Flight ban lifts following volcanic ash scare

Aeronautical engineer Dr Ranjan Vepa comments on the lifting of flight restrictions following Iceland's volcanic eruption and Europe-wide ash cloud.

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Dr Vepa, who is Lecturer in Avionics in the School of Engineering and Materials Science at Queen Mary, University of London, said today;

"To decide on whether or not to lift the ban, one would need definite evidence of reduction in the presence of volcanic ash in the relevant atmosphere, below certain critical levels. The decision to lift the ban should be made entirely based on technical considerations of the aircraft engines' ability to handle ash intake.

"It is better to play safe until such data is definitely available. The point is that at this stage no one can be absolutely sure about:

i) the safe levels of ash that can be allowed into an aircraft engine; this has never been formally established although some initial studies have now been conducted.

ii) the current concentrations of ash (worst case scenario) in the relevant parts of UK airspace.

"A whole range of essential aircraft systems are vulnerable when the airflow is contaminated with volcanic ash including several air data sensors and communication systems. Probably the most significant effect of the presence of volcanic ash would be to aircraft jet engines.

"The presence of volcanic ash increases the mass of the air flowing into the engine. This leads the systems controlling the engine to adjust their intake and compression - possibly causing the compressor to surge or the engine to stall.

"In jet engines, surge can lead to the so called flame-out of the combustor which could reverse the flow or cause chaotic vibrations with catastrophic consequences.

"The presence of volcanic ash also influences the material properties of the engine blades which could lead to blade loss followed by compressor failure. Moreover the presence of a high quantity of particles and dust in the flow can clog ports in the compressor and engine system. If these particles were to enter the hydraulic circuits, they would lead to a total failure of the aircraft's hydraulic systems.

"In future it would be possible, in principle, to filter the air entering the jet engines. However this naturally leads to loss of efficiency, due to the need to expend energy to drive the filter (possibly a centrifuge) and this will eventually result in an increase in operating costs which will be quite significant."