A: Civil aviation has become increasingly aware of the dangers of flying through the microscopic fragments of rock and pumice that make up ash clouds. Jet engines are highly complex machines designed to function in environments free of debris and corrosive gases, and the effects of volcanic ash have severely endangered safety on some flights.

  

The volcanic plume normally travels at altitudes of between 20,000 feet and 55,000 feet. This coincides with the flight levels of almost all commercial jet flights. But flights by smaller propeller planes using visual flight rules are usually permitted because these take place at much lower altitudes.

 

The very fine but extremely abrasive particles present a hazard to the aircraft's airframe and powerplants. They easily scratch and erode paint, aluminum and glass. This damages the wings' leading edges, and has a sandblasting effect on cockpit windscreens and landing lights. Inside the engines, the particles stick to the engine's hot parts, forming a glasslike coating, and grind up turbines, bearings, and other moving parts, restricting air flow through the turbine. This may lead to the immediate loss of thrust and eventually engine failure.

 

The ash can block the pitot tubes and other sensors that supply vital information on speed and outside air pressure. It also can clog air filters, such as the ones through which air flows to the passenger cabin.

 

During the early 1980s, two Boeing 747s were severely damaged by the ash clouds spewing from Galunggung Volcano on Indonesia's Java island. One of these, a British Airways flight, lost power on all four engines. While the pilots managed to restart the engines at a lower altitude, the resulting glide still ranks as one of the longest ever performed by an aircraft not specifically designed as a glider.

 

Another 747 encountered similar problems while flying through the ash clouds over Mount Redbout, near Anchorage in Alaska. And at least 10 jumbo jets and 10 DC-10s suffered multiple engine failures in 1991 from ash from Mount Pinatubo in the Philippines.

 

This is the first time that a volcanic plume has covered an area of dense air traffic. About 20,000 flights take place in Europe each day, and the ash caused about 20 percent of these to be canceled. Usually, the eruptions occur in regions where there is limited airliner traffic, such as the "Ring of Fire" that extends across the Pacific rim from Indonesia to Chile.

 

One of the main dangers posed by ash is that it is not visible at night or in cloud, and that an aircraft's weather radar cannot pick it up. Radar works on the principle of detecting water droplets in clouds, but since the ash clouds are dry, they do not reflect radar signals.

 

Ash particles conduct electrical charges. Often, the first indication that the pilots have of volcanic ash is the appearance of St. Elmo's Fire — a harmless electrical phenomenon that creates a ball of light around the wingtips — created when the charged particles strike the aircraft.