Twice in less than a month, a major airline was paralyzed by a computer outage that prevented passengers from checking in and flights from taking off.

Last month, it took Southwest days to recover from a breakdown it blamed on a faulty router.

On Monday, it was Delta's turn, as a power outage crippled the airline's information technology systems and forced it to cancel or delay hundreds of flights. Delta employees had to write out boarding passes by hand.

Why do these kinds of meltdowns keep happening?

The answer is that airlines depend on huge, overlapping and complex IT systems to do just about everything, from operating flights to handling ticketing, boarding, websites and mobile-phone apps. And after years of rapid consolidation in the airline business, these computer systems may be a hodgepodge of parts of varying ages and from different merger partners.

These systems are also being worked harder, with new fees and options for passengers, and more transactions — Delta's traffic has nearly doubled in the past decade.

"These old legacy systems are operating much larger airlines that are being accessed in many, many more ways," said Daniel Baker, CEO of tracking service FlightAware.com. "It has really been taxing."

The result: IT failures that can inconvenience tens of thousands of passengers and create long-lasting ill will.

Associated Press