 More than 27,000 flights worldwide were delayed and 2,800 were canceled by mid-morning Friday. United and American asked the FAA overnight to alert their pilots in the air that they were experiencing communication troubles. Several U.S. airlines, including American Delta and United, said they were starting to resume flights on Friday morning, but some cautioned many travelers would experience disruptions Friday into Saturday. Delta initially paused flights because the outage had affected back-end systems that ensure planes are safely dispatched.