is able to provide warnings hours to weeks in advance of likely eruptive activity to air-traffic controllers, aviation-weather groups, and the aviation industry so that flights can be routed to avoid ash-cloud encounters. AVO developed a Level of Concern Color Code specifically designed to communicate hazard information to aviation users (for further explanation, see http://www.avo.alaska.edu/avo4/updates/color_code.html). AVO also works with scientists of the Kamchatka Volcanic Eruptions Response Team (KVERT) to detect and report eruptions in the Russian Far East that can affect North Pacific airspace.
In the United States, images with resolutions ranging from 1 to 8 kilometers are available as often as every 15 minutes from sensors on geostationary (GOES) satellites stationed over the equator. Advanced very-high-resolution radiometers (AVHRR) on two polar-orbiting satellites provide data at 1-kilometer resolution about four times per day at the equator and eight times per day at high latitudes (Alaska). Data downloaded from these civilian meteorological satellites operated by the National Oceanic and Atmospheric Administration (NOAA) are readily accessible for a variety of public uses and are widely used for ash-cloud detection by groups both within and outside NOAA.
As volcanic-ash clouds disperse, they intermingle with meteorological clouds in the atmosphere and thus have a similar appearance when observed in infrared and visible images. The best method currently available for distinguishing ash clouds from meteorological clouds utilizes two spectral bands within the thermal infrared range (one at ~ 11 microns wavelength and the other at ~ 12 microns wavelength). Because of the differences in the absorption and scattering properties of ash compared to water, subtraction of the 12-micron data from the 11-micron data discriminates ash (Prata, 1989). Unfortunately, the use of this "split-window" technique, as it is termed, soon will be seriously impaired. The 12-micron channel is being phased out of use on U.S. GOES satellites and replaced by a channel not suitable for ash detection. Efforts by scientific and aviation groups are underway to identify and secure a robust set of channels for volcanic-cloud detection and tracking on the next generation of GOES sensors, but the loss of the split-window capability could last for several years.
The use of airborne sensors to detect ash ahead of an aircraft has been investigated. A prototype of a forward-looking multi-channel infrared radiometer has been tested (Barton and Prata, 1994) with some encouraging results. Such a sensor would be a "tactical" device to use in conjunction with "strategic" flight planning based on knowledge of locations of explosive eruptions and ash-clouds.
Meteorological watch offices (MWOs) include information from ash-dispersion models, when applicable, in their aviation warnings. In the United States, the Volcanic Ash Forecast Transport and Dispersion (VAFTAD) model is widely used by meteorological services in their advisories to aviation (Stunder and others, 1994). The National Weather, for example, issues Volcanic Ash Significant Meteorological Advisories (SIGMETs) that include "outlooks" based on VAFTAD analyses.
The February 2000 Hekla, Iceland, encounter described in a previous section is an important reminder that transport models cannot precisely locate the edges of clouds (Grindle and Burcham, 2002). In that incident, the flight track was well beyond the forecasted edge of the cloud; nevertheless the aircraft flew through the low-concentration yet damaging fringe of the cloud.
Because of the global scope of the ash hazard to aviation, an international approach has been used to address the threat. Under the auspices of the World Meteorological Organization and the International Civil Aviation Organization (ICAO) as part of the International Airways Volcano Watch, nine regional Volcanic Ash Advisory Centers (VAACs) were established in the mid 1990's to provide advisories to international MWOs about the location and movement of ash clouds. The VAACs are located in Anchorage, Buenos Aires, Darwin, London, Montreal, Toulouse, Tokyo, Washington DC, and Wellington. VAACs use volcano-observatory reports, pilot reports, geostationary and polar-orbiting satellite data, and ash-dispersion models as the basis for their advisories. Upon receiving a VAAC ash advisory, a MWO issues a SIGMET. As an ash cloud drifts, responsibility for monitoring it passes from one VAAC to the next.
Carriers that have sustained damage from volcanic ash are the most motivated to provide training; those that fly frequently over active volcanic also are likely to see the need for training. Carriers that believe they operate clear of volcano areas typically will not justify volcanic-ash training, but given the long distance that ash clouds can travel, that perspective may be flawed.
A training video has been developed by the Boeing Company, in cooperation with the Air Line Pilots Association and the USGS ("Volcanic Ash Avoidance Flight Crew Briefing"). ICAO provides translations of the video into French, Russian, and Spanish. Presently, neither that video nor other volcanic-ash information developed by airframe and engine manufacturers is mandated for inclusion in training material for pilots. Minimum requirements should be that every licensed pilot and dispatcher views the training video. More effective training would require the inclusion of information about volcanic ash hazards in ground-school courses required for a pilot's license. An ideal training program would include the required ground school, the video, and a volcanic-ash drill during the simulator portion of pilot and dispatcher training courses. To achieve this, airline management must be convinced that the training programs are necessary.
Volcanic ash will persist as a serious aviation hazard. Air routes over active volcanic regions will continue to be heavily used, and industry changes such as more free-flight routing and extended twin-engine operations will increase the need for ash-hazard information. With free-flight routing, where in-flight changes to an aircraft's route can be made by a pilot rather than air-traffic control, advisories about ash-cloud locations and trajectories will have to take into account greater variability in flight tracks likely to be affected. For extended twin-engine operations on transoceanic flights, many countries require that aircraft be within 207 minutes (about 3.5 hours) flight time of an alternate airport at all times; consequently, airport closures because of volcanic ash (either airborne or deposited) could seriously disrupt air traffic. Moreover, larger engines that operate at higher temperatures are more susceptible to damage from volcanic ash.
Mitigation strategies are working now, but should be strengthened in various
Continual refinement of communication protocols:
Table-top exercises to practice responding to a hypothetical ash-cloud incident have proven helpful in improving information flow among diverse parties. Also, preparation of inter-agency operational plans can help clarify procedures and contact points.
