Last updated: 10:17 AM ET, Wed September 21 2016

Why Can’t Jets Fly Through Volcanic Ash?

Airlines & Airports Paul Thompson September 21, 2016

Why Can’t Jets Fly Through Volcanic Ash?

Photo courtesy of NASA

Volcanoes have the potential to erupt at nearly any given time, in many places around the world. Their ash clouds, towering for miles into the stratosphere, seem relatively innocuous to most people. However volcanic ash is so much more than a “cloud,” and it can be catastrophic to commercial airliners. Let’s have a look at what makes them so dangerous.

Just this week, a volcano in Costa Rica called Turrialba spewed ash into the sky, forcing the closure of the country’s primary commercial airport in San Jose on Monday and Tuesday. A more memorable eruption in recent memory for many is the 2010 eruption of  Eyjafjallajökull in Iceland. That eruption stifled air traffic between Europe and North America for several days.

What is it about volcanic ash that is so dangerous for planes? If you’ve ever felt volcanic ash, it feels soft, much like baby powder. But when a plane flies through it at cruising speed — roughly 600 miles per hour — the ash begins to sandblast the aircraft. As it hits the cockpit windows, it etches into the glass, reducing visibility. As you might imagine, pilots like to be able to see where they’re flying.

READ MORE: Volcanic Eruption Causing Flight Delays, Cancellations in Costa Rica

Many modern planes give pilots the ability to fly with little to no visibility to of the cockpit windows, so visibility isn’t even the scariest part of a volcanic ash cloud. When a plane’s engines ingest volcanic ash, it begins to clog the engines. But like sand, it also melts into glass at high temperatures. Jet turbine engines operate at extremely high temperatures in order to combust fuel during the propulsion process. In a notable British Airways 747 incident over Indonesia in 1982, this is exactly what happened.

When British Airways Flight 9, also known as “Speedbird 9” flew through the ash, the pilots at first noticed when they deemed to be St Elmo’s Fire on the cockpit windscreen. In addition, the cabin began to smell like smoke, though at the time, cigarette smoking was still allowed on flights, so people didn’t think too much of it. The smoke smell eventually took on a sulphuric odor. It was only two minutes before the first of four Rolls-Royce engines shut down. Within the next minute or so, the other three engines had also shut down.

READ MORE: 5 Active Alaskan Volcanoes

The ingested ash had become molten, and the glass actually adhered to the inside of the engines. At that point, they were also beginning to lose cabin pressure, so the pilots were forced to descend toward the ocean, although they had no hope of landing on even a remote island at that point.

The saving grace for the flight was the fact that as the glass cooled, (because the engines were inoperable) it began to break off from the smooth interior surfaces of the engines. Finally, enough of it broke off, and as the pilots began planning to ditch the plane in the ocean, they were able to restart the engines — fourteen minutes after they had failed. After finally landing the plane safely in Jakarta with minimal visibility, the pilots ended up winning awards from the Queen of England for Valuable Service in the Air. For a long time afterward, flight 9 held the record for the longest glide in a non-purpose-built aircraft.

Two other flights also encountered issues from this same ash cloud during its eruption — one by Singapore Airlines, and another by Indonesian airline Garuda. Events such as these continue to serve as lessons-learned, improving aviation safety.