DALLAS (AP) -- American Airlines played catch-up Wednesday, resuming most flights and even adding a handful that weren't on the schedule to help passengers stranded by a massive technology failure that grounded the carrier's entire U.S. fleet a day earlier.
But some cancellations persisted, and delays were still common. About a third of American flights were late as of mid-afternoon.
American's CEO blamed Tuesday's breakdown on a software problem that knocked out computer systems needed for booking flights, tracking bags, loading and fueling planes and more.
"As you'd imagine, we do have redundancies in our system," Tom Horton, chief executive of parent company AMR Corp., said in an apology to passengers posted on YouTube. "But unfortunately in this case, we had a software issue that impacted both our primary and backup systems."
American and smaller-jet subsidiary American Eagle still canceled more than 300 flights by mid-afternoon, according to flight-tracking service FlightAware.com. American said some flights were affected by bad weather in Chicago.
But American's performance was a huge improvement over Tuesday, when the computer failure brought all departures to a halt. Flights already in the air were allowed to continue to their destinations, but planes on the ground could not take off.
American and American Eagle canceled nearly 1,000 flights and delayed another 1,100. Two-thirds of their scheduled flights were late or never got into the air.
On Wednesday, American added seven unscheduled flights to accommodate passengers stranded the day before in Chicago, Dallas and Los Angeles.
As marooned passengers resume their travels, questions lingered about the technology and whether American's systems will be adequate to avoid similar collapses after the company merges with US Airways to form the world's biggest airline.
AMR and US Airways hope to complete their merger by September and create a giant that would surpass current industry leader United in passenger miles. Airline mergers are always difficult, and one of the trickiest parts is combining technology systems.
When the systems of US Airways and America West Airlines were combined in 2007, chaos ensued. Hundreds of check-in kiosks didn't work, ticket agents were swamped, and flights delays lingered for days.
United's reservations system failed on several days last year after it converted to the one run by merger partner Continental Airlines. The CEO was forced to apologize.
Even if flights operate on time, there can be other hitches when systems are merged. At United, upgrades for elite members of the frequent-flier program weren't handled properly, and flights booked with miles weren't always ticketed correctly. Those mistakes were especially upsetting to the airline's most loyal customers. It's one thing for one flight to be late. For many veteran fliers, it's worse to constantly worry about upgrades.
Airline technology is notoriously complicated because so many pieces of the system rely on each other for information, said Emre Serpen, a consultant at InterVistas who works with airlines on technology issues.
Passenger information goes into the system and tells gate workers whether all the passengers have arrived. The same information is used to calculate the weight of the fully loaded plane. Flight dispatchers, who may be in a command center across the country, use the figures to order the right amount of fuel — too much fuel adds weight and reduces mileage.
Besides performing many functions, airline computer systems might be assembled or serviced by different vendors. For American, the complexity will be magnified when it combines reservations, scheduling, frequent-flier and other programs with those at US Airways.
"They have to merge all the data from US Airways over to the American system. There could be huge translation problems" if the two airlines' systems use different coding, said Bill Curtis, senior vice president of CAST, a business software-analysis firm.
"They'll probably have some hiccups," Curtis said. "They may be big ones, or they may be so small that the public never notices."
AMR did not immediately respond to a request to make its chief information officer, Maya Leibman, available for an interview. She has been American's top technology officer for a little more than a year, replacing an executive who resigned shortly after AMR filed for bankruptcy protection in November 2011.
The term "information technology" appeared only three times in AMR's latest annual report filed with the Securities and Exchange Commission, and the company has provided few details on how much it spends to improve and maintain its computer systems, or who does all the work.
AMR said in the annual report that it has "engaged an increasing number of third-party service providers" for many jobs, including information technology hardware and services. The report also said the company will spend at least $80 million this year and $70 million in later years on a single IT-support contract.
US Airways CEO Doug Parker will run the new company after the merger. He has said he would prefer to convert his company's computer systems to American's since American is larger. He has pointed to the technology breakdowns that occurred after the United-Continental and US Airways-America West mergers, when the larger airline adopted the smaller one's systems.
For a second straight day Wednesday, US Airways declined to comment on whether Parker would reconsider his plans considering American's nationwide outage.
Associated Press Airlines Writer Joshua Freed in Minneapolis contributed to this report.
David Koenig can be reached at http://www.twitter.com/airlinewriter .