Airline miscues — such as the computer glitches that grounded hundreds of thousands of passengers this summer — are now the biggest cause of flight delays in the U.S.

Late arrivals triggered by mechanical breakdowns, a lack of flight crews and other factors attributed to the airlines were the largest category of delay last year for just the second time, and by the widest margin, since the government began collecting such data in 2003.

It exceeded what had been traditionally the biggest causes: weather and hiccups in the Federal Aviation Administration's air-traffic system, according according to data provided by the airlines to the Department of Transportation's Bureau of Transportation Statistics. ...

... The system has gotten better at adapting to weather that slows flights, which includes thunderstorms, fog, unusual winds and snow. Flights held up by weather caused about half of all minutes of delay in the early 2000s. That fell to less than one-third the past two years, according to the data.

Prof Vikrant Vaze

Since 2008, most of the overall decline in tardy arrivals has been due to a reduction in the number of flights after bankruptcies and mergers, the introduction of new air-traffic technology and improvements in how the FAA's controllers have adapted to thunderstorms in the summer months, according to John Hansman, a Massachusetts Institute of Technology aeronautics professor who has studied the issue.

It's less clear why the delays attributed to airlines haven't fallen with other categories, said Hansman and Vikrant Vaze, an assistant engineering professor at Dartmouth who has also researched the subject. Airlines aren't required to provide the government with underlying reasons they caused flights to be late.

"'Air carrier delays' -- that category has been an enigma to me," Vaze said.