In a report issued by U.S. Geological Survey, there were 94 confirmed ash-cloud encounters by aircraft between 1953 and 2009. 79 of those produced various degrees of engine or airframe damage. 26 encounters involved significant to very severe damage, and 9 caused engine shutdown during flight.
Two of the most well known incidents involved passenger jets flown by KLM and British Airways. On June 24, 1982, British Airways Flight 9 flying at 37,000 ft. (11,000m) from London to Auckland, New Zealand, with 248 passengers and a crew of 15, entered an ash cloud rising from the erupting Mt. Galunggung volcano in Indonesia. All 4 engines flamed out due to the silica in the volcanic ash melting inside the engines and coating everything with glass. The plane had dropped 23,500 ft. (4,200m) before the crew was able to restart 3 of the engines and make an emergency landing in Jakarta.
On December 15, 1989, KLM Flight 867 from Amsterdam to Tokyo flew through a thick ash cloud from Alaska’s Mt. Redoubt volcano as the 747 started its descent into Anchorage. All 4 engines failed, and the plane lost 14,000 ft. (4,400m) in altitude before the crew could restart the engines and make a safe landing. The ingested ash caused $80 million in damage to the aircraft, including replacement of all 4 engines. The expertise of the air crews in both cases averted what could have been disastrous crashes.
The aviation industry learned from those incidents and started grounding all flights when volcanic ash was present. That’s why most European and North Atlantic flights were cancelled between April 15 and April 20, 2010, when Iceland’s Mt. Eyjafjallajökull erupted, ejecting 250 million cubic meters (330 million cubic yards) of volcanic ash into the atmosphere. The ash cloud drifted west, covering the sky over the North Atlantic and most of Europe. Many thousands of passengers were stranded in European airports for up to 5 days.
Ash clouds are hard to distinguish from moisture clouds either visually or by radar. That’s why aircraft continue to wander into them, and why the United Nations has set up a network of Volcanic Ash Advisory Centers (VAAC). There are 9 centers located around the world, each covering a geographic region. When an eruption produces an ash cloud, the VAAC in that area uses a computer model to predict the path of the cloud at different flight levels and issues an international alert. VAACs are located in Alaska, Argentina, Australia, England, Canada, Japan, France, and Washington, DC. Fewer incidents have been reported since the centers have been in full operation.
On average, 15 major explosive volcanic eruptions powerful enough to eject tons of ash into the stratosphere occur each year. A sudden Mt. St. Helens or Mt. Pinatubo type of super explosion can eject massive amounts of ash into the stratosphere in minutes, creating unexpected hazardous conditions. Air crews must stay ready to act immediately on VAAC ash alerts, and take the necessary evasive action to keep their flights safe and uneventful.