For Illinois, you can only travel by air for essential purposes, same as any other mode of transport:

https://www.chicagotribune.com/coron...6b4-story.html

Can I travel?

Pritzker’s executive order allows “essential travel,” which includes travel into or out of the state if necessary for maintaining essential business operations. Travel to work for what’s considered an essential business or governmental function; to care for an elderly person, minors and dependents, someone with disabilities or other vulnerable people; travel required by law enforcement or court order; travel to return to one’s residence, including out of state; and travel to a school or other educational center to obtain distance learning materials or meals are considered essential travel.

People who continue to use public transportation “must comply with social distancing requirements to the greatest extent feasible.” The order prohibits nonessential travel by a range of means, though, including by car, motorcycle, bicycle, train and plane.