No-one knows how much ash is needed before an engine is damaged or it catastrophically fails. Aircraft radars cannot detect ash clouds and therefore there is a danger that the aircraft will fly through a dense section of cloud. Ash ingested by an engine will become molten in the turbofan's combustor, which operates at up to 2,500°C (4,400°F) and the molten ash will then have an impact on engine blades.
Engine blades have holes to allow cooling air to pass through them. This is necessary as blades normally operate above their melting temperature. But the heat transferred away from the blade by the cooling air enables the blade to survive this harsh environment. When the cooling holes are blocked by molten ash the blades will rapidly heat up and fail. Blade failure will lead to engine failure.
Canfield University's professor of gas turbine engineering Riti Singh says that the particles do not have to be large, but if there are a lot of them that can lead to there being enough molten ash to cause blades to fail in the manner described above.
He adds that there might be particle-free routes to fly through to reach an airport, but only aircraft fitted with laser sensors can detect the cloud's location.
© Finnish air force
The US Geological Survey's (USGS) volcanic hazards programme has studied the potential impact of ash on aircraft. In 2002 the USGS wrote that "an ash cloud eventually dissipates in the atmosphere, and ash concentrations drop. However, the threshold concentration at which ash poses no harm to aircraft is not known, and indeed, may never fully be characterised for all situations involving aircraft. It is usually assumed that ash identifiable on satellite images continues to present a hazard to aircraft. Accordingly, the consensus of the aviation community is that if an ash cloud can be discerned, it should be avoided."
Engine manufacturer Rolls-Royce has made no statement on the issue of volcanic ash.