A federal computer system that processes pilots' flight plans went down for two hours Friday morning, causing a domino effect of problems and hours of delays for travelers.

A spokesman for the Federal Aviation Administration called Friday the worst travel day of the year so far.

But you wouldn't have to tell that to any of the thousands of people spending hours waiting in an airport or strapped into a seat, sitting on the tarmac waiting to take off.

An Atlanta computer system that processes pilots' flight plans and sends them to air traffic controllers for domestic and international flights failed at 6:57 Friday morning, according to Tammy Jones, an FAA spokesman. A mirrored back-up site in Salt Lake City, Utah immediately took over the Atlanta systems' operations, just as planned.

However, veering from plan, the Salt Lake system couldn't take the full weight of the load and began to stagger under the strain. Jones said in an interview with InformationWeek that real-time flight information that should have been moving quickly through the system began to back up and lag.

"It was moving processing very slowly," said Jones. "The air traffic controllers had to process information manually."

That's a lot of hand-written plans. Jones calculated that the system generally handles 50,000 to 55,000 domestic flights every day. That number doesn't include international flights coming in or out of the United States. The computer backup quickly translated into a flight backup. Without the necessary flight plans in order, planes simply couldn't take off.

So the lag continued until the Atlanta system was brought online at 10:52 a.m.

However, New York's airports -- giants La Guardia and John F. Kennedy -- didn't come back online so easily. Jones said the New York airports struggled to regain their connection to the Atlanta processing system, and they didn't get back online until 12:48. That left air traffic controllers at two major U.S. airports trying to move flights in and out with hand-written schedules.

By 5:30 in the afternoon Friday, La Guardia was still experiencing an average of a three-hour flight delay, according to an advisory on the FAA site. JFK flights were delayed for two hours and 42 minutes, while Newark International Airport in neighboring New Jersey was backed up for two and a half hours.

The FAA did not yet have information on how many flights were delayed or how many passengers were affected. The cause of the computer glitch is under investigation.

Jones, however, was quick to point out that the downed computer in Atlanta wasn't the only cause of Friday's travel trouble. A line of harsh thunderstorms stretched from Canada down through Texas for much of Friday, further delaying flights across the mid-section of the country.

"As far as delays, it's a pretty bad day," said Jones. "We can't distinguish at this time what percentage of the delays were caused by the computer system and how many were caused by the weather."