ICAO 9974 PDF

Volcanic ash consists mostly of sharp-edged, hard glass particles and pulverized rock. It is very abrasive and, being largely composed of siliceous materials, has a melting temperature below the operating temperature of modern turbine engines at cruise thrust. A volcanic ash cloud may be accompanied by gaseous solutions of sulphur dioxide (which when combined with water create sulphuric acid), chlorine (which when combined with water create hydrochloric acid) and other chemicals which are corrosive to the airframe and are hazardous to health. Given these facts, it is self evident that volcanic ash in the atmosphere may pose a serious hazard to aircraft in flight. Thus, aircraft should avoid volcanic ash encounters.

Volcanic ash forecasts serve to levy the probability, when performing a safety risk assessment, of the hazard of an aircraft encountering volcanic ash. The risk may be mitigated using effective in-flight procedures. This document establishes guidelines which States may recommend to aircraft operators and regulatory authorities to adopt in order to assess the safety risk of flight operations in areas forecast to be affected by volcanic ash or aerodromes contaminated with volcanic ash.