(Reuters) - A cloud of volcanic ash drifting eastward from Iceland has halted flights across northern Europe, causing widespread disruption.
Below are some questions and answers on the aviation risk.
— What is volcanic ash?
Plumes of dust spewed out by volcanoes usually contain tiny particles of glass, pulverized rock and silicates. The result is a cloud of material resembling sandpaper.
— Why is it a hazard to aviation?
The problem is not visibility but the abrasive affect of the ash, which can both strip vital surfaces and bung up an engine.
Pulverized rock colliding at high speed with a speeding jetliner can blast away surfaces inside the engine.
Due to intense heat in the motor, the particles can fuse together when they penetrate the engine and stop it working.
Aircraft avionics and electronics can also be damaged.
Aviation authorities say clouds of ash are often accompanied by clouds of gas such as sulphuric acid and hydrochloric acid.
— How long will the problem last?
The answer depends on the weather. Prevailing winds have so far spread the cloud eastwards from Iceland toward British and Scandinavian airspace. Britain’s meteorological office says ash may be present over the United Kingdom on Thursday and Friday. British airspace is expected to be closed until at least Thursday evening.
— What happens when volcanic ash is detected?
A 120 nautical mile danger zone is declared around the original plume and flights are banned in contaminated areas as winds move the ash across flight paths. The scale of Thursday’s response is “unprecedented,” a British spokesman told Sky TV.
— What happens if an aircraft does fly into volcanic ash?
* On June 24, 1982, the captain of a British Airways jumbo jet en route from Kuala Lumpur to Perth, Australia, came on the speaker system at around 37,000 feet and calmly told the 247 passengers on board that all four of its engines had failed.
In an incident that went down in aviation history, Captain Eric Moody glided the jet down more than 20,000 feet and managed to restart one engine at 13,000 feet followed by others, according to the Flight Safety Foundation (aviation-safety.net).
It was only later that investigators found the combination of engine failure, an eerie luminous glow around the plane and acrid smoke inside the cabin had been caused by flying into a cloud of volcanic ash from the eruption of Mount Galunggung.
The aircraft landed safely on three engines but the incident prompted new flight procedures and international exercises. * On December 15, 1989, all four engines failed again when a KLM jumbo jet flew into a cloud that turned out to be volcanic ash while en route from Amsterdam to Anchorage, Alaska. The engines resumed working and the damaged plane landed safely.
That incident was blamed on lack of sufficient information provided to the crew, the Flight Safety Foundation says.
— How do controllers plan for such an event?
Partly as a result of these incidents, the International Civil Aviation Organization, a United Nations body, maintains detailed contingency plans that were activated on Thursday.
The incident came just six weeks after European authorities carried out the first of two 2010 exercises for just such an event, aimed at preventing a catastrophe due to volcanic dust.
Reporting by Tim Hepher, Editing by Lin Noueihed