Volcanic ash

Often invisible, fallout from volcanic eruptions poses risks to aircraft at any altitude. The ash can cause widespread flight ops disruption.

Satellite image of the ash plume following the 2008 eruption of Mt Chaitén in Chile. The plume moved across Argentina and out over the southern Atlantic Ocean.

Conversely, the vents of lava domes and composite volcanoes are subject to collapse due to high-viscosity lava that can create substantial plugs. This allows a great deal of pressure to build up before the plug can be forcibly ejected in a massive, explosive eruption.

When these volcanoes erupt, they tend to begin as a series of tremors that geologists can use to estimate the likelihood of an imminent eruption. This is often accompanied by a measured swelling of the volcano itself as the trapped pressure pushes from within. The eruption itself is normally quite sudden and catastrophic, and may last for several days.

While lava doesn't generally get tossed high into the air, explosive eruptions can instantly transport billions of tons of pulverized rock, dirt and tiny particles of rapidly cooled lava, collectively known as tephra, well into the stratosphere.

Simultaneously, debris, gases and lava may combine into a pyroclastic flow that can race downslope at over 100 mph, followed by tumultuous "lahars" or mud flows that can bury the surrounding landscape in several meters of volcanic concrete. It was this process that doomed and entombed the city of Pompeii in 79 AD.

For aviation, however, it is the lofting of those billions of tons of tephra into the sky that poses the danger to flight. In the case of Mt Pinatubo in 1991, the eruption began on June 13, with 2 large eruptions the following day, each sending ash clouds to around 20 km (65,600 ft MSL). But on June 15, the largest eruption in the series sent several billion tons of ash at least 25 km (82,000 ft) high.


Ash itself is a sharp, silica-based particle formed when the magma leaving the vent is blasted into small droplets (generally less than around 2 mm). These droplets solidify as they rise up in the atmosphere, and individually are light enough that they can be easily transported thousands of miles from their source by the prevailing winds.

Because of their high silica content, ash particles are highly abrasive and act as a sand blaster on any aircraft encountering an ash cloud. This can produce substantial damage to forward facing surfaces such as windscreens and fan blades.

When mixed with water, ash can create a very corrosive solution that can damage paint and metal aircraft skins. Given its tiny size, it also has the ability to clog pitot tubes and pressurization ports. Another issue with volcanic ash is that it can conduct a current. This can cause a disruption of communications as the ash strikes exposed antennae. Its conductivity also contributes to the presence of St Elmo's fire where it is flowing around an aircraft.

Eyjafjallajökull's ash plume as seen from the cockpit. An ash cloud can cover thousands of sq km and can produce substantial damage to an aircraft, including the loss of power from all engines.

Importantly, ash has a melting point of roughly 1100° C (2000° F) meaning that the temperatures it would encounter inside a jet engine's hot section – around 1400° C – are sufficient to reliquify the ash and allow it to coat and clog injectors and other essential engine components, resulting in surging and flame-outs. Historically, engines that have been shut down due to ash accretion have been able to be restarted.

This is because once an engine cools, the ash also cools, solidifying and contracting. In doing so, enough of the ash breaks free from the engine to permit a restart. However, if the restart is done while still in the ash cloud, the process may repeat itself as new ash is ingested.

When an ash cloud is visible, most pilots have the good sense to avoid it altogether. However, after its initial expulsion into the atmosphere, ash clouds quickly spread out in the wind, often to the point that they become invisible to the naked eye after traveling a few hundred miles. This is where the cloud becomes most dangerous to aviation. Since we normally try to fly from 1 place to another with as small a deviation as possible, we stay in the (seemingly) clear air away from the visible ash plume.

But even the low concentration of ash particles in the clear air downwind of an eruption can be enough to damage an aircraft. And, since the ash that went up through the atmosphere will eventually drift back down, pilots should expect ash at any altitude below the maximum altitude of the eruption.

Eruptions that send ash only a few km into the sky tend to have plumes that settle out after just a few hundred km. But larger eruptions will have downwind plumes that may make it halfway around the globe and spread hundreds of km wide. In addition, it can take weeks for ash to settle out of the troposphere.

It can take even longer if it has been injected into the stratosphere where it may enter the jet stream circulation as it falls, which may send it even further downwind before it settles out. In this way, major eruptions such as Tambora in 1815, Krakatoa in 1883, and Pinatubo in 1991 all helped to suppress air temperatures around the world for a year or so.

Where are the threats?

A global map of the earth's crust will reveal crustal boundaries where volcanic activity is likely. In particular, the so-called "ring of fire" is the rim of the Pacific Ocean basin, where a number of plates are subducting beneath each other. This ring is dotted with volcanoes that see frequent activity, including in Indonesia, New Zealand, Japan, and all of the Americas from Alaska to Chile. Other likely global hotspots include Iceland, Italy, the Caribbean, and southern Africa.

Tracking ash

Given the potentially catastrophic impact on aviation posed by volcanic ash, the international aviation community implemented a series of 9 Volcanic Ash Advisory Centers (VAACs) charged with monitoring and forecasting volcanic ash plumes and issuing advisories. These VAACs use satellite imagery and numerical dispersion models, such as one called Hysplit, to forecast the likely extent of an ash cloud.

When an eruption is imminent or volcanic ash cloud has been detected, the VAACs will issue a volcanic ash advisory that describes the area and altitudes likely to be affected. These advisories are issued as needed and passed along to local and national aviation and meteorology authorities who may decide to issue a volcanic ash SIGMET.

Like all SIGMETs, a volcanic ash SIGMET indicates that significantly hazardous conditions to flight are or are very likely to be occurring within the defined region. SIGMET areas should be avoided entirely while they are active.


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