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Danger to Aircraft from Volcanic Eruption Clouds and Volcanic Ash
Ash Reduces Engine Performance and May Cause Engine Failure
Ash ingested by jet engines may lead to the immediate deterioration in engine performance and engine failure. The principal cause of engine failure is the deposition of ash in the hot sections of the engine. Glass from melting volcanic ash will coat fuel nozzles, the combustor, and turbine, which reduces the efficiency of fuel mixing and restricts air passing through the engine. This causes surging, flame out, and immediate loss of engine thrust.
Ash Abrades External Components of Airplanes
Volcanic ash is highly abrasive because it consists of hard, sharp rock fragments that easily scratch and erode plastic, glass, and metals. Any forward-facing surface of an airplane is likely to be damaged . . . Cockpit windows may become so abraded and scratched that pilots have extreme difficulty seeing the runway on which to land the plane.
Aero 09 - Volcanic Ash Avoidance
Reduce thrust to idle immediately. By reducing thrust, engines may suffer less buildup of molten debris on turbine blades and hot-section components.
Exit the ash cloud as quickly as possible.
(NSS! ed.) A 180-deg turn out of the ash cloud using a descending turn is the quickest exit strategy.