Volcanic Ash Invisible to Weather Radar presents serious danger to Aircraft 

There have been 90 incidents over the last 30 years in which aircraft have encountered volcanic ash, invisible to weather radar and totally undetectable at night, and several of the encounters created very dangerous situations.

Made up of pulverized rock and glass, the ash gets into virtually every aircraft system and can paralyze engines in minutes. The ash turns molten in a jet engine's combustion chamber and melds with moving parts like spray paint. It blasts cockpit windows, compromising visibility in seconds, and also clogs vital speed sensors, rendering an aircraft's flight computers almost useless. The deposits additionally coat the fuel system's temperature sensors, creating a false and lower temperature reading that causes the automatic monitoring system to pour in more fuel. This makes the engine dangerously hot, damaging the turbine and potentially leading to a shutdown.

The two most famous encounters with volcanic ash involved 747s, one a British Airways flight over Indonesia in 1982 en route to Perth from Kuala Lumpur and another involving a 1989 KLM flight en route from Amsterdam to Anchorage. In both cases, all of the engines failed and eventually were restarted at much lower altitudes but later were scrapped because of the damage. The KLM 747's environmental control system was replaced, its fuel tanks were cleaned and the hydraulic systems needed repair.

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