By Richard Ingham, Agence France-Presse

In the past two decades, there have been 80 recorded encounters between aircrafts and volcanic clouds, according to experts.

These encounters have caused the near-loss of two Boeing 747s with almost 500 people on board as well as damage to 20 other planes, with repair costs totalling hundreds of millions of euro.

'Volcanic ash in the upper troposphere, where jet aircraft fly, can cause jet engine failure, damage to turbine blades and pitot static tubes with the possibility of the loss of the aircraft and lives,' warned journal Natural Hazards last year.

'The increase in passenger air traffic, growing at 5% per year globally, coupled with the inherent unpredictability of volcanic eruptions, make this relatively new volcanic hazard a significant threat to society.'

The threat first made the headlines in 1982 in an episode now known as the 'Jakarta Incident.'

At 11,000m (35,750'), a British Airways jumbo en route from London to Auckland entered a cloud of ash that had been propelled into the air from the Indonesian volcano, Mount Galunggang. All four engines flamed out, sending the plane and its passengers on a glide towards ground before the crew were able to restart the engines at 4,100m (13,325').

The plane made an emergency landing at Jakarta, with the crew flying on manual, following a roughly calculated glide slope. It was impossible to see out because the windscreen had been sandblasted.

In 1989, a KLM 747 bound for Anchorage, Alaska, endured a five-minute powerless descent with 231 passengers on board when it inadvertently entered a cloud of ash blown from Redoubt Volcano, 177km away, which had erupted ten hours earlier.

After the engines were restarted, the plane landed safely at Anchorage. All four engines had to be replaced and the repairs totalled $80m.

'The threat to aviation is very obvious,' said Kjetil Toerseth, Director of Regional and Global Pollution at the Norwegian Institute for Air Research.

'The dust can stop the engines but it can also damage the engines and you have to do a full maintenance check afterwards if you know you have been flying through an ash cloud. The cost means that the airlines do everything to avoid ash clouds.'

One of the most problematic areas is in the Pacific, home of the 'Ring of Fire' where volcanoes are most active, said Mr Toerseth.

In the biggest eruption of recent decades, Mount Pinatubo in the Philippines spewed out an ash cloud on 15 June 1991 which travelled more than 8,000km in less than three days to the east coast of Africa, according to the US Geological Survey.
'This ash cloud damaged more than 20 aircraft, most of which were flying more than 600 miles (1,000km) from the volcano', the agency notes.

The KLM incident led to a global effort to track volcanic clouds and notify aviation of the hazard. There are nine 'Volcanic Ash Advisory Centres' around the world, which are responsible for advising international aviation of the location and movement of clouds of volcanic ash.

There have been two international conferences on the issue since 1991, and there is active debate among scientists in peer-reviewed journals.