In the past 20 years, there have been 80 recorded encounters between aircraft and volcanic clouds, causing the near-loss of two Boeing 747s with almost 500 people on board and damage to 20 other planes, with repair costs totalling hundreds of millions of dollars, say experts.

"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," the journal Natural Hazards warned last year.

"The increase in passenger air traffic, growing at five percent 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 a 1982 episode now known as the "Jakarta Incident."

At 11,000 metres (35,750 feet), a British Airways jumbo en route from London to Auckland entered a cloud of ash disgorged by an Indonesian volcano, Mount Galunggang.

All four engines flamed out, leaving the plane and its terrified passengers on a glide towards doom before the crew were able to restart the engines at 4,100 metres (13,325 feet).

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 to opacity.

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, 177 kilometers (110 miles) away, which had erupted 10 hours earlier.

After the engines were restarted, the plane landed safely at Anchorage. Repairs cost 80 million dollars, for all four engines needed replacing.

"The threat to aviation is very obvious," Kjetil Toerseth, director of regional and global pollution at the Norwegian Institute for Air Research, told AFP.

"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 known 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 Toerseth.

In the biggest eruption of recent decades, Mount Pinatubo in the Philippines spewed out an ash cloud on June 15, 1991 that in less than three days travelled more than 8,000 kilometres (5,000 miles) to the east coast of Africa, the US Geological Survey (USGS) says.

"This ash cloud damaged more than 20 aircraft, most of which were flying more than 600 miles (1,000 kms) 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.

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