Why volcanic ash is more dangerous to jets than most weather hazards

Volcanic ash looks like gray dust from the ground, but inside a jet engine it behaves closer to ground glass mixed with cement. The material is pulverized rock and volcanic glass, rich in silica, and a jet engine's combustion chamber runs hotter than the temperature at which that glass softens and melts. Ash that enters the core melts on contact, flows rearward, and resolidifies as a hard coating on cooler turbine blades. That coating narrows the passages the engine needs for airflow and plugs the small holes that keep blades from overheating. Thrust falls. In severe cases, the engine quits.

Two incidents in the 1980s established the pattern. In June 1982, a British Airways Boeing $BA 747 flying from Kuala Lumpur toward Perth lost all four engines after crossing an ash cloud from Indonesia's Mount Galunggung. The crew glided the aircraft down through thousands of feet before restarting engines below the ash layer and landing in Jakarta with a windshield sandblasted nearly opaque. In December 1989, a KLM 747 approaching Anchorage lost all four engines in ash from Alaska's Mount Redoubt and recovered the same way. Two near-identical outcomes, on opposite rims of the Pacific, showed this was a repeatable physical process rather than a fluke.

The 2010 eruption of Iceland's Eyjafjallajökull turned that engineering problem into a logistical one. Ash drifted across European airspace for days, grounding roughly 100,000 flights, stranding millions of travelers and costing airlines well over a billion dollars in lost revenue. The shutdown forced regulators to replace blanket closures with tiered, concentration-based rules, and it pushed scientists to sharpen the dispersion models that predict where a plume will drift next.

Understanding how volcanic ash and airplanes interact explains a part of aviation safety most passengers never think about until a departure board fills with cancellations. What follows covers the engine chemistry, the reason standard weather radar cannot see an ash cloud, and the global advisory network that tracks eruptions. It also looks at the volcanoes sitting under busy corridors, and why rerouting around ash is almost always cheaper than flying through it.

Illustration: Quartz

Volcanic ash damages a jet engine by melting inside it. Ash is pulverized rock and volcanic glass rather than soot, and silicate material of that kind softens and melts near 1,100 degrees Celsius — about 2,000 degrees Fahrenheit. The hot section of a large turbofan runs above that threshold, so ingested ash liquefies within seconds of entering the combustion chamber.

What follows is a chain reaction. The molten material travels rearward with the gas flow and strikes the turbine, where surfaces are cooler. It freezes there as a hard, glassy layer on the blades and on the nozzle guide vanes that shape the gas path. The deposit alters blade geometry, narrows the area the gas needs to pass through, and raises back pressure inside the engine. Compressor stall and flameout can follow within minutes.

A second failure mode is thermal. Turbine blades survive their environment only because cooling air bleeds through rows of tiny holes drilled across each blade surface, forming a protective film. Glassy deposits plug those holes. The film breaks down, metal temperatures climb past design limits, and blades can distort or crack even while the engine continues to run.

Abrasion accounts for the rest of the damage. Ash particles are angular and hard enough to erode compressor blade edges, strip paint and metal from wing leading edges, and frost a cockpit windshield into something closer to etched glass. The same particles can block pitot tubes and static ports, the small openings that feed airspeed and altitude data to flight instruments, leaving crews with unreliable readings. Fine material also enters bleed air lines and air conditioning packs, which is why crews flying through ash have reported a sulfurous or acrid smell in the cabin.

Low concentration is not the same as safe. A plume thin enough to pass for haze still carries abrasive silicate, and at night it may be invisible from the flight deck. The deposits do not burn off once an aircraft clears the cloud. Affected engines are inspected, cleaned, repaired or replaced on the ground.

Illustration: Quartz

British Airways Flight 9 became aviation's defining ash incident on June 24, 1982, when a Boeing $BA 747 lost power in all four engines over the Indian Ocean after crossing a plume from Indonesia's Mount Galunggung. Nothing in the cockpit warned the crew beforehand.

The aircraft, registered G-BDXH, was flying the Kuala Lumpur to Perth leg of a London–Auckland service at 37,000 feet after dark. Passengers and crew saw a blue-white glow flickering across the windshield and along the engine nacelles, consistent with static discharge inside a charged particle cloud. Smoke carrying a sulfurous smell drifted into the cabin, which crew first took for an electrical fire. The engines then failed one after another until the 747 was gliding.

