A U. S. watchdog is investigating the world’s largest airline United Continental Holdings Inc. (UAL) after it halted flights on tarmacs during Chicago storms, a recent report has uncovered.

On July 13, Chicago’s O’Hare International Airport was hit by varied thunderstorms, as told by the Chicago-based airline. The same had caused shut of the ramp into UAL’s gates, the report finds. Besides, 14 flights being operated by affiliated regional carriers were stranded for more than three hours.

It has been found that two United mainline flights were also in the same line. The investigating agency says that the delays were longer than allowed under a 2010 rule by the U. S. Transportation Department.

The agency adds that a total of 580 flights were affected in one day alone in August as well due to a network failure. And it was confirmed by the airline that incorrect passenger counts had contributed to the same.

Thus, the airline is being urged to repair computer and operations malfunctions so that aircraft preventive-maintenance systems and passenger-service could be improved.

“We have taken aggressive action to improve, including adding spare aircraft and more ground time between flights, and making changes to give employees and customers a more reliable product”, said a spokeswoman for United.