How It Works

Why is volcanic ash a safety risk to planes?


Volcanic ash consists of exceptionally small bits of pulverised rock and glass, no bigger than two millimetres in diameter, created when volcanoes erupt. The formation of volcanic ash can occur in three different ways: gas release under decompression causing magmatic (magma flow) eruptions, thermal contraction caused from quick cooling occurring on contact with water (this causes phreatomagmatic eruptions), and the ejection of entrained particles during steam-based eruption, leading to phreatic eruptions (highly explosive). In any case, the violent nature of the eruptions causes many tiny particles of rock, clay and sand to be projected in an ash plume from the volcano, as solid rock and magma is separated under the dynamic and explosive activity.

Volcanic ash is spread from its initial plume – which consists of a mixture of steam and ash particles – by weather systems, with ash often (as with the recent case in Iceland, Britain and Europe) being carried and sometimes deposited over hundreds of miles. If the ash is distributed in great quantity over one area then a powdery dust-layer is formed, a process referred to as ashfall. Unlike the ash typically formed when combustible materials such as wood are burned, this ashfall is hard and abrasive, and does not dissolve in water. Due to this, inhaling such ash is a severe health risk to humans and animals, with side effects including a liquid-cement substance forming in the lungs.

This image, taken yesterday afternoon by NASA's Terra satellite and published by Dundee University's Satellite Receiving Station, shows the pink-coloured ash of Iceland's Grimsvotn volcano (top-left) driting towards the UK.

Importantly and topically, volcanic ash also affects aircraft in a variety of ways. Firstly, the ash has a sandblasting effect on any aircraft, with the billions of particles colliding with its fuselage and damaging its landing lights and main body.

Secondly, ash can clog many of the aircraft’s sensors, such as the pitot tubes (pressure measurement) and, because the ash’s particles are charged, communication radios. Thirdly, and potentially with the worst consequences, ash can cause combustion power failure, leaving the plane with no engine thrust and minimal chance of landing without a catastrophic crash.

Power Loss

Volcanic ash is a very fine powder that consists of tiny particles of rock and glass. These particles are normally harmless to humans, however when jet engines are involved, they can prove a real danger to the safety of any plane. When the ash is sucked into the jet engine – a constant and large supply of air is needed for the engine to operate properly – its tiny particles are compressed and heated up within the combustion chambers, causing them to melt under the immense internal temperature and form molten glass on the turbine blades, jamming them and causing the plane to lose power.

What happened last year?

In April 2010, when Iceland’s Eyjafjallajokull volcano erupted, air traffic all but ceased over Europe as controllers worried about the effects of volcanic ash on the engines of a plane.


This image shows relatively
normal airspace activity
for Europe (data for the
south of France is absent),
with hundreds of flights
departing and arriving
over a 24-hour period.


However, this image taken on
17 April – during the heart of
the ash crisis – shows how few
flights were in operation, with
only a couple of aircraft managing
to progress as usual.

Present danger

Thousands of travellers across northern England and Scotland are facing flight cancellations as the ash from Iceland’s Grimsvotn volcano makes its way towards the UK. While flight controllers are better prepared for such a situation after last year’s events, currently the grounding of planes is still deemed necessary rather than attempting to fly through or around the ash and potentially encountering problems.

The image below of the eruption on May 21, taken by NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) on its Terra satellite, shows the 12 mile (20 km) high ash cloud spread across Iceland, forcing its largest airport – Keflavik – to close. The ash particles of Grimsvotn are larger than those of Eyjafjallajokull and, for this reason, it is hoped they will more readily settle out of the atmosphere and cause less disruption to air travel. The eruption is Grimsvotn’s biggest since 1902.