How Some Flights Were Exempt From the Ground Stop

Thousands of passengers scrambled to update their travel plans on Saturday as air traffic slowly unsnarled after a nationwide ground stop on Friday, but a number of travelers had already arrived at their destinations on flights left unaffected by the order.

Dozens of international flights took to the skies even as airports across the United States coped with only the third nationwide ground stop issued by the Federal Aviation Administration this century. Certain flights were also permitted to depart from the West Coast to Hawaii during the stoppage.

Here’s why some air travel was allowed to proceed even as flight systems were hampered.

Some international flights were able to depart.

More than 8,200 flights were canceled nationwide on Friday — over 31 percent of those scheduled — according to Cirium, an aviation data firm.

But more than 60 flights bound for overseas left the United States during the stoppage, according to Flightradar24, an aviation tracking site.

There are 21 air traffic control facilities, 20 in the contiguous United States and one in Alaska, that oversee swaths of regions throughout the country, according to the F.A.A.

As some of those centers recovered, flights were “allowed to depart through coordination with the Air Traffic Control System Command Center,” the F.A.A. said in an advisory less than two hours after the ground stop was ordered.