Flight disruptions across much of Europe will continue well into Saturday, with a giant plume of ash sent into the sky by a volcano eruption in Iceland still drifting across the continent. Some 60 percent of flights in Europe have been grounded, and more than half of transatlantic flights have been canceled, according to the BBC.
It may sound like a once-in-a-lifetime phenomenon but, in fact, pilots come up against ash clouds all the time. After 34 years of flying experience and 18,000 flying hours, Robert Schapiro, an international airline captain, has had to deal with dozens of volcano eruptions while at the controls. In many cases, he says, it's possible just to fly over or around them. "It's not an unusual phenomenon at all for long-range pilots. If you fly in Alaska, it's pretty much a monthly routine," he says. In such hot spots, pilots are so comfortable navigating around the plumes that airports are often kept open during eruptions. Other routine trouble zones include the Philippines and Java—essentially anywhere that follows the Ring of Fire around the Pacific—as well as parts of central Africa. What's unusual, Schapiro says, is that the volcano in Iceland is occurring in an area that isn't generally prone to volcanic plumes but that is used to high levels of air traffic.
So what does a pilot do when up against a volcanic cloud? The first trick is to know it's there, which is tougher than it might seem. Volcanic plumes generally are not picked up on weather radar, so pilots have to rely on other tools to spot them, like a color-coded tracking system that assigns every known volcano a color based on its level of activity. Green is normal, yellow raises the threat, and red signifies an imminent eruption. From there, pilots check the wind direction to gauge where the plume will blow and make a call—either over or around—based on their altitude. "You definitely want a good 10,000 feet between you and the cloud, and even then you'd want to be cautious," says Schapiro. Planes generally fly at about 35,000 feet, so small eruptions don't pose a significant problem. Major eruptions, which can reach up to 60,000 feet, are more troublesome. The ash cloud from the Iceland eruption is hovering between 20,000 and 30,000 feet.
The worst-case scenario, of course, would be to fly right into a volcanic cloud. That generally happens at night, when it's tougher for pilots to tell the plume apart from other clouds. But eerie signs resulting from high levels of static electricity in volcanic material immediately indicate that something is amiss. First, an electrical phenomenon called St. Elmo's fire can cause green flashes to appear on the windows. The mouth of the plane's motor may start to glow, and pilots may notice the smell of sulfur. Radio transmissions become less clear, and the plane's windshield can become more opaque because the pulverized lava particles are made of silicates, like glass, that basically sand the front of the plane, including the windows. Those glasslike particles affect the motor, too. If they enter a jet engine, they melt just like glass melts, then solidify, coating the engine and raising its temperature—and potentially destroying it.
But even cruising through the middle of a cloud of electrified, glass-spewing volcanic ash doesn't automatically spell catastrophe. The good news, says Schapiro, is that all pilots are trained how to navigate their way out. "The technique for exiting is literally to get out the way you came in, because you don't know how big the thing is," says Schapiro. As pilots turn around, they try to cool the motors by bringing them back to idle, which means the plane descends at the same time. That actually kills two birds with one stone, since pilots have to descend to restart any motors that have blown out, anyway. Any jet engine exposed to volcanic damage is totaled beyond salvation. Still, in most cases, even if the engines are cooked, they are still capable of doing the one thing that matters at that point: getting you far enough to reach an airport.