Southwest Airlines flights resume following the lifting of a brief nationwide stoppage in Chicago
A Southwest Airlines plane sits idle on the tarmac after Southwest Airlines flights resumed following the lifting of a brief nationwide stoppage caused by an internal technical issue, according to the U.S. Federal Aviation Authority (FAA), at Chicago Midway International Airport in Chicago, Illinois, U. Reuters

Southwest Airlines Co's flights resumed operation Tuesday after a one-hour nationwide stoppage, which the U.S. airline said was caused by a firewall failure.

The Dallas-based carrier said a vendor-supplied firewall went down Tuesday morning and connection to some operational data was "unexpectedly" lost, forcing it to temporarily pause all flights.

Data from flight tracker FlightAware shows 43% of Southwest's flights are delayed, thus far. Many of the delayed flights are expected to be canceled later in the day.

The airline asked those whose flights are canceled to request a refund.

Shares of the airline fell 1.4% in mid-day trade on Tuesday.

Southwest has been under fire ever since a staffing crisis due to bad weather during the Christmas holidays overwhelmed its crew scheduling software, disrupting travel plans for two million customers.

The holiday disruption led to over 16,700 flight cancellations, costing the company more than $1 billion.

A sharp rebound in travel demand from the pandemic lows has exposed the fragility of the aviation system.

The Federal Aviation Administration (FAA) had to halt flights nationwide in January due to a systems outage.

The National Transportation Safety Board is investigating a series of serious close calls, including a near collision in January between FedEx and Southwest planes in Austin, Texas, where the jets came within 100 feet of each other, and a runway incursion at New York's John F. Kennedy Airport involving an American Airlines plane.

The near-miss incidents prompted the FAA last month to issue a safety alert to airlines, pilots and others about the "need for continued vigilance and attention to mitigation of safety risks."