In the eruption of 1980, some planes flew into the ash which was blown east, blanketing much of Eastern Washington.
There were no crashes, but there could have been. Nowadays, airlines and pilots can do a better job of avoiding the ash.
When European airlines had to virtually shut down because of an erupting volcano in Iceland a few weeks ago, it was because European governments shut down large swaths of air space, some covering entire countries.
But in the U.S., the FAA has a different approach. It doesn't close large chunks of air space, so airlines are able to find a way around it.
Volcanic ash is finely ground flying rock. You can see why it's a problem during the most recent eruption of Mount St. Helens in 2004.
The eruption of Mount Redoubt in the state of Alaska just last year at times blanketed the Anchorage airport. You can make a pretty good argument that Alaska Airlines has more experience dealing with ash than other carriers.
Their motto is simple.
"Avoid all ash, that way you know you're 100 percent safe," said Capt. Bob Graves, Alaska Airlines Operations Director.
Ash can clog engines, even melting like glass over fuel nozzles. It can sandblast windshields so pilots can't see out.
Graves was hired on as a flight engineer shortly before Mount St. Helens blew in 1980. It was a different time then.
"We didn't have all these tools to project when we were going to be able to fly, if we were going to be able to fly and where we had a safe track to fly the airplanes," said Graves.
Most of today's tools are accessed online, including volcano cameras and earthquake seismometers. Tools come from the National Weather Service, the Alaska Volcano Observatory and other sources.
"We want to stay 35 miles away from the cloud," said Graves.
The easiest to visualize is a computer model called Puff. Even though Mount St. Helens is not currently erupting, every 6 hours Puff predicts how the winds could carry ash, just in case something blows. Each color, representing a different altitude.
At the airline's operations and dispatch center near Sea-Tac Airport, the models are checked regularly, whether there's an eruption or not. And when Mount Redoubt erupted last year, both dispatchers and crews at Sea-Tac got very busy.
"This is all happening within a couple of minutes. We do a ground stop on all our airplanes that are destination Anchorage," said Jonathan Burton, Acting Director of Dispatch at Sea-Tac Airport.
To keep things safe, many flights had to be canceled. As the eruption changed, so did the airlines' schedule.
Planes that couldn't be flown out or put into hangars were wrapped in plastic. And when the ash cleared, planes had to be inspected by mechanics. Small jets were sent up with Alaska pilots on board to snap photos to put real eyes on the situation.
It's a long way from 1980.
"We can give pilots actual printouts of all these things," said Graves, "And we can give them a route. We know this route's going to be clear.
There are other rules. When the ash is bad, flights that do go are daylight only, so the ash can be seen. And a buffer of 35 miles is put in place.
And at night or in bad weather, it's really hard to see where the ash ends and the clouds begin, so flights get canceled.