WASHINGTON • Behind each of the four big players in the United States airline industry - American, United, Delta and Southwest - there is a tangled computer system pieced together after decades of mergers that married mismatched networks.
Just what went wrong with Delta Air Lines' system on Monday, which caused hundreds of cancellations and delays, is still being sorted out. Was it a power cut, as Delta said? Or was it more likely an internal computer glitch, as Georgia Power, the utility at Delta's Atlanta hub, said?
In either case, aviation and computer specialists said it should not have happened. Computer systems and their electric power sources should have foolproof backups, they added.
Tens of thousands of passengers were delayed as Delta cancelled 451 of its nearly 6,000 daily flights by mid-afternoon on Monday.
"These airlines have long histories - 20 years of cobbled-together systems," said Mr Rick Seaney, the creator of FareCompare.com, a travel website.
"It's difficult to integrate older technology. They have hundreds and hundreds of cobbled-together things that they have to track down to make sure everything is working correctly," he added.
Before Monday, the most recent evidence of airline computer troubles came last month, when Southwest Airlines needed several days to recover fully from a collapse in its systems.
Last year, a United Airlines computer glitch forced the cancellation of dozens of flights and delayed hundreds more.
Southwest absorbed a new and different computer system when it acquired AirTran six years ago. It is investing millions of dollars in the creation of a computer system to replace its ageing network.
Airlines rely on their computer systems to manage a number of operations, including obvious elements such as reservations and ticketing, and behind-the-scenes management of plane movement. They are also used for gate assignments, air-crew scheduling and even the displays on arrival and departure screens at many airports.
Delta's computer system breakdown came at the height of the summer travel season, and on a Monday when many business travellers were heading to airports to begin their work week.