Ash damage to turbine engines and other airplane systems can be very subtle. The hot sections of modern jet engines have very tight operating tolerances and operate very near the temperature limits of the materials involved. Minor loss in cooling efficiency or excess wear could lead to at least a loss of efficiency and increases the possibly of several different engine failure scenarios.

For Commercial Air Transport flights (Airlines) the general rule is to keep the likelihood of a catastrophic failure (one leading to the loss of the airplane) to less than one chance in 1,000,000,000 hours. This focus on safety leads to a very conservative approach to managing the airspace when things like volcanic ash are floating around. Loss of an engine, or even significantly reduced performance from one or more engines is a significant flight hazard.

- Eric