The US will investigate “unacceptable” cancellations by Southwest Airlines

Severe winter weather has caused thousands of flights to be cancelled across the US over the last several days. The majority of those were operated by a single airline: Southwest.

According to tracking service FlightAwareout of 4,006 canceled flights into, within, or out of the US on Dec. 26, some 2,909 of them were operated by Southwest—that’s more than 70 percent. Southwest was putting up similarly strong numbers on Dec. 27, at the time of publishing.

The rash of cancellations has prompted an inquiry by federal regulators. The Department of Transportation tweeted on Dec. 27 that it is “concerned by Southwest’s unacceptable rate of cancellations” and “will examine whether cancellations were controllable and if Southwest is complying with its customer service plan.”

In a statementthe airline blamed “consecutive days of extreme winter weather across our network” that left staff and airplanes stranded. Bob Jordan, Southwest’s CEO, told the Wall Street Journal on Dec. 26 that “this is the largest scale event that I’ve ever seen.”

With so many last-minute changes, the airlines’ staff scheduling system couldn’t keep up to find the right people in the right places at the right times, Jordan said. Over the next few days, he added, the airline will continue to operate a reduced schedule with the aim of getting its people and planes back to where they need to be.

Scheduling issues aren’t new at Southwest. Staff attrition and burnout have been building at the airline for several years. And Southwest, unlike most major US airlines, doesn’t concentrate its operations around a few big hub airports. Instead, it typically keeps its aircraft and staff widely distributed, which can be efficient when the weather is good—but cause the network to quickly collapse when it’s not.

Editor’s note: This article was updated at 5pm US eastern time with the latest FlightAware data.