Captain Eric Moody told the cabin: "Ladies and gentlemen, this is your captain speaking. We have a small problem. All four engines have stopped. We are doing our utmost to get them under control. I trust you are not in too much distress." The crew turned back toward Jakarta and descended for roughly 16 minutes without thrust, dropping from cruise altitude to below 14,000 feet before the engines relit one by one underneath the ash layer. One surged again on approach and was shut down.

The landing brought its own difficulty. The windshield had been scoured close to opaque, and the pilots flew the approach into Jakarta's Halim Perdanakusuma airport through a narrow strip of unabraded glass, with the landing lights clouded as well. No one was hurt.

Investigators traced the cause to an ash cloud that left no radar signature and offered no usable visual cue at night. On December 15, 1989, KLM Flight 867 flew into ash from Alaska's Mount Redoubt while descending toward Anchorage and lost all four engines the same way, recovering after a powerless drop of several thousand feet. Two matching outcomes on opposite sides of the Pacific established ash as a standing hazard rather than a freak event.

Illustration: Quartz

Airborne weather radar is built to detect water — rain droplets, hail, ice crystals — and dry mineral particles reflect almost none of its signal. A cockpit radar display can show clear skies while the aircraft flies toward an ash concentration dense enough to damage engines.

Radar works by sending out pulses and measuring what bounces back, and the strength of that return depends on particle size and water content. Volcanic ash that drifts long distances downwind is fine-grained, typically well below the size of a raindrop, and it carries no moisture to reflect the beam. The return is too weak for the system to read as a hazard, so the screen stays empty.

Visual confirmation fails for similar reasons. A dispersed ash cloud can look like ordinary haze, and at night or inside cloud it is effectively invisible. Crews on both the 1982 and 1989 all-engine-failure flights had no useful warning from either their eyes or their instruments.

Detection therefore moved off the aircraft and onto satellites. Weather satellites can separate ash from water cloud by comparing how the two absorb and emit infrared radiation at different wavelengths, which lets analysts map a plume's extent from orbit. Ground-based volcano observatories, seismic networks and pilot reports fill in what satellites miss, particularly at low altitudes or under thick overcast.

Infrared ash detectors designed to mount on the aircraft itself have been developed and flight-tested, with the aim of giving crews a forward-looking warning in real time. They are not standard equipment across commercial fleets, so pilots still depend mainly on information passed to them before departure and over the radio en route.

That gap matters most where nobody is watching from the ground. Eruptions in thinly populated regions such as the Aleutians or Russia's Kamchatka Peninsula can push ash into busy trans-Pacific corridors hours before anyone on the surface confirms what happened, leaving satellite imagery as the only early signal.

Illustration: Quartz

Nine Volcanic Ash Advisory Centers, or VAACs, split the world into assigned regions and issue the official bulletins airlines and air traffic controllers use to plan around eruptions. The network operates under the International Civil Aviation Organization as part of a program called the International Airways Volcano Watch.

The centers sit in Anchorage, Washington, D.C., Montreal, London, Toulouse, Tokyo, Darwin, Wellington and Buenos Aires, and each is run by the national weather agency of its host country. The U.K. Met Office runs London, Météo-France runs Toulouse, the Japan Meteorological Agency runs Tokyo, and Australia's Bureau of Meteorology runs Darwin. Their assigned areas are drawn to cover the entire globe without gaps, including stretches of ocean where no one on the ground is watching a volcano erupt.

Once an eruption is detected or reported, the responsible center issues a Volcanic Ash Advisory. The bulletin states where the ash is, which altitude bands it occupies, and where the plume is forecast to sit at set intervals over the hours ahead. Each advisory comes with a graphic outlining the ash area at those forecast times. Air navigation service providers turn that material into NOTAMs and SIGMETs, the formal notices that reach dispatchers and cockpit crews.

Before this structure existed, no agreed global method told a flight crew that pulverized rock was drifting across their route. Information moved through scattered pilot reports and local arrangements, part of why the crews in the 1982 and 1989 engine-loss incidents got no warning at all.

