Southwest Airlines (LUV) cancelled 250 flights at Midway International Airport in Chicago, U.S., on February 11, 2018, due to shortage of de-icing fluid. It was the third time in two months that de-icing problems complicated its Chicago operations. The carrier now says it has secured additional de-icing fluid vendors to curb the shortages.

Southwest cancelled nearly all flights at mid-day Sunday, February 11, 2018, when it ran out of de-icing fluid at Midway during snowy weather. As the biggest snowstorm so far this year hit the Midwest, temperatures in Chicago dropped to below 20 degrees Fahrenheit. The carrier had to shut down operations at the airport which resulted in 250 flight cancellations in and out of Chicago over the weekend.

The flights were canceled “due to winter operational issues,” Midway Airport tweeted that night. Southwest also tweeted saying “Due to having to de-ice many of our aircraft because of the weather, we are running low on deicing fluid. Safety will remain our top priority in this circumstance.”

The carrier said the vendor they use ran out of the de-icing fluid glycol and the “safest operational solution” was to cancel the rest of the day’s flights, NBC Chicago writes.

It appears that the carrier ran through most of a several day supply of glycol in one day, and could not access additional fluid because of technical issues with a pump on one tank, Brandy King, a spokesperson for the airline said in an interview for Bloomberg on February 12, 2018.

In a separate interview, Anthony Gregory, vice president of ground operations and provisioning, explained that heavy snows forced Southwest to use an exceeding amount of glycol to remove ice and snow from planes that parked at the airport overnight, exhausting the carrier’s supply.

To make matters worse, air seeped into pumps, preventing from accessing some of the remaining fluid in storage tanks. Unable to get more, and with additional icy weather forecasted, the airline had to cancel most of the day’s flights at Midway.

But is the issue really about the wintry weather or is it about Southwest’s supplies and inventory problem? Gregory told Bloomberg that what occurred in Chicago was because of “the supply challenges” the carrier has had with its vendor. Adding that the airline had to “secure additional vendors” to bring the sufficient supplies to Midway.

But it is not the first time that Southwest has run out of the de-icing fluid at this Chicago airport. After experiencing similar problems in December, 2017, the carrier hired more suppliers, but some of them have also had difficulties securing sufficient quantities of glycol. In addition, new regulations that enforce work limits on truck drivers led to shortage of available drivers to make deliveries, Gregory explained.

Southwest first encountered glycol supply and delivery problems on December 24, 2017. At the same time, a glycol pumping station was left inoperable, adding to shortages. Delivery and supply issues soon arose again, on December 28, 2017, forcing the airline to cancel about 90 flights at Midway, Bloomberg reports.

Depaul aviation expert Joe Schweiterman expressed his surprise that the airline did not act on the de-icing depletion sooner. “This is a major hub for Southwest, this sort of thing should have never happened,” he said. “Preemptive cancellations, notifying passengers that can change reservations none of that appeared to happen so we had just a meltdown situation”, Schweiterman commented for NBC Chicago.

Operations for Southwest were restored at the airport the following day as a supply of the de-icing fluid arrived early Monday, February 12, 2018. “We resumed operations first thing this [Monday] morning and anticipate being close to normal today,” King said at the time. Southwest re-booked passengers on Monday’s flights which are now back on time.

Chicago’s Midway, with about 250 daily departures, is Southwest’s largest airport by number of flights.