In 2010 and 2011 plumes of ash from Icelandic volcaneos caused flight cancellations across Europe. The ash is damaging and a safety issue for high flying jet aircraft.
It raises the question: why if the dust is so fine is it a problem to aircraft? There are two principal reasons: it is very abrasive and it melts.
Dust is Very Fine but the Particles are Highly Abrasive
Although the dust is very fine, which is why it is able to stay in the air, it is sharp edged and very abrasive. Large quantities hitting the windscreen at the nearly 600mph flying speed of modern airliners will act like sand paper and cover the screen with fine scratches making it effectively opaque.
Captain Moody flying a British Airways Boeing 747 (Speedbird 9) in June 1982 inadvertently flew into a volcanic ash cloud over the Indian Ocean and lost all four engines. It also made the windscreen so opaque that they had to land blind using instruments only. They were not able to see sufficiently to even taxi off the runway.
Melts like Glass in the Engines
As the dust is very fine it might be assumed that it would just pass through the engines without causing any harm. As already suggested the ash particles are highly abrasive and that alone would be damaging to modern jet engines which are built to very tight tolerances.
The particles are of volcanic rock and similar to glass. Modern engines operate at very high temperatures, up to 2,000 Celsius which is hot enough to melt the particles. They then form a thick, viscous liquid which clogs the engine and stops it burning fuel and creating power – it “flames-out”.
As Captain Moody and others pilots discovered it has usually been possible to restart the engines by losing height rapidly; the resultant cooling causes the molten ash to solidify and break off sufficiently to allow engine restart.
Although engines might be restarted after ingesting volcanic ash they would certainly be damaged. As gas turbines spin at very high-speed they are very precisely balanced to avoid vibration, according to Rolls-Royce the turbine blade tip may be travelling at 1,000mph. Not all the solidified rock would be expelled from the engine and any remaining on fan and turbine blades would cause vibration. When running engines will throw off lumps of solid ash which will hit turbine blades and cause further damage. It is therefore urgent for any aircraft that has flown through an ash cloud to land and have the engines fully checked and repaired.
Could Aircraft Avoid the Problems of Volcanic Ash?
Theoretically if engines operated at lower temperatures the melting problem could be reduced. Unfortunately modern gas turbine engines rely on very high temperatures to achieve their remarkable efficiency. Lower operating temperatures would increase fuel use, cost and carbon emissions.
The abrasion problem cannot be avoided and due to the high precision of gas turbine engines that abrasion would cause damage through vibration and wear on moving parts. That would be enough to put the engine and aircraft at risk.
In some circumstances it may be possible to fly below the ash cloud. That too reduces aircraft efficiency and cause increased fuel consumption. In any case there will be ash falling from the cloud or being rained out. It might not be as acutely damaging but it would require increased maintenance to ensure engines had not been damaged.
Therefore the authorities’ decision to stop flying is sound and goes to show that despite mankind’s technological abilities nature is still in charge.
A Final Thought
It has been suggested that if flights had continued in such busy airspace there may have been long term flight cancellations because of a potential lack of spares and replacement engines.
First appeared on Suite101