How Do Airlines Operate When There Is No ATC On Duty?

Air traffic controllers are integral to airline operations and instrument flying. Without civilian ATC, pilots would not be allowed to file and fly IFR flight plans, which would preclude flight through clouds, limited visibility, and operations above 18,000 feet. Though airline operations rely on controllers for every flight, there are times when airline pilots operate without direct ATC oversight. Here's an explanation of when and how it happens.