There are many reasons this is a very bad idea.
A number of flights traveling to and from Bali's Denpasar Airport have been canceled due to volcanic activity from the nearby Mount Rinjani. ABC News Australia reports that travelers should check their trip status if they plan to fly in or out of the travel hub.
Most travelers know storms, tornados, and other inclement weather can cause problems for air travel, but many probably haven't considered the disruption an active volcano can have on flight schedules.
In Iceland, the volcanic landscape has wreaked havoc on travel in the past: The eruption of Eyjafjallajökull in 2010 caused closures of airspace over Europe and delayed thousands of flights.
Active volcanoes can throw ash miles into the air, and though volcanic ash may seem relatively harmless from the ground, when aircrafts collide with it at hundreds of miles per hour it's an entirely different story.
Severely Decreased Visibility
Ash is heavy enough to make it incredibly hard to see out of a cockpit window.
This is an expected condition of flying near an active volcano, but pilots are trained to fly in situations with little to no visibility with help from radars. Even so, flying into a giant column of ash is not something pilots do on purpose.
Consider this the second-worst thing that can happen when you're flying through ash. According to Boeing, if caught at the right (that is, wrong) time, the ash can pile up in the engines and cause equipment failures. Boeing has documented a number of these situations.
Ash Can Melt Into Glass
Popular Science has broken down the nightmarish thing that occurs when you put volcanic ash in a hot situation: When ash is introduced to extremely high temperatures—much like the environment of an airplane engine—it melts into glass.
When this happens to an airplane engine, the inside of the pylon (the part that holds the engine on the wing) can become encased in glass. After this happens, the cabin can fill with a sulfuric-smelling smoke.
Travel Pulse recently shared the story of a British Airways plane stuck flying through a column of volcanic ash over Indonesia. That specific flight has since gone down in history for the longest glide by a non-purpose-built aircraft. How did they even manage that? The engines cooled enough—because they weren't inoperation—for the glass to crack and break apart, eventually falling out of the pylon.