The network does not remove the hazard. A VAAC advisory is a forecast built on partial data about how much material a volcano is throwing out and where wind will carry it. What the system delivers is one authoritative source that every carrier crossing a region works from at the same time.

Illustration: Quartz

The 2010 eruption of Iceland's Eyjafjallajökull grounded roughly 100,000 flights over about a week and produced the largest shutdown of European air travel since World War II. The scale of the disruption came less from the eruption itself than from the rulebook in force at the time, which treated any detectable ash as grounds for closing airspace.

That zero-tolerance standard dated to the engine failures of 1982 and 1989, when no one could say how much ash a turbine could survive. The safe answer was none. It held up for decades because eruptions near busy corridors tended to be short-lived or far from traffic. Eyjafjallajökull was neither. Meltwater from the ice cap above the vent shattered the magma into unusually fine particles. Prevailing winds then carried the plume across one of the world's busiest air traffic regions, where it spread thin and lingered instead of clearing.

Applied to a diffuse cloud sitting over most of a continent, a no-detectable-ash rule closed nearly everything. Airlines, engine manufacturers and national regulators spent the following days arguing over whether concentrations at the edges of the plume posed any real threat. No agreed threshold existed to settle the argument.

European authorities replaced the binary rule within weeks. The revised framework sorted forecast airspace into concentration bands. A low-contamination zone stayed open to normal operations. An intermediate zone let carriers fly under a safety case agreed with their regulator and engine maker. A high-contamination zone was treated as closed. Responsibility moved partly onto operators, who had to document their own risk assessment rather than wait for blanket clearance.

The science underneath those bands is still incomplete. Engine tolerance varies with particle size, exposure time and the specific powerplant, and the dispersion forecasts that assign a band carry their own margin of error. What 2010 changed was the structure of the decision, not certainty about where the safe line sits.

Illustration: Quartz

Airlines route around ash advisory zones even when forecast concentrations are low because ash damage builds up across flights and often does not announce itself in the cockpit. A reroute carries a known, bounded cost. Ash exposure carries an open-ended one.

The incidents that made volcanic ash famous were abrupt. Most exposure is not. An engine that crosses a thin, diffuse plume can finish the flight at normal thrust and show nothing unusual on the crew's displays. The wear turns up later, during a borescope inspection or a scheduled teardown, after the aircraft has flown the route several more times.

That gap between exposure and evidence drives the conservative posture. A dispatcher weighing a longer track around a forecast plume is comparing extra fuel burn and crew hours against an unscheduled engine removal. For a large turbofan, a shop visit runs into the millions of dollars and can keep the aircraft out of service for weeks. Maintenance slots and aircraft rotations are planned well in advance, and an engine pulled early disrupts far more than a single flight.

Operators have three practical responses when an advisory sits across a planned route. They can fly a longer lateral track that stays clear of the forecast area. They can request a cruising altitude above or below the layer where the advisory places the ash. Or they can delay the departure, or cancel it, until the plume disperses or the wind carries it off the corridor.

Regulation and liability push the same way. Tiered airspace rules can permit operations in low-concentration zones under defined conditions, but the burden of justifying that decision sits with the carrier, not the regulator. An airline that enters an advisory area and later files an engine damage report answers to its safety regulator, its insurers and its engine manufacturer. Staying clear is the easier position to defend, which is why it remains the industry default.

Illustration: Quartz

An eruption near a flight path triggers work on the ground first. Airline dispatchers pull the latest volcanic ash advisory, check it against NOTAMs for the region, and run the planned route through company flight-planning software to see whether the aircraft would cross forecast ash at any point in its timeline.

Timing matters as much as geography. A plume sitting over a corridor at 6 a.m. may have drifted hundreds of miles east by the time a long-haul flight reaches it eight hours later, so dispatchers plan against the forecast track rather than the current position. Altitude is the second variable. Advisories specify flight levels, and a route unusable at 35,000 feet may be clear at 28,000 — though cruising lower burns more fuel, which can force a payload cut or a fuel stop. When no acceptable profile exists, the choices narrow to a longer lateral routing around the advisory area, a delayed departure until the next forecast cycle, or cancellation.

