A Federal Aviation Administration computer outage early Thursday morning caused nearly 2,000 more flight delays than occur on a typical day, according to data compiled by FlightAware.com and reviewed by ABC News.
The problem began shortly after 5 a.m. ET this morning when a single circuit board failed in an FAA computer center in Salt Lake City. That meant air traffic controllers around the nation were no longer receiving information about flights electronically.
The FAA immediately notified the Department of Homeland Security and U.S. military, which, since Sept. 11, 2001, is supposed to receive FAA radar images showing the location of every plane in the air.
But ABC News has learned the military was not getting some of that information during the computer outage and, as a precaution, began using an AWACS surveillance plane as an extra eye in the sky.
Controllers reached by ABC News said during the outage they had to rely on phones to receive flight information and manually enter data, such as aircraft call signs, requested altitudes and flight routes, into their computers. The result was a slowdown of takeoffs and landings at airports around the country.
"It's time-consuming and cumbersome," National Air Traffic Controllers Association vice president Victor Santore told ABC News of the process. "It was a step back years from the technology we use today, where the flight plans are all displayed on a computer screen."
The airlines' flight plans tell air traffic controllers the anticipated route and altitude of each flight after taking off.
Controllers say the computer systems were also not receiving weather information, as normally occurs.
"Anytime any piece of equipment that we use fails, it's a little bit unnerving," said Santore.
While flights continued to take off and land during the outage, they did so at a delayed pace, causing planes to back up on the ground and in the air. Passengers whose flights were cancelled were left scrambling for travel options.
Ripple Effects Highlight Problems With Current Air Traffic System
About 3,500 flights were delayed an average of 50 minutes or more today, by one calculation, and dozens more were cancelled. Only 1,390 flights were delayed an average of 35 minutes Wednesday.
Southwest Airlines -- the nation's largest domestic airline -- told ABC News the company delayed about 150 flights, while American Airlines set back nearly 300.
AirTran, which is based in Atlanta, cancelled 22 flights and delayed dozens more this morning. Delta Air Lines also experienced delays and cancellations at its Atlanta hub.
John F. Kennedy International Airport in New York City operated at 40 to 50 percent efficiency in part because of the glitch this morning. The FAA said some of the delays in the busy Northeast air traffic corridor were weather-related.
This was the second time in a little over a year that a computer glitch nearly brought the nation's airlines and airports to a halt. In August 2008, a similar outage caused massive flight delays across the country.
Thursday morning's glitch was fixed within four hours, but what concerns many air travel experts and members of Congress is the havoc it caused.
"Why did it take four hours to locate a seemingly small technical problem, and why did it have a system-wide effect?" asked Rep. Jerry Costello, D-Ill., member of the House Transportation Committee, in a statement. The committee is expected to ask the Department of Transportation to investigate today's incident.
The FAA says a contractor -- the Harris Corporation -- is responsible for maintaining the equipment which included the failed circuit board, and investigators will likely focus on the group's role in the nation's air traffic system.
Mark Raimondi, a Harris Corp. spokesman, told ABC News, "We're working with the FAA to evaluate the interruption in order to prevent similar outages in the future... Safety and security are the highest priority."
Experts say today's incident also represents a failure of redundancy in the system, which is supposed to prevent outages from being debilitating.
"I think a single point of failure is a concern," said Santore. "You need to ensure that as more and more technology takes over for the human beings, that there are fail-safes in place…. All these different computer systems need to be replaced. They need to be updated. They need to be modernized."
The FAA is developing plans to transition from a ground-based system of air traffic control to a satellite-based system of air traffic management. But that system is not expected to be implemented until 2018.
ABC News' Harvey Goldberg contributed to this report.