It was a malfunction with the F.A.A.'s automated system that collects flight plans that snarled air traffic nationwide, caused major delays and cancellations.
The problem started early Thursday morning, shortly after 5 o'clock eastern time. The glitch impacted mostly flight plan, but also traffic management.
During the outage, dispatchers had to send flight plans to controllers, who then had to manually enter them into computers. Airlines reported there was no danger to flights in the air.
The problem occurred in an Atlanta-based computer system. Flight plans are collected by the F.A.A. for traffic nationwide at two centers, one near Salt Lake City and the other, Atlanta.
Hartsfield-Jackson Atlanta International Airport, the world's busiest airport, was hit particularly hard by delays. It was the second time in 15 months that a glitch in the flight plan system caused delays.
In august 2008, a software malfunction delayed hundreds of flights around the country.
Low clouds along the East Coast compounded the problem, causing even more delays. Passengers are being asked to check the status of their flights online before heading to the airport.