In Britain, we are very lucky to have our spot on the globe. We
have no active volcanoes and any earthquakes are absolute tiddlers
and hardly ever cause any structural damage. Our weather is also
very gentle as we very rarely get extremes of temperature.
Minus 10C (50F) is considered to be very cold here in the UK,
but it would be a mild day in Moscow. If temperatures reach just
over 30C (86F), Brits start running for the shade whilst people in
North Africa would be reaching for their jumpers.
This explains why almost any kind of natural event that impacts
on us here in the UK tends to be blown a bit out of proportion.
This because we are used to things being quite mild here in the
UK.
Travelling aerosols
Iceland is the closest highly volcanic region to Britain and it
was this that caused chaos in the skies over Britain this spring.
In fact, the Icelandic volcanoes are a lot closer than most people
think and do give us a pummeling at pretty regular intervals,
geologically speaking that is. They are only about 1609Km (1000
miles) away. This is about the same distance as the Costa Brava in
Spain, about two hours by jet!
We also regularly get material from the Sahara blown up as far
as Britain. If you've ever found a thin layer of dust on the car in
the morning, then you have just seen a bit of the Sahara
Desert.
Aerosols can travel many thousands of miles. An aerosol in this
case is not something you squirt under your arms to make you smell
less like a buffalo, but any kind of fine particles that can be
carried high into the atmosphere. Volcanoes can be really good at
producing aerosols. Eyjafjallajökull (a mouthful for a volcano eh!)
was really good at it because it started to erupt under a
glacier.
Did we experience an Icelandic aerosol?
All volcanoes produce ash. Pompeii was buried under tens of
metres of volcanic ash, but it was made of large, heavy particles
that fell out of the air after travelling only a few miles from the
volcano.
To make really fine ash that travels hundreds miles you need
water, and Eyjafjallajökull has loads of it! As the volcano came to
life under a glacier the heat started to melt the ice and boil the
water. The steam gets hotter and hotter until the pressure gets so
great that it blasts its way out of the glacier. Very
spectacular!
Also, it's potentially deadly. As the melted glacier fell down
the mountain, it turned into a river as big as the Amazon.
Thankfully, no towns were in the way or they would have been
instantly destroyed.
The ash formed was incredibly fine, the same size as flour
grains. The lava that was blasted to pieces is made mostly of
Silica. This is the same stuff that sand and window glass is made
of. So the dust that settled over Britain in the weeks that
followed the eruption was really very fine glass particles!
Why is this so dangerous to aircrafts?
Actually it's only dangerous for a jet aircraft. Old fashioned
piston engine aircraft can fly quite happily. Just like a car
engine, they have air filters that clean out any potentially
damaging particles before they can pass into the engine. A jet
sucks in so much air that you could never filter the particles out.
In fact jet engines are incredibly robust. They are one of the most
reliable machines ever made. An airline pilot that flies every day
for twenty years may never experience a single engine failure.
Amazing! The safest form of travel really is the aeroplane.
So why all the panic?
Jet engines can cope with quite staggering amounts of rubbish
being pushed into them. A large jet can swallow several birds the
size of chickens, chew them up and spit them out the back without
incident. So why should a little bit of ash cause problems?
The reason is that the ash is made of these fine glass particles
and glass has a very low melting point. You can soften a glass rod
by sticking it into the hottest part of Bunsen burner flame. The
temperature inside a jet engine is a lot hotter than that, over
1500C (2732F). This is well above the melting point of some glass.
What happens is that the volcanic ash is drawn into the engine,
melts and then sticks to the turbine blades. This can cause the
engines to shut down.
Speedbird 9
This is what happened to a British Airways flight over Indonesia
in 1982. It was flying at 37,000 feet, about 6 miles up, when it
flew through an ash cloud produced by the Galunggung volcano on the
Island of Java. Nothing appeared on the planes radar as the ash
particles were too fine to reflect any radio waves back.
The first time the crew knew that something strange was going on
was when they noticed a terrific display of 'St.Emlo's' fire on the
outside of the plane. This is a form of static electricity and it
isn't unusual to see this at high altitudes.
But this time it was because the aircraft was flying through a
cloud of very fine ash particles that were charging up due to the
friction with the aircraft.
On that day though, there was worse to come. Smoke started to
appear though the ventilation system and there was a strong smell
of sulphur. A few minutes later the number 4 engine shut down.
Within a minute or so the other three engines did the same.
Speedbird 9 was the biggest glider the world had ever seen.
Luckily, the captain calculated that from 37,000ft, the aircraft
could glide for about 23 minutes and cover 146Km (91 miles). There
were two clear options: clear the high Indonesian mountains and
make it to an airfield, or a sea landing. But a sea landing was
something that had never been attempted in a 747 before.
As the cabin pressure failed, it was clear that the aircraft
would have to descend rather quicker to keep the 248 passengers
breathing. All this time they had been trying to restart the
engines. At about the 15th attempt one of the engines restarted
followed shortly by the other three. Speedbird 9 landed safely
sometime later. This is why aviation people tend to be a little
touchy about volcanic ash clouds.
Was it a stroke of luck?
Remember that it is a coating of molten glass that causes jet
engines to fail. As the plane descended to lower altitudes, masses
of cold air rushing into the engines cooled the glass and caused it
to solidify. As it froze solid it contracted and fractured,
shattering into thousands of tiny pieces. This unlocked their grip
on the engines and they could fire up again. It is an amazing story
and one that should not need to be repeated!