Regional U.S. air traffic control centers communicate using a complex network that’s “one of the largest telecommunications systems in the country,” and disruptions like those seen on Friday with an hourslong, nationwide grounding of flights can reverberate widely, according to an aviation expert. Michael McCormick, a former airspace manager with the Federal Aviation Administration, said it could take days for airlines — many of which had to divert flights — to resume normal operations after officials lifted the grounding order. The order was lifted late Friday. Here’s a look at what happened, how regional air traffic control centers work, how controllers communicate with pilots and why a problem on the ground can affect flights thousands of miles away, among other issues: