Whilst there are a number of potential effects of volcanic ash encounters on aircraft, the ones that warrant serious attention are those affecting aircraft engines. These effects are attributable almost entirely to the dominant silica particles. Their extreme hardness means that they have an abrasive effect on any surface impacted at a significant relative speed and this includes the interior of aircraft engines. However, their more significant characteristic, as far as aircraft engines are concerned, is the fact that their melting point is well below the core temperatures which typically are sustained in high by-pass jet engines operating at above flight idle thrust. Ingested silicate ash melts in the hot section of the engine and then fuses onto the high pressure turbine blades and guide vanes. This drastically reduces the throat area and both static burner and compressor discharge pressures rapidly increase and cause engine surge. Transient and possibly terminal loss of thrust can occur in the most severe cases with a successful engine re-start only possible if clear air can be regained.
The ash loading at which this process affects normal engine operation is not established beyond the awareness that relatively high ash densities must exist. Whether this silica-melt risk remains at the much lower ash densities characteristic of downstream ash clouds is currently unclear.
What makes this matter of extreme importance is that since this sequence may interfere with the normal function of aircraft engines and perhaps cause them to run down completely, a similar effect can be anticipated on all the engines on an aircraft. This is therefore a serious safety hazard which invites preventive risk management strategies in line with other comparable aviation risks.
Abrasion damage to aircraft engines caused by ash impact, whilst not affecting their continued normal function, cannot be repaired and will permanently reduce their operating efficiency thereafter. However, this is an economic issue rather than a safety one and establishing what knowledge would better inform the management of that economic risk is not the primary focus of this article.