A major portion of the decades-old national air traffic control system used to manage thousands of commercial and general aviation takeoffs and landings every day in the United States has crashed multiple times under the 20-year strain of its 24/7 operations.

As a result, industry analysts and a number of former Federal Aviation Administration staff members believe there is heightened likelihood of a major air traffic stoppage, as was demonstrated twice in the last two weeks by the crash of the system head in Atlanta. They also believe there is a new, increasing vulnerability to terrorist cyber-attacks.

The Aug. 26 event in which a corrupt file of some sort entered the system and rendered it useless for about 90 minutes during a high-traffic period was not an isolated incident, as the FAA's chief administrator originally had told the media.

Hundreds of flights were delayed and thousands of passengers were thrown off schedule by the system crash, which lasted only 90 minutes but caused widespread havoc.

The NADIN (National Airspace Data Interchange Network) system, which processes an average of 1.5 million messages per day, has a history of technical issues, and resulting travel disruptions are not out of the ordinary, according to knowledgeable air industry sources.

The FAA originally had reported Aug. 27 that the breakdown of the automated system was the first of its kind. The crash apparently had baffled FAA officials, who then conducted a technical investigation to determine the cause.

The 90-minute system crash, which pretty much affected all the major airports in the nation, later was blamed on a single corrupt file-most likely a virus-that had entered the system and somehow torpedoed it into uselessness.

The second NADIN system in Salt Lake City, to its credit, continued normally in handling all the West Coast flight plans. But when Atlanta crashed, all the East Coast data switched over immediately to Salt Lake City, which could not handle the extra data traffic-even though it was designed to handle 125 percent of normal load in the event of an emergency.

Commercial aircraft of any type cannot take off with having filed a valid flight plan, one that includes destination, estimated flight speed, description of cargo, estimated altitude, weather conditions and a number of other data points.

So, for a part of the afternoon of Aug. 26, pilots at about 40 U.S. airports were forced to manually type their flight plan information into the system, causing long delays in takeoffs. Chicago's O'Hare International, one of the two or three busiest airports in the world, and nearby Midway Airport were among the most directly affected.

"We've just never seen it fail in this manner," Hank Krakowski, the chief operating officer for the FAA's Air Traffic Organization, said in his media remarks.

However, a look at the record shows it had indeed failed several times before, including only five days prior to the Aug. 26 crash.