Crews already airborne get updates through air traffic control and company datalink messages from dispatch. If ash is confirmed or suspected ahead, standard practice is to leave the area rather than continue. Manufacturer and ICAO guidance for an inadvertent encounter is consistent: reduce thrust toward idle to lower the temperature in the engine core, turn back along the path already flown, descend if terrain allows, and switch on engine ignition and anti-ice systems. Crews are told not to climb through a plume, since the cloud can extend well above cruise altitude.

Larger carriers supplement official advisories with in-house meteorology desks or contracted weather providers that turn a regional bulletin into route-specific guidance. Operators with heavy Pacific, polar or Southeast Asian networks tend to run more developed procedures, since their routes cross active volcanic zones regularly.

For a closely watched volcano with established observatory coverage, the move from eruption to a signed-off alternate flight plan happens quickly. Eruptions in thinly monitored terrain leave dispatchers working with coarser data and wider margins.

Illustration: Quartz

Ash forecasts come from atmospheric dispersion models. They begin with an estimate of how much ash a volcano is putting into the air, and at what height, then carry that material through the same wind fields used for ordinary weather prediction. The London Volcanic Ash Advisory Centre, run by the U.K. Met Office, uses a dispersion model called NAME for this work.

The hardest part is the starting figure. Forecasters need what the field calls eruption source parameters: plume height, mass eruption rate, particle size distribution and how long the eruption lasts. Plume height can be estimated from satellite imagery, from weather radar near the volcano, or from ground observers watching the column. Mass eruption rate is usually inferred from plume height rather than measured directly. An error at the source then travels through every hour of forecast that follows.

Altitude matters as much as volume. Winds at 15,000 feet often blow in a different direction from winds at 35,000 feet. One eruption can send ash down two divergent paths at once. Models output concentration fields in separate flight levels rather than one flat map, and advisories describe the cloud in altitude bands for the same reason. Flight planners need that vertical detail, since a route can sometimes be flown above or below a drifting layer.

Satellites help correct a model while it runs. Infrared channels can distinguish ash from ordinary water cloud. Sulfur dioxide, which is easier to detect from orbit, serves as a marker for where volcanic material has traveled, though ash and sulfur dioxide can drift apart over long distances. That makes the signal a guide rather than a direct measurement of ash concentration.

Uncertainty concentrates at the edges of a plume, where concentrations thin out toward the thresholds that decide whether airspace stays open. Measurements recovered from the Eyjafjallajökull plume have been used since 2010 to test these models. Research aircraft sampling and ground-based lidar readings gave forecasters a cloud documented closely enough to check model output against.

Illustration: Quartz

Four regions — Indonesia, Alaska, Japan and Iceland — generate most ash-related rerouting, because frequent eruptions there sit under heavily used air corridors. A volcano in an empty stretch of ocean matters far less to airlines than a moderate one beneath a route carrying hundreds of flights a day.

Indonesia lies along the Pacific Ring of Fire and holds more than 100 active volcanoes. Eruptions at Mount Merapi on Java and Mount Raung in East Java have repeatedly forced Indonesian airports to close, and ash from Mount Agung on Bali shut Ngurah Rai International Airport several times in late 2017 and 2018, stranding tens of thousands of travelers. Routes linking Australia to Southeast Asia and Europe funnel through this airspace, so one Javanese eruption can ripple through an entire network.

Alaska's volcanoes line the Aleutian arc, directly beneath the great-circle tracks connecting North America with East Asia, including much of the trans-Pacific cargo traffic. Redoubt, Augustine and Pavlof have all erupted in recent decades. Many of these peaks rise in terrain with few people living nearby, which leaves fewer eyes on the ground to report a plume in its first minutes.

Japan manages near-constant low-level activity at Sakurajima, on the southern island of Kyushu. The volcano vents ash often enough that Kagoshima Airport treats falling ash as a routine operational matter rather than a crisis. Japanese airspace carries some of the busiest domestic and international traffic in Asia, so even modest plumes demand coordination between controllers and carriers.

Iceland matters out of proportion to its size because the North Atlantic tracks between Europe and North America run close by. Katla, beneath the Mýrdalsjökull ice cap, and Grímsvötn, which erupted in 2011 and briefly disrupted flights across northern Europe, stay under close watch. Eruptions under ice fragment magma into finer particles that travel farther on the wind, which is part of why Icelandic ash reaches so much of the continent.