Augmented volcano monitoring:
Early eruption reporting helps with flight planning and satellite tracking of ash clouds. Volcano observatories should receive the resources to instrument more high-risk volcanoes with real-time monitoring networks.
Development of new satellite-based sensors:
The loss of the split-window capability on GOES satellites needs to be remedied, and the specific capability for ash-cloud detection should be designed into future sensors.
Improvement of ash-transport models:
Government and academic scientists need to improve modeling with better dispersion algorithms and improved input regarding the eruption source conditions.
Development of on-board sensors to detect ash:
On-board sensors could augment the current mitigation strategy by providing a "last-minute" means to avoid encounters, including encounters with low-concentration clouds that might not otherwise be recognized as causing engine damage.
A contradictory aspect of effective mitigation is that the prevention of bad outcomes can lead to unwarranted complacency that the underlying hazard has been eliminated. Should our ability to prevent aircraft encounters with ash clouds improve to the point that there are no incidents, we should take this not as evidence that no threat exists, but rather as a reason for continued vigilance and support of broad-based mitigation capabilities.
The Internet has greatly improved the sharing of ash-hazard information. Some useful entry points to a variety of volcano and ash-cloud information include:
Highly recommended print publications are the ICAO Manual on Volcanic Ash, Radioactive Material, and Toxic Clouds published in 2001 (Doc 9691-AN/954, call +1-514-954-8022 to order); the Proceedings of the First International Symposium on Volcanic Ash and Aviation Safety published in 1994 (Casedevall, 1994) by the USGS (Bulletin 2047); and the World Map of Volcanoes and Principal Aeronautical Features published in 1999 (Casadevall and others, 1999) by the USGS (Map I-2700). Call 303-202-4700 to order USGS publications. The Boeing training video is available through the company's Flight Standards Section.
Boeing, 2000, Advances in Volcanic Ash Avoidance and Recovery: Aero Volume 9, p. 18-27.
Casadevall, T.J., Thompson, T. B., and Fox, T., 1999, World Map of Volcanoes and Principal Aeronautical Features: U.S. Geological Survey Map I-2700.
Casadevall, T. J. (ed.). Volcanic Ash and Aviation Safety - Proceedings of the First International Symposium on Volcanic Ash and Aviation Safety: U.S. Geological Survey Bulletin 2047 (1994): 450 p.
Casadevall, T. J., 1993, Volcanic Ash and Airports - Discussion and Recommendations from the Workshop on Impacts of Volcanic Ash on Airport Facilities: U.S. Geological Survey Open-File Report 93-518, 52 p.
Dunn, M. G., and Wade, D. P., 1994, Influence of Volcanic Ash Clouds on Gas Turbine Engines, in Casadevall, T. J. (ed.). "Volcanic Ash and Aviation Safety - Proceedings of the First International Symposium on Volcanic Ash and Aviation Safety" U.S. Geological Survey Bulletin 2047, p. 107-118.
Federal Aviation Administration, 2002, Aeronautical Information Manual - Official Guide to Basic Flight Information and ATC Procedures (Feb. 21, 2002). Online at http://www.faa.gov/atpubs
Grindle, T., J., and Burcham, Jr., F. W., 2002, Even Minor Volcanic Ash Encounters Can Cause Major Damage to Aircraft: ICAO Journal, v. 57, n. 2.
International Civil Aviation Organization, 2001, Manual on Volcanic Ash, Radioactive Material, and Toxic Chemical Clouds: Doc 9691-AN/954.
Miller, T. P., and Casadevall, T. J., 2000, Volcanic Ash Hazards to Aviation, in Encyclopedia of Volcanoes, Sigurdsson, H. (ed.), Academic Press, San Diego, California, USA, p. 915-930.
Prata, A. J., 1989, Infrared Radiative Transfer Calculations for Volcanic Ash Clouds: Geophysical Research Letters, v. 16, p. 1293-1296.
Rossier, R. N., 2002, Volcanic Ash: Avoid at All Costs: Business and Commercial Aviation, Feb., p. 70-75.
Schneider, D. J., Rose, W. I., and Kelley, L., 1995, Tracking of 1992 Eruption Clouds from Crater Peak, Mount Spurr, Alaska, Using AVHRR, in Keith, E. C. (ed.): U.S. Geological Survey Bulletin 2139, p. 27-36.
Self, S., and Walker, G. P. L, 1994, Ash Clouds - Characteristics of Eruption Columns, in Casadevall, T. J. (ed.)., Volcanic Ash and Aviation Safety - Proceedings of the First International Symposium on Volcanic Ash and Aviation Safety" U.S. Geological Survey Bulletin 2047, p. 65-74.
Simkin, T., 1994, Volcanoes - Their Occurrence and Geography, in Casadevall, T. J. (ed.)., Volcanic Ash and Aviation Safety - Proceedings of the First International Symposium on Volcanic Ash and Aviation Safety" U.S. Geological Survey Bulletin 2047, p. 75-80.
Stunder, B. J. B., and Heffter, J. L., 1994, Modeling Volcanic Ash Transport and Dispersion, in Casadevall, T. J. (ed.)., Volcanic Ash and Aviation Safety - Proceedings of the First International Symposium on Volcanic Ash and Aviation Safety" U.S. Geological Survey Bulletin 2047, p. 277-282.
Acknowledgements: David Schneider, Thomas Miller, and Terry Keith of the USGS and Leonard Salinas and Shawn Vinson of United Airlines gave helpful reviews of this paper. Dave Schneider and Tom Miller also generously provided images for the power-point presentation. This work was supported by the USGS Volcano Hazards Program and the Air Line Pilots Association.
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