How does an ash cloud stop jet engines?

[MFgeo]

There are 3 major ways in which volcanic ash particles damage jet engines:

1. Many of the ash particles are volcanic glass that melts at temperatures around 600-800 degrees Celsius, well below the combustion temperatures in modern jet engines. The melted material cools rapidly while traversing the turbine chamber, and end up deposited on the turbine blades, disrupting gas flow through the high-pressure turbine stages. This, by itself can lead to engine stoppage. This behavior of volcanic ash is distinctly different than conventional sand particles, which are mostly quartz and melt at around 1600 degrees Celsius.

2. Turbine blades, especially in the first stage would melt if not for a continuous flow of cooling air through an array of tiny holes distributed along the blade. The ash particles are small enough to enter the plenums and plug these holes, which can cause blade failures, or leave the blades operational but with greatly reduced service life, requiring expensive inspection and replacement activities well before normal service intervals.

3. The ash particles have extremely sharp edges, with hardness values similar to, in some cases higher than, the metals in the compressor and turbine blades. These particles passing through the engine cause rapid wear on the leading edges of blades, shortening service life, and in extreme cases disrupting air flow sufficiently to stop the engine. The ash particles are much more abrasive than conventional sand because most sand particles are highly rounded by the very geologic processes that produce sand.

In addition, the ash cloud may contain substantial amounts of gasses such as CO2 and/or SO2 which may reduce the O2 levels to below those needed for the engine to sustain combustion. However, the gasses disperse to sufficiently low concentrations so as not to interfere with combustion much closer to the source than do the ash particles.