It's best just to stay way, way back. Because no two eruptions are the same, no one can say how far the eruption will reach or where it will go. Stratovolcanoes like Mount St. Helens are symmetrical, steep-sided peaks that erupt in explosive bursts, rather than seep lava like the shield volcanoes that formed Hawaii. The difference is in the silica content of the molten rock fueling the volcanoes. Magma in stratovolcanoes is viscous due to its high silica content.
Gases build in the thick molten rock, escaping with a blast.
In contrast, magma in shield volcanoes has less silica, allowing the easy escape of gas and quieter eruptions. Ash causes engines to stall by clogging air filters and damaging moving parts. In airplanes, ash sucked into an engine will melt and make a glassy coating on turbines, causing them to quit. Wet ash also conducts electricity, causing short circuits and power outages following eruptions. Clouds of ash can disrupt radio communication by scattering or absorbing signals.