The Calbuco volcano in Chile spews as high into the sky in 2015. (GIAN REPETTOGIAN REPETTO/AFP/Getty Images)

More than 3,000 passengers on Alaska Airlines were grounded on Monday after the Pavlof volcano erupted and launched ash 37,000 feet into the air. “We simply won’t fly where ash is present,” said John Ladner, Alaska Airlines director of operations, in a press release.

The decision to ground flights around eruptions is a simple one. Volcanic ash is a huge risk for jet engine airplanes.

In at least three cases, volcanic debris stopped all four engines of Boeing 747s, temporarily turning the airliners into the world’s largest gliders. Fortunately in all of these cases the pilots managed to restart at least some of the engines and make safe landings.

Obsidian, otherwise known as "volcanic glass," is formed when a body of melted rock (magma) deep under the Earth's surface is forced to the surface, where it either oozes out as a lava flow, or explodes out. (USGS) Obsidian, otherwise known as “volcanic glass,” is formed when a body of melted rock (magma) deep under the Earth’s surface is forced to the surface, where it either oozes out as a lava flow, or explodes out. (USGS)

Volcanic ash isn’t anything like the soft ashes that are left after you put out a campfire. Instead, it consists of various kinds of rock that the eruption has broken into millions of tiny pieces, many of which harder than steel and have sharp edges.

When the ash hits airplane windshields, it makes them too opaque to see through, which can be considered a minor problem compared with what volcanic ash does to jet engines.

Once inside an engine, volcanic ash melts on the hot components, and then solidifies as a layer of dirty glass, which can cause engines to flame out.

The good news for pilots and their passengers is that this stuff becomes very brittle when it cools after stopping an engine.

In the three cases of the Boeing 747 gliders, the pilots’ repeated attempts to re-start the engines broke off enough of the ash glass to allow at least some of the engines to restart, and the 747s to safely land.

On the night of June 24, 1982, British Airways Flight 9 was cruising at 37,000 feet above the Indian Ocean when it flew into an ash cloud from the Mount Galunggung volcano in Indonesia. When all four engines failed, the jet glided over 20,000 feet down to 13,500 feet above the ocean before the crew were able to restart the engines and land in Jakarta, Indonesia.

Three weeks later, a Singapore Airlines 747 flew into ash from the same volcano and lost power from all four engines. Again, the pilots managed to restart them in time to make a safe landing.

On December 15, 1989 a KLM Boeing 747 was descending to land in Anchorage, Alaska. It was around 27,000 feet when it flew into volcanic ash from the Redoubt Volcano, approximately 150 miles away. All four engines failed with the airplane above the Talkeetna Mountains, which are as high as 11,000 feet. The airplane glided down to a frightening 13,300 feet before the engines restarted and the jet landed in Anchorage.

So why don’t pilots just fly around volcanic clouds?

Sometimes a high-flying pilot might see pyrocumulus clouds erupting from a volcano below and avoid it.  But, many times, the ash-filled cloud merges with ordinary clouds, or spreads out, with no signs it contains ash.  Since the ash particles are so small they fall very slowly and can mix into clouds far from the volcano without changing the cloud’s appearance.

The particles are so small that radar, both on the ground and aboard airplanes, does not detect them.

NOAA’s Volcanic Advisory Centers at the Center for Weather and Climate Prediction in College Park, and in Anchorage, Alaska, along with seven similar centers in other nations, keep a close watch on earth’s volcanoes to issue alerts to pilots and airline dispatchers as well as the military.

While scientists are working on systems that airplanes could use to detect volcanic ash, nothing is being used operationally at this point. “We are aware of the development of volcanic ash avoidance systems like AVOID (Airborne Volcanic Object Identifier and Detector) is being used on an experimental basis by EasyJet, together with its partners Airbus and Nicarnica Aviation,” said Raul Romero in an email, a meteorological technical officer at the International Civil Aviation Organization in Montreal.

Interestingly, President Obama is no stranger to flight delays due to volcanic ash. In April 2010, the eruption of Mount Eyjafjallajokull prevented President Obama and other national leaders from flying to Poland for the funeral of Polish President Lech Kaczynski, his wife, and other Polish officials who had died in a plane crash.

The President’s trip to Indonesia in November 2010 was originally in question, and then ended up cut short so Air Force One could take off early and avoid ash from Indonesia’s Mount Merapi.

And in May 2011 the President cut short a visit to Ireland to fly to London for an highly publicized state visit before ash another Icelandic volcano, Grimsvotn, grounded Air Force One.

Video shows Alaska's Pavlof Volcano emitting a cloud of ash 20,000 feet into the air after an eruption on March 27. (Federal Aviation Administration)