The Guardian's live blog tracks a major US aviation systems failure that grounded all flights for over four hours — the second nationwide ground stop since 9/11 — before the FAA lifted it, leaving chaos at airports and delays rippling worldwide. Sean Duffy acknowledged a technology failure, but no clear cause was known; FlightAware counted 8,553 delays and 6,816 cancellations, and the Guardian's Ben Doherty reported flights held on the tarmac as far away as Sydney.