Why can't planes fly through volcanic ash?

Dozens of airports around the southern hemisphere have been closed and flights cancelled because of the volcanic ash spewed from Chile. Cars and buses are driving around The Southern hemisphere, so why can't jets fly through volcanic ash clouds?

Commercial jets fly through clouds all the time. Why can't they fly in volcanic ash clouds?

The short answer is that volcanic ash is made up of tiny pieces of glass that can shut down a jet engine. A jet engine sucks in huge amounts of air that mixes with the fuel, ignites, and releases energy.

When these bits of volcanic glass get drawn into a gas turbine jet engine, they melt and fuse to parts of the engine. The melting point of volcanic ash is about 1,100 Celsius. But a jet engine operates at temperatures about 300 degrees hotter. The bits of glass tend to melt onto the fuel nozzles and turbine blades, rather than simply passing through the engine. The result: the jet engine (or engines) may quit.

The volcanic ash tends to be concentrated at the high altitudes where commercial airliners fly. But near the ground, it's dispersed, and doesn't have the same effect on cars, trains, or ship engines.

Boeing, one of the world's largest manufacturers of commercial aircraft, says there have been about 90 incidents of aircraft being damaged by volcanic ash over the past three decades. One of the worst was the case of British Airways Flight 9 from London to Auckland, New Zealand. On June 24, 1982, the Boeing 747-200 flew through volcanic ash spewing from Mount Galunggung in Indonesia. All four jet engines quit within one minute. The plane dropped from 36,000 feet to 13,000 feet before the crew was able to restart each of the engines.

Pilots have found that jet engines can often be restarted once the aircraft drops to a lower altitude, as the glass on the engine parts hardens, shatters, and falls away.

In 1982, British Airways and Singapore Airways jumbo jets lost all their engines when they flew into an ash cloud over Indonesia. Reports said that the ash sandblasted the windscreen and clogged the engines, which only restarted when enough of the molten ash solidified and broke off.

A KLM flight had a similar experience in 1989 over Alaska. Stewart John, a fellow of the Royal Academy of Engineering and former president of the Royal Aeronautical Society, explained that the ash can cause severe damage. "This dust really is nasty stuff," he told BBC News. "It's extremely fine and if it gets into a jet engine, it blocks up all of the ventilation holes that bleed in cooling air.

"Jet engines operate at about 2,000C, and the metals can't take that. The engine will just shut down."

In the case of the 1982 British Airways flight, Mr John explained, when the plane emerged from the cloud, the pilot repeatedly tried and failed to restart the engines. "They were going down and down, and had just about accepted that they would have to ditch. But, at the last minute, one engine started. By repeatedly turning the engine over and having a clean airflow going through, he managed to blow the ash out."

Dr Rothery explained that as a result of those incidents, emergency procedure manuals for pilots were changed. "Previously, when engines began to fail the standard practice had been to increase power. This just makes the ash problem worse," he said. "Nowadays, a pilot will throttle back and lose height so as to drop below the ash cloud as soon as possible. The inrush of cold, clean air is usually enough to shatter the glass and unclog the engines. Even so, the forward windows may have become so badly abraded by ash that they are useless, and the plane has to land on instruments."

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