Domestic flights in the US were given the go-ahead to restart at around 1400 UK time, after a nationwide grounding caused by what the Federal Aviation Administration (FAA) described as an IT problem.
"Normal air traffic operations are resuming gradually across the US following an overnight outage to the Notice to Air Missions system that provides safety info to flight crews," the FAA said.
The update followed a near five-hour grounding after the agency ordered airlines to pause all domestic departures to enable it to "validate the integrity of flight and safety information".
During the grounding, the FAA said it would provide "frequent updates" as it sought to restore the IT system, some functions of which it said had come back on line before the restoring of permission to fly.
During the outage, United Airlines said it would issue an update to the public when it "learns more" from the FAA. American Airlines and Southwest Airlines said separately they were "closely monitoring" the situation.
According to aviation analytics business Cirium, more than 21,000 flights were scheduled to take off in the US on January 11.